SSSAJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 1 May 2008
Published in Soil Sci Soc Am J 72:670-676 (2008)
DOI: 10.2136/sssaj2007.0110
© 2008 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Agricola
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Related Collections
Right arrow Structure and Properties
Right arrow Carbon Sequestration
Right arrow Tillage

SOIL & WATER MANAGEMENT & CONSERVATION

Tillage Effects on Aggregate Turnover and Sequestration of Particulate and Humified Soil Organic Carbon

G. Yooa and Michelle M. Wanderb,*

a Korea Environment Institute, 613-2 Bulgwang-Dong, Eunpyeong-Gu, Seoul, 122-706 Korea
b Dep. of Natural Resources and Environ. Sci., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801

* Corresponding author (mwander{at}uiuc.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The seasonal dynamics of soil organic carbon (SOC) fractions and dry aggregates were investigated to determine whether aggregate turnover rates explain the variable influence of no-till (NT) practices on SOC accrual in Illinois. Soils were collected (0–15 cm) of two 17-yr-old trials where NT and conventional tillage (CT) practices had variable effects on SOC sequestration in the 0- to 30-cm depth. Soil was fractionated into loose particulate organic matter (LPOM), macroaggregate-occluded particulate organic matter (OPOM), and humified fractions (HFs) associated with microaggregates and minerals. Dry aggregate mean weight diameter (DMWD) was used to evaluate structure expressed in the field. Within-season changes in fraction concentrations indicate that C derived from LPOM and plant roots was transferred to OPOM at both sites. At DeKalb (silty clay loam), NT practices had less influence on SOC sequestration than at Monmouth (silt loam). DeKalb HF contents increased under NT while OPOM contents were unchanged. At Monmouth, NT practices increased SOC sequestration by increasing OPOM concentrations. Site-based differences in SOC accrual were related to aggregate turnover rates, which were increased by NT practices at Monmouth but were unaffected by tillage at DeKalb. Trends in DMWD and aggregate turnover suggest that when aggregate turnover is slow, as at DeKalb, OPOM formation is limited. Slow turnover in the heavier soil permitted accumulation of HF because residues had longer contact times with particle surfaces. When aggregate turnover is faster, as in the Monmouth NT soil, there is greater chance for residue incorporation into aggregates and accumulation of OPOM. Rapid aggregate turnover limits opportunities for mineral affiliation and HF accrual. Aggregate dynamics predicted site-based differences in the form and rate of SOC accrual caused by the use of NT practices.

Abbreviations: CT, conventional tillage • DMWD, dry aggregate mean weight diameter • HF, humified fraction • LPOM, loose particulate organic matter • NT, no-till • OPOM, occluded particulate organic matter • POM, particulate organic matter • SOC, soil organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
No-till practices are advocated as a strategy to prevent soil degradation and increase soil organic matter sequestration (Phillips et al., 1980; Lal et al., 2007). The ability of NT practices to increase SOC reserves in arable systems is known to vary among systems, locations, and soil depths, and with the methods used to evaluate SOC stock size (VandenBygaart and Angers, 2006). To manage individual soils effectively, we must improve our understanding of the mechanisms that control C dynamics.

Efforts to predict the influence of reduced tillage on SOC sequestration differ substantially in the way they describe physical controls over organic matter dynamics. Arrouays et al. (2006) suggested that practices like NT, which can increase SOC stocks, should be targeted to sites with high organic matter levels because these soils have greater silt plus fine clay contents and therefore higher saturation capacities. Work conducted by Denef et al. (2004) suggested that NT practices promote sequestration by increasing microaggregate protection of soil organic matter. Plante et al. (2006) found that microaggregate protection of SOC decreased with increasing clay plus silt contents at two sites with large texture gradients. The particulate organic matter (POM) fraction has been found to accumulate in surface depths when tillage frequency is reduced, and rates of accrual appear to be greater in finer textured soils (Amado et al., 2006; Needleman et al., 1999). Subdivision of POM into loose and aggregate-protected components can provide insight into tillage dynamics (Magid et al., 1996; Wander and Yang, 2000). Several studies have shown that tillage reduces the amount of LPOM in surface soils (Puget, 1997; Wander and Bidart, 2000; Yang and Kay, 2001). Liang et al. (2002) found that infrequent tillage did not significantly reduce LPOM C contents in Brown and Dark Brown Chernozemic soils in Canada. Plante et al. (2006) found no effect of texture on unprotected POM.

