The impact of land clearing and agricultural practices on soil organic C
fractions and CO
2
efflux in the Northern Guam aquifer
P.P. Motavalli
a,∗, H. Discekici
b, J. Kuhn
caDepartment of Soil and Atmospheric Sciences, The School of Natural Resources, University of Missouri, Columbia, MO 65211, USA bAgricultural Experiment Station, College of Agriculture and Life Sciences, University of Guam, Mangilao, GU 96923, USA
cNatural Resources Conservation Service, U.S. Department of Agriculture, Maite, GU 96927, USA
Received 11 March 1999; received in revised form 11 August 1999; accepted 22 October 1999
Abstract
The importance of the Northern Guam aquifer as a source of drinking water for the tropical Pacific island of Guam has stimulated public interest in the impact of forest clearing and conversion to agriculture on the region’s environment. The objectives of this study were to determine the effects of land clearing, tillage, and fertilization of tropical secondary forest on soil organic and organic C fractions in the shallow, calcareous soil that overlies most of Northern Guam. A field experiment was established on a secondary forest site in Northern Guam to simulate land clearing, cultivation and fertilization with two separate applications of N, P and K fertilizer or leucaena (Leucaena leucocephala(Lam.) de Wit) leaves. Initial aboveground biomass of secondary forest was relatively low in comparison to that of other moist tropical forest sites, possibly because of poor soil fertility, shallow soil depth, and frequent natural disturbance from tropical storms. Rates of litterfall were also affected by the high winds associated with storm activity. Clearing, cultivation and fertilization over a 325-day period significantly reduced microbial biomass C. Soil surface CO2efflux was characterized by short-term flushes shortly after tillage and was affected by
soil moisture content and possibly by the proportion of active organic C contained in the soil. A comparison of commercial fields with continuous cultivation histories of 1–26 years and forest sites in Northern Guam showed approximately a 44% decrease in soil organic C within 5 years after conversion of secondary forest to continuous cultivation. Further information is needed on the effectiveness of minimum tillage, application of organic amendments, or improved crop residue management to maintain soil organic C in Northern Guam. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Land clearing; Soil CO2efflux; Tillage; Soil organic C fractions; Active organic C pool
1. Introduction
Changes in soil organic C in the tropics due to land management can have an impact on soil
phys-∗Corresponding author. Tel.:+1-573-884-3212; fax:+1-573-884-5070.
E-mail address:[email protected] (P.P. Motavalli)
ical and chemical properties (Woomer et al., 1994) and potentially affect global C cycling (Scholes and van Breeman, 1997). The tropical Pacific island re-gion of Micronesia represents a total land area of
approximately 2707 km2 (Karolle, 1991) and
there-fore, alterations in soil C storage in this region are not likely to have a large impact on global C cycling. However, rapid land development in the region,
cluding activities, such as phosphate mining and con-version of secondary forest to agricultural and urban purposes, have affected soil organic C and N (Man-ner and Morrison, 1991; Motavalli and McConnell, 1998). Decreasing soil organic C can lower soil wa-ter and nutrient-retention capacity, structural stability, infiltration rates, and accelerate runoff and erosion; thereby, reducing the natural resource base and soil productivity (Lal and Kang, 1982).
In the humid tropics, losses of soil organic C after forest clearing and conversion to agriculture are ap-proximately 20–40% of the soil C within the first 1 or 2 years following soil disturbance (Davidson and Ackerman, 1993). Losses of soil organic C due to land clearing may result from several processes including decreased inputs and changes in composition of plant litter and increased rates of soil organic matter de-composition and soil erosion (Lugo and Brown, 1993; Feller and Beare, 1997). In addition, tillage increases the rate of soil organic matter decomposition by bury-ing surface residues, disruptbury-ing soil aggregates, aerat-ing the soil, and exposaerat-ing new surfaces to microbial attack (Brown et al., 1994). Therefore, the method of forest clearing and the type of agricultural land use will affect the amount of soil organic matter loss or gain (Lal and Kang, 1982; Lugo and Brown, 1993).
Rapid initial losses of soil organic C following forest clearing and conversion to agriculture are pri-marily losses of the biologically-labile or active soil organic C pool (Lugo and Brown, 1993; Brown et al., 1994). In soils from Hawaii, the active soil organic C pool accounted for 0.7–4.3% of total C (Townsend et al., 1997). Changes in the active organic C pool can
be monitored by measurement of rates of soil CO2
efflux or soil respiration, although other methods in-cluding biological, chemical, physical, and isotopic procedures have also been proposed to distinguish active from more stable organic C pools (Motavalli et al., 1994; Townsend et al., 1995).
