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Published online 18 June 2008
Published in Soil Sci Soc Am J 72:1132-1139 (2008)
DOI: 10.2136/sssaj2007.0265
© 2008 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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SOIL & WATER MANAGEMENT & CONSERVATION

Indications for Soil Carbon Saturation in a Temperate Agroecosystem

Haegeun Chunga,*, John H. Groveb and Johan Sixa

a Laboratory of the Agroecology Group, Dep. of Plant Sciences, Univ. of California, One Shields Ave., Davis, CA 95616
b Univ. of Kentucky, Dep. of Plant and Soil Sciences, 1100 Nicholasville Rd., Lexington, KY 40546-0091

* Corresponding author (hgchung{at}ucdavis.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The soil C saturation concept postulates that there is an upper limit to the equilibrium soil C level of mineral soils even when soil C input is increased. To test this concept, we analyzed the relationship between steady-state soil C input and soil organic C (SOC) concentration in a temperate corn (Zea mays L.) agroecosystem experiment located in Lexington, KY. In this experiment, a gradient in plant productivity and consequent C input to the soil was produced with four N application rates (0, 84, 168, and 336 kg N ha–1 yr–1) under two disturbance regimes, no-till (NT) and moldboard plowing (MP). We examined whether physical protection of organic matter by soil aggregation and chemical protection by association with silt and clay particles led to SOC saturation. We used wet sieving to physically separate SOC pools that differ in C stabilization potential, and determined the C sequestration in each fraction. Total SOC was positively related to C input, and this was primarily due to C stabilization in small macroaggregates. In both tillage systems, however, microaggregate C and silt-plus-clay C did not increase with greater C input. Within the small macroaggregates, coarse particulate organic matter C and microaggregate C increased with C input, but there was no increase in silt-plus-clay C. Our results indicate that soil fractions with low C stabilization potential exhibit C saturation behavior. Apparent C saturation of some of the fractions indicates that SOC pools have a limited capacity to stabilize added C and that such a limit to C stabilization will constrain the ecosystem services provided by these SOC pools.

Abbreviations: MP, moldboard plowing • MWD, mean weight diameter • NT, no-till • POM, particulate organic matter • SOC, soil organic carbon • SOM, soil organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils are the largest terrestrial C pool, harboring approximately two-thirds of the C in ecosystems (Schimel et al., 1994). Moreover, SOC pools have the slowest turnover rates in terrestrial ecosystems (Trumbore, 1997), and thus C sequestration in soils has the potential to mitigate CO2 emission to the atmosphere (Paustian et al., 1997). Higher soil C stabilization also serves other important ecosystem functions, such as improved soil structure and fertility, increased water and nutrient holding capacity, and greater provision of energy for soil biota (Rasmussen et al., 1998; Lal, 2004; Robertson and Swinton, 2005). Agroecosystem management practices such as reduced tillage disturbance, increased N input, and residue incorporation to soil can significantly increase soil C (Ndayegamiye et al., 1997; Salinas-Garcia et al., 1997; Thomsen and Christensen, 2004). It remains to be determined, however, whether SOC stocks increase infinitely with greater C input or if there is a limit to how much C can be stabilized in soils (Six et al., 2002a).

Most of the soil organic matter (SOM) models assume a linear increase in SOC levels with increasing C input (Paustian et al., 1997). A number of studies in long-term agricultural experiments that continued for >30 yr indeed show that the amount of C sequestered is linearly related to C input (Rasmussen and Parton, 1994; Huggins et al., 1998); however, it has also been observed that in some soils, no extra C is sequestered with a further increase in C input. For instance, in a 30-yr old study, Campbell et al. (1991) reported that N inputs did not increase SOC in soils already rich in SOC. Similarly, in a Canadian barley (Hordeum vulgare L.) field high in organic matter, SOC concentrations remained unchanged even when the C input increased due to 15 yr of N inputs and straw additions (Solberg et al., 1997). In a grassland ecosystem exposed to a gradient of CO2 from 200 to 550 µmol mol–1, the photosynthetic rate increased linearly with CO2 concentration but the change in total SOC exhibited an asymptotic increase with CO2 levels (Gill et al., 2002). These studies suggest that the capacity of soils to stabilize extra C is dependent on how much C is already in the soil rather than the level of C input (Hassink, 1996; Six et al., 2002a). Furthermore, these results indicate that soils can become saturated with C and not stabilize any more C even when soil C input continues to increase (Six et al., 2002a). If this is the case, most of the SOM models that assume a linear increase in SOC levels with greater C input (Paustian et al., 1997) may not be adequate for predicting changes in SOC levels when C input levels increase, especially in high-C soils.