The influence of tillage practices on OPOM contents is less consistent and might provide insight into site and soil type differences. Work conducted at the sites we considered in this study has shown that the use of NT practices for a decade increased OPOM contents in the surface (0–5 cm) of the silt loam soil but had no influence on OPOM contents of the silty clay loam (Wander and Bidart, 2000). Yang and Kay (2001) observed a similar trend, where NT practices increased the proportion of SOC stored as OPOM (0–15 cm) in a sandy loam soil but not in a clay loam soil. They also showed that the use of NT practices caused C accrual in humified forms in the heavier soil. Differences in structure, understood through measures of bulk density, the least limiting water range, and soil pore characteristics, help explain site-based differences in SOC sequestration (Yoo et al., 2006; Yoo and Wander, 2006). These results and others (Strong et al., 2004) suggest that aeration and physical controls on biological activity may have as much or more influence on a soil's sequestration potential as does particle surface area.

The dynamics of macroaggregation and the fate of POM could account for the variable influence of tillage practices on organic matter equilibrium levels. Six et al. (2000) proposed cycles of formation, stabilization, and degradation of stable macroaggregates (>250 µm) separated by wet sieving, and suggested that incorporation of loose residues into macroaggregates was the first step before POM occluded within a macroaggregate humified and became affiliated with clay and silt particles and microaggregates. Plante and McGill (2002a,b) have noted that even though the physical protection of SOC within soil macroaggregates may be short term, aggregation promotes long-term SOC retention in soil by increasing the contact time between decaying residues and mineral surfaces. Their work considered water-stable macroaggregates, which constitute only a portion of the soil. Natural peds separated by physical fragmentation may provide a better surrogate for in situ soil structure than wet-sieved aggregates (Perfect et al., 1993; Yang and Wander, 1998; Nissen and Wander, 2003). Several studies suggest that peds obtained by dry sieving have functional relevance. Linquist et al. (1997) showed that dry aggregate size influenced P sorption by determining the reactive surface mass actively participating in chemical reactions. Blackwood et al. (2006) showed that the eubacterial community differed in the surface and centers of peds obtained by dry sieving. The mass distribution of soil among aggregate classes influences physical protection in bulk soil. Soils containing a larger proportion of large-sized peds are expected to contain more SOC because labile fractions within these aggregates would be more protected from decay. Assuming that soil aggregation is maintained by a dynamic equilibrium, Plante and McGill (2002a) argued that seasonal variation of aggregate wet mean weight diameter could be used to compare the relative rates of formation and destruction of aggregates. They found that more rapid turnover of water-stable aggregates increased the rate of residue incorporation. Based on this, one might expect OPOM to accumulate with increased ped turnover. Soil HF concentrations are likely to be little influenced by dry aggregate turnover because formation of HF occurs mostly inside stable macroaggregates, which have longer half-lives than dry aggregates. The objectives of this experiment were to: (i) develop a mechanistic understanding of the relationship between aggregate and SOC fraction dynamics by characterizing the influence of tillage practices on seasonal changes in LPOM, OPOM, HF, and aggregate DMWD; and (ii) relate DMWD and dry aggregate turnover rates to SOC fraction dynamics.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
This work was conducted at a study site established at two University of Illinois agronomy research centers located in DeKalb and Monmouth, IL, where soils are a poorly drained Drummer silty clay loam (fine- silty, mixed, mesic Typic Haplaquoll) and a somewhat poorly drained Muscatine silt loam (fine-silty, mixed, mesic Aquic Hapludoll), respectively. Similar CT and NT practices have been in place at both sites since 1985 and corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] have been continually grown in rotation. At each experimental location, main tillage treatments were laid out in a completely randomized block with three replicate blocks per location and a total of six plots per location. Plots were 9.1 m wide by 36.6 m long in Monmouth and 6.1 by 36.6 m in DeKalb. Both corn and soybean phases of the rotation were grown each year. The CT treatment consisted of moldboard plowing to a 20- to 25-cm depth after corn and chisel plowing to a 10- to 15-cm depth after soybean. Fertilization and pest control practices were consistent with the University of Illinois's recommendations, with late fall injection of anhydrous NH3 to a depth of 15 to 20 cm after soybean. Corn stalks were chopped in the fall.

