Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue8-9.Aug2000:

(1)

In¯uence of tillage on the dynamics of loose- and

occluded-particulate and humi®ed organic matter fractions

Michelle M. Wander

a,

*, Xueming Yang

b

a

Department of Natural Resources and Environmental Sciences, College of Agriculture, University of Illinois-Urbana, 1102 South Goodwin Avenue, Urbana, IL 61801, USA

b

Department of Land Resource Science, University of Guelph, Richards Building, Guelph, Ont., Canada, N1G 2W1

Received 19 May 1999; received in revised form 11 October 1999; accepted 3 March 2000

Abstract

This study was carried out at a site (®ne silty mixed mesic Argiaquic Argialboll) where use of no-tillage (NT) practices, for over a decade, had not increased soil organic carbon (SOC) sequestration relative to plots that had been moldboard plowed (MP). Even though the total SOC contents of these soils were known to be similar, we expected input and decay rates of residue-derived C, conservation of root-derived C, and the importance of aggregate protection of particulate organic matter (POM) to dier among these tillage treatments. Corn (Zea maize L.) was pulse-labeled with13CO2 repeatedly during the 1995

growing season to allow the fate of residue-derived C retained in loose-POM (LPOM), aggregate-occluded POM (OPOM) and in mineral associated humi®ed (HF) fractions, to be tracked through April 1997. Tillage practices were related to fundamental dierences in the depth and rate at which residues decayed and the distribution of those residues among SOC fractions. In December 1995, approximately 50% of the C derived from labeled residues was recovered in the LPOM, OPOM, and HF fractions of the NT plots, while only 22% was recovered in those fractions from MP plots. After initial rapid losses of label-derived C, C turnover rates were relatively slow in the MP plots compared to C turnover rates observed at the surface of the NT plots. As a result, after 1.5 years the MP and the NT plots retained similar amounts (110%) of label-derived C in the 0±20 cm depth. Shifts in the percent label recovery suggest that newly assimilated C was rapidly lost from the LPOM fraction as it accumulated in the OPOM and HF fractions. Increases in the fractional abundance of label-derived C in the OPOM and HF fractions accounted for approximately half of the label lost from LPOM. Trends in both the fractional abundance and percent label recovery in the OPOM and HF fractions indicated that C derived from 1995-residues was concentrated at 0±5 cm depth in NT plots and was more evenly distributed in the MP plots. In December 1995, the fractional abundance of OPOM and HF was greater in the root than shoot labeled plots, indicating that root-derived C was incorporated into SOC more rapidly than shoot-derived materials. By spring, the fractional abundance of OPOM and HF had increased in tilled plots amended with labeled shoots. Our fractionation scheme revealed the in¯uence of aggregation on the decay dynamics of C introduced by newly incorporated residues and identi®ed fundamental dierences in the depth, decay dynamics and distribution of C, newly assimilated into the SOC fractions of NT and MP soils.72000 Elsevier Science Ltd. All rights reserved.

Keywords:C sequestration; Tillage; Physical protection; Particulate organic matter;13Carbon

1. Introduction

In situations where erosion is not a major factor,

the use of no-tillage (NT) practices for a decade or more has not always increased soil organic carbon (SOC) contents relative to conventionally tilled soils (Havlin et al., 1990; Franzluebbers and Arshad, 1996; Angers et al., 1997; Alvarez et al., 1998; Wander et al., 1998). This atypical eect of NT practices on SOC sequestration indicates that either C inputs are reduced

Soil Biology & Biochemistry 32 (2000) 1151±1160

www.elsevier.com/locate/soilbio

* Corresponding author. Tel.: +1-217-333-9471; fax: +1-217-244-3219.

(2)

and/or net C mineralization rates are similar to those occurring in tilled soils. Cold and wet soil conditions, soil compaction, and root diseases can reduce crop productivity in soils under NT management (Grith et al., 1988) and therefore reduce the quantity of C returned to the soil. Additionally, the manner in which residues are incorporated into the soil can in¯uence fate of C. Frequently, SOC concentrations increase at the surface of NT soils as a result of residue concen-tration and erosion abatement. In some cases, decay rates of residues placed at the soil surface are slow compared to residues that are incorporated (Beare et al., 1993; Parton et al., 1994). However, surface resi-dues appear to decay rapidly at the surface of soils where moisture and nutrient status are non-limiting (Scott et al. 1996; Alvarez et al., 1998). Yang and Wander (1999) found that in the spring after soybean production in a corn soybean rotation, the quantity of residues did not dier signi®cantly among plots that had been under MP and NT management for over a decade. Disappearance of surface residues might have resulted from decay or their incorporation by earth-worm activity.

