Effects of Soil Texture on
Below ground Carbon and Nutrient
Storage in a Low land Amazonian
Forest Ecosystem
Wh en d ee L. Silver,
1,2* Jason Neff,
3Megan McGrod d y,
1Ed Veld kam p ,
4Mich ael Keller,
2an d Raim u n d o Cosm e
51Dep artm en t of En viron m en tal S cien ces, Policy, an d Man agem en t, Un iversity of Californ ia, 151 Hilgard Hall, Berk eley,
Californ ia 94720, US A ;2T h e In tern ation al In stitu te of Trop ical Forestry, US DA Forest S ervice, Call Box 25000 Rio Pied ras,
Pu erto Rico 00928, US A ;3Dep artm en t of Biological S cien ces, S tan ford Un iversity, S tan ford , Californ ia 94305-5020, US A ; 4In stitu t fu er Bod en k u n d e u n d Wald ern aeh ru n g, Un iversitaet Goettin gen , Bu esgen w eg 237077 Goettin gen , Germ an y;
5EMBRA PA A m az oˆn ia Orien tal, S an tare´m , Para´, Braz il
A
BSTRACTSoil textu re plays a key role in below grou n d C storage in forest ecosystem s an d stron gly in flu en ces n u trien t availability an d reten tion , particu larly in h igh ly w eath ered soils. We u sed field data an d th e Cen tu ry ecosystem m odel to explore th e role of soil textu re in below grou n d C storage, n u trien t pool sizes, an d N flu xes in h igh ly w eath ered soils in an Am azon ian forest ecosystem . Ou r field resu lts sh ow ed th at san dy soils stored approxim ately 113 Mg C h a-1to a 1-m depth versu s 101 Mg C h a-1 in clay soils. Coarse root C represen ted a large an d sign ifican t ecosystem C pool, am ou n tin g to 62% an d 48% of th e su rface soil C pool on san ds an d clays, respectively, an d 34% an d 22% of th e soil C pool on san ds an d clays to 1-m depth . Th e qu an tity of labile soil P, th e soil C:N ratio, an d live an d dead fin e root biom ass in th e 0–10-cm soil depth de-creased alon g a gradien t from san ds to clays, wh ereas th e opposite tren d w as observed for total P, m in eral N, poten tial N m in eralization , an d den itrification
en zym e activity. Th e Cen tu ry m odel w as able to predict th e observed tren ds in su rface soil C an d N in loam s an d san ds bu t u n derestim ated C an d N pools in th e san ds by approxim ately 45% . Th e m odel predicted th at total below grou n d C (0–20 cm depth ) in san ds w ou ld be approxim ately h alf th at of th e clays, in con trast to th e 89% w e m easu red. Th is discrepan cy is likely to be du e to an u n derestim a-tion of th e role of below grou n d C allocaa-tion w ith low litter qu ality in san ds, as w ell as an overestim a-tion of th e role of ph ysical C proteca-tion by clays in th is ecosystem . Ch an ges in P an d w ater availability h ad little effect on m odel ou tpu ts, w h ereas addin g N greatly in creased soil organ ic m atter pools an d produ ctivity, illu stratin g th e n eed for fu rth er in tegra-tion of m odel stru ctu re an d tropical forest biogeo-ch em ical cyclin g.
Ke y w o rd s: roots; soil carbon ; cen tu ry m odel; soil
textu re; biogeoch em istry; tropics.
I
NTRODUCTIONSoil textu re exerts a stron g in flu en ce on m an y h ydrologic an d biogeoch em ical processes in forest
ecosystem s by affectin g th e ability of soils to retain C, w ater, an d n u trien t ion s (Jen n y 1980). For th ese reason s, soil textu re is also a key param eter in m odels of terrestrial biogeoch em istry, w h ich gen er-ally sh ow th at soil organ ic m atter (SOM) in creases lin early w ith clay con ten t at region al an d global scales (Parton an d oth ers 1993; Sch im el an d oth ers
Received 3 March 1999; accepted 27 Au gu st 1999.
*Corresponding author; e-mail:w silver@n atu re.b erk eley.ed u
r2000 Sprin ger-Verlag
1994). Despite th e w ell-dem on strated im portan ce of soil textu re, m an y qu estion s rem ain abou t th e role of textu re in below grou n d C allocation , soil C storage, ion exch an ge capacity, an d ecosystem scale processes, su ch as prim ary produ ctivity an d decom -position . Th is is especially tru e for low lan d tropical forests, w h ich exh ibit con siderable spatial h eteroge-n eity ieteroge-n soil textu re at both local aeteroge-n d regioeteroge-n al scales (Cu evas an d Medin a 1986, 1988; Matson an d Vi-tou sek 1987; Moraes an d oth ers 1995).
Th e in teraction s of soil textu re an d biogeoch em i-cal cyclin g are com plex. Clay soils can facilitate th e form ation of passive C pools w ith slow tu rn over tim es du e to th e ph ysical protection of SOM by clay m in erals (Ch risten sen 1992). Clay soils also ten d to h ave h igh er cation exch an ge capacity, n et prim ary produ ctivity (NPP), an d litter decom position rates in th e tropics u n der n atu ral con dition s (Ueh ara 1995). San dy soils are often associated w ith h igh fin e root biom ass in tropical forests du e to greater C alloca-tion to roots for n u trien t an d w ater captu re (Klin ge 1973b, 1975; Cu evas an d Medin a 1988). San dy soils m ay also h ave slow er litter tu rn over rates du e to n u trien t an d w ater lim itation s on decom position (Cu evas an d Medin a 1986).
Soil textu ral properties vary in respon se to w eath -erin g rates an d th e in itial m in eralogy of th e paren t m aterial or th e deposition of m aterial from stream s, ru n off, an d erosion . In gen eral th erefore, soil tex-tu re ten ds to ch an ge at local scales alon g topo-graph ic gradien ts an d at lan dscape or region al scales associated with ch an ges in paren t m aterial or weath -erin g rates. Th e n atu re of soil-form in g processes th at lead to spatial h eterogen eity in soil textu re are also gen erally associated w ith differen ces in m icroan d m acroclim ate, vegetation , icroan d h ydrologic in -pu ts an d exports m akin g it difficu lt to con trol for textu re w h ile h oldin g several oth er en viron m en tal variables con stan t.
In th is stu dy, w e u se both an em pirical an d a m odelin g approach to exam in e th e relation sh ip of soil textu re to pattern s in below grou n d C an d n u trien t pools an d N tran sform ation rates in a low lan d Am azon ian forest. Th e Am azon basin is a geologically old region th at is diverse w ith regards to soil textu re (Fu rch an d Klin ge 1978; Moraes an d oth ers 1995). Pattern s in biogeoch em ical an d eco-logical processes w ith soil textu re h ave been de-scribed alon g topograph ic gradien ts in Am azon ian forests (Cu evas an d Medin a 1986, 1988; Matson an d Vitou sek 1987; Livin gston an d oth ers 1988; Vitou sek an d Matson 1988; Medin a an d Cu evas 1989), an d larger, region al-scale gradien ts (McKan e an d oth ers 1995; Moraes an d oth ers 1995). In th is stu dy, w e w ere in terested in h ow below grou n d
processes varied in relation to soil textu re w h ile h oldin g lan dscape position , clim ate, an d cover type relatively con stan t. Ou r site occu rred at th e con tact betw een a relic deposition al su rface an d an u plan d lan dscape, all of w h ich is n ow u plan d, Tierra Firm e forest w ith little or n o topograph ic variation .
We w ere also in terested in u sin g a m odelin g approach to test ou r u n derstan din g of th e processes con tribu tin g to pattern s in below grou n d C an d n u trien t pools in tropical forests. Soil textu re is often u sed as a prim ary param eter con trollin g SOM stabilization in biogeoch em ical m odels. To exam in e th e role of textu re in greater detail, an d to test m ech an ism s by wh ich textu re in flu en ces th e biogeo-ch em istry of a m oist low lan d tropical forest, w e u sed th e Cen tu ry biogeoch em istry m odel (Parton an d oth ers 1987) to sim u late th e forest on th e differen t soil textu ral classes at ou r site. Th e Cen tu ry m odel origin ally w as developed for u se in tem perate grasslan ds an d agricu ltu ral system s bu t h as sin ce been exten ded an d re-param eterized for u se in tem perate an d tropical forests (San ford an d oth ers 1991; Com in s an d McMu rtrie 1993; Vitou sek an d oth ers 1994, Raich an d oth ers 1997), as w ell as for an alyses of global biogeoch em ical dyn am ics (Sch im el an d oth ers 1997). Wh ereas th e applicability of Cen tu ry h as been tested exten sively for tem perate ecosystem s, less testin g of th e m odel h as been don e for tropical ecosystem s (bu t see Vitou sek an d oth ers 1994; Gijsm an an d oth ers 1996; Raich an d oth ers 1997). Ou r m otivation for u sin g Cen tu ry is tw ofold. First, w e exam in e th e ability of th e m odel to captu re th e tren ds w e observe alon g ou r soil textu re gradi-en t. Th is exercise is h elpfu l both becau se it allow s u s to evalu ate ou r u n derstan din g of textu ral con trols on tropical forest biogeoch em istry, an d becau se it represen ts a validation test for a key param eter in flu en cin g th e beh avior of th is m odel. Secon d, w e exam in e th e poten tial role of feedbacks am on g textu re, n u trien ts, an d w ater in th e m odel by exam in in g m odel sen sitivity to ch an ges in N, P, an d w ater availability.