Soil aggregation promotes SOC stabilization by providing physical barriers between decomposers and SOM (Tisdall and Oades, 1982; Elliott, 1986), and if a maximum level of aggregation exists, C sequestration may not increase even when C input to soil increases (Six et al., 2002a). In the Tisdall and Oades (1982) model of hierarchical aggregate organization, various sizes of structural units are bound by distinct binding agents, which makes each soil size fraction unique in C stabilization potential and C turnover rate. Macroaggregates are mainly bound by temporary binding agents such as roots and fungal hyphae and have the highest C concentrations and the fastest C turnover rates. In most cases, organic matter is first incorporated into macroaggregates, and then becomes associated with microaggregates when further decomposed (Oades, 1984; Angers et al., 1997). Therefore, microaggregates are bound by resistant aromatic compounds and have slower C turnover rates. Sequestration of C in microaggregates occluded in macroaggregates can be an important C stabilization mechanism, and studies have shown that C is predominantly stabilized in this soil fraction when tillage ceases or a site is afforested (Six et al., 2002b; Denef et al., 2004). Due to their differences in C stabilization mechanisms, we hypothesized that the C saturation behavior of these aggregate fractions would differ, i.e., microaggregates would be saturated at a lower C input level than macroaggregates.

Chemical association of SOC with silt and clay particles can also impose a limit to the soil's C stabilization potential due to a limit in the surface area of the silt-plus-clay fraction in a given soil (Mayer, 1994; Hassink, 1997; Six et al., 2002a). In a study comparing C stabilization in native grassland and adjacent arable soils, Hassink (1997) found that the greater SOC content of the grassland soils was due to higher SOC concentrations in particles >20 µm, and that the SOC concentrations of particles <20 µm did not differ. These results suggest that the silt-and-clay-associated SOC pool can be C saturated at a lower C input level than larger SOC pools due to a smaller C sequestration potential and slower C turnover. Although observations that suggest soil C saturation at the whole-soil level have been made in a few studies (Campbell et al., 1991; Solberg et al., 1997), we are only beginning to understand the distinct modes of C saturation in SOC pools that are unique in their C stabilization potentials and turnover rates (Kool et al., 2007).

Our objective was to test the soil C saturation concept in a temperate agroecosystem that has a gradient of steady-state soil C input under two disturbance regimes. A corn agroecosystem experiment established in Lexington, KY, in 1970 provided an excellent opportunity to determine if SOC pools that are protected by distinct physical mechanisms exhibit saturation. In this experiment, different disturbance and N input regimes are represented and the long duration of the experiment has led to soils differing in their steady-state C input rates, albeit under similar environmental conditions. We hypothesized that the whole soil and each soil aggregate fraction would saturate at high C input rates relative to low input rates, and that the modes of C saturation would differ across SOC pools due to their distinct C stabilization potentials and turnover rates. More specifically, we hypothesized that smaller soil size fractions will saturate at lower C inputs than larger soil size fractions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Design and Soil Sampling
The experiment was established at the Kentucky Agricultural Experiment Station farm "Spindletop" near Lexington, KY, in 1970. The site had been a bluegrass (Poa pratensis L.) pasture for about 50 yr before the start of the experiment (Frye and Blevins, 1997). In this experiment, corn is grown for grain and winter annual cereals are planted for winter cover between the summer seasons. The experimental design is a split-block design with four replications. Each block (11.0 m by 48.8 m) was split vertically for the tillage treatments, NT and MP, which were randomly assigned to the two halves of each block. Furthermore, each block was split horizontally for randomized allocation of four N input treatments (0, 84, 168, and 336 kg N ha–1 yr–1).

The tilled plots were moldboard plowed to a depth of ~20 cm in mid-April each year, about 1 to 2 wk before planting corn. For secondary tillage of the plowed plots, two passes were made with a tandem disk harrow, cutting to a depth of about 8 cm. Shredded corn stalks were left on the soil surface following corn harvest (Ismail et al., 1994). The N source, NH4NO3, was broadcast over the soil surface about 1 wk after planting. The soil is a moderately weathered Maury silt loam (fine, mixed, semi-active, mesic Typic Paleudalf) with a clay fraction containing high concentrations of amorphous, noncrystalline Fe and Al oxides. The clay fraction also contains both 1:1 (kaolinite) and 2:1 clay minerals (vermiculite) (Six et al., 2000).

