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a Dep. of Ecology and Evolutionary Biology, University of Connecticut, 75 N. Eagleville Rd., U-3043, Storrs, CT 06269
b Dep. of Plant Science, University of Connecticut, 1376 Storrs Rd., U-4067, Storrs, CT 06269
* Corresponding author (bethanie.hooker{at}uconn.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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13C analysis in cornfields (Zea mays L.) established in 1972 at the University of Connecticut Research Farm. The plots have been under no-till (NT) or conventional till (CT) management, with residues returned (+) or residues removed (–) within each tillage treatment. After 28 yr of continuous management, NT increased SOC significantly by 48.3 ± 9.9 g C m–2 yr–1 over CT in treatments with residue returned (i.e., NT+ compared with CT+), and by 60.1 ± 13.8 g C m–2 yr–1 over CT in treatments with residue removed (i.e., NT– compared with CT–). Residue return in NT+ plots did not increase SOC relative to NT– plots (23.0 ± 12.7 g C m–2 yr–1, N.S.), but residue return to the CT+ plots resulted in a significant increase in SOC of 34.8 ± 11.1 g C m–2 yr–1 over SOC content in CT– plots. The greatest difference in SOC content was found between the CT– and NT+ treatments (83.1 ± 10.7 g C m–2 yr–1). Our results indicate that there may be a rapid cycling of the aboveground C4-C back to the atmosphere as CO2 or lost as dissolved organic C from the soil profile. Such a rapid cycling of returned C4-C suggests that the annual return of aboveground biomass may not increase soil C storage over the long term once soils have reached a steady-state SOC level.
Abbreviations: CT–, conventional tillage with residues removed • CT+, conventional (moldboard plow) tillage with residues returned • NT–, no till with residues removed • NT+, no till with residues returned • SOC, soil organic C
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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It is anticipated that equilibrium SOC levels would be lower when corn stover is removed for ethanol production, but it is unclear how strongly this stover removal would influence SOC storage, even with current research and modeling efforts (Mann et al., 2002; Wilhelm et al., 2004). While it has been well established that tillage and residue management regimes can influence SOC in cropland, continuous long-term experiments that investigate these practices are rare. Such experiments are essential for exploring processes whose results accrue slowly in soils. At the University of Connecticut Plant Science Research Farm, we have a unique set of cornfields that have been maintained in silage corn (stover removed each year) or grain corn (stover is left to decompose on site) under no-till (NT) or conventional till (CT) management regimes, with three replicates, for over 30 yr. In this paper, we present results from this long-term experiment testing how stover removal affects SOC levels under different tillage regimes.
Our long-term agricultural plots provide an excellent opportunity to test whether the long-term goals of SOC maintenance and harvest of stover for ethanol production can be compatible. In complex soil systems, treatment-induced patterns and mechanisms underlying long-term soil C dynamics are dependent on soil moisture, texture, nutrient status, and climate. The objective of this study was to determine the effects of tillage and residue management on SOC, turnover of relic forest-derived C, and cycling of new C4 (corn)-derived C in a cornfield after 28 yr of continuous management.
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METHODS AND MATERIALS |
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TOPABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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In 1972, the on-going, long-term experiment was initiated and was designed to test for differences in yield between CT and NT management, and between grain corn (shoot residue returned annually) and silage corn (no residue returned annually). The experiment is a split plot design with three replicates; tillage is the main plot (NT and CT), and corn residue is the subplot (silage and grain corn). The area of each of the twelve plots is approximately 0.03 ha, with dimensions of 7.3 by 36.5 m. Residue treatments are aboveground corn stover returned (+) and aboveground corn stover harvested (–).
The CT plots are moldboard plowed to a depth of 20 to 25 cm and disked to a depth of 10 to 15 cm in late April or early May of each year. The CT plots are cultimulched to firm the soil prior to planting. The NT plots are undisturbed until planting. Corn is planted in the entire field in late May to early June using a John Deere no-tillage planter in 76-cm wide rows. The seeding rate is set to grow a stand of approximately 65000 plants ha–1. Available records indicate that the typical hybrids used were Agway 590 X (1978–1989), Funks G 4309 (1990–1992), Funks G 4292 (1993–1998), and Funks G 4286 (1997–2000 with exception of 1999 when DeKalb KD 378RR was planted). There have been no fall tillage practices.