M
ETHODSSite Descrip tion
lim ited drain age n etw ork form ed on sedim en ts of th e Barreiras form ation . Th e u n derlyin g bedrock geology con sists of slan ted beds of sedim en tary rocks deposited as a flu vial-lacu strin e sequ en ce. Th e soil textu ral differen ces at th is site m ost likely reflect th e u n derlyin g sedim en tary bedrock th at con sists of altern atin g san dy an d clayey beds.
Soil textu ral classification of th e larger stu dy area w as determ in ed by field in spection of sam ples from 551 con trol poin ts spaced at 50-m in tervals in a regu lar pattern th rou gh ou t th e 1000-h a area (5 x 2 km ). In total, tran sects covered approxim ately 28 km . A lim ited n u m ber of poin ts w as skipped be-cau se of th e difficu lty of access in clu din g a sm all area of poorly drain ed soil. Soils w ere divided in to tw o categories: (a) th ose h avin g clay an d clay loam textu re (fou n d at 375 poin ts); an d (b) th e rem ain der h avin g coarser textu res (m ain ly san d an d san dy loam fou n d at 176 poin ts). Assu m in g th e regu lar tran sect pattern is represen tative of th e area, approxi-m ately 68% of th e area su rveyed is on clays or clay loam s (u ltisols an d oxisols) an d 32% on san ds an d san dy loam s (u ltisols).
Field Sam p lin g
We sam pled su rface soils (0–10 cm depth ), forest floor, an d fin e root biom ass alon g six 10 x 60–m tran sects located perpen dicu larly to a 400-m gradi-en t from san d to clay soils. Tran sects w ere located at 0 an d 50 m (san ds), 150 an d 200 m (san dy loam to clay loam ), an d 350 an d 400 m (clays) alon g th e textu ral gradien t (Figu re 1). Addition al sam ples of forest floor an d fin e root biom ass w ere taken on san ds an d clays to facilitate com parison of th ese tw o textu ral extrem es (see below ). To estim ate soil ch em ical an d ph ysical properties an d root C, N, an d P to 1 m depth , w e took sam ples from five large 3 x 1 x 1–m qu an titative soil pits ran dom ly located at 0, 50, 200, 350, an d 400 m alon g th e textu re gradien t an d 18 addition al 1 x 1 x 1–m soil pits w ith n in e each on san d an d clay soils.
Su rface Soil Ph ysical an d Ch em ical
Prop erties alon g th e Tran sects
Soils w ere sam pled from th e 0–10-cm depth by u sin g a 2.5-cm diam eter soil corer. We took m u ltiple core sam ples at each of five ran dom ly selected poin ts alon g each tran sect yieldin g 30 aggregate soil sam ples. Sam ples w ere refrigerated im m ediately after collection an d u n til an alyses cou ld be con -du cted (approxim ately 3–7 days). Roots an d litter w ere m an u ally rem oved from field m oist sam ples, w h ich th en w ere passed th rou gh a 2-m m sieve. Soil textu re was determ in ed u sin g a Bou you cos h ydrom -eter in a soil su spen sion of 50 g of soil in 1 L of H2O (Gee an d Bau der 1986). Sam ples w ere pretreated
w ith h eat an d H2O2to rem ove organ ic m atter an d w ith NaHMP as a dispersal agen t to m in im ize foam in g. Soil pH w as determ in ed on fresh soils in a slu rry of 4 g in 8 m L deion ized H2O.
We m easu red den itrification en zym e activity (DEA) accordin g to Tiedje (1994) w ith som e m in or ch an ges becau se of th e h igh activity m easu red. A 10-g sam ple of field m oist soil w as placed in a 250-m L Erlen m eyer flask an d 25 m L of a solu tion con tain in g 1 m M glu cose, 1 m M NaNO3an d 1 g L-1 ch loram ph en icol w as added. Th e flasks th en w ere evacu ated an d flu sh ed six tim es w ith He to produ ce an an aerobic en viron m en t. Acetylen e (m ade from calciu m carbide) w as added to a fin al con cen tration of 10 kPa. Th e flasks w ere in cu bated du rin g a 30-m in an aerobic assay on a rotary sh aker (100 rpm ). Head space gas w as sam pled after 10, 20, 30, an d 40 m in u tes an d an alyzed for N2O on a gas ch rom atograph w ith electron captu re detector (Keller an d Rein ers 1994). Th e DEA w as calcu lated
from th e lin ear in crease in N2O takin g th e dissolved N2O in to accou n t by u sin g th e Bu n sen relation sh ip. Poten tial n et N m in eralization an d n et n itrifica-tion were m easu red u sin g aerobic laboratory in cu ba-tion s (Hart an d oth ers 1994). Tw o 15-g su bsam ples of field m oist soils w ere w eigh ed in to plastic Nal-gen e bottles. On e set of su bsam ples w as im m edi-ately extracted w ith 100 m L of 2 M KCl; th e oth er set w as covered w ith perforated parafilm an d in cu -bated at am bien t tem peratu res (approxim ately 25oC) for 7 days before extraction . Net n itrification an d N m in eralization w ere an alyzed an d calcu lated accord-in g to Hart an d oth ers (1994).
Sam ples for oth er ch em ical an alyses w ere air-dried an d grou n d to pass th rou gh a 2-m m sieve. Approxim ately 5-g sam ples w ere extracted sepa-rately w ith 55 m L 1 M NH4Cl for exch an geable cation con cen tration s an d w ith 50 m L of NH4F for extractable P (Olsen an d Som m ers 1982) by u sin g a vertical vacu u m extractor (Joh n son an d oth ers 1991). Sam ples w ere an alyzed for exch an geable Ca, Mg, K, Al, Mn , Fe, an d P on a Direct Cu rren t Plasm a Spectraspan V spectroph otom eter, Fu llerton , CA USA at th e In tern ation al In stitu te of Tropical For-estry (IITF). For total C an d N, sam ples w ere regrou n d to a fin e pow der an d an alyzed u pon com bu stion by u sin g a Fison s CN an alyzer, Su ffolk, UK. Stan dard referen ce m aterial, procedu ral blan ks, an d replicate sam ples w ere ru n for qu ality assess-m en t at th e rate of 1 per 10 saassess-m ples. Su bsaassess-m ples of all soils w ere dried at 105oC to determ in e m oistu re con ten t. All data reported h ere are on an oven dry soil basis. Bu lk den sity of th e 0–10-cm depth w as determ in ed at th ree ran dom poin ts on each of th e six tran sects alon g th e gradien t (n518) by u sin g a
kn ow n volu m e bu lk den sity corer. Sam ples w ere dried at 105oC to a con stan t w eigh t an d w eigh ed for bu lk den sity determ in ation s.
Forest Floor Mass an d Elem en tal Con ten t
Th e forest floor w as sam pled at five ran dom loca-tion s alon g each tran sect (n 5 30), an d at 10
addition al ran dom location s each on clay an d san dy soils by u sin g a 15 x 15–cm tem plate (in side area). Th e forest floor con tain ed recen t litter an d h u m ified organ ic m atter. Sam ples were dried at 65oC, weigh ed to determ in e m ass, grou n d in a Wiley m ill, an d redried. Grou n d sam ples w ere predigested in H2O2 an d th en digested in con cen trated HNO3by u sin g a block digestor (Lu h Hu an g an d Sch u lte 1985) an d an alyzed at Boston Un iversity for Ca, Mg, K, Al, P, Fe, an d Mn at IITF an d for total C an d N. Stan dard referen ce m aterial (citru s an d apple leaves), proce-du ral blan ks, an d replicate sam ples w ere ru n for qu ality assessm en t at th e rate of 1 per 10 sam ples.
Fin e Root Biom ass
Fin e root stan din g stocks (0–10 cm depth ) w ere sam pled from five ran dom location s alon g each tran sect (n 5 30) by u sin g a root corer of 4.1 cm
in side diam eter (Vogt an d Perrson 1991; Silver an d Vogt 1993). Cores w ere refrigerated u n til th ey w ere processed by sortin g live an d dead roots by size class (less th an or equ al to 2 m m diam eter; greater th an 2–5 m m diam eter) from w ash ed sieves. Th is tech -n iqu e yielded few sam ples i-n th e greater th a-n 2–5-m m size class, so th ese data are n ot reported h ere. In stead, w e u se data from th e large qu an tita-tive pits described below to estim ate th e coarse root fraction . Root sam ples w ere dried at 65oC an d w eigh ed to determ in e m ass. We took 10 addition al ran dom ly located sam ples for fin e root stan din g stocks (less th an or equ al to 2 m m diam eter) on both san d an d clay soils (n 5 20). To ch aracterize
pattern s in fin e root stan din g stocks by depth , w e took th ree root cores from both th e 20–30-cm an d th e 30–40-cm depth s each on san d an d clay soils (n512). All cores w ere processed as above. Root C
an d N w ere m easu red on a C.E. In stru m en ts CN an alyzer at U.C. Berkeley, an d root P w as m easu red at IITF after a H2O2-H2SO4 digest (Parkin son an d Allen 1975).