Four soil cores, 5.5 cm in diameter and 20 cm in length, were collected from each plot in April 2005. Each soil core was split into 0- to 5- and 5- to 20-cm depth increments, and then composited for separate analyses. We focused on the upper 20 cm of soil because C saturation of this part of the soil would have the most relevance in determining whether soils may serve as an unlimited sink of C or not. At 0 to 5 cm, the bulk density (±1 SE) was 1.27 (±0.02) Mg m–3 under NT and 1.47 (±0.02) Mg m–3 under CT. The bulk density was 1.33 (±0.01) Mg m–3 under NT and 1.42 (±0.01) Mg m–3 under CT at the 5- to 20-cm depth.

Soil Aggregate Fractionation
The soil aggregate fractions were separated using a wet-sieving method adapted from Elliott (1986). A series of three sieves was used to separate large macroaggregates (>2000 µm), small macroaggregates (2000–250 µm), microaggregates (250–53 µm), and the silt-plus-clay fraction (<53 µm). An 80-g subsample of 8-mm sieved and air-dried soil was immersed in water for 5 min on top of a 2000-µm sieve for slaking. Water-stable aggregates were isolated by manually moving the sieve up and down 3 cm for 50 times in 2 min. Aggregate fractions were transferred to aluminum pans, dried in an oven at 50°C, and weighed. As a measure of soil structural stability, the mean weight diameter (MWD) was calculated according to

Formula
where di is the mean diameter and wi is the weight proportion of each aggregate size fraction.

Physical Fractionation of Macroaggregates
Small macroaggregates were further fractionated to particulate organic matter (POM, 2000–250 µm), which is a labile SOC pool that consists mainly of materials with visibly plant-like structure and is not associated with mineral components of a soil (Cambardella and Elliott, 1992), microaggregates (250–53 µm), and the silt-plus-clay fraction (<53 µm) according to the method described by Six et al. (2000). Large macroaggregates were not subject to further separation because the weight proportion of this fraction was <10% of the total soil weight in some samples. Ten grams of small macroaggregates were slaked in water for 20 min before microaggregate isolation to facilitate the process. Macroaggregates were then immersed in deionized water on top of a 250-µm mesh screen inside a cylinder, and were reciprocally shaken with 50 stainless steel beads (diam. 4 mm) for 8 min. Upon complete disruption of the macroaggregates, materials <250 µm were immediately flushed onto the 53-µm sieve with deionized water so that breakup of microaggregates was minimized. Water-stable microaggregates were isolated from this <250-µm material by regular wet sieving using a 53-µm sieve. Aggregate fractions were washed into aluminum pans, oven dried at 50°C, and weighed.

Soil Carbon Analysis
Whole soil, each aggregate fraction from the bulk soil, and fractions within the macroaggregates were ground and analyzed for their C concentrations using a PDZ Europa 20–20 Stable Isotope Analyzer (Europa Scientific, Crewe, UK) at the University of California-Davis Stable Isotope Facility. Carbon associated with whole soil and each fraction was considered to be entirely SOC because no carbonates were present.

Determination of Cumulative Soil Carbon Input
We determined the cumulative soil C input from 1970 to 2005 by converting the grain dry weight (GDW) (Mg ha–1) from each plot to aboveground residue (AGR) and belowground residue (BGR), and then adding the AGR and BGR for each year. The corn residues were estimated using the following equations, which are regression models based on reported corn yield data from agroecosystem experiments in the United States (Intergovernmental Panel on Climate Change, 2006, Table 11.2). In the regression model estimating BGR, rhizodeposition was not included (Intergovernmental Panel on Climate Change, 2006, Table 11.2):

Formula

Formula
To convert cumulative aboveground and belowground residues to cumulative C inputs, we used a C concentration of 40.9% for corn residue (Clay et al., 2006). The treatment effects on cumulative soil C input are shown in Table 1 .


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  		Table 1. Cumulative soil C input under tillage and N input treatments ( in kg N ha–1 yr–1). The effect of N input treatment was significant (P < 0.01), but the tillage (P = 0.44) and tillage by N input interaction (P = 0.18) effects were not significant.