According to Linden and Clapp (1998), enhanced earthworm activity in NT soils can increase SOC as a result of increased casting, or decrease SOC as a con-sequence of enhanced mineralization of residues within earthworm gut and casts. In tilled soils, residues intro-duced from above ground are mixed into the plow depth, while the soil typically is simultaneously loo-sened and oxygenated. It is well known that this prac-tice stimulates the mineralization of both residues and native SOC. However, the short- and long-term in¯u-ences of disturbance on C mineralization are complex and may vary among soils. Hu et al. (1995), Fran-zleubbers and Arshad (1996) and Alvarez et al. (1998) found net C mineralization rates to be greater in NT than in tilled soils. Tillage's ability to accelerate or-ganic matter mineralization may be tempered by edaphic factors. The stimulatory eects of tillage on C mineralization can be minimized when residues were buried in fall in cool or moist regimes (Franzleubbers and Arshad, 1996; Angers et al., 1997). Greater knowl-edge of the C dynamics occurring after tillage are needed to understand and accurately predict its eects on SOC sequestration.

Balesdent and Balabane (1996) concluded that roots were major contributors to soil carbon storage in maize cultivated soils. They argued that root contri-butions to SOC were greater than shoot contricontri-butions because root matter decayed more slowly than shoot matter. The slower decay of root-derived SOC was attributed to its composition and greater physical pro-tection by soil minerals. Roots may be greater contri-butors to SOC in NT than in tilled soils receiving large inputs of C as incorporated residues. W.J. Gale and

C.A. Cambardella (National Soil Tilth Lab, Iowa, per-sonal communication) have shown that root inputs play a disproportionately large role in the formation and stabilization of aggregates in non-disturbed soil systems such as NT. The increased physical protection of SOC by aggregates in NT may explain how NT soils frequently maintain SOC contents equal to or greater than amounts maintained in tilled soils (Beare et al., 1994). According to Angers (1998), aggregate protection of particulate or macro organic matter plays a particularly important role in the C equili-brium levels achieved by ®ne-textured soils with inher-ently high SOC contents.

Our purpose was to identify factors controlling C turnover in a site where use of NT practices for over a decade had not increased SOC sequestration (Yang and Wander, 1999). Findings from that and other stu-dies (Wander et al., 1998; Needelman et al., 1999) suggest that application of NT practices to Illinois soils frequently increases SOC concentrations in the top few cm of the soil while depleting the SOC con-tents of the lower rooting zone. To gain insight into the C dynamics of NT and MP plots, we labeled corn shoot and root residues in situ by pulse-labeling grow-ing corn with 13CO2. We anticipated that: the inputs

and decay rates of crop-derived C would be lesser in the NT than in MP plots, that root-derived C would be a greater contributor to SOC in the NT than in MP plots, and that aggregate protection of macroorganic matter would be greater in the NT than in MP plots.

2. Materials and methods

2.1. Labeling and soil sampling

This experiment was conducted at a long-term tillage experiment established in 1986 that is located on the Agricultural Engineering Research Farm of the Uni-versity of Illinois at Urbana, IL. The soil is a Thorp silt loam (US Taxonomy: ®ne-silty, mixed, mesic Argiaquic Argialboll; FAO classi®cation: Orthic Grey-zem). This work was carried out in NT and MP plots randomized in four experimental blocks in plots pro-ducing corn (Zea mays L.) in 1995 and soybean [ Gly-cine max(L) Merr.] in 1996. The only soil disturbance in the NT treatment occurred during planting oper-ations. After corn harvest, the MP plots were mold-board-plowed (20±25 cm deep) followed by spring disking (7.5±10 cm deep). After soybean harvest, the MP plots were fall chisel-plowed (30±35 cm) followed by spring disking.

Corn was 13C pulse-labeled by enclosing sections of a plant-row (microplots) with a portable chamber and introducing 13CO2 during the 1995 growing season.

(3)

during the summer, crops in microplots were exposed to 99-atom% 13CO2 (Cambridge Isotopes, Cambridge,

MA) using a portable chamber. The chamber was loosely modeled after the design of Berg et al. (1991). A 1 m2 PVC frame, adjustable in height, was covered with a tedlar bag equipped with a gas-tight septum (US Plastics). The frame was attached to a metal base that was forced into the soil (5 cm). The base was equipped with ports that attached to a recirculating cooling system. The entire system was constructed to ®t between 75 cm crop rows. The quantity of 13CO2

introduced on each date was based on estimates of canopy leaf area and maximum photosynthetic rate. Plots for ``reciprocal transfer'' of unlabeled surface residues were established adjacent to labeled plots. These reciprocal transfer plots were treated identically, except that non-labeled CO2 was vented through the

chamber. Labeling was carried out between 11:00 h and 14:00 h on all dates; the labeling sequence was varied to avoid bias. Four replicates were established for each tillage and labeling treatment (16 plots).