Coarse Roots
A total of 23 large qu an titative pits w ere excavated for coarse root biom ass. We sam pled th e five pits located at 0, 50, 200, 350, an d 400 m alon g th e soil textu re gradien t, as w ell as 18 1 x 1 x 1–m pits w ith n in e each on san d an d clay soils. Each pit w as sam pled for root biom ass at th e 0–10-cm , 10–40-cm , an d 40–100-10–40-cm depth s. In th e 0–10-10–40-cm depth , w e separately w eigh ed th e m ass of fin e roots th at cou ld be easily separated from soil in th e field. Th is tech n iqu e u n dou btedly exclu ded a proportion of th e very fin e (less th an 1 m m ) roots an d root fragm en ts, w h ich are better sam pled u sin g th e corin g tech n iqu e m en tion ed above. At th e tw o deeper depth s, fin e root biom ass w as low, so w e took sam ples of total root biom ass com bin in g all size classes togeth er. Roots w ere clean ed of adh erin g soil an d w eigh ed in th e field by u sin g a sprin g balan ce (650 g), an d su bsam pled for m oistu re determ in
Soil Ch em ical an d Ph ysical Prop erties
to 1 m Dep th
Soil textu ral properties, bu lk den sity, soild15N, an d soil cation , P, C, an d N con ten t w ere determ in ed in 1-m deep qu an titative soil pits located alon g th e soil textu re gradien t an d in addition al pits located on san ds an d clays. An aggregate soil sam ple w as collected at th e 0–10-cm , 30–40-cm , an d 90– 100-cm depth s from pits located at 0, 50, 200, 350, an d 400 m alon g th e gradien t (n515) for textu ral
properties, exch an geable cation con cen tration s, ex-tractable an d total P, an d pH. Soil pH w as deter-m in ed in w ater as above, an d in a slu rry of 4 g of soil in 8 m L of 1 M KCl. Soil textu re, exch an geable cation s, an d extractable P w ere m easu red as above. To estim ate total P con cen tration s, w e digested approxim ately 5 g of soil in H2O2an d con cen trated H2SO4 by u sin g a block digestor (Parkin son an d Allen 1975). Solu tion s w ere an alyzed at IITF. Stan -dard referen ce soils, replicate sam ples, an d blan ks w ere u sed for qu ality con trol.
Total C an d N w ere estim ated from su bsam ples collected at 10-cm -depth in crem en ts (0–100 cm ) from th e pits at 0 m (san d), 200 m (loam ), an d 350 m (clay) alon g th e textu re gradien t. We also took sam ples from six addition al pits each on clay an d san d soils at 7 cm , 40 cm , an d 100 cm depth . We fit a logarith m ic cu rve to th ese valu es an d in terpolated th e soil C an d N con cen tration s for th e in term ediate 10cm depth in crem en ts. Th e m ean squ ared residu -als for fitted C cu rves w ere 0.98 (60.01) for clays
an d 0.94 (60.03) for th e san ds, an d for N th ey w ere
0.95 (60.02) for both san ds an d clays. All sam ples
w ere an alyzed on a CN an alyzer at U.C. Berkeley an d in clu ded replicate sam ples an d referen ce soils. We in terpolated total an d extractable P con cen tra-tion s by depth by u sin g th e valu es from th e 0–10-cm , 30–40-0–10-cm , an d 90–100-0–10-cm sam ples from pits alon g th e textu re gradien t. Total P did n ot vary sign ifican tly w ith depth in th e clays, so w e u sed a m ean valu e for all depth s. Total P follow ed a decreasin g pattern w ith depth in th e san ds (r2 5
0.8460.01). Total P data reported h ere represen t a
sm all sam ple size (n54) an d th u s sh ou ld be view ed
w ith cau tion .
Bu lk den sity by depth w as sam pled on san ds (50 m ) an d clays (350 m ) at 10-cm -depth in cre-m en ts by u sin g a kn ow volu cre-m e bu lk den sity corer. Roots an d rocks w ere rem oved, an d th e sam ples w ere dried at 105oC to a con stan t w eigh t an d w eigh ed to determ in e m ass. To estim ate th e m ass of soil C, N, an d P to a depth of 1 m , w e m u ltiplied th e con cen tration s by depth w ith th e bu lk den sity estim ation s in san d an d clay soils. Valu es w ere
su m m ed by depth to produ ce pool estim ates for specific depth in tervals. We h ad n o bu lk den sity estim ates for su bsu rface loam soils, an d so th ese soils w ere exclu ded from th is an alysis.
Soils w ere sam pled ford15N from vertical profiles at 0 m (san d), 200 m (loam ), an d 350 m (clay) alon g th e gradien t. We took on e sam ple from th e su rface to 2-cm , 2–5 cm , 5–10 cm depth s an d su bsequ en t 10-cm -depth in crem en ts to 1 m in depth . Addi-tion al sam ples (n 5 4–6) w ere taken from th e
su rface an d 90–100-cm depth s in th e san ds (0 m ) an d clays (350 m ). Soils w ere air-dried im m ediately after field sam plin g an d again at 60oC to a con stan t w eigh t. Soils th en w ere grou n d to a pow der an d an alyzed for d15N at th e Natu ral Resou rce Ecology Lab at Colorado State Un iversity on a VG Isoch rom Mass Spectrom eter, Fran klin , MA, USA w ith a dedicated Carlo Erba sam ple preparation system . Th e 15N abu n dan ce is expressed in delta u n its relative to th e 15N/14N ratio (R) of atm osph eric N (15N air/14N air50.0036765). Th ed15N is calcu lated asd15N5[(R
sam ple/ Rstan dard)-1] x 1000].
Mod el Ap p lication
We u sed th e Cen tu ry biogeoch em istry m odel (Par-ton an d oth ers 1987) for ou r m odelin g exercise. Cen tu ry h as a th reecom partm en t SOM m odel con -sistin g of active, slow an d passive pools of organ ic m atter. Tem peratu re, m oistu re, an d n u trien t con -strain ts con trol th e m ovem en t of C from plan t pools to SOM pools, as w ell as betw een SOM pools. In addition , th e m ovem en t of C from th e slow to passive organ ic m atter pools in creases proportion -ally w ith clay con ten t. We in itialized Cen tu ry by u sin g param eters from tropical forest sim u lation s don e by Vitou sek an d oth ers (1998) an d Raich an d oth ers (1997) w ith rain fall an d tem peratu re data from th e TNF. Th e tropical forest param eterization s differ from th e origin al grasslan d an d forest m odels in th e stoch iom etry of C:N ratios an d th e relatively h igh er N in pu ts. On e of th e m ain biogeoch em ical differen ces between tem perate system param eteriza-tion s an d th e TNF is th e lack of paren t m aterial P in Brazil. For th ese ru n s, w e in itialized th e m odel w ith n o paren t m aterial P an d h igh P sorption capacity to sim u late th e very low P con dition s of th is low lan d tropical forest.
th e m ost sen sitive aspects of th e m odel is th e stron g poten tial N lim itation to prim ary produ ctivity (Vi-tou sek an d oth ers 1998). By adju stin g N in pu ts, w e cou ld alter th e steady state con ten t of SOM for th e clay soil to ach ieve a good fit to ou r observed data. Th e equ ilibriu m valu es of N addition s requ ired to obtain reason able SOM C an d N valu es for th e clay site w ere addition s of 2.2 kg N h a-1 y-1. Wh en th e m odel in pu ts w ere set for th e clay site, w e altered on ly th e bu lk den sity an d textu re to reflect th e con dition s in th e loam an d san dy soils.
On ce Cen tu ry w as param eterized for th e clay site, w e th en com pared th e equ ilibriu m soil an d root C, N, an d P con ten ts from th e m odel to m easu red valu es for san d an d loam soils. To system atically explore th e m odels sim u lated lim itation s to plan t grow th , an d con cu rren tly exam in e th e sen sitivity of Cen tu ry to w ater an d n u trien t availability in th e san ds, w e ran experim en ts w h ere w e dou bled N in pu ts (from 2.2 to 4.4 kg h a-1y-1), P in pu ts (from 1.1 to 2.2 kg h a-1y-1), an d rootin g depth (60–120 cm depth ). Becau se w e cou ld n ot adju st w ater u se by vegetation in th e m odel, w e u sed rootin g depth as a rou gh su rrogate for w ater access an d u se, becau se rootin g depth h as been sh ow n recen tly to correlate w ith ecosystem -level w ater u se in season ally dry tropical forests (Nepstad an d oth ers 1994).
