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Published online 1 January 2007
Published in Soil Sci Soc Am J 71:145-154 (2007)
DOI: 10.2136/sssaj2005.0231
© 2007 Soil Science Society of America
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SOIL BIOLOGY & BIOCHEMSITRY

Corn-Soybean Sequence and Tillage Effects on Soil Carbon Dynamics and Storage

D. R. Huggins*

USDA-ARS, Washington State University, Pullman, WA 99164

R. R. Allmaras (deceased) and C. E. Clapp

USDA-ARS, Dep. of Soil, Water and Climate, University of Minnesota, St. Paul, MN 55108

J. A. Lamb

Dep. of Soil, Water and Climate, University of Minnesota, St. Paul, MN, 55108

G. W. Randall

Southern Minnesota Res. and Ext. Center, University of Minnesota, Waseca, MN, 56093

* Corresponding author (dhuggins{at}wsu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil organic carbon (SOC) in agroecosystems is regulated by crop rotation and soil disturbance. We assessed crop sequence and tillage effects on SOC dynamics and storage using natural 13C abundance of corn (Zea mays L.) and soybean [Glycine max (L.), Merr.]. Treatments consisted of tillage: moldboard plow (MP), chisel plow (CP), and no-tillage (NT); and crop sequence: continuous corn (CC), continuous soybean (SS), and alternating corn–soybean (CS). Soil samples were collected after 14 yr in each treatment and in fallow alley-ways and were analyzed for SOC, {delta}13C, bulk density, and pH. Tillage by crop sequence interactions occurred as treatments with MP and SS as well as fallow averaged 135 Mg SOC ha–1 (0- to 45-cm depth), while CP treatments with corn (CC and CS) and NT with CC averaged 164 Mg SOC ha–1. Crop sequence effects on SOC (0- to 45-cm depth) occurred when tillage was reduced with CP and NT averaging 15% greater SOC in CC than SS. In addition to less C inputs than CC, SS accelerated rates of SOC decomposition. Tillage effects on SOC were greatest in CC where CP had 26% and NT 20% more SOC than MP, whereas SOC in SS was similar across tillage treatments. Up to 33% of the greater SOC under CC for CP and NT, compared with MP, occurred below tillage operating depths. Substantial losses of SOC were estimated (1.6 Mg SOC ha–1 yr–1) despite lowering SOC decay rates with reduced tillage and high levels of C inputs with CC.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tillage and crop rotation are major regulators of SOC cycling, flow, and storage in agroecosystems (Blevins and Frye, 1993; Reicosky et al., 1995; Paustian et al., 1997). Tillage controls both SOC substrate availability and decomposition rates by: (1) affecting the quantity and distribution of C derived from plant residues and roots in the soil profile (Lal, 1976; Doran, 1980; Dick, 1983; Rasmussen and Rohde, 1988; Campbell et al., 1995; Potter et al., 1997; Wander et al., 1998); and (2) influencing edaphic factors such as aeration and water content (Doran, 1980; Linn and Doran, 1984; Betz et al., 1998; Rasmussen, 1999), temperature (Phillips et al., 1980), soil reaction (Blevins et al., 1977; Dick, 1983), and soil aggregate properties (Angers et al., 1992; Beare et al., 1994; Six et al., 1999). Overall, shifts from high to lower soil disturbance often promotes the accumulation of otherwise labile SOC that is less available to microbial attack and subject to edaphic controls that slow rates of C decomposition (Paustian et al., 1997). Accumulating greater SOC is dependent on the capacity of agroecosystem practices to increase the stability of highly labile C inputs (Huggins et al., 1998a). Many studies have documented SOC increases in reduced as compared with intensive tillage systems (Bauer and Black, 1981; Havlin et al., 1990; Halvorson et al., 2002). In some studies, however, tillage effects on total SOC have been limited or absent (Powlson and Jenkinson, 1981; Carter and Rennie, 1982; Karlen et al., 1994; Angers et al., 1997; Wander et al., 1998).

Crop rotation can regulate C substrate availability and decomposition rates by: (1) controlling the quantity, quality, and periodicity of C inputs via residues and roots; and (2) modifying soil and crop canopy environmental conditions as resources (i.e., water, nutrients) are consumed, crop biomass accumulates and residues and roots are cycled to soil. The amount of C derived from crop residues, root biomass and rhizodeposition, and the timing of C return to soil varies considerably among crops (Kay, 1990; Franzluebbers et al., 1995; Paustian et al., 1997; Allmaras et al., 2000; Molina et al., 2001). Despite differences in the quality and source of residue- and root-derived C, SOC is often linearly correlated with C inputs (Larson et al., 1972; Rasmussen et al., 1980; Zielke and Christenson, 1986; Havlin et al., 1990; Huggins et al., 1998a). Exceptions occur, however, and lack of correlation between C inputs and SOC has been attributed to cropping system (i.e., residue quality and soil disturbance) and edaphic controls (i.e., clay content, aggregate formation and turnover) over SOC availability and decomposition rates (Bloom et al., 1982; Campbell et al., 1991; Huggins et al., 1998a; Soon, 1998).