Corn residues were harvested in November from labeled and unlabeled plots before tillage. Composites of the eight labeled and unlabeled residues were weighed, clipped into 5±15 cm lengths, divided into eight equal parts (425 g plot), and transferred recipro-cally to establish `shoot' and `root' labeled plots. We assumed the mass of shoot and root-derived residues were equal (Buyanovsky and Wagner, 1986) and, based upon the work of Gregorich et al. (1995) and Wander (unpublished data), that the isotopic compo-sition of shoots and roots were equal. Soil samples were collected after fall ®eld operations on 7 December 1995, before seeding soybean on 20 May 1996, and again before corn planting on 21 April 1997, using a splitable soil sampler of 4.9 cm diameter. Sampling intensity increased from one sample collected from each plot in 1995 to three random replicates per plot in 1996 and, further to nine replicates in 1997. All samples were divided into 0±5 and 5±20 cm incre-ments, weighed, sieved (8 mm) and air-dried. Sub-samples were dried at 1058C to determine soil moisture for bulk density determination.

2.2. Soil organic matter fractionation

Particulate organic matter (POM), not protected by aggregates (LPOM), was separated from samples using a modi®ed version of the method described by Golchin et al. (1994). The method was adapted to maximize recovery of macroaggregates. A 20 g soil sample was placed in a 250 ml Oakridge tube, to which 50 ml sodium polytungstate (1.6 g cmÿ3; Geoliquids, Chi-cago, IL) was added. The tube was orbitally shaken at low speed (200 oscillations minÿ1) for 30 min. Particles adhering to the side of the tube were rinsed free with

10 ml of sodium polytungstate, and the solution was allowed to stand overnight. Tubes were then centri-fuged. Materials recovered from the supernatant on 0.5 mm polycarbonate ®lters were rinsed with 50 ml of deionized water and dried at 808C. To obtain the aggregate occluded POM (OPOM) and the relatively humi®ed, mineral associated fraction (HF), the soil remaining in the tube was further shaken at high speed (350 oscillations minÿ1) for 60 min. The suspension was ®ltered though a 53 mm polyester mesh (Gilson, Columbus, OH). The OPOM captured on the mesh and the HF fraction passing through the 53 mm sieve were collected, dried in an oven at 808C, weighed and ground with a disk mill.

2.3. Analysis

Carbon and carbon isotope ratios of whole soil and SOC fractions were determined at the University of Saskatchewan, Canada with a Continuous Flow Iso-tope Ratio Mass Spectrometer (CF-IRMS) using a TracerMass mass spectrometer interfaced with a RoboPrep combustion system (Europa Scienti®c, Crewe, UK). The isotopic composition of whole soil and SOC fractions were expressed in delta `d_{' units}

where d_=[(_R_Sample_/_R_PDB_ÿ₁₎ _{1000], when} _R_Sample _is

the sample ratio of 13C/12C, and RPDB=0.0112372

(based on the Pee Dee Bee standard) (Wolf et al., 1994). Soil samples that were collected from unlabeled plots between residue harvest and amendment were fractionated as described previously. Those fractions, which included the 1994 inputs of non-labeled corn roots, were used to establish control values to deter-mine the fractional abundance of C derived from labeled residues d_LRES_-₉₅₁₅₉:36- in SOC fractions.

The fractional abundance in the LPOM, OPOM, and HF fractions fLPOM, fOPOM and fHF was taken to be

equal to d_Sample_ÿd_Control_)/(d_LRES_-₉₅_ÿd_Control _{(Wolf et}

al., 1994). The 1995 d _{values of individual unlabeled}

fractions were used as the controls for 1995 and 1996 samples. During that period, there were no other or-ganic matter inputs. For the 1997 controls, we sub-tracted 1.4- from all SOC fraction values to re¯ect the inputs of soybean. The magnitude of this correc-tion factor was based upon thed _{of SOC fractions}

col-lected in Spring of 1994, before corn was seeded, and on the magnitude of change observed in the isotopic content of the whole soil. The percent recovery of labeled residues in LPOM, OPOM and HF fractions was computed by multiplying fractional abundance by the C concentration in the fraction, fraction concen-tration in soil, soil bulk density and the volume of soil amended with residue: 30,000 cm3 in the 0±5 and 90,000 cm3 in the 5±20 cm depth. This sum was then divided by the quantity of C applied in labeled resi-dues and then expressed as a percentage. Tillage, date,

(4)

Table 1

Probability values summarizing the in¯uences of tillage, depth, date, and label origin (shoot versus root) on the isotopic compositiond13_{C), carbon contained in (mg C g fraction), and fractional} abundance (fLPOM) and percent recovery (REC) of C derived from label in loose particulate organic matter (LPOM), occluded-particulate organic matter (OPOM), and humi®ed material (HF) <53mm. The letter D identi®es non-signi®cant interactions that were dropped from the model

Factor LPOM OPOM HF

d13C mg C g fractionÿ1 fLPOM RECLPOM d13C mg C g fractionÿ1 fOPOM RECOPOM d13C mg C g fractionÿ1 fHF RECHF

P value

Tillage 0.841 0.284 0.840 0.460 0.467 0.284 0.450 0.032 0.911 0.956 0.824 0.633

Depth 0.336 0.001 0.340 0.307 0.335 0.001 0.335 0.0001 0.175 0.153 0.183 0.0001

Tillagedepth 0.093 0.001 0.097 D 0.180 0.0001 0.183 0.448 0.026 0.137 0.028 0.077