Th e Cen tu ry m odel ou tpu t u ses a fixed depth of 0–20 cm . To com pare th e Cen tu ry m odel ou tpu t w ith th e field data, w e u sed ou r m easu red bu lk den sity, C an d N valu es, an d th e m easu red an d
in terpolated NH4F-P an d total soil P con cen tration s (see above). For th e loam soil, w e u sed th e 0–10-cm bu lk den sity m easu rem en t becau se th ere w ere n o data for th e deeper soil depth s. On ly clay an d san dy soils w ere m easu red for root C, N, an d P. Coarse root data w ere collected from th e 0–10-cm , 10–40-cm , an d 40–100-cm depth s. We estim ated th e m ass of coarse root C, N, an d P for th e 0–20-cm depth by addin g on e-th ird of th e valu es for th e 10–40-cm depth to th e valu es for th e 0–10-cm depth . Becau se root biom ass ten ded to decrease w ith depth , th is is likely to be a sligh t u n derestim ation of th e pool sizes of root C, N, an d P.
Statistical An alyses
Statistical an alyses w ere perform ed u sin g Systat (Wilkin son 1990). Data w ere log tran sform ed w h en n ecessary to m eet th e assu m ption s for an alysis of varian ce (ANOVA). On e-w ay an d tw o-w ay ANO-VAs w ere u sed to determ in e sign ifican t differen ces w ith in variables alon g th e gradien t, an d w ith tex-tu re an d depth . Pairw ise com parison s u sin g th e Least Sign ifican t Differen ces protocol w ere per-form ed to determ in e w h ere sign ifican t differen ces occu rred. Pearson correlation s w ere u sed to exam -in e relation sh ips am on g soil textu re an d soil an d root properties. We also u sed sim ple an d stepw ise m u ltiple lin ear regression s to iden tify tren ds w ith soil textu re. We perform ed t-tests to exam in e differ-en ces in soil, root, an d forest floor C, N, an d P betw een san d an d clay. Residu als from all an alyses
Table 1. Exch an geable Cation s, Extractable P, Total C an d N, an d Soil Ph ysical Propertiesain th e Tapajos
Nation al Forest, Para, Brazil
Soil Prop erty
M eters alon g th e Textu re Grad ien t
0 50 150 200 350 400
Bu lk den sity (g cm23) 1.34 (0.05) ab 1.16 (0.11) abc 1.36 (0.04) a 1.14 (0.07) bc 1.23 (0.09) ac 1.02 (0.02) c
San d (% ) 80 (0.9) a 80 (0.7) a 52 (1.7) b 52 (0.6) b 37 (1.0) c 38 (1.1) c Clay (% ) 18 (0.4) a 18 (1.1) a 46 (1.7) b 45 (0.8) b 60 (1.1) c 60 (1.3) c Silt (% ) 2 (0.8) 3 (0.5) 2 (0.2) 3 (0.4) 3 (0.5) 2 (0.5) pH (H2O) 3.90 (0.03) ac 3.89 (0.08) ac 3.62 (0.14) a 3.79 (0.12) a 4.26 (0.18) bc 4.19 (0.13) c
Total C (% ) 2.81 (0.54) 2.19 (0.43) 2.18 (0.08) 2.11 (0.13) 2.66 (0.11) 2.26 (0.06) Total N (% ) 0.15 (0.02) 0.13 (0.03) 0.15 (0.02) 0.15 (0.01) 0.18 (0.01) 0.17 (0.01) Extractable P (µ g g21) 9.50 (1.44) a 6.50 (0.51) b 4.00 (0.49) c 4.28 (0.11) c 3.96 (0.25) c 3.01 (0.18) d
Ca (cm ol1
kg21
) 0.13 (0.04) 0.09 (0.02) 0.05 (0.01) 0.06 (0.02) 0.07 (0.01) 0.06 (0.01) Mg (cm ol1
kg21) , , , , , ,
K (cm ol1
kg21) 0.08 (0.01) a 0.06 (0.01) a 0.07 (0.01) a 0.06 (0.01) a 0.11 (0.01) b 0.08 (0.01) a
Mn (µ g g21) 8.06 (2.39) a 2.93 (0.74) b 2.13 (0.27) b 1.76 (0.36) b 1.72 (0.20) b 1.74 (0.29) b
Al (cm ol1
kg21) 0.85 (0.14) a 0.58 (0.03) b 1.96 (0.06) c 1.74 (0.06) c 3.25 (0.13) d 2.56 (0.25) e
a0–10 cm depth.
w ere ch ecked for n orm ality an d h om ogen eity of varian ces (Steel an d Torrie 1980). We report sign ifi-can t differen ces at th e 95% level u n less oth erw ise n oted. Valu es reported in th e text are m ean s fol-low ed by stan dard errors.
R
ESULTSPh ysical an d Ch em ical Prop erties
of Su rface Soils
Th e percen t san d an d clay varied sign ifican tly in th e 0–10-cm depth alon g th e textu re gradien t (Table 1 an d Figu re 2) ran gin g from 80% (61) san d to 60%
(61) clay. Extractable soil P con cen tration s w ere
sign ifican tly n egatively correlated w ith clay con ten t (r250.70, P,0.01; Table 2), an d th e com bin ation
of clay con ten t an d exch an geable K con cen tration s (r250.81, P,0.01). Th e correlation w ith K m ay be
du e to som e K occlu sion by th e 1:1 clays, alth ou gh K is likely to be sorbed less stron gly th an P (Grah am an d Fox 1971). A sign ifican t n egative relation sh ip also occu rred with P an d exch an geable Al con cen tra-tion s (r2 5 0.57, P, 0.01). Extractable soil P w as
very low in clays soils (3.0 µ g g-1), an d in creased by a factor of 3 in th e san ds (Table 1).
Th ere w ere n o sign ifican t tren ds in total C or N con cen tration s alon g th e soil textu ral gradien t in th e 0–10-cm soil depth , bu t th e C:N ratio decreased sign ifican tly from 18.4 in th e san ds to 13.6 in th e clays (Figu re 2). Total C an d N in th e soil w ere positively an d sign ifican tly correlated alon g th e gradien t (r2 5 0.73; P,0.01). Th e soil C:P
extractable an d N:Pextractable ratios both in creased dram atically from san ds to clay alon g th e textu re gradien t (Fig-u re 2), an d w ere sign ifican tly positively correlated w ith clay con ten t (Table 2). Total C pools in th e su rface soils ran ged betw een 23 (60.6) an d 38
(67.2) Mg C h a-1 an d did n ot follow a sign ifican t tren d w ith soil textu re.
Nitrogen pools an d flu xes varied sign ifican tly w ith soil textu re. Am m on iu m -N con cen tration s w ere sign ifican tly low er in th e loam soils th an in th e san ds (P50.05), w h ereas NO3-N w as sign ifican tly
h igh er in th e loam soils th an in th e san ds (Figu re 3). Net n itrification rates in creased sign ifican tly from 0.68–0.72 µ g g-1 d-1 in th e san ds to 1.91–2.29 µ g g-1d-1in th e clays (Figu re 3). Net n itrification rates
Figu re 2. Soil ch em ical an d ph ysical properties in th e 0–10-cm depth alon g a soil textu re gradien t in th e Tapajos Nation al Forest, Brazil. (A) Textu re; (B) C:N ratio; (C) N:P ratio; (D) C:P ratio. To calcu late ratios, w e u sed total C an d N an d extractable P.
Table 2. Correlation Coefficien ts of th e Relation sh ip of Soil Ch em ical an d Ph ysical Properties, Fin e Root Biom ass, an d Forest Floor Biom ass to Clay Con ten t in th e Tapajos Nation al Forest, Para, Brazil
Variab le Clay (% )
Ca cm ol1
kg21 2
0.32 K cm ol1
kg21 0.47
Mn µ g g21 20.57
Al cm ol1
kg21 0.91*
P µ g g21 20.84*
C (% ) 0.21
N (% ) 0.49*
pH 0.35
Live roots (g m22) 20.47
Dead roots (g m22) 20.56*
Forest floor (kg h a21) 20.06
C:N 20.73*
C:P 0.88*
N:P 0.93*
NH4-N (µ g g21) 20.26
NO3-N (µ g g
21) 0.60
Net n itrification (µ g g21d21) 0.70*
Net N m in eralization (µ g g21d21) 0.56
DEA (n g g21h21) 0.86*
Bu lk den sity (g cm23) 20.24
explain ed m ost of th e tren d in n et N m in eralization rates, w h ich also gen erally in creased sign ifican tly from san ds to clays, w ith th e exception of th e last clay soil site alon g th e gradien t (Figu re 3). Den itrifi-cation en zym e activity in creased sign ifican tly from 77 (623) n g g-1h-1in th e san ds to 317 (624) n g g-1 h-1 in th e clays an d w as sign ifican tly positively correlated w ith in itial NH4-N con cen tration s an d clay con ten t (Figu re 3 an d Table 2).