Soil Sampling and Preparation for Analysis
Soils were collected from plots seeded to corn immediately after planting, in midsummer, in early fall, and after fall tillage in 2002 (May, July, September, and November or December) and in early spring of 2003. The sampling depth was limited to 15 cm, which is where the influence of tillage on SOC concentrations is most apparent (Diaz-Zorita and Grove, 2002; Wright and Hons, 2004). For information on storage within the upper 30 cm, see Yoo and Wander (2006). A splittable core sampler with a 4.7-cm i.d. and 30 cm in length (Forest Supply, Jackson, MS) was used to collect two cores per plot. Each core was divided into two depths (0–5 and 5–15 cm) before samples from each plot were combined in a plastic bag in the field. A subsample was taken to determine gravimetric moisture content, and bulk density was determined by the core method (Krzic et al., 2000). Within a day after sampling, soil samples were weighed and then passed through a 2.5-cm sieve to fragment soil before drying.

Soil Fractionation
Soil samples collected on four dates were used for SOC fractionation. A 20-g sample of whole soil was generated by combining aggregates <8 mm in proportion to their relative mass. Aggregates >8 mm were excluded to ensure that the aliquot of soil used in subsequent analyses was well mixed. Separate analysis of SOC and organic matter fractions contained in dry aggregate classes has shown that the quantity and composition of SOC in 8-mm aggregates is representative of bulk soil (Yoo et al., 2006; and other preliminary studies conducted by this lab group), and thus we assumed that exclusion of material in aggregates 8 mm and larger would not bias results.

Loose particulate organic matter was separated from samples using the method described by Wander and Yang (2000). This fraction was obtained using density to separate material recovered after gentle shaking. Briefly, a 20-g soil sample was placed in a 250-mL tube to which 50 mL of sodium polytungstate (1.6 g cm–3; Geoliquids, Chicago, IL) was added. The tube was orbitally shaken at low speed (200 oscillations min–1) for 30 min. As is typical for procedurally based methods (for a review, see Wander, 2004), the timing and energy combination used was selected after preliminary work to identify conditions that would liberate light organic matter without destroying stable structure. Particles adhering to the side of the tube after shaking were rinsed with 10 mL of sodium polytungstate and the solution was allowed to stand overnight. Tubes were then centrifuged at 5000 rpm for 30 min. Materials recovered from the supernatant on 1-µm polycarbonate membrane filters were rinsed with 50 mL of deionized water and dried at 80°C. This fraction was assumed to have little or no physical protection.

The POM remaining was considered aggregate occluded (OPOM) and may approximate POM retained in all sizes of water-stable aggregates. This was separated from the relatively humified, mineral-associated organic material (HF) after the residue was shaken at high speed (350 oscillations min–1) for 60 min with 50 mL of deionized water. The suspension was filtered through a 53-µm polycarbonate mesh (Gilson Co., Columbus, OH). The OPOM retained on the mesh was dried at 80°C. The LPOM, OPOM, and bulk soil were ground using a disk mill before C and N were determined using dry combustion (Carlo Erba NA 1500 C/N, Carlo Erba, Milan). The quantity of SOC in the HF fraction was calculated by subtracting the sum of the C contents in LPOM plus OPOM from total SOC (Wander and Yang, 2000). The HF is considered to be a mixture of organo-clay, organo-mineral, and biochemically recalcitrant materials.

Dry Aggregate Dynamics
Soil samples collected on the four dates in 2002 and spring 2003 were used for dry aggregate analysis. Dry aggregates were obtained by placing 100 to 200 g of air-dry soil on the top of a stack of six screens (25-cm diameter, 8-, 4-, 2-, 1-, 0.25-, 0.05-mm openings) and sieving for 2 min with a Ro-tap sieve (W.S. Tyler, Mentor, OH). This combination of soil mass and timing has been found to distribute the soil among sieves without abrading them. After soils were dry sieved, soil remaining on each sieve was collected and weighed. The dry aggregate size distribution was expressed in terms of DMWD (Youker and McGuiness, 1957):

Formula 1[1]
where Xi is the mean diameter of the size fraction or size class midpoint, and Wi is the proportion of the total sample retained on the sieve.

We assumed that at these study sites, soils were at steady state in reference to aggregation because the experimental treatments had been in place for 17 yr, which is long enough for the influence of tillage to be expressed (Plante and McGill, 2002a). Seasonal variation in DMWD primarily reflects the net change in buildup and breakdown of larger peds and was used to calculate the turnover rate of dry aggregates. The dry aggregate turnover rate is defined as the absolute value of the difference in weighted averages of DMWD in the 0- to 15-cm depth measured on adjacent dates divided by the number of days between two dates:

Formula 2[2]
where DMWD(ti) and DMWD(ti+1) are the weighted averages of DMWD in the 0- to 15-cm depth measured on adjacent dates.