The objectives of this study were to determine the effects of land clearing, tillage, and fertilization of tropical secondary forest over time on soil organic C and organic C fractions in a shallow, calcareous soil located on the northern half of the Pacific island of Guam. The region of Northern Guam is experiencing rapid development and is of environmental and eco-nomic importance because it overlies the sole fresh water aquifer for the island.
2. Materials and methods
2.1. Field experiment
A field experiment was established in September 1996 on a secondary forest site located at the Yigo
Agricultural Experiment Station (13◦31′N, 144◦52′E,
elevation of between 145 and 155 m above sea level) in Northern Guam. The soil underlying the site was the Guam soil series (U.S. Soil Taxonomy: clayey, gibb-sitic, nonacid, isohyperthermic Lithic Ustorthents; FAO Soil Classification: Rendzic Leptosols). The de-sign of the experiment was based on a standardized protocol to examine soil organic matter dynamics in tropical environments proposed by the Tropical Soil Biology and Fertility Programme (Woomer and In-gram, 1990). The objective of the experimental design was to simulate long-term changes in soil organic matter due to forest conversion and agricultural prac-tices. However, the influence of crop plants on these changes was excluded. Plots measured 2.4 m width by 2.4 m length.
Treatments were arranged in randomized com-plete block design with three replications and con-sisted of: (1) soil left with the existing vegetation, (2) soil cleared of aboveground vegetation, (3) soil cleared of aboveground vegetation and large roots and tilled once a month with a rototiller, (4) same as treatment #3 but with addition of approximately
20 Mg ha−1 (dry basis) leucaena leaves and stems,
and (5) same as treatment #3 but with addition of
400 kg N ha−1 and 1106 kg K ha−1 in the form of
KNO3 and 131 kg P ha−1 as triple superphosphate.
Root barriers consisting of galvanized steel sheets were placed to a depth of 30 cm around all borders of plots receiving treatments #2, #3, #4 and #5. Fertilizer and leucaena treatments (treatments #4 and #5) were applied and incorporated twice over the course of the experiment on 28 October 1996 and 5 May 1997. For
the first application of treatments, 19 Mg ha−1 (dry
basis) leucaena leaves (424.8±6.9 g organic C kg−1,
40.0±2.4 g total N kg−1) were added and for the
sec-ond set of treatments, 21 Mg ha−1(dry basis) leucaena
leaves (457.1±30.2 g organic C kg−1, 33.28±0.78 g
total N kg−1) were applied. Additional tillage using a
periodic application of the herbicide, glyphosate, at a
rate of 3 litres active ingredient ha−1. Litter falling on
cleared plots from surrounding forest vegetation was removed every 4 weeks using a rake. Soil samples were also collected before treatments were imposed and periodically up to 15 September 1997 to a depth of 12.5 cm using a stainless steel push probe and mixing 20 subsamples per plot. Half of the collected
sample was stored moist at 4◦C for determination of
soluble organic C and microbial biomass C and the other half air-dried for determination of total organic C and particulate organic matter C. All soil samples were passed through a 2 mm sieve before analysis.
2.2. Aboveground biomass and litter characterization
Litter traps measuring 76 cm (length)×61 cm
(width)×15 cm (height) were constructed from steel
rebar and aluminum netting with 1.5 mm square open-ings in order to characterize aboveground organic C inputs into the secondary forest ecosystem of Northern Guam. Ten traps were placed randomly within sec-ondary forest vegetation located over a 6 ha area at the Yigo Agricultural Experiment Station. The traps were placed at a height of approximately 30 cm above the soil surface. Litter, consisting primarily of leaves and fine branches, was collected monthly over a 13-month period beginning 13 June 1996, weighed and dried at
70◦C for moisture determination, and ground through
a Wiley-Mill with a 1 mm screen before analysis. To estimate aboveground biomass of the secondary forest vegetation, vegetation that was cleared and re-moved from the plots from the field experiment was weighed, fed through a chipper, and a subsample taken
for moisture determination after drying at 70◦C.
Sub-samples were also ground through a Wiley-Mill with a 1 mm screen for chemical analysis.