Con cen tration s of exch an geable Al in creased sign ifican tly from san ds to clays (Table 1), an d clay con ten t alon e explain ed 84% of th e variabil-ity in exch an geable Al (Table 2). Exch an geable n u trien t cation con cen tration s w ere very low in th e su rface soils alon g th e textu re gradien t (Table 1). Exch an geable Mg w as below th e detection lim it of th e an alytical in stru m en tation (1.6 µ g g-1 or 0.013 cm ol1 kg-1). Pool sizes (kg h a-1) of m ost elem en ts follow ed sim ilar tren ds as th e elem en tal con cen tration s alon g th e gradien t. Soil pH w as sign ifican tly low er in th e loam soils th an in th e clays (Table 1).
Forest Floor an d Root Biom ass
Forest floor m ass alon g th e textu re gradien t aver-aged 6.9 (60.7) Mg h a-1 (Table 3). Alon g th e gradien t, forest floor C con cen tration s w ere greater on th e san ds th an on th e oth er soil types, an d forest floor N an d P con cen tration s decreased sligh tly bu t sign ifican tly from san ds to clays. Wh en com parin g ju st th e tw o textu ral extrem es (n540), forest floor
C, N, an d P con ten t w ere sign ifican tly greater on san ds th an on clays (Table 4). Forest floor C:N an d C:P ratios w ere very sim ilar on san d an d clays soils. Stan din g stocks of total fin e root biom ass (live plu s dead) in th e 0–10-cm depth decreased sign ifi-can tly from approxim ately 4.5 (60.4) Mg h a-1in th e san dy soils to less th an 1.9 (60.4) Mg h a-1in clays alon g th e textu re gradien t (Figu re 4). Both live an d dead fin e root biom ass follow ed th e sam e tren d w ith soil textu re; live fin e root biom ass w as very low in all soil types (0.05–0.4 Mg h a-1). San dy soils h ad sign ifican tly greater live an d dead fin e root biom ass to a depth of 40 cm th an th e clays (Figu re 4). Total fin e root biom ass w as sign ifican tly n egatively corre-lated w ith N m in eralization rates an d positively correlated w ith extractable P pools (r2 50.48; P,
0.05). Fin e root C, N, an d P con ten t w ere sign ifi-can tly greater on san dy soils th an on clays in th e top 10 cm of soil (Table 4).
Total root biom ass to a depth of 1 m ran ged from 11 to 188 Mg h a-1in th e five pits alon g th e textu re gradien t (Figu re 4) an d w as greatest in th e 0–10-cm depth at all sites. Th ere w as a gen eral pattern of in creasin g total root biom ass alon g th e gradien t from san ds to clays. Th e exception w as at m eter 50 (san ds) w h ere w e en cou n tered an extrem ely h eavy root (u p to 15 kg m-2 w et w eigh t) th at pen etrated from n ear th e su rface to 1 m in depth . Com parin g all 22 pits on san d an d clay soils (in clu din g th e tw o clay an d tw o san d pits alon g th e textu re gradien t), th ere w as sign ifican tly m ore coarse root C, N, or P in clays to a depth of 10 cm , an d greater root P in 40–100-cm depth on clays. Th e ratios of root C:N an d C:P w ere h igh er on th e san ds th an on th e clays for all soil depth s (Table 4). Th e total root C pool to 1 m depth w as approxim ately 26 6 8 Mg h a-1 on san ds an d 1762 Mg h a-1on clays.
Soil Prop erties to 1 m Dep th
Total soil C pools to 1 m depth w ere very sim ilar on clays (80 Mg C h a-163) an d san ds (81 Mg C h a-16
4; Table 4). Th ere w as sign ifican tly m ore C in th e su rface 20 cm of th e clays bu t greater C in low er 50 cm in th e san ds (P , 0.10). Total N follow ed a
differen t pattern th an C (Table 4). Th ere w as sign ifi-can tly m ore total soil N in clays to 1 m (8.5 Mg N h a-160.4) th an in th e san ds (6.6 Mg N h a-160.4),
du e prim arily to th e sign ifican tly greater pools of N in th e top 50 cm in th e clays. Total C an d N con cen tration s in th e su rface soils w ere low er w h en sam pled from th e large qu an titative pits th an from th e core sam ples. It is u n clear w h at cau sed th ese differen ces, bu t th ey cou ld resu lt from th e sm aller sam ple size an d differen ces in th e area sam pled. We estim ated a total below grou n d C pool (forest floor plu s roots plu s soil) of 113 Mg h a-1C on th e san ds to 101 Mg h a-1C on th e clays (Table 4).
Total soil P con cen tration s an d pools in th e 0–10 cm depth follow ed th e reverse tren d of extractable P. Alon g th e soil textu re gradien t, total soil P con cen tration s in creased by a factor of 4 to 5 from san ds to clays (Table 5). Total P con cen tration s ten ded to be greater at th e deeper soil depth s in th e san ds bu t follow ed n o tren d in th e loam or clays soils (Table 5).
Th ere w ere few tren ds in soil textu re w ith depth alon g th e textu ral gradien t (Table 5). San d con ten t decreased sligh tly (-10% ) w ith depth in th e san d pits w ith a correspon din g in crease in clay con ten t. Exch an geable n u trien t cation an d extractable P con cen tration s gen erally decreased w ith depth in th e soil (Table 5).
Delta 15N valu es ran ged from 6.2 to 12.7. San ds h ad sign ifican tly low er d15N th an clays in both th e su rface soils (Table 6). Th ere w ere n o statistically sign ifican t pattern s ind15N betw een th e su rface an d 1 m depth an d n o stron g pattern s w ith depth in th e san d or loam soils. In th e clays, th e top 10 cm of m in eral soil w ere sligh tly depleted ind15N relative to deeper soil depth s (Table 6).
Mod el Resu lts
Th e Cen tu ry m odel sim u lation of th e soil textu re gradien t predicted greater soil C, N, an d P in clays th an in loam s or san ds, respectively (Table 7). Cen tu ry u n derestim ated th e soil C an d N pools by 45% an d 44% , respectively, on th e san ds, bu t captu red th e tren ds of low er C an d N in th e san ds th at w e fou n d w ith ou r sam ples from th e large qu an titative pits. Cen tu ry h as several P pools in clu d-in g m d-in eral P, SOM P, secon dary P, occlu ded P, an d paren t P. It is som ew h at difficu lt to com pare th ese m odel pools directly to data from P fraction ation sch em es (Gijsm an an d oth ers 1996), an d w e lack th e data for a com preh en sive evalu ation of th e Cen tu ry P m odel; h ow ever, th ere are several in ter-estin g pattern s w orth n otin g. We report th e m od-eled SOM P fraction from Cen tu ry in com parison to ou r NH4F extractable P, forest floor P, an d total P valu es. In terestin gly, th e vast m ajority of th e P in th e m odel sim u lation s w as located in th e SOM P pool rath er th an in secon dary or occlu ded P. Field m easu rem en ts of NH4F extractable P an d forest floor P sh ow ed approxim ately th ree tim es m ore labile P in th e san ds th an in th e clays. In con trast, m odeled P m in eralization (an in dex of labile P) follow ed th e opposite pattern an d ran ged from 0.15 g P m2y-1in th e san ds to 0.2 g P m2y-1in th e clays. Th e m odel sim u lation predicted a th reefold in crease in SOM P from san ds to clay, sim ilar to th e 2.7-fold in crease w e m easu red for total P in san ds versu s clays at th is depth . Th e average pool size of total P m easu red on th e clays (398 kg h a-1) w as con siderably greater th an th e m odel SOM P ou tpu t (129 kg h a-1), an d in clu sion of paren t, occlu ded, or secon dary P pools in th e m odel do n ot ch an ge th is pattern .
Table 3. Forest Floor Mass an d Elem en tal Con cen tration s alon g a Textu re Gradien t in th e Tapajos Nation al Forest, Para, Brazil
Forest Floor Prop erty
M eters alon g th e Textu re Grad ien t
0 50 150 200 350 400
Mass (g m22) 839 (165) 840 (213) 652 (174) 564 (66) 538 (193) 720 (190)
C (% ) 50 (0.5) a 51 (0.4) a 43 (2.6) b 46 (1.3) ab 42 (1.6) b 47 (1.6) ab N (% ) 1.89 (0.2) ac 2.69 (0.3) b 2.28 (0.2) ab 2.06 (0.1) abc 1.66 (0.1) c 1.77 (0.3) c C:N 27 (2.5) 20 (1.8) 19 (1.7) 23 (1.7) 27 (2.1) 31 (8.0) P (m g g21) 0.49 (0.04) ab 0.67 (0.10) a 0.55 (0.06) a 0.53 (0.06) a 0.53 (0.03) a 0.37 (0.05) b
Ca (m g g21) 3.97 (0.86) 5.78 (1.41) 6.02 (0.66) 7.88 (0.94) 5.45 (0.82) 4.49 (0.79)
Mg (m g g21
) 1.19 (0.11) 1.73 (0.37) 1.25 (0.10) 1.47 (0.14) 1.67 (0.18) 1.74 (0.45) K (m g g21) 1.50 (0.38) 1.81 (0.31) 1.63 (0.17) 2.53 (0.35) 1.62 (0.25) 1.16 (0.12)
Mn (m g g21
) 0.22 (0.05) 0.42 (0.10) b 0.51 (0.08) 0.24 (0.02) 0.25 (0.02) 0.33 (0.13) Al (m g g21) 0.46 (0.11) a 0.75 (0.33) ab 2.07 (0.45) c 1.87 (0.74) bc 6.31 (0.82) d 3.66 (0.66) cd
Th e m odel gen erally estim ated fin e an d coarse root C con ten t w ith in 20% of th e m easu red valu es, except for overestim atin g fin e root C in th e clays by 67% . Th e m odel closely predicted fin e root N an d P con ten t in clays, bu t u n derestim ated th ese in th e san ds. We m easu red greater coarse root N an d P in both clay an d san dy soils th an Cen tu ry predicted, bu t Cen tu ry accu rately captu red th e tren ds of greater coarse root N an d P in clays th an in san ds (Table 7). Cen tu ry u n derestim ated th e total below grou n d C (root plu s soil) in th e san ds by 53% . Abovegrou n d biom ass C on san ds w as predicted to be 20% low er th an on clays, w h ereas th e total forest C pool w as 30% low er on san ds th an on clays. Cen tu ry pre-dicted th at total forest C w ou ld in crease from san ds to loam s to clays (Table 7).