 
Statistical Analyses
We performed analysis of variance to test for the effect of tillage disturbance and N input on the measured variates. To determine the relationship between cumulative C input and SOC concentrations, we performed linear regression analysis using JMP IN 5.1 (SAS Institute, Cary, NC), and significance was accepted at the {alpha} = 0.05 level of significance. For linear regression analysis, we performed a separate analysis of data from each of the two disturbance regimes because the tillage effect was significant (P < 0.05) on the SOC concentration of most soil fractions. Analysis of covariance was performed to compare the linear regression slopes of NT and MP, and significance was accepted at {alpha} = 0.05.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Aggregation and Soil Organic Carbon Concentrations
Small macroaggregates occupied the greatest weight proportion of the whole soil in both tillage systems. The proportion of macroaggregates was significantly lower in the MP soils, and the proportions of the microaggregate and silt-plus-clay fractions were increased (Table 2 ). As a result, MWD was significantly lower under MP in both the 0- to 5- (P < 0.01) and 5- to 20-cm (P = 0.02) depth increments. At 0 to 5 cm, MWD (±1 SE) was 1.4 (±0.1) mm under NT and 0.6 (±0.0) mm under MP. At 5 to 20 cm, it was 1.3 (±0.1) mm under NT vs. 0.9 (±0.1) mm under MP.


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  		Table 2. The weight proportion and soil organic C concentration of soil aggregates under the no-till (NT) and moldboard plowing (MP) systems.

 
Organic C concentrations (±1 SE) for the whole soil were significantly lower under MP than under NT in both the 0- to 5- (23.9 ± 1.1 g C kg–1 soil under NT vs. 11.6 ± 0.4 g C kg–1 soil under MP) and 5- to 20-cm (13.9 ± 0.3 g C kg–1 soil under NT vs. 12.3 ± 0.4 g C kg–1 soil under MP) depth increments. Moldboard plowing also decreased the SOC concentrations of the aggregate size fractions. At 0 to 5 cm, this was the case for all of the aggregate size fractions, and at 5 to 20 cm, this was the case for the microaggregate and silt-plus-clay fractions (Table 2).

Within small macroaggregates, occluded microaggregates accounted for >50% of the weight (Table 3 ). The proportion of microaggregates within the macroaggregates significantly decreased under MP and, in return, the proportion of POM and silt-plus-clay fractions within macroaggregates increased under MP (Table 3). Moldboard plowing significantly lowered the SOC concentration of fractions isolated from small macroaggregates in the 0- to 5-cm soil depth increment (Table 3). At the 5- to 20-cm depth, the SOC concentrations of the POM (P = 0.23), microaggregate (P = 0.07), and silt-plus-clay (P = 0.50) fractions were not different between the two tillage treatments (Table 3).


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  		Table 3. The weight proportion and soil organic C concentration of fractions within small macroaggregates under the no-till (NT) and moldboard plowing (MP) systems.

 
Relationship between Soil Organic Carbon and Soil Carbon Input
Total soil C increased with C input under both disturbance regimes, and this was the case at both the 0- to 5- and 5- to 20-cm depth increments (Table 4 ). Although total soil C exhibited a significant positive relationship with C input in both NT and MP systems, there was a greater increase (P < 0.01) in SOC concentration per unit of C input under NT than under MP at the 0- to 5-cm depth. At 5 to 20 cm, however, the increase in SOC concentration per unit of C input was not significantly influenced (P = 0.18) by tillage regime.


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  		Table 4. Relationships between cumulative C input during 35 yr (Mg ha–1) and soil organic C (SOC) concentrations (g C kg–1 bulk soil) in whole soil and aggregate fractions [SOC = (C input)slope + intercept]. For whole soil and all soil fractions, n = 16. Aggregate fraction relationships are shown in Fig. 1.

 

Figure 1
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  		Fig. 1. The linear relationship between cumulative C input during 35 yr and soil organic C concentration in soil aggregate fractions separated from the bulk soil: (A) no-till (NT), 0 to 5 cm; (B) moldboard plowing (MP), 0 to 5 cm; (C) NT, 5 to 20 cm; (D) MP, 5 to 20 cm. The linear regression equations are given in Table 4. LM = large macroaggregates, SM = small macroaggregates, m = microaggregates, and SC = silt-plus-clay fraction.

 
The increased total SOC with greater C input was primarily due to C sequestration in small macroaggregates (Fig. 1 ). Carbon associated with small macroaggregates (±1 SE) accounted for most of the total soil C under both tillage systems at 0 to 5 cm (59.0 ± 1.6% under NT and 51.8 ± 2.3% under MP) and at 5 to 20 cm (61.9 ± 1.6% under NT and 51.0 ± 1.7% under MP). Small macroaggregate C was positively related to C input in both NT and MP systems (Table 4). There was a larger increase in SOC concentration per unit of C input under NT than under MP in small macroaggregates (P < 0.01 for both 0–5- and 5–20-cm depth increments).