2.3. Measurement of soil CO2efflux
Rates of soil CO2efflux and changes in soil
temper-ature in the field experiment were determined using
a portable infrared CO2 analyzer (LI-6200, LI-COR,
Inc. Lincoln, NE, USA) fitted with a soil respira-tion chamber (Model 6000-09) and a soil temperature probe (Garcia et al., 1997). Total system volume for the
chamber and analyzer was 991 cm3. During soil CO2
efflux measurements, the chamber was fitted on top of a 10.2 cm diameter PVC plastic soil collar inserted 2 cm into the soil surface. The soil temperature probe was placed 5 cm into the soil. Each plot had two soil collars spaced approximately 45 cm apart in the center of the plot. A composite of five soil subsamples was also collected from each plot to determine soil mois-ture content during soil respiration measurements.
To determine soil CO2efflux, ambient CO2
concen-trations were first measured. The CO2 concentration
in the chamber headspace was then scrubbed below ambient levels while the chamber was fitted on the soil collar. The scrubber was then switched off and soil
CO2 efflux and soil temperature logged over a 3-min
period. A single soil CO2 efflux was then calculated
for the ambient CO2concentration by interpolating
ef-flux values over the measurement period. Average soil temperature was also calculated over the sampling pe-riod. A comparison of this dynamic chamber method
for measuring soil surface CO2 flux with the static
chamber method using alkali traps has been presented by Jensen et al. (1996).
At the end of the field experiment, an additional
comparison was made of diurnal soil surface CO2
ef-flux among undisturbed forest sites, and adjoining sites at the Yigo Agricultural Experiment Station which were initially cleared and cultivated from forest or had been cultivated for 1 or 7 years. A description of soil properties of the cultivated 7 year site is found in Motavalli and McConnell (1998). The sites cleared and cultivated for 1 year were from the field exper-iment. The initially cleared and cultivated sites were from forested areas around the field experiment.
Base-line soil surface CO2 efflux was measured over a
3-h period at all sites and then all cultivated sites
were tilled once with a rototiller. Subsequent soil CO2
efflux was then measured periodically over a 51-h
pe-riod. At each sampling time, two soil CO2efflux
mea-surements were made for each site and duplicate soil samples composited from five subsamples were taken to a depth of 10 cm for soil moisture determination.
2.4. Survey of farmers’ fields in Northern Guam
Fig. 1. Map of the island of Guam showing locations of farm, forest and field experimental sites. Dashed line indicates southern boundary of the Northern Guam aquifer. Inset shows approximate location of Guam in relation to the Philippines and the west coast of North America.
horticultural crops, including eggplant, cucumber, tomato and long beans. Cropping histories for each field were collected from farmers and only fields of varying years (0.6–26 years) of continuous cropping after clearing from secondary forest were selected. In addition, three secondary forest sites located within
(Karolle, 1991). The primary soil underlying the sam-pled sites was the Guam soil series (clayey, gibbsitic, nonacid, isohyperthermic Lithic Ustorthents) and all sites were located over the Northern Guam aquifer (Fig. 1). Soil samples were collected at each field to a depth of 12.5 cm by using a stainless steel push-probe and by mixing 30 subsamples per field. Surface litter or crop residue was removed before sampling soil at all sites. Samples were air-dried, ground in a hammer mill, and sieved (2 mm) before analysis.
2.5. Chemical and physical analysis
Soil particle size analysis was determined using the hydrometer method (Bouyoucos, 1962). Soil bulk den-sity was determined by the soil core method (Blake and Hartge, 1986) and the proportion of stones with a diameter >2 mm measured by sieving air-dried soil. Soil moisture content under 0.033 MPa suction was determined using the pressure plate method (Klute, 1986). Soil pH was measured in water (1 soil : 1 water
w/v) and soil P by extraction with 0.5M NaHCO3.
Ex-changeable Ca and Mg were determined by extraction
with 1M NH4OAc and atomic absorption
spectropho-tometry. Determination of the proportion of CaCO3in
soil was determined by an acid titrimetric procedure (Rowell, 1994). Total organic carbon of soil and or-ganic materials was determined using a heated dichro-mate oxidation method (Nelson and Sommers, 1975) and total nitrogen by a micro-Kjeldahl digestion pro-cedure (Lachat Instruments, 1992).
Microbial biomass C was determined on field-moist soils from the field experiment and on air-dried soils
from farmer fields using the CHCl3fumigation–direct
extraction method (Vance et al., 1987) with a 3-day
fu-migation and a conversion factor (kEC) of 0.35
(Spar-ling et al., 1990). Air-dried soils were wetted to their 0.033 MPa moisture content and pre-conditioned with a 7-day incubation before fumigation. Soil organic C
extracted in 0.5M K2SO4of the unfumigated soils was
considered a measure of soluble organic C. Particu-late organic matter (POM) C was measured by a wet
sieving procedure (53mm sieve) using sieved (2 mm),
air-dried soil (Cambardella and Elliott, 1992). The proportion of total extractable polyphenolics in litter and cleared vegetation was determined by ex-traction in hot 50% methanol using tannic acid as a standard (Anderson and Ingram, 1993). Lignin
con-tents of organic materials was determined using the acid detergent fiber method (Goering and Van Soest, 1970).
2.6. Data analysis
Analysis of variance (ANOVA) by PROC GLM (SAS Institute, 1988) was used for determining the effects of land clearing and cultivation on soil
charac-teristics and soil CO2efflux. The multiple comparison
test used was Fisher’s (protected) LSD at a 0.05 sig-nificance level. Pearson linear correlations were calcu-lated among soil characteristics and soil organic C and organic C fractions of samples collected from farmer fields and secondary forest sites using PROC CORR (SAS Institute, 1988).
The nonlinear regression model used for analysis of the relationship between soil organic C in farmers fields and time of cultivation was:
Y = C1e−k1t +C2e−k2t
whereYis the soil organic C andtis the time of
cul-tivation. The coefficientsC1andC2 give an estimate
of the active and stabilized C pools, respectively. The
coefficientsk1andk2are rate constants for each
cor-responding C pool.
3. Results and discussion
3.1. Soil characteristics
Table 1
Selected average chemical and physical soil properties of the field experiment of the Guam soil
Soil texture Stones >2 mm Moisture 0.033 Organic CaCO3 pH NaHCO3−extraction diameter (g kg−1) MPa (g g−1) C (g kg−1) (g kg−1) (1 : 1 water) P (mg kg−1) Sand Silt Clay
435 248 307 360 0.392 51.7 374.8 7.3 2.8
(87)a (68) (143) (324) (0.064) (9.9) (249.9) (0.2) (1.2)
aNumbers in parentheses are the standard deviations.
this soil is characteristic of tropical forest ecosystems and often limits net primary production (Vitousek, 1984).
3.2. Aboveground biomass and litter in secondary forest
Forest vegetation was typical of the limestone forest community described by Moore (1973) and
in-cluded pago (Hibiscus tiliaceusL.), lulujet (Maytenus
thompsonii(Merr.) Fosb.),Allophylus ternatus(Forst.)
Radlk., ifit (Intsia bijuga(Colebr.) Ktze.), kafu (
Pan-danus tectorius Sol. ex Park) and the fern species,
sword fern (Nephrolepsis hirsutala(Forster f.) Presl.)
(Motavalli and McConnell, 1998). The average weight of aboveground biomass cleared and removed from
plots in the field experiment was 36.6±18.4 Mg ha−1
(dry weight basis±standard deviation) with an
aver-age organic C content of 409.2±16.6 g kg−1 and a
total N content of 6.7±1.6 g kg−1. The aboveground
biomass of this secondary forest site is relatively low in comparison to that of other moist tropical forest sites (Vitousek and Sanford, 1986), possibly because of poor soil fertility, shallow soil depth, and frequent natural disturbance from tropical storms. In contrast to other tropical forest regions where cleared vegeta-tion is burned on site, cleared vegetavegeta-tion in Northern Guam is primarily hauled away or less commonly left in the field (Motavalli, unpublished data). There-fore, organic carbon inputs from secondary vegeta-tion after clearing are mainly from roots and surface litter.
The rate of litterfall under the secondary limestone forest vegetation of Northern Guam averaged 40.0
±49.1 kg ha−1 per day (dry weight basis±standard
deviation) over a 13-month period (Fig. 2B). This rate of litterfall falls within the range of litterfall from
5–25 Mg ha−1 per year reported by Greenland et al.
Table 2
Soil organic C, soil organic C fractions and soil bulk density at the initiation and conclusion of a 325-day field experiment
Treatment Bulk density Total organic C Soluble organic C Microbial biomass C POM C
Mg m−3 g kg−1 soil g kg−1 of total organic C
Initial 0.88 51.7 3.31 27.15 329.1
After 325-days:
Forest 0.93 56.6 3.09 27.70 267.4
Cleared 0.86 59.9 2.22 21.08 80.9
Cultivated 1.02 48.2 2.72 22.80 214.7
Leucaena-treated 0.82 51.1 2.60 21.68 62.5
Fertilizer-treated 0.86 48.8 2.86 18.24 126.8
LSD(0.05) 0.16 NSa NS 8.18 248.1
aNS=not statistically significant atp≤0.05.