Table 4. Forest Floor, Root, an d Total Soil C, N, an d P Pools by Depth in San d an d Clay Soils in th e Tapajos Nation al Forest, Para, Brazil
Dep th San d s Clays
P,
0.05
Forest floor
Litter C 4.39 (0.47) 3.10 (0.47) * Litter N 0.18 (0.02) 0.13 (0.02) * Litter P 4.44 (0.60) 3.13 (0.50) * Litter C:N 24.80 (2.03) 25.70 (2.65) Litter C:P 1030 (102) 1067 (92.8) 0–10 cm
Soil C 12.07 (0.68) 16.46 (1.25) * Soil N 0.89 (0.06) 1.48 (0.07) * Soil P 67.0 (5.00) 212.5 (19.5) * Soil C:N 13.03 (0.87) 9.95 (0.42) * Fine root C 1.48 (0.11) 1.06 (0.10) * Fine root N 0.05 (0.004) 0.03 (0.003) * Fine root P 1.59 (0.11) 0.80 (0.07) * Coarse root Ca 6.00 (3.25) 6.77 (1.02) * Coarse root N 0.09 (0.05) 0.18 (0.03) * Coarse root P 2.99 (1.42) 4.49 (0.64) * Root C:N 72.37 (3.58) 38.72 (0.52) * Root C:P 1888 (116.3) 1515 (59.8) * 10–40 cm
Soil C 29.78 (1.84) 32.02 (1.69) Soil N 3.44 (0.21) 3.79 (0.27) * Soil P 277.5 (5.30) 569.0 (18.94) * Soil C:N 11.66 (1.04) 8.53 (0.55) * Total root C 13.88 (4.46) 6.35 (0.92) Total root N 0.20 (0.06) 0.17 (0.02) Total root P 5.18 (1.67) 5.22 (0.75) Root C:N 69.67 (0.07) 37.64 (0.09) * Root C:P 2682 (10.1) 1216 (0.10) * 40–100 cm
Soil C 39.28 (2.21) 31.68 (1.76) * Soil N 3.44 (0.21) 3.74 (0.26) Soil P 563.4 (0.51) 894.8 (5.46) * Soil C:N 11.56 (1.06) 8.55 (0.52) * Total root C 6.00 (2.44) 3.71 (1.13) Total root N 0.09 (0.04) 0.07 (0.02) Total root P 0.93 (0.38) 2.01 (0.61) * Root C:N 63.94 (0.04) 53.99 (0.04) * Root C:P 6523 (32.4) 1848 (1.40) * 0–100 cm
Total
below-ground C 112.88 — 101.15 — Total
below-ground N 8.38 — 9.59 — Total
below-ground P 923.06 — 1691.90 —
aP50.05. C and N values are in Mg ha21; values for P are in kg ha21. Fine roots
are#2 mm diameter; coarse roots are .2 mm diameter. Asterisks signify statistically significant differences between soil texture types by using a 2 sample t-test. Standard errors are in parentheses.
Figu re 4. Th e distribu tion of fin e an d coarse root biom ass in th e Tapajos Nation al Forest, Brazil. A Th e proportion of live an d dead fin e roots in th e su rface soils alon g th e soil textu re gradien t. Error bars represen t 61 SE. B Th e
Th e Cen tu ry m odel also in clu des param eteriza-tion s of losses du e to leach in g an d volatilizaeteriza-tion . Nitrogen losses gen erally follow ed th e sam e tren ds as for total SOM N pools w ith h igh er rates of N
Th e Effects of Soil Textu re on Soil
Nu trien t Pools
Th e pool sizes an d distribu tion of soil P an d N varied sign ifican tly alon g th e 400-m soil textu re gradien t in th is forest. San dy soils h ad m ore extractable P th an th e loam an d clay-rich soils. In w ell-aerated soils su ch as th ese, P is easily com plexed w ith exch an geable Al an d Fe in th e m in eral soil, sh ow n by th e sign ifican t n egative correlation of exch an ge-able Al an d P alon g th e gradien t as clay con ten t in creased. Th ere w as sign ifican tly greater total P w ith in creasin g clay con ten t alon g th e gradien t, probably du e to th e form ation of Fe an d Al ph os-ph ates an d organ ically bou n d P, n eith er of w h ich
Table 5. Soil pH, Textu re, Exch an geable Cation s, Extractable (Pex) an d Total P by Depth in Five Large Qu an titative Soils Pitsa
Sand (% ) 86.25 84.34 58.81 50.56 37.70 Clay (% ) 13.75 15.18 36.87 47.04 60.36 Silt (% ) 0.00 0.48 4.32 2.40 1.93
Sand (% ) 75.22 80.48 51.52 44.79 37.12 Clay (% ) 22.39 19.52 48.48 54.25 60.00 Silt (% ) 2.39 0.00 0.00 0.96 2.88
Sand (% ) 75.27 73.32 50.14 44.91 39.31 Clay (% ) 23.77 22.38 49.86 52.69 61.69 Silt (% ) 0.95 4.30 0.00 2.40 0.00
Values represent one pooled sampled per depth. Samples differ from those in Table 1.,signifies below detection of the analytical instruments. na5 not available.
Table 6. Mean Delta15N by Depth in Th ree Soil Pitsaalon g a Soil Textu re Gradien t in th e Tapajos
Nation al Forest, Para, Brazil
are likely to be extracted w ith NH4F. Ph osph oru s is cited frequ en tly as a lim itin g elem en t in low lan d tropical forests an d h as been sign ifican tly correlated w ith rates of fin e root grow th , decom position , an d n u trien t u se efficien cy by vegetation (Cu evas an d Medin a 1986, 1988; Medin a an d Cu evas 1989; Silver 1994). Tiessen an d oth ers (1994) did a de-tailed an alysis of differen t P fraction s alon g a topo-graph ic gradien t in th e Ven ezu elan Am azon an d fou n d th at resin extractable P (th eir in dex of th e plan t available fraction ) in creased from ridges to valleys, correspon din g to a gradien t from m ore fin ely textu red soils (17% clay) to m ore coarsely textu red soils (2% clay). Th ey also fou n d m ore total an d stron g acid-extractable P in th e fin er textu red soils on ridges th an in san dier soils.