Aggregate turnover rates calculated from the seasonal change in DMWD were verified with the first-order model of Plante et al. (2002) describing the flow of materials among four aggregate size compartments (>4, 2–4, 1–2, and <1 mm). The model was fit to the observed data by the method of least squares using the Excel equation editor (Microsoft Office Excel 2003).

Statistical Analyses
Analysis of variance was performed using SAS PROC MIXED on C concentrations in LPOM, OPOM, total POM (TPOM), which is the sum of LPOM and OPOM, and HF; the OPOM/HF ratio; DMWD; and the dry aggregate turnover rate (SAS Institute, 2001). Tillage, site, depth, and date were the fixed effects and block (site) was the random effect. Significant differences among least square means of LPOM, OPOM, HF, OPOM/HF ratio, DMWD, and aggregate turnover rate were tested at a 0.05 significance level. Correlation coefficients were calculated using the CORR procedure of SAS among DMWD, bulk density, SOC, and soil water content. Multiple regression was implemented using the REG procedure of SAS (SAS Institute, 2001) to predict DMWD values from bulk density, soil water, and SOC contents. The mean DMWD was plotted against OPOM/HF ratios and HF contents to see the influence of dry aggregate turnover rate on accumulations of OPOM and HF. Correlation coefficients were calculated using the CORR procedure of SAS (SAS Institute, 2001) between seasonal mean DMWD and mean concentration of soil HF C.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Organic Carbon Fractions
Seasonal variation in LPOM C concentrations was similar in all soils (Table 1 , Fig. 1a ), with concentrations decreasing from May to September and then increasing between September and early winter. Seasonal trends in OPOM C concentrations varied with tillage and depth (Table 1, Fig. 1b). Soil OPOM C concentrations remained unchanged from May to July regardless of tillage treatment or depth. For the period after July, C concentrations in OPOM increased in all cases except for the NT soil at the 5- to 15-cm depth. In the 0- to 5-cm depth (Fig. 1), the increase in OPOM C concentrations from May to September was 1.67 g C kg–1 soil in the NT and 0.88 g C kg–1soil in the CT soils. In the 5- to 15-cm-depth soils, there was no change in the NT soil and an increase of 0.46 g C kg–1 soil in the CT soil. The increase in soil OPOM C concentrations seen during the growing season (May–September) is due in part to the transfer of C from LPOM to OPOM. This trend is consistent with other observations (Golchin et al., 1994; Puget et al., 1995, 2000; Wander and Yang, 2000; Bossuyt et al., 2002). Increases in OPOM C concentrations, however, exceed losses from LPOM reservoirs, which were 0.35 g C kg–1soil on average. It is probable that increases in OPOM C are the result of root inputs, which are important contributors to OPOM (Wander and Yang, 2000; Puget and Drinkwater, 2001). Studies using 13C natural abundance indicated that midseason increases in POM concentrations in fields in corn are due to root inputs (unpublished data, 2004). The biggest discrepancy between LPOM loss and OPOM gain occurred in the 0- to 5-cm depth of the NT soil. Similar results have been noted by Wander and Yang (2000) and Gale and Cambardella (2000). The failure of NT practices to increase OPOM concentrations below the surface was probably due in part to lack of mixing of aboveground residues (Balesdent et al., 1998). Soil HF C contents did not vary seasonally (Table 1). This is not surprising, as the HF constitutes a relatively large proportion (typically 80–90%) of total C and consists of more recalcitrant material.


View this table:
[in this window]
[in a new window]

  		Table 1. Variance analysis of loose (LPOM), occluded (OPOM), and total particulate organic matter (TPOM), the humified fraction (HF), the OPOM/HF ratio, aggregate dry mean weight diameter (DMWD), and dry aggregate turnover rate.

 

Figure 1
View larger version (10K):
[in this window]
[in a new window]

  		Fig. 1. Seasonal change of C concentrations in (a) loose (LPOM) and (b) occluded particulate organic matter (OPOM). Means not followed by the same letter differ at P < 0.05.