Date 0.001 0.020 0.001 0.006 0.001 0.025 0.009 0.007 0.001 0.008 0.0001 0.0001

Datedepth 0.805 0.770 D 0.403 0.613 0.770 0.612 0.085 0.713 0.089 0.722 D

Datetillage 0.459 0.961 D 0.342 0.009 0.961 0.009 0.224 0.027 0.064 0.028 0.097

Datedepthtillage 0.116 0.026 D D 0.012 0.062 0.012 0.005 0.008 D 0.009 0.008

Origina D 0.712 0.390 0.211 0.244 D 0.320 0.291 0.049 0.827 0.052 0.447

Tillageorigin D D D D 0.466 D 0.615 0.369 0.043 0.661 0.033 D

Dateorigin D D D D 0.007 D 0.001 D 0.438 D 0.448 D

Depthorigin D D D 0.077 D D D D 0.713 0.432 D D

Dateorigintillage D D D D 0.026 D 0.046 D 0.038 D 0.052 D

Tillagedepthorigin D D D D D D D D 0.028 0.087 D D

a

Label was of corn shoot or root origin.

M.M.

W

ander,

X.

Yang

/

Soil

Biology

&

Biochemistry

32

(2000)

1151±11

(5)

label origin, soil depth and their interactions were the main eects included in statistical models assessing variables: C concentration, d_, _f_{, and percent of}

label-derived C recovered. Plot was included as a random factor and depth was a repeated ®xed-eect. All stat-istical analyses were performed using the SAS software package PROC MIXED (SAS Institute, 1996). When possible, non-signi®cant factor interactions were dropped from statistical models. Signi®cance was reported at 5% probability level.

3. Results

3.1. Total carbon and isotopic composition

The results that follow are based upon the statistical analyses summarized in Table 1. The quantity of C in LPOM declined over time in the NT 0±5 and MP 5± 20 cm depths, while in the NT 5±20 and MP 0±5 cm depths, concentrations increased between Dec. 95 and May 96, and then declined by April 97 (Fig. 1a). Car-bon concentrations in OPOM were consistently highest in the surface depth of the NT plots (Fig. 1b). Between Dec. 95 and May 96, the quantity of C in OPOM increased in both depths of the MP and in the 5±20 cm depth of the NT plots. By the following spring, OPOM C concentrations had fallen in those plots to

values equal to or slightly greater than Dec. 95 (MP 0±5 cm) values. Tillage treatments did not interact with depth to in¯uence the quantity of C in the HF (Table 1). The HF C concentration, which was greater in the surface than in the subsurface depth and was not in¯uenced by tillage practices, increased and then declined to amounts equal to (0±5 cm) or below (5±20 cm) the Dec. 1995 concentration (Fig. 1c, inset). Tem-poral trends in total SOC, which is the sum of all measured SOC fractions, echo tillage by depth eects on the POM fractions (Fig. 1c). Very coarse residues and roots removed by sieving (8 mm) and soluble or-ganic C lost during soil fractionation are not re¯ected by total SOC values. Total SOC contents were greatest in the NT 0±5 cm depth immediately after corn har-vest. In all other treatment by depth combinations, SOC increased between Dec. 1995 and May 1996 as roots and residues were fragmented and decayed. Total SOC declined in all fractions during the soy-bean-year (May 1996±April 1997).

The isotopic composition of whole soils, corn shoot residues and soil organic matter fractions are listed in Table 2. After soybean production in 1994, the d _of

whole soil was ÿ18.29-. Corn production in 1995 increased soil d _{values to} _ÿ_16.85- in the unlabeled plots. Labeling corn with 13CO2markedly increased d

of corn shoot residues and further increased soil d

values to 0.58-. Labeling increased the d _{of the}

Fig. 1. Temporal change in the quantity of C in the (a) loose-particulate organic matter, (b) occluded-particulate organic matter, and (c) total or-ganic matter (the sum of all measured fractions) and humi®ed mineral associated fractions (HF) recovered from the 0±5 and 5±20 cm depths of moldboard plowed (MP) and no-tilled (NT) plots. For individual fractions, dierences between letters identify means that dier atP< 0.05.

(6)

LPOM by 38.7-, the OPOM by 3.3-, and of the HF by 0.5-. By spring 1996, mean d_OPOM _and d_HF _had

increased while d_LPOM _{had decreased. In spring 1997,} d_LPOM _andd_OPOM _{still exceeded values obtained for the}

unlabeled control fractions; d_HF_-₁₉₉₇ _{was similar to} d_HF_-₁₉₉₅_{in both the labeled and unlabeled plots.}

3.2. Percent recovery and fractional abundance of label-derived C

Approximately 40% of the label applied to plots was recovered in SOC fractions in Dec. 1995 (Fig. 2a). Despite the relatively small quantity of C contained in LPOM (Fig. 1), the majority of the label was recov-ered from that fraction (Fig. 2b). The fractional abun-dance of label in LOPM was high. Both the recovery and the fractional abundance of the LPOM, which was initially 125%, had greatly declined by May 1996. By April 1997, recovery in the LOPM had declined further but fLPOM and therefore its isotopic

composition had not changed. The percent recovery and fractional abundance of label-derived C increased in OPOM and HF between Dec. 1995 and May 1996. By April 1997, fractional abundance did not change in either the OPOM or HF fraction while the percent recovery in the HF decreased slightly.

Averages of d_OPOM _and _f_LPOM_{, which were ranked}

NT 0±5 cm > MP 5±20 cm > MP 0±5 cm > NT 5±20, re¯ected residue placement patterns (Table 3, Fig. 3a, inset). Heterogeneity in the isotopic composition and relatively limited sample numbers in 1995 prevented observation of any treatment-based dierences in the LPOM that might have existed (n= 32, mean=26.5

-Fig. 2. Temporal change in (a) percent recovery and (b) the frac-tional abundance of label-derived C in loose- and occluded-particu-late organic matter (LPOM and OPOM), and in mineral associated fractions (HF). For individual fractions, dierences between letters identify means that dier atP< 0.05.

Table 3

Tillage and depth eects on the loose particulate organic matter (LPOM) isotopic composition and fractional proportion of C derived from labeled residues. Values are means from all dates

SOC fraction MP NT

0±5 (cm) 5±20 (cm) 0±5 (cm) 5±20 (cm)

d13C

Loose POM

Mean of all dates ÿ32aba _3.37ab _9.73a _ÿ_3.67b

Occluded POM

1995 ÿ18.09a ÿ16.99ab ÿ14.01c ÿ15.48ab 1996 ÿ12.16d ÿ14.76bc ÿ14.79bc ÿ14.51bc 1997 ÿ17.33ab ÿ15.25b ÿ15.41b ÿ17.66bc Humi®ed OM (HF)

1995 ÿ17.13bc ÿ16.87c ÿ16.28cd ÿ16.99bc 1996 ÿ15.47d ÿ15.73cd ÿ16.20d ÿ15.98d 1997 ÿ17.93abc ÿ17.65abc ÿ17.66b ÿ18.31a

a

Means comparisons were made across tillage by depth (LPOM) and tillage by depth by date (OPOM and HF) combinations; means within fraction categories not followed by the same letter are signi®-cantly dierent atP< 0.05.

Table 2

The isotopic composition of unlabeled and13C-labeled corn residues, bulk soils, and loose-particulate organic matter (LPOM), occluded-particulate organic matter (OPOM), and the humi®ed fraction (HF)

Material Isotopic composition

Control-unlabeled 13C-labeled

1995 1995 1996 1997

d13_{C (}

-)

Corn residues ÿ11.70ba _{+159.36a ±} _±

Bulk soil ÿ16.85ab ÿ16.28a ÿ16.36a ÿ17.05b SOC fractions

Loose-POM ÿ15.26 +23.45ab _ÿ_6.56b _ÿ_13.13b

Occluded-POM ÿ19.45 ÿ16.15a ÿ14.06b ÿ16.41a HF ÿ17.31 ÿ16.82a ÿ15.85b ÿ17.89a

a

Values within rows followed by dierent letters were signi®cantly dierent atP< 0.05.

b

(7)

, std=47.5-). Tillage and depth interacted with time to aect d_OPOM _and _f_OPOM _{(Table 1, Fig. 3). Initially,}

thed_OPOM _and _f_OPOM _{were ranked NT 0±5 cm}rNT 5± 20 cmrMP 5±20 cmrMP 0±5 cm and both d_OPOM

and fOPOM were greater in the NT 0±5 cm than in

either MP depth and were greater in the NT 5±20 cm than in the MP 0±5 cm depth. By spring 1996, d_OPOM

and fOPOM had increased signi®cantly in the MP 0±5

cm and slightly in the MP 5±20 cm depths and had declined (0±5 cm) or remained unchanged (5±20 cm) in the NT plots. The following spring, d_OPOM _and _f_OPOM

of the MP 5±20 and NT 0±5 cm depths remained unchanged but had declined in the MP 0±5 and NT 5± 20 cm depths. The decline ind_OPOM_{in the NT 5±20 cm}

depth was not signi®cant. Temporal trends in the d_HF

and fHF were generally similar to trends in OPOM

(Table 3, Fig. 3). Values for bothd_HF _and_f_HF _were

in-itially greatest in the 0±5 cm depth of NT and least in the 0±5 cm depth of MP treatments. By May 1996,

d_HF _and _f_HF _{had not changed in the NT 0±5 cm depth}

and had increased in the other samples; the increase in

fHF in the MP 5±20 cm depth was not signi®cant. As

was true for OPOM, the relative increase in d_HF _and

fHF that occurred during the ®rst winter was greatest in

the 0±5 cm depth of the MP plots. After soybean pro-duction, the d_HF _{of all samples and the} _f_HF _{of the NT}

5±20 cm depth had declined signi®cantly.

Our ability to recover label within the LPOM frac-tion did not vary signi®cantly among tillage treat-ments, soil depths or sampling dates. The in¯uence of those factors on C dynamics is revealed by label recov-ery within the OPOM and HF fractions (Table 1, Fig. 4). Shortly after harvest, percent recovery in the OPOM and HF from the 0±5 cm depth of the MP plots was less than that recovered in fractions from other treatment by depth combinations. The amount of label-derived C recovered as OPOM and HF increased between Dec. 1995 and May 1996 in both depths of the MP and in the 5±20 cm depth of the NT plots. Within a year, the amount of label recovered in

Fig. 4. Temporal change in tillage and depth eects on the percent recovery of label-derived C in the loose- and occluded-particulate or-ganic matter (LPOM and OPOM), and humi®ed (HF) fractions. Dierences between lower case letters identify OPOM means and upper case letters identify HF means that dier atP< 0.05. Fig. 3. Temporal change in the fractional abundance of label-derived

C recovered in the (a) loose- (fLPOM, see inset) and occluded-particu-late organic matter (fOPOM), and (b) humi®ed fractions (fHF) of the 0±5 and 5±20 cm depths of moldboard plowed (MP) and no-tilled (NT) plots. For individual fractions, dierences between letters ident-ify means that dier atP< 0.05.

(8)

the 0±5 cm depth of the MP and 5±20 cm depth of the NT plots had declined to 1995 values, while recovery in the OPOM and HF from the MP 5±20 and NT 0±5 cm depths remained unchanged. In general, about 60% of the label initially recovered was lost between Dec. 1995 and spring 1997; a much greater proportion of the label was initially recovered in the NT plots than was lost from the MP plots.

Temporal trends infOPOM andfHF were in¯uenced by

the interaction between tillage practices and label ori-gin (Fig. 5). Initially, fOPOM and fHF were greatest in

the root-labeled NT plots. By May 1996, fOPOM had

increased in all MP and in the shoot-labeled NT plots, while it declined in the root-labeled NT plots. Simi-larly, the fHF of the shoot-labeled MP and NT plots

increased during that time period. The fHF of the

root-labeled plots had increased, but not signi®cantly, by May 1996. Only fOPOM and fHF of shoot labeled MP

plots declined signi®cantly during the 1996±1997 sea-son.

4. Discussion

4.1. Labeling and fractionation of total, loose and occluded SOC

During the 1995 growing season whole soil d

increased by 1.4±16.3- in unlabeled plots due to corn production. The increase in 13C isotopic abundance is consistent with the magnitude of change (1.6-) observed after one corn crop by Qian and Doran (1996). Pulse labeling only increased the whole soil d

values by an additional 0.58- but increases in

d_{corn residue} _and d_LPOM _{were sucient to allow}

short-term C dynamics (1.5 years) to be assessed.

Our fractionation scheme revealed the in¯uence of aggregate protection on the short-term decay dynamics of C derived from newly-incorporated residues. Reduced label recovery in the LPOM between Dec. 1995 and May 1996 coincided with increased total SOC recovered in fractions and increased label recov-ery in the OPOM and HF fractions. The temporal trends in all fractions were in¯uenced by the large sieve size (8 mm) used to initially process the soil. Macroorganic matter typically removed from soils before quanti®cation of SOC was a focal point of this study. The May 1996 peak in OPOM, HF and total C in the MP and NT 5±20 cm depths likely re¯ects the movement of C, that was in residues too large to measure in Dec. 1995, into measured particulate and humi®ed forms. Additionally, there was a net transfer of label-derived C during the winter from LPOM and crop residues, not yet incorporated into SOC fractions in Dec. 1995, into the OPOM and HF fractions. Tem-poral changes in d_,_f_{, and percent label recovery}

indi-cate that LPOM decayed rapidly while OPOM and HF were less dynamic. These ®ndings are consistent with those of Beare et al. (1994), Gregorich et al. (1995), Besnard et al. (1996) and Jastrow et al. (1996) who showed that aggregate occluded-particulate or-ganic matter (POM) has a slower turnover rate than does POM that is loose and not protected by mineral aliation.

4.2. Tillage and C dynamics

At the onset of this experiment, we had anticipated that the inputs and decay rates of crop-derived C would be lesser in the NT plots. This assumption was not supported by our ®ndings. In Dec. 1995, approxi-mately twice as much label-derived C was recovered from NT than MP plots, suggesting that less residue-derived C had been added to the MP plots. The amount of residue-derived C contained in the MP plots could have been underestimated if residues were buried below the depth of sampling in Dec. 1995. Chi-sel plowing of those plots in Dec. 1997 is likely to

(9)

have further diluted our microplots with unlabeled soil. Rapid mineralization of residues following their incorporation by plowing could also explain the low recovery of label-derived C from the MP plots (Prior et al., 1997). By mixing residues into the soil in a fairly concentrated mass, plowing can shift microbial decom-poser communities from autochthonous, high eciency species to more zymogenous populations with lower metabolic eciencies (Bradley and Fyles, 1995). Regardless of whether lower C-additions to the MP plots were caused by mineralization or dilution of C by unlabeled soil, less residue-derived C was initially accounted for in the 0±20 cm depth of the MP plots. During the next 1.5 years, more of the label initially recovered was ultimately lost from the NT plots, indi-cating that during that period residue decay rates in the top 20 cm depth were actually faster than rates occurring in the MP plots.

4.3. The in¯uences of tillage, physical protection and roots on SOC

We had expected that aggregate protection of macroorganic matter and root contributions to SOC would be greater in the NT than in MP plots. Our ®ndings indicate that the placement pattern, compo-sition or origin of residue interacted with physical pro-tection to in¯uence SOC sequestration. Initially, more root- than shoot-derived C was incorporated into OPOM and HF fractions at the surface of the NT plots. This ®nding is consistent with the results of Balesdent and Balabane (1996), who observed that SOC derived from corn root-derived C was 1.5 times that of stalks + leaves. They attributed this to enhanced belowground production and to relatively slow biodegradation of root-derived materials. In our work, the relatively low fractional abundance of root-derived C in the OPOM and HF fractions recovered in Dec. 1995 from tilled plots might re¯ect dilution by mixing of label rich surface soil by depleted subsoil or relatively low root productivity. The lowfOPOM andfHF

of root-derived C in the 5±20 cm depth of the NT plots at that time likely re¯ects limited inputs by roots. The relatively highfOPOM and fHF in the 0±5 cm depth

of the NT plots then, probably re¯ect root concen-tration and shoot contributions to SOC. Between May 1996 and April 1997, the fractional abundance of root-derived C in OPOM and HF fractions did not decline. ThefOPOM andfHF of shoot-derived C increased

signi®-cantly by May 1996 in MP and to a lesser degree in NT plots and declined again by April 1997 to equal 1995 values. While Richter et al. (1990) found that root-derived C was a dominant factor in the C balance of tilled soils and argued that tillage induced SOC de-pletion was associated with the preferential loss of root-derived C, our data suggest that the tilled soil

failed to conserve shoot-derived C. This could stem from the incorporation of shoot residues as a mass, as the proportion of newly-added C conserved in SOC has been shown to decrease as the concentration of C added increases (Bremer and Kuikman, 1994; Jans-Hammermeister et al., 1997). Collectively these results suggest shoot- and root-derived residues move between SOC fractions at dierent rates, that root-derived ma-terials are more rapidly occluded by aggregates and are likely to contribute to humic materials where roots are concentrated. Besnard et al. (1996) showed that aboveground and root residues were probably not in-corporated homogeneously in organic matter fractions and that more root-like macroorganic matter was pre-served within aggregates than was retained in the free POM. Overall, our ®ndings suggest that root-derived SOC in OPOM and HF fractions may be more persist-ent in the long-term. Even though the total amount of residue-derived C retained after 1.5 years did not dier in the MP and NT soils, there were fundamental dierences in the depth, decay dynamics and distri-bution of C newly-assimilated into SOC fractions.

References

Alvarez, R., Russo, M.E., Prystupa, P., Scheiner, J.D., Blotta, L., 1998. Soil carbon pools under conventional and no-tillage systems in the Argentine Rolling Pampa. Agronomy 90, 138±143. Angers, D.A., 1998. Water-stable aggregation of Quebec silty clay

soils: some factors controlling its dynamics. Soil and Tillage Research 47, 91±96.

Angers, D.A., Bolinder, M.A., Carter, M.R., Gregorich, E.G., Drury, C.F., Liang, B.C., Voroney, R.P., Simard, R.R., Donald, R.G., Beyaert, R.P., Martel, J., 1997. Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada. Soil and Tillage Research. 41, 191±201.

Balesdent, J., Balabane, M., 1996. Major contribution of roots to soil carbon storage inferred from maize cultivated soils. Soil Biology & Biochemistry 28, 1261±1263.

Beare, M.H., Pohlad, B.R., Wright, D.H., Coleman, D.C., 1993. Residue placement and fungicide eects on fungal communities in conventional and no-tillage soils. Soil Science Society of America Journal 57, 392±399.

Beare, M.H., Cabrera, M.L., Hendrix, P.F., Coleman, D.C., 1994. Aggregate-protected and unprotected organic matter pools in conventional- and no-tillage soils. Soil Science Society of America Journal 58, 787±795.

Berg, J.D., Hendrix, P.F., Cheng, W.X., Dillard, A.L., 1991. A label-ing chamber for13_{C enrichment of plant tissue for decomposition}

studies. Agriculture Ecosystems and Environment 34, 421±425. Besnard, E., Chenu, C., Balesdent, J., Puget, P., Arrouays, D., 1996.

Fate of particulate organic matter in soil aggregates during culti-vation. European Journal of Soil Science 47, 495±503.

Bradley, R.L., Fyles, J.W., 1995. A kinetic parameter describing soil available carbon and its relationship to rate increase in C miner-alization. Soil Biology & Biochemistry 27, 167±172.

Bremer, E., Kuikman, P., 1994. Microbial utilization of

14

C[U]Glucose in soil is aected by the amount and timing of glucose addition. Soil Biology & Biochemistry 26, 511±517. Buyanovsky, G.A., Wagner, G.H., 1986. Post-harvest input to

crop-land. Plant and Soil 93, 57±65.

(10)

Franzluebbers, A.J., Arshad, M.A., 1996. Soil organic matter pools with conventional and zero tillage in a cold, semiarid climate. Soil and Tillage Research 39, 1±11.

Golchin, A., Oades, J.M., Skjemstad, J.O., Clarke, P., 1994. Study of free and occluded particulate organic matter in soils by solid state13C CP/MAS NMR spectroscopy and scanning electron mi-croscopy. Australian Journal of Soil Research 32, 285±309. Gregorich, E.G., Ellert, B.H., Monreal, C.M., 1995. Turnover of soil

organic matter and storage of corn residue carbon estimated from natural 13_{C abundance. Canadian Journal of Soil Science}

75, 161±167.

Grith, D.R., Kladivko, E.J., Mannering, J.V., West, T.D., Parsons, S.D., 1988. Long-term tillage and rotation eects on corn growth and yield on high and low organic matter, poorly drained soils. Agronomy Journal 80, 599±605.

Havlin, J.L., Kissel, D.E., Maddux, L.D., Claassen, M.M., Long, J.J., 1990. Crop rotation and tillage eects on soil organic carbon and nitrogen. Soil Science Society of America Journal 54, 448± 452.

Hu, S., Coleman, D.C., Beare, M.H., Hendrix, P.H., 1995. Soil carbohydrates in aggrading and degrading ecosystems: in¯uences of fungi and aggregates. Agriculture Ecosystems and Environment 45, 77±88.

Jans-Hammermeister, D.C., McGill, W.B., Izaurralde, R.C., 1997. Management of soil C by manipulation of microbial metabolism: daily versus pulsed C additions. In: Lal, R., Follett, R.F., Stewart, B.A. (Eds.), Soil Processes and the Carbon Cycle. Lewis Publishers, Boca Raton, pp. 321±333.

Jastrow, J.D., Boutton, T.W., Miller, R.M., 1996. Carbon dynamics of aggregate-associated organic matter estimated by carbon-13 natural abundance. Soil Science Society of America Journal 60, 801±807.

Linden, D., Clapp, C.E., 1998. Eect of corn and soybean residues on earthworm cast carbon content and natural abundance isotope signatures. In: Lal, R., Follett, R.F., Stewart, B.A. (Eds.), Soil Processes and the Carbon Cycle. Lewis Publishers, Boca Raton, pp. 345±351.

Needelman, B.A., Wander, M.M., Bollero, G.A., Boast, C.W., Sims, G.K., Bullock, D.G., 1999. Interaction of tillage and soil texture: biologically active soil organic matter in Illinois. Soil Science Society of America Journal 63, 1326±1334.

Parton, W.J., Ojima, D.S., Cole, C.V., Schimel, D.S., 1994. A gen-eral model for soil organic matter dynamics: sensitivity to litter chemistry, texture, and management. In: Bryant, R.B, Arnold, R.W. (Eds.), Quantitative Modeling for Soil Forming Processes. Soil Science Society of America, Madison, pp. 147±167.

Prior, S.A., Rogers, H.H., Runion, G.B., Torbert, H.A., Reicosky, D.C., 1997. Carbon dioxide-enriched agroecosystems: in¯uence of tillage on short-term soil carbon dioxide eux. Journal of Environmental Quality 26, 244±252.

Qian, J.H., Doran, J.W., 1996. Available carbon released from crop roots during growth as determined by carbon-13 natural abun-dance. Soil Science Society of America Journal 60, 828±831. Richter, D.D., Babbar, L.I., Huston, M.A., Jaeger, M., 1990. Eects

of annual tillage on organic carbon in a ®ne-textured Udalf: the importance of root dynamics to soil carbon storage. Soil Science 149, 78±83.

SAS Institute, 1996. SAS/STAT, Vers. 6.11, SAS Inst. Inc., Cary, NC.

Scott, N.A., Cole, C.V., Elliott, E.T., Human, S.A., 1996. Soil tex-tural control on decomposition and soil organic matter dynamics. Soil Science Society of America Journal 60, 1102±1109.

Wander, M.M., Bidart-Bouzat, G., Aref, S., 1998. Tillage impacts on depth distribution of total and particulate organic matter in three Illinois soils. Soil Science Society of America Journal 62, 1704± 1711.

Wolf, D.C., Legg, J.O., Boutton, T.W., 1994. Isotopic methods for the study of soil organic matter dynamics. In: Weaver, R.W., Angel, J.S., Bottomey, P.S. (Eds.), Methods of Soil Analysis, Part II. Soil Science Society of America, Madison, pp. 865±906. Yang, X.M., Wander, M.M., 1999. Tillage eects on soil organic