Con trary to th e gen eralized pattern s for P, N su pply gen erally is con sidered adequ ate for plan t grow th in th e low lan d tropics (Vitou sek an d San -ford 1986; Vitou sek an d Matson 1988). Notable exception s to th is in clu de qu artz san ds (psam -m en ts) in Brazil (Livin gston an d oth ers 1988) an d Ven ezu ela (Cu evas an d Medin a 1986, 1988). Alon g th e textu re gradien t, clays exh ibited h igh er in itial con cen tration s of NO3-N an d low er NH4-N th an san ds an d greater rates of poten tial n et NO3produ c-tion . Rates of poten tial n et N m in eralizac-tion an d n itrification w ere strikin gly sim ilar to th ose
re-ported by Livin gston an d oth ers (1988) an d Vi-tou sek an d Matson (1988) for Am azon ian soils alon g a topograph ic an d soil textu re gradien t n ear Man au s, Brazil. Th ey fou n d th at san dy soils of th e low er topograph ic position s exh ibited low poten tial n et NO3 produ ction an d relatively low levels of recovery of added 15NO
3com pared w ith ridge an d slope soils (Vitou sek an d Matson 1988). In th is stu dy, h igh er poten tial n itrification rates in clay soils w ere cou pled w ith greater DEA an dd15N in dicatin g th at N losses via den itrification an d/ or leach in g also m ay be affectin g N pools in th e clays. Alth ou gh den itrification is spatially an d tem porally dyn am ic, DEA gen erally is th ou gh t to be an in tegrative in dex of lon ger-term (an n u al) den itrification rates be-cau se en zym es can persist in th e soil lon ger th an actu al den itrifier activity (Groffm an an d Tiedje 1989). Nitrou s oxide flu xes an d den itrification rates h ave been sh ow n to decrease sign ifican tly w ith in creasin g san d con ten t in tem perate an d tropical forests (Matson an d Vitou sek 1987; Livin gston an d oth ers 1988; Groffm an an d Tiedje 1989), presu m -ably in part becau se san dy soils often h ave better drain age an d low er w ater h oldin g capacity leadin g to better aeration . In th is stu dy, as in th e stu dy n ear Man au s, san dy soils also h ad low er n et n itrification rates an d NO3con cen tration s, w h ich also are likely
Table 7. Cen tu ry Model Sim u lation an d Measu red Valu es of th e Below grou n d C, N, an d P Pools an d N Trace Gas Flu x in th e Tapajos Nation al Forest, Brazil
Pools or Flu xes
San d s San d y Loam s Clays
CENTURY M easu red CENTURY M easu red CENTURY M easu red
Soil organ ic m atter C Mg h a21 12.7 22.9 24.0 31.4 31.2 29.8
Soil C 23N Mg h a21 26.3 n a n a n a n a n a
Soil C 23P Mg h a21 13.1 n a n a n a n a n a
Soil C 23w ater Mg h a21 13.1 n a n a n a n a n a
Soil organ ic m atter N Mg h a21 0.96 1.70 2.12 2.44 2.90 2.79
Soil organ ic m atter P kg h a21 44.1 5; 148 94.6 8; 305 129.1 5; 398
Fin e root C Mg h a21
1.78 1.94 2.11 n a 2.24 1.34
Fin e root N Mg h a21 0.02 0.08 0.03 n a 0.03 0.03
Fin e root P kg h a21
0.84 2.07 0.99 n a 1.06 1.01
Coarse root C Mg h a21 8.63 10.63 10.2 n a 10.9 8.89
Coarse root N Mg h a21 0.06 0.16 0.07 n a 0.07 0.24
Coarse root P kg h a21 1.25 4.72 1.48 n a 1.58 6.23
Below grou n d C Mg h a21 23.1 35.5 36.3 n a 44.3 40.0
Abovegrou n d C Mg h a21 69.8 n a 83.0 n a 88.6 n a
Total forest C Mg h a21 92.9 n a 119.3 n a 132.9 n a
Trace gas loss kg N h a21y21 1.07 n a 1.31 n a 1.42 n a
to redu ce den itrifier activity (Livin gston an d oth ers 1988).
Th e d15N sign atu re of soils provides addition al in form ation abou t th e ecosystem N cycle. Du rin g decom position , SOM ten ds to becom e en rich ed in
d15N relative to plan t litter du e to fraction atin g losses of 14N du rin g m in eralization (n itrification ) an d du rin g trace gas an d leach in g losses (Blackm er an d Brem n er 1977; Mariotti an d oth ers 1982; Hea-ton 1986; Nadelh offer an d Fry 1988). Delta 15N valu es often in crease w ith depth as in creasin gly decom posed m aterial m oves dow n th rou gh th e soil profile in leach ate an d as isotopically ligh t N is lost to trace gases or leach ate. Th e d15N valu es reported h ere are sim ilar to th ose reported for oth er Am azo-n iaazo-n forest soils, bu t depth profiles, aazo-n d particu larly th e san d an d loam profiles, sh ow ed less en rich m en t th an oth er m easu rem en ts in th e Am azon basin (Piccolo an d oth ers 1994). Th e reason for th e lack of en rich m en t w ith depth in th e san d an d loam soils is n ot clear, bu t it cou ld be th e resu lt of faster dow n -w ard m ovem en t of organ ic m atter th rou gh th e san ds versu s th e clays. Th e h igh erd15N in th e clays relative to san ds is likely to resu lt from greater rates of fraction atin g N losses via den itrification an d N leach in g, w h ich is con sisten t w ith th e h igh er DEA an d n itrification rates w e observed. Both den itrifica-tion an d NO3 leach in g occu r preferen tially for 14N rath er th an 15N an d leave th e residu al SOM en -rich ed in 15N (Karam an os an d Ren n ie 1980; Kim an d Craig 1993).
Un like th e pattern s for N an d P, pools of exch an ge-able Ca an d Mg did n ot vary sign ifican tly alon g th e textu re gradien t an d w ere very low in th e su rface soils. Low exch an geable cation con ten t is com m on in Am azon ian soils (Stark 1971; Klin ge 1977; Fu rch an d Klin ge 1978; Uh l an d Jordan 1984) an d m ay be partially m oderated by h igh litter in pu ts an d tu rn -over an d rapid an d efficien t cyclin g of n u trien ts from th e forest floor (Stark an d Spratt 1977; Stark an d Jordan 1978; Cu evas an d Medin a 1986, 1988). In th is stu dy, forest floor Ca an d Mg con ten ts w ere greater th an th e exch an geable Ca an d Mg con ten t (in kg h a-1) in th e top 10 cm of m in eral soil an d are likely to be im portan t sou rces of n u trien ts to plan t roots. Th e lack of pattern alon g th e gradien t su ggests th at oth er factors, besides soil textu re, m ay exert a stron g in flu en ce on soil properties.
Pattern s in Live an d Dead Fin e Root Biom ass
Plan ts often respon d to ch an ges in n u trien t an d w ater availability by alterin g th e allocation of C to root biom ass (Cu evas an d Medin a 1988; Jackson an d oth ers 1990). High fin e root biom ass in su rface soils facilitates ecosystem n u trien t con servation
th rou gh rapid an d efficien t n u trien t captu re an d im m obilization in tissu es (Wen t an d Stark 1968; Stark an d Spratt 1977; Cu evas an d Medin a 1988; Silver an d Vogt 1993). Fin e root stan din g stocks alon g th e textu re gradien t w ere sim ilar to or greater th an valu es reported for oth er tropical forests (Klin ge 1973a, 1975; Gow er an d Vitou sek 1989; San ford 1989; Cu evas an d oth ers 1991; Silver an d Vogt 1993) an d decreased sign ifican tly from san ds to clays in su rface soils an d to 40 cm depth . We fou n d th at fin e root stan din g stocks w ere n egatively corre-lated w ith N m in eralization rates an d positively correlated w ith extractable soil P con cen tration s. Alth ou gh fin e root stan din g stocks can n ot be m ean -in gfu lly extrapolated to produ ctivity, oth er stu dies in th e Am azon basin h ave sh ow n th at root grow th respon ded to N addition s on san dy soils an d to P addition s on clays (Cu evas an d Medin a 1986, 1988). Addition ally, periodic w ater stress cau sed by rapid drain age an d low w ater h oldin g capacity in san ds m ay lead to greater fin e root biom ass th rou gh -ou t th e soil profile (Nepstad an d oth ers 1994).
Th e Distribu tion of Below grou n d C, N, an d P
Recen t stu dies h ave poin ted to th e im portan ce of estim ates of total (fin e an d coarse) root biom ass for u n derstan din g th e ecosystem C cycle an d th e rela-tive im portan ce of deeper roots (greater th an 10 cm depth ) in tropical forests (Nepstad an d oth ers 1994; Tru m bore an d oth ers 1995; Jackson an d oth ers 1996; Cairn s an d oth ers 1997). In th is stu dy, total root biom ass C in th e 0–10-cm soil depth am ou n ted to u p to 62% of th e su rface soil C pool (Table 6). Dow n to 1 m depth , root C am ou n ted to 22% of th e soil C pool on clays an d 34% on san ds. Few stu dies h ave m easu red coarse root biom ass in tropical forests, an d even few er h ave attem pted to con sider th e spatial h eterogen eity of coarse roots by takin g m u ltiple sam ples. Th e data from th e five large qu an titative pits alon g th e textu re gradien t w ere stron gly in flu en ced by on e particu larly h eavy root fou n d in th e san ds. Even w ith 11 large qu an titative pits, th ere w as still con siderable spatial variability in coarse root biom ass in san ds. Ou r data illu strate th e poten tially large con tribu tion of coarse roots to soil organ ic C an d N an d th e spatial h eterogen eity in h eren t in th is ecosystem com partm en t.
on clays m ay be able to access P n ot n orm ally th ou gh t of as biologically available or th at plan ts on clay soils h ave h igh er P-u se efficien cy th an plan ts on san dy soils. In terestin gly, th e forest floor C:N an d C:P ratios w ere sim ilar on san ds an d clays su ggestin g th at th e big differen ces in soils m ay be a resu lt of in tern al soil processes, or in pu ts from root tissu es rath er th an a direct resu lt of th e ch em ical com posi-tion of th e stan din g litter crop.
Mod elin g th e Effects of Soil Textu re
on Below grou n d C an d Nu trien t Pools
Soil textu re adds an extra layer of com plexity to th e ecosystem from both an em pirical an d m odelin g perspective. Th e Cen tu ry m odel captu res tren ds an d pool sizes in soil C for tem perate an d tropical grasslan ds on a variety of soil textu ral classes (Parton an d oth ers 1993). Ou r com parison of th e Cen -tu ry m odel ou tpu t an d th e field collected data from th e TNF dem on strates th at th e Cen tu ry m odel is relatively su ccessfu l at captu rin g th e gen eral tren ds in C an d N as w ell as th e distribu tion of C an d N betw een SOM an d below grou n d plan t pools. How -ever, th e m odel appears to be overly sen sitive to th e effect of textu re on SOM stabilization in th e TNF.
Cen tu ry represen ts th e role of textu re in SOM stabilization in tw o w ays. First, th e flow of carbon from slow tu rn over SOM to passive SOM is in -versely scaled to san d con ten t. In sites w ith h igh clay con ten t, m ore passive SOM is form ed reflectin g th e role of in creasin g soil su rface area on C protec-tion . Secon d, th e efficien cy of C tran sfers is affected by textu re w ith m ore CO2 lost du rin g tran sform a-tion s betw een pools in h igh ly san dy soils. Both of th ese m ech an ism s lead to in creased C con ten t in clays an d scale lin early w ith clay con ten t.
We m easu red su bstan tially m ore C in th e san ds th an Cen tu ry predicted for th e site based on ou r sen sitivity an alysis. Th e reason s for th is m ay be sim ple an d direct in th at Cen tu ry m ay overstate th e in flu en ce of textu re on passive SOM form ation an d efficien cies of C tran sform ation s. Altern atively, th e param eters th at govern th ese relation sh ips in Cen tu ry m ay sim ply fail to take in to accou n t n on m in -eral preservation of organ ic m atter (Hedges an d Oades 1997). Th e differen ces also cou ld be in flu -en ced by com plex in teraction s an d feedbacks be-tw een textu re, n u trien t, an d w ater availability an d forest ecoph ysiology. Plan t-soil feedbacks, su ch as in creased below grou n d C allocation togeth er w ith decreased root litter qu ality, cou ld act to m ain tain h igh er below grou n d C pools an d C reten tion tim e in san dy soils at th e TNF. Th ese pattern s n eed to be in terpreted w ith som e cau tion becau se th is is n ot a detailed stu dy of SOM stabilization in tropical soils.
In ou r m odel an alysis, w e tu n ed th e m odel to clays an d th en adju sted it to represen t san ds. Had w e ch osen th e opposite approach of tu n in g first to san ds an d th en predictin g clay elem en tal con ten t, ou r resu lts w ou ld sh ow an overestim ation of ele-m en t storage in th e clay soils. Regardless of w h eth er th e m odel is u n derrepresen tin g C storage in san ds or overestim atin g storage in clays, it is clear th at th e overall sen sitivity of C storage to textu re is sign ifi-can tly differen t in th ese tropical soils relative to th e stan dard m odel represen tation .
We tested th e sen sitivity of th e m odel to several m ech an ism s th at m ay be im portan t in determ in in g th e relation sh ip of ecosystem C an d n u trien t cyclin g to soil textu re. First, forests grow in g on san ds m ay h ave differen t stan d-level w ater-u se efficien cies. If w ater availability con strain s grow th an d/ or decom -position du rin g th e dry season , th en w ater-u se ch aracteristics m ay be im portan t in determ in in g SOC dyn am ics. As a prelim in ary assessm en t of th e m odel sen sitivity to ch an ges in w ater availability, w e in creased th e depth to w h ich plan ts can access w ater in th e m odel (for exam ple, fu n ction al rootin g depth ). Dou blin g th e fu n ction al rootin g depth from 60 cm to 120 cm resu lted in a sm all in crease in equ ilibriu m SOM con ten t, bu t th e ch an ge is n ot large an d does n ot accou n t for th e differen ces in m easu red an d m odeled SOM for th e san ds (Table 7). Th ere is good eviden ce th at season ally dry Am azon forest m ay h ave roots th at exten d several m eters in depth (Nepstad an d oth ers 1994). Deep roots m ay be qu ite im portan t for w ater u ptake an d m igh t act to m ain tain h igh er levels of produ ctivity (an d th ere-fore SOM con ten t) th an m igh t be expected for forests w ith on ly sh allow roots. Cu rren t version s of Cen tu ry an d m ost oth er ecosystem m odels do n ot h ave option s for sim u latin g deep rootin g an d as a resu lt m ay n ot in corporate an im portan t m ech a-n ism by w h ich tropical produ ctivity is m aia-n taia-n ed th rou gh th e dry season s an d du rin g drou gh t years.
dem an d for N an d P. Wh ereas th is approach m ay w ork in N-lim ited tem perate system s, it cau ses poten tial problem s in P-lim ited tropical soils. Th e in teraction s of P w ith textu re, SOM, an d produ ctiv-ity in Cen tu ry are relatively com plicated. Cen tu ry in clu des m odeled lin kages betw een P m in eraliza-tion an d N/ P availability, an d w h en plan t or m icro-bial P availability in th e m odel is low, P is m in eral-ized. On ce P en ters th e Cen tu ry organ ic pools, it can be (an d is) lost as dissolved organ ic P or sorbed in to secon dary form s in a m an n er in depen den t of biotic dem an d for P (see Vitou sek an d oth ers 1998 for a discu ssion of th ese losses). In ou r m odeled system s, Th e com bin ation of h igh rates of P m in eralization , fertility in depen den t P losses, an d n o paren t P cau ses th e system to crash w ith ou t su bstan tial atm osph eric in pu ts of P. We fou n d th at w e cou ld m ain tain reason able rates of produ ctivity as lon g as P in pu ts (from th e atm osph ere) exceeded P losses th rou gh dissolved organ ic P an d PO4 sorption , bu t th at below th is th resh old valu e, produ ctivity w ou ld approach zero. Th ese resu lts h igh ligh t th e discrepan cies betw een th e treatm en t of tropical biogeoch em -istry in sim u lation m odels an d cu rren t con ceptu al m odels of th e factors th at sh ou ld regu late tropical produ ctivity.
Th e data an d m odel sim u lation s from th e Tapajos textu ral gradien t h igh ligh t several issu es th at w ill be im portan t for evalu atin g th e role of soil textu re in tropical forest biogeoch em istry. Clearly, m ore in for-m ation is n eeded on th e relative ifor-m portan ce of both ch em ical an d ph ysical stabilization of SOM to refin e ou r u n derstan din g of th e relation sh ip of soil textu re to biogeoch em ical cyclin g. Th is m ay be critically im portan t in tropical ecosystem s w h ere m u ch of th e soil n u trien t capital is in organ ic m atter rath er th an paren t m aterial (San ch ez 1976; Tiessen an d oth ers 1994). For P, th is distin ction m ay be especially im portan t becau se of cu rren t difficu lties in determ in -in g ecologically m ean -in gfu l -in dices of labile an d organ ic P (Gijsm an an d oth ers 1996). Th e m odelin g exercises em ph asized th e relation sh ips am on g n u tri-en t availability an d SOM con ttri-en t. Alth ou gh , th is particu lar m odel w as m ost sen sitive to N, th e role of SOM in cyclin g of P an d oth er n u trien ts also are likely to be im portan t becau se SOM m ay be th e dom in an t reservoir for N, P, an d base cation s in h igh ly w eath ered tropical soils. Ou r resu lts also h igh ligh t th e im portan t con tribu tion of C allocation to roots in estim ates of below grou n d C pools. Coarse roots frequ en tly are ign ored in estim ation s of ecosys-tem C cyclin g, bu t th ey clearly con stitu te a sign ifi-can t below grou n d C pool.
Models of biogeoch em ical cyclin g w ill n eed to be fu rth er evalu ated if th ey are to be u sed su ccessfu lly
in tropical en viron m en ts, an d ou r resu lts clearly dem on strate th e n eed to better lin k con ceptu al an d sim u lation m odels of tropical elem en t cyclin g. Virtu -ally all th e m ajor biogeoch em ical m odels h ave th eir origin s in tem perate system s an d h ave been m ost exten sively tested in th ese en viron m en ts. Greater atten tion n eeds to be paid to variation in basic biogeoch em ical an d ph ysical factors, su ch as tex-tu re, P cyclin g, w ater u se, an d th e role of base cation s, for th ese m odels to be accu rate in tropical en viron m en ts.
A C K N O W LE D G E M E N TS
Th is stu dy was part of th e Brazil-led Large Biosph ere-Atm osph ere Experim en t in Am azon ia an d w as fu n ded by NASA’s Office of Earth Scien ce. Addi-tion al su pport w as provided by a gran t from th e A.W. Mellon Fou n dation to W. L. S., th e US Depart-m en t of Agricu ltu re In tern ation al In stitu te of Tropi-cal Forestry, an d a Nation al Scien ce Fou n dation Predoctoral Fellow sh ip, an d a NASA Earth System Scien ce Fellow sh ip to J.N. We w ish to th an k IBAMA an d EMBRAPA for h elp w ith logistics, fieldw ork, an d lab an alyses in Brazil, an d Su san Rain ey an d An dy Th om pson for h elp in th e lab at Un iversity of Californ ia at Berkeley. Peter Vitou sek, Vern Cole, Pam Matson , an d an an on ym ou s review er provided h elpfu l com m en ts on earlier drafts.
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