(1992) for humid tropical forest sites. The average rate
of organic C input was 16.0±18.1 kg ha−1 per day.
However, litterfall on Guam is affected by the peri-odic tropical storm activity in the region and the high winds associated with those storm events. For exam-ple, the rate of litterfall as a result of Typhoon Dale was approximately 4–5 times greater than the average, but after Typhoon Isa, a storm with weaker winds, lit-terfall was not significantly higher than the average (Fig. 2A and B). Although not measured in this study, the high winds of Typhoon Dale also caused a sub-stantial felling of trees and large branches. Guam ex-periences tropical storms with winds greater or equal
to 33 m s−1 every 3.5 years and storms with winds
approaching 67 m s−1 approximately every 10 years
(Karolle, 1991).
No significant seasonal variations in litter compo-sition were observed (Fig. 2C). Total N content of
litter averaged 15.7±5.6 g kg−1; lignin content
av-eraged 191.9±55.6 g kg−1; the C : N ratio of litter
averaged 29.2±10.2; the lignin : N ratio averaged
12.9±4.1; and the polyphenolic : N ratio averaged
0.9±0.8. Litter composition or quality can affect
subsequent rates of C and N mineralization and the partitioning of organic C into active and stabilized C pools (Woomer et al., 1994; Scholes et al., 1997). Based on the results of Constantinides and Fownes (1994) who examined the relationship between litter quality from several tropical agroforestry species and soil N mineralization, decomposition of litter from the secondary forest sites in Northern Guam would be expected to result in net N immobilization due to
the low total N content (<20 g N kg−1) and relatively
high lignin content (>100–50 g lignin kg−1). The
atively higher lignin content would also reduce rates of initial C decomposition.
3.3. Soil organic C and organic C fractions
The effects of clearing, cultivation, and fertilization on soil organic C, organic C fractions and soil bulk density are shown in Table 2. Compared to soil under secondary forest vegetation, microbial biomass C was significantly reduced by the fertilizer treatment (Table 2). This result suggests that nutrient deficiency may limit rates of C decomposition in this soil. Significant reductions in particulate organic matter C were also observed in the cleared and leucaena-treated plots (Table 2), although an explanation for this result is not clear. Reductions in particulate organic matter (POM) have been observed with long-term tillage (Cambardella and Elliott, 1992), possibly due to de-struction of soil aggregates and exposure of organic
Fig. 4. Precipitation following application of the first set of treat-ments (A) or second set of treattreat-ments (B) in the field experiment. Arrows indicate precipitation associated with tropical storm events. Names of individual tropical storm events are also given.
matter to microbial decomposition (Brown et al., 1994). Removal of the top forest litter layer, which often occurs in mechanical clearing, the reduction in litter inputs, and the lack of mixing of dead roots by tillage may have resulted in lower soil POM C in the cleared plots during this 325-day period. There is no clear explanation for the reduction of POM C in the leucaena-treated plots.
3.4. Soil surface CO2efflux
Application and incorporation of the first treatment
of approximately 19 Mg ha−1 (dry basis) leucaena
leaves in the field experiment significantly increased
soil surface CO2 flux up to 34 days after treatment
(Fig. 3A). The field site received high initial precip-itation up to approximately 93 days (Fig. 4A). This high initial precipitation was followed by an extended
drought period up to the last soil surface CO2 flux
measurement at 169 days after treatments were applied (Fig. 4A). Higher soil moisture content at the forest site compared to other treated plots may account for
the significantly higher CO2flux measured in the
for-est sites after 140 and 169 days (Fig. 3A). Short-term
increases in soil CO2 flux were also observed
per-sisting less than 1 day after tillage events (Fig. 3A).
This pulse of increased CO2 flux immediately after
tillage has also been observed in temperate soils and is dependent on the interactive effects of changes in soil moisture, soil temperature, soil structure, and soil organic matter availability caused by tillage (Hendrix et al., 1988; Reicosky and Lindstrom, 1993).
Soil CO2 efflux was also elevated following a
sec-ond treatment of leucaena and this effect persisted throughout the measurement period (Fig. 3B).
Signif-icantly higher CO2 efflux was also observed in the
forest plots compared to the cleared/cultivated plots up to approximately 41 days after the start of the sec-ond treatment period. This same period was an inter-val of low precipitation (Fig. 4B) and suggests that higher soil moisture in the forest sites compared to the cleared and cultivated sites during dry periods
pro-motes higher soil CO2efflux.
Short-term changes in soil CO2efflux and soil
mois-ture after a tillage event were examined for sites with
Fig. 6. Effects of years of continuous cultivation on soil total organic C of Northern Guam commercial farm fields. Soil total organic C of secondary forest sites are also shown.
varying previous periods of continuous cultivation and compared to a forest site (Fig. 5A and B). All
cul-tivated sites had higher soil CO2 efflux immediately
after cultivation, but this effect only persisted over the 51 h period of measurement in the recently cleared and cultivated forest site (Fig. 5A). Initial rapid efflux of
CO2is possibly released from soil pores and from
min-eralization of exposed labile compounds (Prior et al.,
1997). Soil CO2efflux was lowest in the site that had
been cultivated continuously for 7 years (Fig. 5A) sug-gesting increasing loss of active organic C with longer periods of continuous cultivation. An additional factor
affecting reduced soil CO2efflux in the sites that had
been cultivated was the lower soil moisture content of those sites compared to the forest site (Fig. 5B).
3.5. Soil organic C status of farmers’ fields and secondary forest sites in Northern Guam
A comparison of commercial farm fields with con-tinuous cultivation histories of 1–26 years and forest sites in Northern Guam showed approximately a 44% decrease in soil organic C within 5 years after con-version of secondary forest to continuous cultivation leveling off at an equilibrium level of approximately
Table 3
Soil organic C and soil organic C fractions of commercial farm fields and forest sites in Northern Guam
Total organic C (g kg−1 soil) Soil organic C fractions (g kg−1 of total organic C)
Soluble organic C Microbial biomass C POM C
Averagea 36.2 6.01 8.12 56.00
Standard 10.7 1.70 4.02 37.71
Minimum 18.3 3.10 2.10 16.60
Maximum 56.0 9.00 15.80 187.60
an=26.
3.10 to 9.00 g kg−1 of total organic C; microbial
biomass C from 2.10 to 15.80 g kg−1 of total
or-ganic C; and POM C from 16.60 to 187.60 g kg−1
of total organic C (Table 3). Soil total organic C significantly negatively correlated with soil clay
con-tent (r= −0.74***,n=26) and positively correlated
with soil exchangeable Ca (r=0.64***, n=26) and
Mg (r=0.77***,n=26). However, soil clay content
also had a significant negative correlation with soil
exchangeable Ca (r= −0.72***, n=26). The
stabi-lizing effects of soil Ca content on decomposition of organic C has been noted by Oades (1989) and may be a more important factor than soil clay in stabiliz-ing soil organic C in the limestone-derived soils of Northern Guam. No significant trends were observed between years of cultivation and soil soluble C, soil microbial biomass C, or POM C (data not shown).
4. Conclusions
An environmental impact of clearing of secondary forest vegetation and conversion of land to agriculture in Northern Guam on soil organic C has been a rapid loss of active organic C within 5 years after first clear-ing and cultivation. This loss has been affected by sev-eral interacting factors including reduction in organic C inputs from forest litter and from periodic felling of aboveground vegetation from tropical storms, inten-sive conventional tillage, periodic fertilization, and the stabilizing effects on soil organic C decomposition of high soil Ca in the limestone-derived soil of Northern
Guam. Measurements of soil CO2efflux indicate that
loss of active organic C occurs in short-term flushes shortly after tillage and are affected by soil moisture content and possibly the proportion of active organic C
contained in the soil. A portion of this short-term CO2
efflux may be also from loss of CO2from soil pores.
Further investigations on the use of minimum tillage, application of organic soil amendments, and improvements in crop residue management may be needed to understand how to reduce soil organic C losses in Northern Guam. Additional information may be of assistance to landowners and land use planners to facilitate adoption of sustainable land use practices in this rapidly developing region.
Acknowledgements
We greatly appreciate the technical assistance pro-vided by Treasa Chopp, Rosenilda Marasigan, and Edwin Paulino. Research sponsored by a grant from the U.S. Department of Agriculture under the Trop-ical and SubtropTrop-ical Agricultural Research Program (Grant No. 95-34135-1768).
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