Unlike small macroaggregates, no additional SOC was sequestered in large macroaggregate, microaggregate, or silt-plus-clay fractions (Fig. 1, Table 4). Under both tillage systems, there was no significant relationship between C input and the SOC concentrations of these fractions (P > 0.05, Table 4). This was the case for both the 0- to 5- and 5- to 20-cm depth increments (Table 4).

The SOC concentration of microaggregates within small macroaggregates increased with higher C input under both disturbance regimes at 0 to 5 cm (Fig. 2 , Table 5 ), and the increase in SOC concentration per unit of C input was greater (P < 0.01) under NT than under MP. Soil organic C associated with microaggregates occluded in macroaggregates increased with greater C input only under NT at 5 to 20 cm (Fig. 2, Table 5). Carbon residing in microaggregates within macroaggregates (±1 SE) accounted for most of the SOC in small macroaggregates under both disturbance regimes at 0 to 5 (72.9 ± 3.0% under NT and 55.7 ± 1.3% under MP) and 5 to 20 cm (65.3% ± 0.8% under NT and 60.8 ± 1.0% under MP).


Figure 2
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  		Fig. 2. The linear relationship between cumulative C input during 35 yr and soil organic C concentration in fractions separated from the small macroaggregates: (A) no-till (NT), 0 to 5 cm; (B) moldboard plowing (MP), 0 to 5 cm; (C) NT, 5 to 20 cm; (D) MP, 5 to 20 cm. The linear regression equations are given in Table 5. POM-SM = particulate organic matter within small macroaggregates, m-SM = microaggregates within small macroaggregates, and SC-SM = silt-plus-clay fraction within small macroaggregates.

 

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  		Table 5. Relationships between cumulative C input during 35 yr (Mg ha–1) and soil organic C concentrations (g C kg–1 bulk soil) in fractions within small macroaggregates. For all soil fractions, n = 16. Small macroaggregate fraction relationships are shown in Fig. 2.

 
The silt-plus-clay fraction within small macroaggregates exhibited no increase in SOC with greater C input in either tillage system; this was the case at both 0- to 5- and 5- to 20-cm depth increments (Fig. 2, Table 5). The SOC concentration of the POM fraction within small macroaggregates increased with greater C input under both disturbance regimes at 0 to 5 cm. On the other hand, SOC concentrations of POM within small macroaggregates did not increase in response to greater C input at 5 to 20 cm in either tillage system (Fig. 2, Table 5).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Determining whether soils have a limited capacity to stabilize C will have important implications for management of SOM to combat global warming and to maximize soil fertility. Our study demonstrates that small soil size fractions, due to their low C sequestration potential, are the fractions that show C saturation. Apparent saturation of small size fractions, including the free microaggregates, the free silt-plus-clay fraction, and the silt-plus-clay fraction within small macroaggregates, suggests that there is a limit to physical and chemical protection of SOC in these soil fractions. If the same constraints also apply to larger soil fractions such as macroaggregates, the whole soil will probably reach a limit in C stabilization at higher soil C input (Kool et al., 2007; Gulde et al., 2008).

Carbon Saturation of Small Soil Size Fractions
No increase in SOC associated with the free microaggregate or free silt-plus-clay fractions in response to greater C input indicates that these SOC pools have reached their maximum capacity to sequester C. The smaller size fractions are likely to reach C saturation potential at lower C inputs than larger soil size fractions due to aggregate hierarchy (Tisdall and Oades, 1982) and limits in total surface area (Hassink, 1997). Because of the long duration of this cultivation study (35 yr), it is unlikely that there was not enough time for microaggregates and the silt-plus-clay fraction to incorporate extra SOC with increasing C input. Statistical modeling of C sequestration in SOC pools also showed that the silt-plus-clay fraction reached C saturation at the lowest C input level due to a small C sequestration potential and the low SOM decomposition rate associated with this fraction; C saturation of microaggregates and macroaggregates followed at higher C input levels (Kool et al., 2007).

The increase in total soil C with greater C input was mainly due to increased SOC associated with small macroaggregates, which suggests that new C is being preferentially sequestered in larger soil size fractions because smaller size fractions are saturated with C at a lower C input level. Comparable results have been observed in a California agroecosystem study where the C stabilization potential of soils under different management practices was determined (Kong et al., 2005). In that study, performed 10 yr after the initiation of the field experiment, SOC increased linearly with C input; however, the observed SOC increase was mainly due to SOC stabilization in small macroaggregates, and there was no increase in SOC associated with the free microaggregate or free silt-plus-clay fractions. Similar findings have been reported for a 17-yr-old study on a Brazilian agroecosystem, where the effect of N input on C stabilization was investigated (Diekow et al., 2005). When SOC levels in the sand-, silt-, and clay-sized fractions were compared across a broad range of total soil C values (5–14 Mg C ha–1) in that study, the SOC in the clay-sized fraction exhibited an asymptotic increase, whereas SOC associated with sand- and silt-sized fractions increased linearly with greater total soil C. Altogether, these results suggest that smaller size fractions exhibit C saturation at lower C input levels due to their lower C sequestration potential and that, on saturation, C will be allocated to SOC pools with higher C sequestration potential if C input to the system increases.

The silt-plus-clay fraction within small macroaggregates did not exhibit greater SOC concentration with higher C input, which also indicates C saturation of this fraction. Among the three fractions within the small macroaggregates, SOM in the silt-plus-clay fraction exhibited the lowest C/N ratio, followed by that of the microaggregates and then that of the POM (data not shown). Because POM has a higher C/N ratio than organic matter associated with the soil minerals due to greater N conservation during decomposition (Gregorich et al., 2006), it is likely that C associated with POM and the microaggregates is younger than C found in the silt-plus-clay fraction. This needs to be confirmed, perhaps with a laboratory incubation study using residues labeled with a known stable isotope signature. The increase in small macroaggregate SOC in response to greater C input was mainly due to C sequestration in microaggregates within small macroaggregates, but C saturation in the silt-plus-clay fraction within small macroaggregates indicates that the microaggregates within macroaggregates could also be saturated with SOC at higher C input rates.

Taking the soil C saturation concept explicitly into account will significantly improve the accuracy in predicting C stabilization, and therefore the influence of increased C input on the ecosystem services provided by generally higher SOM distributed in functionally different SOC pools. Soil C sequestration has the potential to offset 5 to 15% of global CO2 emissions from fossil fuel burning (Lal, 2004). Moreover, an increase in SOC is often accompanied by enhanced soil biota activity, better water infiltration, and higher plant productivity (Bauer and Black, 1994; Bruce et al., 1995; Bundt et al., 2001), making efforts to increase SOC stocks meaningful from an ecosystem service standpoint. If not all SOC pools increase linearly in response to C input, however, as is generally assumed by SOC models (Paustian et al., 1997), the extent of C stabilization in soils with increased C input will be overestimated for those SOC pools.

Effect of Physical Disturbance by Tillage on Carbon Sequestration
Due to decreased physical protection of SOM indicated by lower MWD with MP, which has been observed in other tillage studies as well (Six et al., 2000; Mikha and Rice, 2004; Pikul et al., 2007), the maximum level of C stabilization under MP is likely to be lower than that under NT. In the MP system, there was a smaller increase in SOC concentrations per unit of soil C input in the whole soil, small macroaggregates, and microaggregates within macroaggregates than in the NT system, which suggests that faster decomposition with tillage (Reicosky and Archer, 2007) will hinder soils from reaching their maximum C sequestration potential. Tilled and no-till soils in an experiment exhibited two distinct asymptotic curves when the relationship between soil C input and SOC in the whole soil was analyzed (Stewart et al., 2007). These results indicate that soils with the same physiochemical properties could exhibit different levels of maximum C sequestration potential depending on the management practices applied to them. Our results extend these findings and demonstrate that the different relationships between soil C input and SOC in the whole soil under tillage and NT are due to lower soil aggregation in the tilled soil, as indicated by the decreased weight proportions of macroaggregates and microaggregates within small macroaggregates under tillage.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
We have demonstrated that SOC pools with low C sequestration potential do not stabilize extra C even when soil C input increases, which indicates C saturation of these fractions. Carbon saturation of some SOC pools could constrain predictions of CO2 mitigation potential, nutrient retention capacities, and SOM-derived soil fertility improvements in response to greater C inputs through N addition, organic amendments, or higher plant productivity. Consequently, the C saturation levels of functionally different SOC pools need to be considered to determine changes in ecosystem services mediated through SOM in response to higher soil C input.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication July 16, 2007.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
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