Studies evaluating the combined influence of crop and tillage management on SOC often conclude that crop rotation effects are primarily due to differences in C additions, while major tillage effects are on SOC decomposition rates and losses via soil erosion (Rasmussen et al., 1980; Havlin et al., 1990; Potter et al., 1997). Under semiarid conditions, reduced soil disturbance has increased SOC only in conjunction with intensification of crop rotation and elimination of fallow periods (Campbell et al., 1995; Potter et al., 1997; Schomberg and Jones, 1999; Halvorson et al., 2002). Few studies, however, have examined the interactive effects of tillage and crop rotation on SOC under subhumid and humid environments. Yang and Kay (2001) hypothesized that reduced tillage would accentuate crop rotation effects on SOC storage; but effects of rotation combinations including corn, soybean, wheat (Triticum aestivum L.), and barley (Hordeum vulgare L.) on SOC in the 0- to 40-cm depth were comparable for chisel and MP tillage in a cold, humid environment. These results agreed with those reported by Angers et al. (1992) under similar climatic conditions. In a warm, subhumid climate, Havlin et al. (1990) concluded that rotation effects on SOC were primarily due to differences in C inputs; however, significant tillage by rotation interactions were apparent at the site with finer textured soil. Here, no differences in SOC occurred between the CP and disk tillage as compared with NT for the SS rotation, but significant increases in SOC occurred with NT in the continuous sorghum (Sorghum bicolor L.) rotation. These data suggest that rotation effects on SOC can be significant when soil disturbance is reduced.

Cropping systems of the northern Corn Belt are currently dominated by corn and soybean production under a variety of tillage practices. Highest levels of returned C for annual crops are typically produced by corn, which can exceed C levels returned by grain legumes (e.g., soybean) by over 1.5 times (Buyanovsky and Wagner, 1986; Paustian et al., 1997; Allmaras et al., 2000). Decomposition of crop residues and SOC could be accelerated by soybean as compared with corn as soybean: (i) reduces soil aggregate size, stability, and C content (Fahad et al., 1982; Bathke and Blake, 1984; McCracken et al., 1985; Ellsworth et al., 1991); (ii) uses less soil water (Allmaras et al., 1975); (iii) provides less surface residue cover; (iv) undergoes finer physical break-up of residues (Buyanovsky and Wagner; 1986); and (v) generally has residues with lower C/N ratios and lignin contents (Buyanovsky and Wagner, 1986; Broder and Wagner, 1988; Torstensson, 1998).

Overall, we hypothesize that corn and soybean sequences will have greater influence on SOC storage and dynamics under low versus high soil disturbance. Specifically, as tillage is reduced, we expect that soybean sequences in comparison with CC will: (i) accelerate SOC decomposition rates; and (ii) result in significantly lower overall SOC storage than can be explained by a reduction in C inputs. Furthermore, we hypothesize that reducing tillage will be insufficient to maintain or increase SOC to native levels despite large C inputs under annual cropping. Considering these hypotheses, our objectives are to: (1) evaluate tillage practice (MP, CP and NT) and crop sequence (CC, SS and CS) effects on SOC storage after 14 yr; (2) use natural 13C abundance to quantify interactive effects of tillage and crop sequence on SOC dynamics; (3) identify causal factors and present multiple working hypotheses that aid the evaluation of treatment effects on observed SOC storage and estimates of SOC decomposition; and (4) simulate long-term effects of tillage and crop sequence on SOC storage, and the C inputs and SOC dynamics required to maintain SOC.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plot Establishment and Field Measurements
A 14-yr field study with three tillage treatments and three crop sequences was initiated in 1980 on a Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquoll) at the University of Minnesota Southern Research and Outreach Center, Waseca, MN (44°04' N, 93°32' W, 351 m elevation). Soil particle-size composition in the 5- to 10-cm layer is 322, 347, and 331 g kg–1 of sand, silt, and clay, respectively (Betz et al., 1998). The study area, in the northwestern Corn Belt, has a subhumid, continental climate with an annual precipitation of 824 mm and average annual air temperature of 6.3°C.

The conversion of native tall-grass prairie to agricultural use in the area dates back to 1870. Before 1973, the study area was managed as a general enterprise farm with dairy and likely planted to corn-small grain-alfalfa (Medicago sativa L.) based rotations with manure applications. Moldboard plow-based tillage systems with CC were used between 1973 and 1980, and in the spring of 1976, artificial subsurface drainage was installed 1.2-m deep at 23-m spacing.

The experiment was a randomized split-plot design with four replications. Main plots consisted of tillage treatments and subplots were crop sequence treatments. Plots were 6.1 by 16.7 m with eight crop rows, each 76-cm wide. Tillage treatments consisted of MP, CP, and NT and crop sequences were CC, SS, and CS. Before fall tillage, corn stalks were chopped, whereas soybean residue required no treatment. Primary tillage with MP (25- to 30-cm deep) or CP (15-cm deep) was performed in the fall and was followed by spring disking in the CP treatment and tine-type field cultivation in the MP treatment. Tillage depths are further detailed in Betz et al. (1998). No primary or secondary tillage was conducted in NT. The same planter was used in all tillage systems and wheel traffic was controlled to impact only one side of the row. Interrow cultivation following planting was performed in MP and CP treatments; traffic was consistent with that at planting. Soil-test recommended rates of P and K were band applied with the planter and 225 kg N ha–1 as NH4NO3 was spring broadcast in treatments with corn. Weeds and insects were chemically controlled with recommended materials and application methods.

Soils were sampled in June 1994 following procedures of Allmaras et al. (1988) to measure soil bulk density and obtain material for SOC analyses. Composites of 12 soil cores (1.8 cm diam.) from four depths (0–7.5, 7.5–15, 15–30, and 30–45 cm) were collected from the center of three non-tracked inter-rows (four cores per inter-row). Soil sampling was centered in the interrow to minimize effects of current crop root biomass, and located at least 1 m from the plot boundary to avoid contamination from neighboring treatments. All treatments and replications were sampled including both phases of the CS sequence. In addition, three alley-ways adjacent to the plots, fallowed since 1980, were soil sampled using the same procedure. The fallowed alley-ways were chisel plowed and repeatedly field cultivated as needed to control weeds.

Carbon and Carbon-13 Natural Abundance Analyses
Source C inputs from above- and belowground biomass were estimated from grain yield. Grain yield of each plot was determined after physiological maturity by harvesting 15 m of two center rows with a plot combine. Grain yield was not measured during the 1990–1993 seasons; yields were estimated using the mean response to experimental treatments (1980–1989) and the long-term (1980–1993) mean annual yields of soybean and corn at the Southern Minnesota Research and Outreach Center. Therefore, these estimates included adjustments for treatment and year effects.

Shoot biomass was estimated from the harvest index (0.48 for corn and 0.29 for soybean) measured from a nearby experiment during the same time period (Crookston et al., 1991). Shoot biomass and C concentrations were used to determine C returned to the soil from aboveground biomass. As in other studies (Huggins et al., 1998b; Collins et al., 1999) an estimate of C additions from roots of soybean was based on a root/shoot ratio of 0.38 and root C concentrations reported by Buyanovsky and Wagner (1986), but our estimates of root/shoot (including grain) ratio for corn (0.53) was derived from Brye et al. (2002). Carbon inputs were cumulated over the 14-yr period and expressed as total estimated C return per treatment. Although both CS and soybean-corn (SC) sequences were sampled and analyzed, no significant differences in soil variables occurred and the treatment mean, indicated as CS, was compared to CC and SS sequences.

Representative samples of aboveground corn and soybean dry matter and sieved (2 mm) soils from each plot were ball-milled and analyzed for C and 13C/12C ratio with an elemental C-N analyzer interfaced with an isotope ratio mass spectrometer (Carlo Erba, model NA 1500 and Fisons, Optima model; Fisons, Middlewich-Cheshire, UK). Coarse organic matter retained on the 2-mm sieve was separated from the soil samples, dried, ground, and returned to the sample before ball-milling. Soil pH was determined in a 1:5 soil/0.01 M CaCl2 suspension. Inorganic C was removed using a H3PO4 method (Follett et al., 1997) modified to remove both carbonates and sulfates (Wilts et al., 2004). Soil samples above 15 cm required no treatment for removal of carbonates. The C isotope ratios were expressed as {delta}13C values:

Formula 1[(1)]
where Rsam = 13C/12C ratio for the sample, and Rstd = 13C/12C ratio of a working standard. Urea ({delta}13C of –18.2{per thousand}) and soil ({delta}13C of –17.6{per thousand}) served as working standards and {delta}13C values were calculated relative to Pee Dee Belemnite as an original standard.

The SOC was partitioned into C3 and C4 sources using a mixing equation:

Formula 2[(2)]
where {delta}m = {delta}13C of SOC from a mixture of C4 and C3 sources, {delta}a = {delta}13C from C4 source (corn residue = –12.0{per thousand}), {delta}b = {delta}13C from C3 source (soybean residue = –26.4{per thousand}), f = fraction of organic C from C4 source, and (1– f ) = fraction of organic C from C3 source. All of the SOC did not originate from corn- or soybean-derived C; however, we assumed that {delta}13C labeling with C4– and C3–derived C was similar over time. Soil C was expressed on a volume basis using soil bulk density measured at the time of sampling. Analysis of variance was used to determine significant (0.05 probability level) crop sequence effects on SOC variables (SAS Institute, 1996).

Carbon Modeling
The dynamics of C3– and C4–derived SOC were evaluated using a two-C-pool model consisting of Pool F, in which decomposition is rapid, and Pool S, with a slower rate of decomposition (Henin and Dupuis, 1945; Jenkinson, 1988; Huggins et al., 1998b). Annual C inputs (A) enter Pool F where labile C is rapidly decomposed and respired as CO2 (mineralization) and where a fraction, h (the humification rate constant), enters Pool S each year. A fraction, k, of SOC in Pool S is also decomposed each year. The change in SOC over time is:

Formula 3[(3)]
where Cs is the SOC in Pool S, and A is the C input (Henin and Dupuis, 1945). Solving Eq. [3] for Cs = C0 when t = 0 gives:

Formula 4[(4)]
where C0 is the initial SOC in Pool S (Jenkinson, 1988). When A = 0 (as in fallow), Eq. [4] simplifies to:

Formula 5[(5)]
Turnover time (tt), half-life (t1/2), were defined for SOC in each pool: for Pool F, ttF = 1/(1– h) and tF1/2 = 0.693ttF; for Pool S, ttS = 1/k and tS1/2 = 0.693ttS. Carbon inputs required to maintain C0 (AE) were also estimated:

Formula 6[(6)]
Decay rate constants (k) for C4– and C3–derived SOC (0–45 cm) were calculated for treatments and fallow, where A = 0 using Eq. [5]. Carbon additions were 0 for the C4–derived SOC under fallow and SS treatments. For the C3–derived SOC, C additions were 0 for fallow and CC treatments. No soil samples were collected at the initiation of the experiment (1980), consequently initial levels of C4– or C3–derived SOC (C0) were unknown. To estimate C0, simulations of C parameters (h, k) were made as a function of C0 ranging from 50 to 100 Mg SOC ha–1 for both C4– and C3–derived SOC. Within treatments, h was nearly insensitive to C0, while k increased linearly (data not shown). Criteria for selecting C0 were: (1) simulations where k > 0 for the most restrictive tillage treatment (CP, C0 > 80 Mg SOC ha–1 for C4– and C3–derived SOC); (2) C0 was greater than Cs measured in any treatment (the largest amount of C4– and C3–derived SOC in any treatment at the time of sampling were 95 and 85 Mg SOC ha–1, respectively); and (3) k for C4– and C3–derived SOC were similar under fallow (Eq. [5]). Using these criteria, C0 levels were estimated as 100 Mg SOC ha–1 for C4–derived SOC and 87 Mg SOC ha–1 for C3–derived SOC (0–45 cm). The estimated C0 values gave a k for both C4– and C3–derived SOC under fallow of 0.028 yr–1, similar to reported values for soil with relatively high SOC under annual cropping in the tall-grass prairie region (Huggins et al., 1998a).

The estimated C0 values were applied in Eq. [5] under CC treatments to calculate k for C3–derived SOC and under SS treatments to calculate k for C4–derived SOC. The k values were then used along with A and measured Cs to determine h for both C4– and C3–derived SOC for all tillage and crop sequence treatments using Eq. [4]. One final criterion was added following these calculations: a lower limit of h for soybean C inputs was set at 0.1 yr–1, approximately equal to the concentration of lignin (Broder and Wagner, 1988; Andriulo et al., 1999). This lower bound for h prevented calculation of negative h values that occurred for SS under CP and NT. When h values reached the lower bound during the fitting procedure, h was held constant and k values were adjusted using Eq. [4], thereby estimating ‘priming’ effects of C inputs on SOC decay rates.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil {delta}13C
Crop sequence effects on soil {delta}13C were significant for all tillage treatments in the 0- to 7.5-cm depth with greatest values occurring in CC (–17.83 {per thousand}), lowest in SS (–19.21 {per thousand}) and intermediate values in CS (–18.67 {per thousand}) and fallow (–18.55 {per thousand}; Table 1). At depths below 7.5 cm, crop effects on soil {delta}13C diminished, but there were significant differences between CC and sequences with soybean in MP (all depths) and in NT (30- to 45-cm depth). Significant tillage effects on soil {delta}13C were limited to intermediate depths in CC where soil {delta}13C in MP was greater than in reduced tillage treatments (Table 1).


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  		Table 1. Crop sequence and tillage effects on soil {delta}13C ({per thousand}) by depth-increment.

 
Distinct patterns of soil {delta}13C with depth occurred within each tillage and crop sequence, as well as fallow (Table 1). In CC, soil {delta}13C were relatively large near the surface for all tillage treatments; however, {delta}13C in MP remained large and uniform with depth, whereas {delta}13C in CP and NT decreased significantly at the 7.5- to 30-cm depths and then increased at the 30- to 45-cm depth. In contrast to CC, soil {delta}13C in SS were lowest at surface depths and generally increased with depth in all tillage treatments.

Differences in soil {delta}13C values arise from multiple, often confounding processes and treatments (Boutton and Yamaski, 1996; Huggins et al., 1998b). In this study, the annual rate of soil {delta}13C change between CC and SS of 0.1 {per thousand} is similar to that found by Huggins et al. (1998b) and arises from in situ labeling of SOM with C additions from residues of either corn ({delta}13C of –12.0 {per thousand}) or soybean ({delta}13C of –26.4 {per thousand}). Soil incorporation of naturally labeled surface residue and shallow root C in CP and MP results in tillage-specific redistributions of residue (Staricka et al., 1991) and distinct depth patterns of soil {delta}13C (Huggins et al., 1998b; Layese et al., 2002).

Soil {delta}13C differences occurred at depths beyond physical mixing by tillage (30 to 45 cm) in MP and NT (Table 1). These data provide evidence that the influence of crop sequence and/or tillage can occur below tilled depths. Differential contributions of C at depths below tillage are expected from soybean versus corn due to dissimilarities in root distributions (Allmaras et al., 1975a, 1975b). A confounding factor is that SOC pools of differing stability are likely not uniformly labeled with 13C, due to historic variations in native prairie vegetation, crops produced, and atmospheric {delta}13C (Huggins et al., 1998b). Consequently, if tillage or crop sequence differentially influences SOC decomposition, changes in soil {delta}13C can occur independently of C inputs. For example, MP is the only tillage treatment where a significant crop sequence effect on soil {delta}13C occurred at all depths (Table 1). This difference in soil {delta}13C could have resulted not only from continued mixing of crop C inputs throughout the tillage zone with MP, but also from a large turnover of SOC and increasing influence of more stable SOC and/or more recent C additions on proportions of C3– and C4–derived SOC. The similar soil {delta}13C in CP for 7.5- to 45-cm depths across all crop sequences suggests less SOC turnover and dilution influence of C additions from the most recent 14 yr. Further interpretation of tillage and crop sequence effects on SOC can be derived from combining soil {delta}13C with soil bulk density and SOC data.

Soil Organic Carbon
Total Soil Organic Carbon
Soil bulk density was not influenced by either tillage or crop sequence and averaged 1.08, 1.39, 1.40 and 1.59 g cm–3, respectively for the 0- to 7.5-, 7.5- to 15-, 15- to 30- and 30- to 45-cm depths. Therefore, treatment effects on SOC resulted from differences of soil C concentration. Fallow alley-ways had greater soil bulk density than adjacent plot areas (data not shown). The soil bulk density data differ from many studies where increased densities were found under NT as compared with tilled treatments (Gantzer and Blake, 1978; Linn and Doran, 1984; Rasmussen, 1999; Balesdent et al., 2000; Kushwaha et al., 2001). Little effect of tillage or cropping treatments on bulk density, however, has also been reported (Campbell et al., 1995; Potter et al., 1997; Sainju et al., 2002). Previous measurements at this site showed marked increases of soil bulk density in wheel tracks of NT (Betz et al., 1998), which were avoided in our sampling procedure.

Total SOC was lowest in MP and greatest in CP, but significant crop sequence by tillage interactions occurred (Table 2). The NT and CP treatments had 15 and 16%, respectively, greater SOC in CC than in SS. In contrast, no crop sequence effects on SOC occurred in MP, where overall SOC was the lowest and no different from SOC in continuous fallow. Tillage effects on SOC were greatest in CC, where CP had 26% and NT 20% more SOC than MP. In CS, SOC averaged 23% greater in CP than MP, while SOC in NT was more than MP, but not significantly different. Continuous soybean was the only crop sequence where tillage had no influence on SOC. Interactive effects of tillage and crop sequence resulted in marked differences in SOC (0–45 cm). The CP treatments with corn and NT with CC averaged 164 Mg SOC ha–1, compared with all MP and SS treatments, and NT treatments with soybean that averaged 137 Mg SOC ha–1.


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  		Table 2. Total soil organic C, C4–, and C3–derived C for sampled soil profile (0–45 cm).

 
The SOC for Webster clay loam under native tall-grass prairie vegetation was reported as 138 Mg C ha–1 (0–30 cm) as compared with 88 Mg ha–1 under annual cropping with MP at a nearby site (Huggins et al., 1998b). Similarly, Collins et al. (1999) reported a decrease of SOC from 154 Mg C ha–1 (0–20 and 25–50 cm) at the same native prairie site (Normania loam) to 79 Mg C ha–1 of SOC at a site with long-term cultivation. The SOC levels of 127 to 174 Mg C ha–1 (Table 2) are therefore more comparable with native prairie than long-term cultivated sites. This is likely due to historic management practices that favored SOC storage (e.g., small grain, alfalfa, and corn rotation, manure applications, and lack of artificial subsurface drainage) in an imperfectly drained, fine-textured soil. Consequently, in contrast to many SOC studies, this study evaluates treatment effects on SOC where initial levels are high, rather than situations where initial SOC is significantly depleted relative to native conditions. More intense disturbance (annual MP starting in 1973) coupled with improved drainage (1976) likely accelerated SOC decomposition rates just before the initiation of the experiment (1980). Reduced tillage (CP and NT) and large C inputs (CC) may not increase SOC, as found under depleted SOC situations, but might maintain SOC or reduce declines relative to MP and sequences with low C inputs (SS). The overall influences of tillage and crop sequence on SOC are likely a consequence of the following factors.

Factor (a): Carbon Input Differences Among Tillage Treatments and Crops
No significant differences in estimated crop inputs of C occurred among tillage treatments (Fig. 1 ); however, estimated C inputs from corn averaged 1.8 times larger than soybean. Linear relationships between total SOC and estimated source C (both corn and soybean derived) occurred for all tillage treatments, however, the slope parameter was not statistically significant for NT and the equation is not given (Fig. 1a). A slope of 0.58 for CP (Fig. 1a) is unlikely as a direct response of SOC to C inputs as this would require C humification rates about three times greater than reported values (Paustian et al., 1997; Huggins et al., 1998b). These data, however, represent the combined influence of C inputs and decomposition on SOC. The C4–derived SOC also had a linear response to C inputs (Fig. 1b) with slopes of 0.18 for CP and 0.14 for MP, representing C humification constants typical of many studies (Huggins et al., 1998b); The C3–derived SOC, however, was not related to C3 inputs and had slopes not different from zero (Fig. 1c). While reduced tillage would be expected to slow decomposition compared with MP, SS appears to accelerate SOC decomposition [see Factor (b)], in addition to supplying less C inputs. Similar results are evident in the data of Havlin et al. (1990) in their comparisons of SS, sorghum-soybean and continuous sorghum sequences.


Figure 1
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  		Fig. 1. Tillage and crop sequence effects on total soil organic C (0–45 cm), C4–derived SOC (0–45 cm), C3–derived SOC (0–45 cm) and relationship to estimated C return. Regression equations were omitted for relationships where parameters (slope) were not statistically significant from 0.

 
Depth distributions of SOC provide further data on interactive effects of tillage and differences in crop C inputs associated with crop sequence (Table 3). Under reduced tillage (CP and NT), the SOC in CC was significantly greater than in SS at all depths with the exception of the 15- to 30-cm depth in NT. In MP, however, no significant crop sequence effects occurred for SOC throughout sampled depths. Therefore, despite large differences in C input for corn versus soybean, crop sequence effects on total SOC were not expressed until tillage was reduced.


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  		Table 3. Crop sequence and tillage effects on soil organic C by depth-increment.

 
Tillage effects on SOC would be expected to arise from differences in crop residue incorporation and SOC decomposition in the tillage zone. This was observed in CS where greater SOC in NT as compared with MP occurred only in the surface 7.5 cm, while CP resulted in greater SOC than MP in 7.5- to 30-cm depths. Interestingly, however, in CC, 24 and 55% of sampled differences in SOC for CP and NT, respectively, as compared with MP, occurred below tillage operating depths (30–45 cm). These differences could arise from more favorable rooting environments at deeper depths for CP and NT (Betz et al., 1998) or from reduced tillage slowing SOC decomposition at relatively deep depths. Furthermore, these data indicate that tillage effects on SOC in CC treatments were not completely evaluated with a 45-cm sampling depth.

Factor (b): Soil Organic Carbon Decomposition Response to Carbon Quality
Continuous soybean reduced total SOC across all tillage treatments as compared with CP and NT under CC (Table 2, Fig. 1a). In addition, levels of C3– and C4–derived SOC under SS were similar across tillage treatments and to fallow areas for the sampled soil profile (Table 2). In contrast, under CC, CP had 21% greater C4–derived SOC and 34% more C3– derived SOC (despite no additions of C3–C) than in MP. These data, in combination with Fig. 1c, show that soybean accelerated decomposition rates to the same extent as the greater soil disturbance and residue mixing with MP tillage or under fallow. The higher N and lower lignin concentrations found in residues and roots of legumes as compared with cereals (Buyanovsky and Wagner, 1986; Broder and Wagner, 1988) coupled with decreased aggregate stability [see Factor (d)] or saturation of physical protection mechanisms [see Factor (c)], would speed decomposition of C under soybean as compared to corn, particularly if soil N is limiting.

Factor (c): Tillage-Induced Redistribution of Carbon and Soil Organic Carbon Decomposition Response
Comparisons of C4– and C3– derived SOC depth-distributions show evidence of tillage-induced differences in C dynamics (Tables 4 and 5). Increases in surface SOC (0–7.5 cm) under reduced tillage and CC were negated by the inclusion of soybean. Under SS, no significant effects of NT or CP on C3– and C4–derived SOC as compared with MP occurred in the surface 0 to 7.5 cm (Tables 4 and 5). Tillage influences on C4–derived SOC (0- to 7.5-cm depth) only occurred in CC where NT had 17% greater SOC than MP, but comprised only 11% of the overall tillage difference in profile SOC (Tables 2 and 4). Tillage also had a relatively minor influence on surface C3–derived SOC where significant increases only occurred in CS for CP and NT, compared to MP (Table 5).


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  		Table 4. Crop sequence and tillage effects on C4–derived soil organic C by depth-increment.

 

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  		Table 5. Crop sequence and tillage effects on C3–derived soil organic carbon by depth-increment.

 
Several hypotheses can be formulated to explain these results: (1) surface residues conserved soil water and improved environmental conditions for surface SOC decomposition; (2) SS did not supply sufficient C inputs to maintain surface SOC; (3) SS accelerated surface SOC decomposition by alleviating N limitations; (4) mineral-organic complexes were C saturated under high initial levels of surface SOC, leaving surface residue and root additions with limited physical protection from microbial attack (Hassink and Whitemore, 1997); (5) adverse soybean effects on soil aggregation [see Factor (d)] were intensified when residues were placed near the surface, thereby decreasing physical protection mechanisms; and (6) contrasting effects of CC and SS on soil pH [see Factor (e)].

Hassink and Whitemore, (1997) presented a relationship between the capacity of soil to physically protect SOC (X; saturation constant; g C kg–1 soil) and clay (particles < 2 µm; g kg–1) where: X = 21.1 + 0.0375 (clay). Using this equation, the protective capacity of clay-organic matter complexes for soil in this study would saturate at 33.5 g C kg–1 soil. Given a soil bulk density of 1.08 g cm–3, for the surface 0 to 7.5 cm, saturation would occur at 3.6 Mg C ha–1 cm–1, very close to measured amounts under MP or SS (Table 3). Consequently, physical protection of C from clay-SOC interactions could be near saturation, limiting further sequestration of surface SOC with SS regardless of tillage treatment.

Tillage effects on SOC were prominent at 7.5- to 30-cm depths, where C3– and C4–derived SOC were often significantly greater in CP than MP, while NT was only greater than MP in the CC sequence (Tables 4 and 5). The comparatively greater SOC accumulation in the 7.5- to 30-cm depth of CP, compared with MP, across all crop sequences is likely a consequence of reduced SOC decomposition combined with higher concentrations of incorporated residues at 8- to 20-cm depths (Staricka et al., 1991). Interestingly, under SS, CP increased relative amounts of C4–derived SOC at 7.5- to 30-cm depths as compared with NT and MP (Table 4). These data indicate that as surface SOC saturates physical protection mechanisms, shallow incorporation of soybean residue may limit adverse effects of soybean on SOC storage observed in both MP and NT.

In CC, C4–derived SOC was 66% greater under NT than MP in the 30- to 45-cm depth, comprising 33% of the total profile difference in SOC (Tables 2 and 4). In addition, the C3–derived SOC (30–45 cm) under CC, tended to be greater with NT and CP as compared with MP (Table 5). In contrast, no significant tillage effects on SOC occurred with SS or CS at 30- to 45-cm depths (Tables 4 and 5). These data indicate that under CC, increases in root contributions and/or reduced SOC decomposition at depths below tillage were major contributors to greater SOC under NT as compared with MP. Furthermore, interactions of tillage with crop sequence were still apparent at the 30- to 45-cm depth as lower C3– and C4–derived SOC occurred in SS and CS as compared with CC only under NT and CP (Tables 4 and 5).

Factor (d): Crop Effects on Soil Aggregation Properties
Treatment effects on soil aggregation were not evaluated; however, corn and soybean have contrasting influences on soil structure. Following corn, soil aggregate stability is often improved while soybean decreases aggregate size, stability and C content (Fahad et al., 1982; Bathke and Blake, 1984; McCracken et al., 1985; Ellsworth et al., 1991). Therefore, soybean would tend to counteract positive effects of less tillage disturbance on aggregate formation, turnover and soil C stabilization. A possible related factor is the greater and earlier soil water uptake by soybean at shallow depths as compared with corn (Allmaras et al., 1975a, 1975b). Enhanced drying near the soil surface followed by precipitation could induce greater macro-aggregate disruption and turnover due to more frequent dry-wet cycles (Denef et al., 2001).

Factor (e): Tillage and Crop Effects on Soil pH
Both CC and SS in the NT treatments had significantly lower soil pH than CP and MP throughout sampled depths (data not shown). Stratification of soil pH with increased acidification near the surface in no-tilled as compared with tilled treatments is not uncommon (Blevins et al., 1977; Dick, 1983; Rasmussen, 1999). In treatments with corn, decreased soil pH is primarily due to the use of ammonical fertilizers, while acidification under soybean is due to N2–fixation and the accumulation of organic acids associated with surface residues. Reduced surface (0–7.5 cm) soil pH under SS with NT (pH of 6.4) may have enhanced biological activity and stimulated SOC decomposition as compared to surface soil pH of 7.1 to 7.6 as found with CP and MP with SS. In contrast, greater surface acidification in NT under CC (pH of 4.8) as compared to CP under CC (pH of 6.0) and MP under CC (pH of 7.2) would reduce rates of SOC decomposition (Rasmussen et al., 1998) and contribute to potential crop sequence by tillage interaction effects on SOC.

Carbon Modeling
Carbon Inputs
Estimated annual additions of C from crop shoot and root biomass (A) were not significantly affected by tillage treatment and averaged 6.55 Mg C ha–1 for corn and 3.59 Mg C ha–1 for soybean (Table 6). These values are within the range reported by other studies and sufficient to maintain SOC levels reported for annual cropping systems in the Corn Belt (Larson et al., 1972; Barber, 1979; Buyanovsky and Wagner, 1986; Huggins et al., 1998a, 1998b). Lack of tillage effects on crop yield in this study may have been due to high levels of SOC, which would buffer adverse treatment effects on soil nutrient cycling and physical properties.


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  		Table 6. Tillage and fallow effects on annual additions of shoot and root C (A), C humification (h) and decay (k) rate constants, turnover time (tt), half-life (t1/2), and C additions required to maintain initial SOC (AE) for C4– and C3–derived C of a Webster clay loam in a subhumid climate of the northern Corn Belt.

 
Soil Organic Carbon Decomposition Rates
The decay rate constants (k) for C4–derived SOC were calculated Eq. [5] from SS treatments and were lowest for CP (0.017 yr–1) and NT (0.022 yr–1) increasing to 0.028 yr–1 for fallow and 0.030 yr–1 for MP (Table 6). Similar values of k among tillage treatments occurred in the C3–derived C where values ranged from 0.024 yr–1 for NT and CP to 0.028 yr–1 for fallow and 0.033 yr–1 for MP. Decay rate constants are dependent on levels of C input, disturbance, and amounts of labile SOC (Huggins et al., 1998a). Estimates of k following cultivation of tall-grass prairie have been reported from 0.03 to 0.05 yr–1, due to the rapid depletion of labile SOC pools (Huggins et al., 1998a). The relatively high and similar k values estimated for MP in CC and fallow show that C inputs were unable to maintain SOC when cultivated. Reduced tillage slowed decomposition but not sufficiently to maintain th e estimated initial SOC. Given the levels of C input under CC, k would need to be further reduced to 0.007 yr–1 to maintain SOC. Achieving these low C decay rates in soils with substantial labile C may not be achievable with reduced tillage alone.

Interestingly, the linear relationship between C inputs (A) and k developed for the Morrow Plots (soils with 27% clay) in Illinois (Huggins et al., 1998a) provided good estimates for k in this study. Under CC and MB at the Morrow Plots, k = 0.0009 + 0.0038A, gave an estimated k of 0.026 yr–1 in this study. In the corn-oat-hay rotation (less tillage) k = 0.0002 + 0.0028A, and gave an estimated k of 0.019 yr–1. The slightly lower estimates of k are due to greater quantities of labile SOC in this study. These data support the concept of expanding simple C models to include relationships between k and A.

The two-C-pool model Eq. [4] gave estimates of h derived from C4 sources that increased as soil disturbance decreased, ranging from 0.17 yr–1 for MP to 0.23 yr–1 for NT. In contrast, h derived from C3 sources decreased as soil disturbance decreased ranging from 0.16 yr–1 for MP to 0.10 yr–1 for CP and NT. The h values for CP and NT represent the lower bound placed on h based on the lignin content of soybean. The humification rate estimates were close to the range of 0.11 to 0.20 yr–1 reported for cultivated systems (Larson et al., 1972; Barber, 1979, Buyanovsky et al., 1987; Huggins et al., 1998a, 1998b). The decrease in humification rates under SS for CP and NT as compared with MP, however, was unexpected and could arise from residue placement effects as previously discussed.

Turnover time in the fast pool (ttf) was more rapid in MP (averaging 1.21 yr) than either CP (averaging 1.27 yr) or NT (averaging 1.31 yr) for C4– and C3–derived C and similar to reported values (Huggins et al., 1998b). In the slow C pool, tts for CP was the slowest (averaging 37.2 yr), NT intermediate (averaging 33.3 yr) with MP (averaging 27.3 yr) and fallow (averaging 29.0 yr) having the most rapid C turnover time. Soil C turnover time decreases as labile C pools increase (Huggins et al., 1998a). The tts are three to four times faster than for soils where SOM levels are relatively low (Huggins et al., 1998b), are representative of soils with large labile C pools, and illustrate the rapid depletion of labile C that can occur with changes in management.

Soil Organic Carbon Storage
In CC, annual C additions required to maintain estimated initial SOC (187 Mg C ha–1) were 33 Mg C ha–1 under MP, five times greater than achieved, and about two times greater than under CP or NT (Table 6). In SS, maintenance of initial SOC would require annual additions of 43 Mg C ha–1, in all tillage treatments, 12 times greater than achieved. Consequently, soils with high levels of labile C were extremely vulnerable to SOC depletion when annually cropped, regardless of tillage, and would sustain SOC only with large additions of organic materials (e.g., animal manures).

Simulations of treatment effects on SOC from the initiation of the experiment and projected over a 30-yr period showed decreased levels of SOC for all treatments (Fig. 2 ). Longer simulations were not warranted as SOC dynamics would slow as labile C pools were depleted. After 30 yr, the SOC in fallow and MP SS was predicted to decline by 56% (105 Mg C ha–1 or 3.5 Mg C ha–1 yr–1). The magnitude of these losses in SOC is similar to those that occurred following the conversion of native prairie to cultivated agriculture (Haas et al., 1957). The smallest SOC declines were predicted for CP and NT CC where initial SOC was depleted by 19 and 27%, respectively. Therefore, reduced tillage and high-yielding annual crops still resulted in substantial estimated losses in SOC over time.


Figure 2
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  		Fig. 2. Thirty year simulation of tillage and crop sequence effects on total soil organic C, C4–derived SOC and C3–derived SOC. Solid lines depict 14-yr period of study while dashed lines show continued simulations from 15 to 30 yr.

 
Positive crop rotation effects on SOC were accentuated over time under CP and NT as compared with MP (Fig. 2). Under MP, annual losses of SOC decreased from 3.5 to 3.2 to 3.1 Mg C ha–1 for SS, CS and CC, respectively. Under CP, however, crop rotation decreased annual losses of SOC from 2.6 to 1.8 to 1.2 Mg C ha–1 for SS, CS and CC, respectively. Estimated SOC differences between MP SS and CP CC were 68 Mg C ha–1 after 30 yr. Simulations of C3–derived SOC over time showed the least decreases occurring for CC under CP despite no additions of C3–derived C (Fig. 2c). The CC treatment for CP and NT show less loss of C3–derived C as compared with SS despite additions of C3–derived C under SS. In contrast under MP, C3–derived SOC declined less rapidly under SS as compared with CC. These simulation results support the conclusion that soybean accelerated SOC decomposition as compared with corn as tillage was reduced, independent of crop differences in C inputs (Fig. 2, Table 6).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Tillage and sequences of corn and soybean had marked interactive effects on SOC dynamics and storage under a subhumid climate in the Corn Belt. Fallow, and treatments with MP or SS decreased SOC from an estimated 187 Mg C ha–1 to an average of 135 Mg ha–1 (0- to 45-cm depth) over 14 yr, an annual loss of 3.7 Mg C ha–1, while CP treatments with corn (CC and CS) and NT with CC averaged 164 Mg ha–1, an annual loss of 1.6 Mg C ha–1. Significant contributions to greater SOC under CC for CP and NT, as compared with MP, occurred from C storage below tillage operating depths (30- to 45-cm). Lower SOC under SS occurred as C inputs were 55% of CC and SOC decomposition rates were accelerated. Under the best scenario of CC and reduced tillage, stabilization of initial SOC would require reducing decay rates by over 50% or doubling C inputs, both of which are improbable. We concluded that the potential to approach SOC levels of native sites is limited with annual cropping and reduced tillage. The combination of low C decay rates, high C inputs and a large proportion of potentially labile soil C may not be achievable in agroecosystems characterized by annual cropping, little or no tillage and artificial drainage, without large additional C inputs (e.g., manures, more perennial cropping).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Abbreviations: A, carbon additions; AE, carbon additions required to maintain soil carbon; C0, initial soil organic carbon; CC, continuous corn; CP, chisel plow tillage; Cs, current soil organic carbon; CS, alternating corn–soybean sequence; F, fast soil organic carbon pool; h, humification rate constant; k, decay rate constant; MP, moldboard-plow tillage; NT, no-tillage; S, slow soil organic carbon pool; SOC, soil organic carbon; SS, continuous soybean; t1/2, half-life; tt, turnover time.

Received for publication July 14, 2005.


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




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