 
Average concentrations of LPOM C were similar at the two sites and there was a significant tillage x depth interaction (Fig. 2a ). The LPOM C concentration was greater in the surface depth of NT than CT soils and was similar in the 5- to 15-cm depth. Chan et al. (2002) also found that increases in LPOM concentrations were restricted to the surface (0–5-cm) depth. Removal of LPOM from total POM made tillage, site, and depth-based interactions more apparent (Table 1, Fig. 2b). At Monmouth, OPOM C concentrations were greater in the NT than CT soils in the 0- to 5-cm depth. This result is consistent with other studies showing that NT practices increase SOC contents by protecting labile soil organic matter (Wander and Yang, 2000; Chan et al., 2002; Liang et al., 2002). At DeKalb, tillage practices did not alter OPOM C concentrations at either depth. The influence of tillage practices on average HF C concentrations also varied by site (Fig. 2c), with HF C concentrations only being increased under NT at DeKalb. Tillage-based differences in HF C contents were not apparent at DeKalb in 1996, when treatments had been in place for a decade (Wander et al., 1998). This may be due to the soil's compressibility, which is known to be correlated with clay contents up to 33% (Imhoff et al., 2004). Thus with contents of ~40% high-activity clay, the DeKalb soil may have been resistant to compaction and this slowed the rate of change in soil properties under NT that could reduce C mineralization by increasing physical protection of SOC. Clay content (26.5%) and clay activity are both lower at Monmouth than at DeKalb.


Figure 2
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 2. Seasonal averages of C concentrations in (a) loose (LPOM), (b) occluded particulate organic matter (OPOM), and (c) the humified organic matter fraction (HF). For individual fractions, means not followed by the same letter differ at P < 0.05.

 
Aggregate Dynamics and Soil Organic Carbon Fractions
The influence of tillage practices on mean DMWD varied by site, date, and depth (Table 1, Fig. 3 ). At DeKalb, NT DMWD was higher than in CT soils only in the 0- to 5-cm depth, whereas NT DMWD was greater than CT treatments in both soil depths at the Monmouth site. This is consistent with the findings of others (Mrabet et al., 2001; Unger and Fulton, 1990; Yang and Wander, 1998).


Figure 3
View larger version (24K):
[in this window]
[in a new window]

  		Fig. 3. Seasonal changes in dry mean weight diameter (DMWD) from May 2002 to April 2003 in (a) DeKalb and (b) Monmouth. Values are depth-weighted averages. Means that are not followed by the same letter differ at P < 0.05.

 
Tillage practices had different effects on seasonal variation of aggregate DMWD (0–15 cm) at the two sites (Table 1, Fig. 4 ). At DeKalb, the DMWD of NT soils declined from May to July, slowly returned to the value observed in May during the growing season, and then remained stable during the winter. In the CT soil at that site, DMWD remained unchanged throughout the period of observation. At Monmouth, the NT treatment DMWD declined from May to July before returning to the value observed in May, where it remained until late fall when the value then again declined. In tilled soils at both sites, DMWD values did not change during the growing season and declined only slightly between fall and spring. Armbrust et al. (1982) also observed reductions in the percentage of larger dry aggregates (>0.84 mm) during the winter.


Figure 4
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 4. Seasonal averages of dry mean weight diameter (DMWD). Within-depth means that are not followed by the same letter differ at P < 0.05.

 
Aggregate DMWD was positively correlated with soil water contents and bulk density, with Pearson correlation coefficients of 0.295 (P < 0.001) and 0.439 (P < 0.001), respectively. This relationship has been reported by others (Zobeck and Popham, 1990; Nissen and Wander, 2003). We found no correlation between DMWD and total SOC contents when data from all time points was considered. This is probably because DMWD varies with moisture and root activity. Multiple regression showed that total SOC, bulk density, and soil moisture contents [DMWD = –5.047 – 0.036(SOC) + 0.230(gravimetric moisture content) + 7.381(bulk density)] explained ~42% of the seasonal variation of DMWD. Other research on similar soils (Yang and Wander, 1998; Wander and Bollero, 1999; Nissen and Wander, 2003) have suggested that the relationship between SOC and DMWD varies as a result of the spatial and temporal scale of comparison and field conditions. The limited ability of our multiple regression models to predict seasonal changes in DMWD may be due to having only three composite samples per site for each sampling date. Aggregate dynamics are known to be influenced by dry–wet and freeze–thaw cycles (Caron et al., 1992b; Layton et al., 1993; Staricka and Benoit, 1995; Lehrsch, 1998) and by biological factors (Angers and Caron, 1998; Denef et al., 2002). The decrease in DMWD from May to July in the NT soils at both sites is probably due to the loosening effect of root growth (Angers and Caron, 1998; Caron et al., 1992a; Denef et al., 2002). The process of aggregate fragmentation by roots is believed to be particularly important in undisturbed or NT soils (Angers and Caron, 1998) and may not be apparent where soils are subject to tillage. The DMWD fluctuated less in CT soils, where bulk density was lower. Notable reductions in aggregate DMWD in NT soils occurred between May and July, when bulk density was relatively high (data not shown). There may be a threshold in soil bulk density beyond which roots will have a fragmenting effect.