Fertilizer is applied based on recommendations from the University of Connecticut Soil Testing Laboratory. The forms of fertilizer used have been urea, triple superphosphate, and muriate of potash (potassium chloride). The fertilizer is applied in the spring before planting. In some years (1992, 1993, and 1994) N fertilizer was also applied when the corn plants of each plot were about 15 cm tall, based on the presidedress nitrate test (Magdoff, et al., 1990). All plots are sprayed with herbicides soon after planting, conforming with accepted agronomic practices. The herbicides atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine, CIBA-GEIGY Corp., Greensboro, NC) and Lasso [2-Chloro-2'-6'-diethyl-N-(methoxymethyl)-acetanilide, Monsanto Co., St. Louis, MO] have been used since the beginning of the experiment. In the past 15 yr, Roundup (N-phosphonomethyl glycine, Monsanto Co., St. Louis, MO) has also been used to control emergent weeds.
Corn is harvested by hand from 6.1-m lengths of two center rows of each plot for a total harvested area of approximately 0.00093 ha. Grain and silage yields were determined in NT and CT plots for either corn stover returned (+) or corn plants harvested (–) from 1972 to 1985. In 1990, 2000, and 2001, grain and stover yields were determined. After harvesting to determine yields, corn stalks are removed from the silage plots. Yields for the 28-yr experimental period were extrapolated using the available harvest data. Grain is handpicked from the grain plots and the stover is pushed over to remain on the soil surface. Silage is harvested when the grain is between 1/4 and 3/4 milk line, and grain is harvested after physiological maturity.
The soil in the Peters' Field is described as a Paxton soil by the Soil Conservation Service Soil Survey (USDA-SCS, 1981). The Paxton soil is a well-drained, loamy soil formed in subglacial till, and it is a coarse-loamy, mixed, active, mesic Oxyaquic Dystrudept. The soil in this area is derived from an Aeolian mantle over loamy basal till derived from granite, gneiss, and schist bedrock (Pelletier, 1982). The fine sandy loam Ap horizon extends to 20 cm, below which is a three-layered B horizon (fine sandy loam) extending to 71 cm. Below 71 cm, a firm C horizon extends to 350 cm. The C horizon is compact basal till, a relatively impermeable, densic contact in the soil profile. During periods of prolonged rain and during spring thaw, the soil often remains wet because of the reduced drainage capacity caused by the compact basal till.
Measurements, Analyses, and Calculations
Soil samples were collected in early July 2000 when corn plants were 15 to 20 cm high. Three replicate samples were taken from the 0- to 5- and 5- to 15-cm profiles in each plot; each replicate was a composite of 14 soil cores (1.3-cm diam.). Over the course of the experiment, erosion and soil transport has created a gradient in topsoil thickness across each plot. We sampled within the middle half of the length of each plot to avoid particularly deep, and particularly thin, topsoil at each end of the field. Because tilling mixes soil to approximately 20 cm, and we only sampled the upper 15 cm, it is possible that some C from upper layers was mixed into soil below our 15-cm sampling depth, making our estimates of SOC content in CT plots potentially slightly low. Soil samples were mixed, air dried, and sieved to pass through a 2-mm mesh. Sample 13C and total C concentration were analyzed by the Isotope Laboratory at the University of California, Berkeley with an isotope ratio mass spectrometer (Europa Scientific Limited, Crewe, UK) coupled with a combustion system (ANCA-SL).
Bulk density was measured in July 2002, when corn plants were 15 to 20 cm high, using the core method (Bertrand, 1965). Samples were taken from two depths, 0 to 6 and 6 to 12 cm, and four samples were taken for each depth in each plot. The core diameter was 5.4 cm and the height was 5.9 cm. Of the four samples at each depth, two were taken along the corn rows and two were taken in between the corn rows to account for vehicle compaction. Sample locations corresponded with the location of the soil C samples.
To calculate the contribution of new corn residue C (C4 source) and of the relic forest C (C3 source), we used the 13C values for corn residues (–11.7o/oo) and for an adjacent C3 forest soil (–26.9 and –25.4o/oo for 0 to 5 and 5 to 15 cm, respectively). Although initial soils samples from 1972 would serve as a better baseline for analyzing changes in SOC during this experiment, soil samples were not archived over the years. Thus, adjacent forest soils are used as a proxy for the initial soil state. A simple mixing model was used to calculate the amount of C3-derived C still present in these soils, the amount of new C4-C that has built up through time, and the half-life of the older SOC (Cardon et al., 2001). The proportion of C derived from corn residues, X%, was calculated as:
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= 13C of whole soil, f = 13C value of whole sample from the forest soil, and cr = 13C value of corn stover.
The mass of SOC is often determined as the product of the concentration, bulk density, and layer thickness. However, Ellert and Bettany (1995) demonstrated that agricultural management techniques could lead to differences in soil mass, affecting comparisons of SOC storage. We accounted for management-induced variations in soil mass by calculating the mass of SOC in an "equivalent soil mass" (i.e., the mass of soil in a reference layer), using the mass of the lightest soil (NT+) as the equivalent soil mass. For each treatment, bulk density was calculated as the mean of both depths, because there was no difference in bulk density between depths. Equivalent SOC masses were calculated using the equation:
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As noted above, the initial C content of the soil in 1972 is unknown, so it is not possible to quantify overall changes in SOC for any single treatment over the 28 yr. However, it is possible to examine differences among treatments in their long-term effects on SOC contents. We calculated the increase in C in soils under NT treatments when compared with CT treatments, both with residues removed and returned, as well as the increase in C in soils under residue return management when compared with residue removal, both under NT and CT management. To facilitate comparisons with a recent review by West and Post (2002), these differences in SOC among treatments can be divided by 28 yr to give an amount of SOC "stored" per year in any treatment relative to any other. This calculation, however, assumes a constant rate of change over the 28 yr, and we suspect the accumulation rate is not constant.
The relative turnover times of the relic (forest-derived) C3-C were calculated by assuming that the initial organic C (designated Ao here) decayed exponentially with time (t) (i.e., At = Aoe–kt). An adjacent forest soil was used as a proxy for the initial SOC. C3-C at At was calculated using the measured 13C values and Eq. [1]. The calculated k for each treatment and depth was then used to calculate a half-life of C3-C by setting At in the above equation to A0/2.
The experiment was set out in a split-plot design with three replicates. Tillage treatments (NT vs. CT) were main plots and corn residue management treatments (residue returned or residue removed) were the subplots. Depth was analyzed as a repeated measure within each combination of tillage and corn residue management. Blocks and the block-tillage interaction were considered as random effects in the model. All data were normally distributed and were analyzed by using the mixed model procedure (PROC MIXED) in SAS (SAS Institute, 1991). Differences between means were further tested with the DIFF option of the LSMEANS statement. Statistical significance was assessed at the 0.05 level.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Since both addition of new C and loss of older C influence SOC, we separated the contributions of older relic C3-C from the forest and newer C4-C from corn to the SOC. There are no archived soils from the beginning of the experiment in 1972, so we collected soil from 0- to 5-cm and 5- to 15-cm depths in an adjacent, continuously forested area to provide the baseline 13C and C content for the initial C3 forested state in our mixing model (Eq. [1]). The C content of the forest soils was 120.2 ± 21.5 g C kg–1 and 60.7 ± 7.9 g C kg–1 for the 0- to 5- and 5- to 15-cm soil layers, respectively. Table 1 shows 13C values for the various treatments and for the forest, as well as the C4-C contribution to the soil C pools calculated using the mixing model.
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In the entire sampled profile (0–15 cm), more C3-C remained in the NT plots over time than in the CT plots (Fig. 2b). There was no effect due to residue addition on the mass of C3-C in either tillage system.
The half-life of C3-C, expressed as Mg C ha–1 soil, in the upper 5 cm is influenced by tillage, with NT having longer half-lives than CT (Table 1). Under CT management, removing residues resulted in shorter half-lives of C3-C in the upper 5 cm of the soil. In both NT and CT plots, the C3-C is lost at a greater rate in the upper layer than in the lower profile (Table 1). In the lower profile (5–15 cm), tillage management influenced the half-life of C3-C; C3-C is lost at a greater rate in the CT system compared with the NT system. There was no effect due to residue management in the lower soil profile.
Cycling of C4-C Added to Soil
C4-C declined in the upper 0 to 5 cm with tillage and with stover removal (Fig. 3a,b)
. Patterns are more complex for the lower layer of soil. When C4-C pools are expressed in either g C4-C kg–1 or Mg C4-C ha–1 soil, tillage has no effect on incorporation of new C4-C at 5 to 15 cm, but plots where stover is retained have more C4-C in lower soil layers than plots where stover is removed (Fig. 3a,b).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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In a survey of 14 long-term, continuous corn experiments of varying length, West and Post (2002) report an increase of 44 ± 27 g C m–2 yr–1 resulting from NT compared with CT management, with nearly 85% of the increased C sequestered in the upper 7 cm of soil. Although West and Post (2002) call this difference between CT and NT plots a "C sequestration rate", this terminology is potentially confusing. As West and Post note, without information about initial SOC content, it is not possible to know whether net C sequestration is occurring. No till plots, for example, may be losing less C than CT plots, leading to a relative but not an absolute C "sequestration" in NT compared with CT plots at any given measurement time. The varied lengths of the 14 experiments reported by West and Post necessitated that they standardize C changes per year by dividing by the length of the experiment, but the potential for C content to reach a saturation point (Six et al., 2002a) indicates it would be useful to measure changes in C with time in various treatments.
Other comprehensive surveys have similarly reported increases in SOC when NT is compared with CT management. For example, Six et al. (2002b) present a mean SOC sequestration rate of 33 ± 11 g C m–2 yr–1 for tropical and temperate regions, Eve et al. (2002) report a mean sequestration rate of 20 to 52 g C m–2 yr–1 across regions of the conterminous USA, and Lal (2004) presents a world-wide SOC sequestration range of 10 to 100 g C m–2 yr–1. Our long-term data reveal that NT management resulted in an increase of 48 ± 10 g C m–2 yr–1 in the upper 15 cm of the soil over SOC content in CT when residues were returned to soil, and an increase of 60 ± 14 g C m–2 yr–1 when residues were removed. Interestingly, returning stover residues to the NT and CT systems resulted in a significant increase in SOC pools only in the CT system, where SOC content in CT+ increased by 35 ± 11 g C m–2 yr–1 compared with CT–. An increase of 23 ± 13 g C m–2 yr–1 in NT+ compared with NT– plots was not statistically significant. These data show that the increases in SOC in NT compared with CT plots is larger, whether residues are returned or not, than the increases in SOC associated with residue return, under either tillage system. The greatest difference in SOC among the treatments was found, not surprisingly, between the CT– and NT+ treatments (83 ± 11 g C m–2 yr–1).
Many studies have emphasized the need to calculate the mass of SOC, rather than the concentration, in making comparisons in SOC storage (Ellert and Bettany, 1995). Our data suggest that using soil C mass (Fig. 1b) was critical to revealing that the effects of residue removal and tillage on total SOC were not as drastic as might have been suggested by changes in the concentration of SOC. For example, when our data were calculated as mass per unit area, SOC storage was not influenced by crop residue additions under NT or CT management in the upper 5 cm of soil (Fig. 1b). However, when our data were presented as soil C concentration, stover addition increased SOC concentration in the upper 5 cm of soil (Fig. 1a).
At our long-term site, we found a net gain in SOC in the 0- to 15-cm horizon when residues were returned to the CT plots (Fig. 1b). However, other research has suggested that intensive tillage (e.g., moldboard plow) would eliminate any net gain in SOC due to residue additions (Reicosky et al., 2002). Our findings are also different from a similar experiment in Minnesota where there was little difference in SOC due to residue management in CT plots, but an increase in SOC when stover was returned to NT plots after 13 yr of continuous management (Clapp et al., 2000). We found no increase in SOC in the upper 15 cm of soil when residues were returned to the NT plots (Fig. 1b).
Comparisons of data from other sites to ours underscores the need for long-term data from various regions, especially given the current interest in the use of corn stover residues as a renewable biofuel (Mann et al., 2002; Lal et al., 1998; Lal, 2004). In the U.S. Cornbelt, grain corn is predominantly grown, and corn stover is left on the ground after harvest of grain. Harvesting of stover, however, could negatively affect SOC levels, soil quality, and C sequestration. Data from our site, located in a climatic zone similar to the Northern Cornbelt, suggests that removing crop residues will adversely affect the amount of SOC storage only in the CT system. Interestingly, additions of residue for 28 yr under NT did not increase SOC storage, suggesting that removing stover residues in a NT system may not affect SOC (Fig. 1b).
A few studies have reported no increase in soil C content with increased C inputs (Campbell et al., 1991; Solberg et al., 1997). Six et al. (2002a) have proposed a conceptual SOM model, suggesting that the storage capacity of soils may become saturated with respect to SOC. At our site, we did not see a significant increase in SOC due to residue management in the NT plots, suggesting that the NT plots are closer to a steady-state SOC level, or SOC saturation level, than the CT treatments (Fig. 1b). Thus, it is important to understand the status of the SOC with regard to saturation levels in making decisions regarding residue removal, and its effect on SOC levels.
Turnover of Relic C3 Carbon
The decline in SOC following cultivation has been well documented (Lal et al., 1998; Follett, 2001; Mann et al., 2002), and several studies have investigated the rate of relic C loss with varying results. For example, in a survey of temperate soils, relic C turnover was reduced under NT relative to CT (Six, et al., 2002b). At two sites in southern Ontario, tillage did not affect the turnover of C3-C in the upper 15 cm of soil (Wanniarachchi et al., 1999). In a study where corn residues were removed annually, meadow-derived (i.e., relic) C decreased at the same rate regardless of tillage treatment (Angers et al., 1995).
In our 28-yr experiment, the relative half-life of relic C3-C was strongly influenced by tillage in the upper 15 cm of the soil (Table 1), with C3-C being retained longer under NT management. A decreased decomposition rate of C derived from original vegetation under NT compared with CT has been suggested as one possible mechanism for C sequestration under NT (Six et al., 2002b). In the Minnesota study with a similar experimental setup to ours, but with a shorter duration, relic C was also retained longer under NT management relative to moldboard plow (Clapp et al., 2000). Under either tillage system, Clapp et al. (2000) reported a significant increase in the half-life of relic SOC when stover was returned to the soil and N fertilizer was mixed with the stover. In contrast, the half-life of C3-C in our system was affected by residue management only in the upper 5 cm of the CT plots (Table 1).
Our half-life values fall within the half-life ranges presented by others. In our experimental plots, half-lives ranged from 13 to 19 yr in the surface horizon, and from 24 to 60 yr in the deeper horizon (Table 1). In other temperate regions, half-lives ranging from 13 to 88 yr have been reported in the literature (Balesdent et al., 1990; Gregorich et al., 1994; Angers et al., 1995; Clapp et al., 2000). Although our values for absolute half-lives of relic C may be slightly shifted because we used the adjacent forest soil C as a proxy for initial soil C values, the half-lives can be compared across treatments since all treatments began at the same time, on land treated similarly.
Cycling of C4-C Added to Soil
The amount of corn-derived C added to and retained in soil over time has been reported in a number of studies. In studies where corn stover was returned to the soil each year, 10 to 37% of the added C4-C has been reported to be retained (Gregorich et al., 1996; Balesdent and Balabane, 1996; Clapp et al., 2000). In our plots where stover was returned, 12 to 15% of the added corn-C remained after 28 yr under NT or CT, which is within the range of reported values (Table 1). When stover was harvested, as in a silage system, greater percentages of corn-residue C have been shown to remain in the soil. For example, 29 to 67% of added C4-C has been reported to remain in the soil where corn residues were removed (e.g., Balesdent and Balabane, 1996; Angers et al., 1995; Clapp et al., 2000). Our estimated values of 41% for NT– and 28% for CT– are in the middle of the range of reported values where corn residues were harvested.
Our long-term experiment allows us to measure changes in C4-C pools under differential tillage intensity and residue return regimes in the same system. It is striking that the percentage of added C4-C that remains in the soil of our plots where stover was removed (28–41%) is much greater than the percentage of added C4-C that remains in the plots where stover was returned (15%). It is particularly interesting that our estimates of C4-C input show that about four times more C4-C has been added to the NT+ plots over the 28 yr than to the CT– plots, but only two times more C4-C has accumulated in the NT+ soils (Fig. 3b). Also, it is remarkable that the amount of C4-C in the NT– and CT+ plots is indistinguishable (Fig. 4) even though four times as much C4-C was applied to the CT+ plots.
Upon examining the contribution of C3-C and C4-C to the total SOC pool in the upper 15 cm, interesting trends emerge. While residue additions result in an increase in C4-C under either tillage system (Fig. 3b), the amount of C3-C is not affected by residue additions (Fig. 2b). Six et al. (2002a) propose a "non-protected" SOC pool that is composed of newly derived plant materials and that is more susceptible to decomposition than the various protected pools. As noted above, the amount of aboveground C4-C added to the system is much greater than what is stored in the soil, under either NT or CT management. Our data suggest that the C4-C may contribute to a non-protected SOC pool, which seems to be approaching its saturation level in NT (Fig. 3b). The relic C3-C is likely part of a more protected SOC pool and is more resistant to decomposition (Fig. 2b). The loss of C4-C indicates that there may be a rapid cycling of the aboveground corn-derived C back to the atmosphere as CO2 or lost as dissolved organic C from the soil profile.
The difference between the amount of corn-derived SOC remaining in our plots where stover was harvested and where stover was returned suggests the possibility that root and shoot tissues decompose at different rates. Corn root tissue has been shown to be 1.6 times more resistant to decomposition than shoot tissue (Balesdent and Balabane, 1996). More recent research suggests that root- and shoot-derived residues are incorporated into SOC at different rates (Wander and Yang, 2000). Although we did not directly measure the contribution of roots to SOC in our experiment, a slower decay of corn roots could explain why approximately 59 to 72% of total C is lost when only root mass contributes to SOC each year, whereas approximately 85 to 88% of total C is lost when stover and root mass are contributed. Results from our experiment as well as others highlight the necessity for more information about the role of aboveground versus belowground residue on the sequestration of SOC (Follett, 2001).
Surface residue decay has been shown to be rapid where moisture and nutrient status are non-limiting (Scott et al., 1996; Alvarez et al., 1998; Green et al., 1995). Our plots have been fertilized annually at accepted agronomic rates, resulting in typical corn production yields, which suggests that moisture and nutrients have been available in adequate amounts. Research conducted to evaluate a new soil test for N at an adjacent cornfield to Peters' Field indicates that corn yields are not significantly reduced when N fertilizer is applied at about 50% of the rate applied to the Peters' Field (Guillard et al., 1999). Thus, it is possible that excess N has contributed to a more rapid decomposition rate of the corn residues.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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At Peters' Field, our results indicate that there may be a rapid cycling of the aboveground corn-derived C to the atmosphere as CO2 or lost as dissolved organic C from the soil profile. The total amount of C4-C remaining in the NT+ plots is double the amount of C4-C in the 0- to 15-cm layer of the CT– plots, but four times as much C4-C has been added to the NT+ plots. This result contrasts with the statistically indistinguishable amounts of C4-C in the NT– and CT+ plots (Fig. 3b, 4). Such a rapid cycling of returned C4-C suggests that the annual return of aboveground biomass may not increase soil C storage over the long term once soils have reached a steady-state SOC level.
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ACKNOWLEDGMENTS |
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Received for publication January 7, 2004.
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