On average, the relative turnover rates of dry-sieved aggregates calculated from the DMWD's seasonal change were faster in the NT than CT soils at Monmouth but not DeKalb, where bulk density was low and clay contents were relatively high (Table 2 ). Estimates of the turnover rates of individual aggregate size classes suggest that differences in DMWD were due to the faster turnover of larger aggregates in the NT soil and that this difference was more pronounced in the silty clay loam at Monmouth (Table 3 ).


View this table:
[in this window]
[in a new window]

  		Table 2. Summary of aggregate dynamics and changes in C concentrations in occluded particulate (OPOM) and humified (HF) organic matter fractions and the ratios of OPOM to HF.

 

View this table:
[in this window]
[in a new window]

  		Table 3. Turnover rates of dry aggregates calculated using least squares to fit a first-order model to describe the flow of materials among size classes of >4, 2 to 4, 1 to 2, and <1 mm.

 
Soils with larger mean DMWD maintained higher SOC concentrations, with increases expressed in HF concentrations at DeKalb and OPOM concentrations at Monmouth. Differences in the form of SOC accrued are reflected in the proportion of soil organic matter in OPOM (OPOM/HF ratio), which was increased by the use of NT practices at Monmouth, where aggregate turnover rates were high. Slow aggregate turnover rates in both CT and NT soils at DeKalb explain why the proportion of soil organic matter in the OPOM failed to increase under NT at this site (Table 2). If dry aggregate turnover is slow, as at DeKalb, the potential for OPOM to increase is limited because there is less opportunity for fresh residues to be incorporated into aggregates where OPOM formation occurs; however, OPOM that is protected inside aggregates with longer contact times is further stabilized by incorporation into microaggregates or association with clay or silt particles. In this situation, aggregate size has a positive influence on the accumulation of HF C. Seasonal means of HF C and DMWD were positively related at DeKalb (r = 0.58, P = 0.09, data not show). In contrast, when dry aggregate turnover is relatively fast, as in the NT soils at Monmouth, opportunities for OPOM formation are increased by more frequent incorporation of fresh organic materials into aggregates. In this instance, reduced protection of OPOM by frequent exposure limits HF accrual even as DMWD increases.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Inherent differences in soils alter the influence of tillage practices on the physical protection of SOC. Seasonal dynamics of SOC fractions revealed that LPOM was first formed and then incorporated into OPOM and that OPOM accumulated within a season was probably the result of C inputs from roots. Failure of NT practices to increase OPOM C concentrations at the 5- to 15-cm depth reflects the lack of mixing of plant residues into the subsurface in NT systems. Climatic factors and biological activity limited the ability of multiple regression models based on soil moisture, bulk density, and SOC contents to predict seasonal trends in DMWD. Slower turnover of aggregates allowed longer contact time between residues and particle surfaces, permitting OPOM to be humified within aggregates. Aggregate size may have a significant influence on the accumulation of humified organic matter only when aggregate turnover rates are slow. Faster aggregate turnover, as seen at Monmouth, may renew POM fractions and even lead to accumulation of OPOM without resulting in accumulation of humified SOC. This study is one of the first attempts to explain the variable influences of tillage on SOC fractions by combining the influence of aggregate size and turnover. Results support the idea that there are threshold rates of aggregate turnover that will protect rather than release SOC and that these vary with texture. Future work is needed to generalize the conceptual model proposed and investigate the relationship between texture and aggregate turnover rates, and the form of SOC sequestered.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication March 20, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Agricola
Right arrow Articles by Yoo, G.
Right arrow Articles by Wander, M. M.
Related Collections
Right arrow Structure and Properties
Right arrow Carbon Sequestration
Right arrow Tillage

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome