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SOIL BIOLOGY & BIOCHEMISTRY

Tillage and Cover Cropping Effects on Aggregate-Protected Carbon in Cotton and Tomato

^a Dep. of Agronomy, 1025 Agronomy Hall, Iowa State Univ., Ames, IA 50011
^b Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616-8627
^c Dep. of Plant Sciences, Univ. of California, Davis, CA 95616

^* Corresponding author (veenstra{at}iastate.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Conservation tillage (CT) and cover cropping (CC) are agricultural practices that may provide solutions to address water and air quality issues arising from intensive agricultural practices. This study investigated how CT and CC affect soil organic matter dynamics in a cotton(Gossypium hirsutum L.)–tomato (Lycopersicon esculentum Mill.) rotation in California's San Joaquin Valley. There were four treatments: conservation tillage, no cover crop (CTNO); conservation tillage with cover crop (CTCC); standard tillage, no cover crop (STNO); and standard tillage with cover crop (STCC). After 5 yr, the top 30 cm of soil in CTCC had an increase of 4500 kg C ha^–1, compared with an increase of 3800 kg C ha^–1 in STCC from initial soil C content in 1999. To enhance our understanding of C dynamics in CT systems, we pulse-labeled cotton with ¹³CO₂ in the field and followed the decomposition of both the roots and the shoots through three physical fractions: light fraction (LF), which tends to turnover quickly, and two relatively stable C pools—intraaggregate LF (iLF) and mineral-associated carbon (mC). Soil under CT treatments retained more of the cotton-residue-derived C in LF and iLF than ST 3 mo after placement in the field. These differences disappeared after 1 yr, however, with no discernable differences between CT and ST regardless of CC. In California's Mediterranean climate, CT alone does not accumulate or stabilize more C than ST in tomato–cotton rotations, and the addition of cover crop biomass is more important than tillage reduction for total soil C accumulation.

Abbreviations: CT, conservation tillage • CTCC, conservation tillage with cover crop • CTNO, conservation tillage, no cover crop • iLF, intraaggregate light fraction • LF, light fraction; mC, mineral-associated carbon • SOM, soil organic matter • ST, standard tillage • STCC, standard tillage with cover crop • STNO, standard tillage, no cover crop

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The soil C sequestration potential of reduced tillage or CT has been extensively studied in other parts of the country, but there has been very little CT research under arid, irrigated agriculture systems, like those in California. California cropping systems are intensively managed and require tillage operations to address furrow irrigation needs. Diverse crop rotations and crops untested for reduced tillage regimes have complicated the development of CT approaches in this highly productive agricultural region.

Reducing tillage of agricultural soils may improve agricultural sustainability by reducing fossil fuel consumption, labor needs, equipment maintenance, and soil erosion, and increase soil water conservation and soil C sequestration (Unger et al., 1997; Lal, 2001). Soil C sequestration potential is dependent on a number of factors including climate and parent material. California agricultural soils often have high clay content (loam to loamy clay) because of the depositional environment of the San Joaquin Valley. As a result, these soils may have a greater potential to promote organo-mineral interactions and stabilize C through aggregation; however, irrigation combined with California's warm climate (near-surface soil temperatures often averaging between 25 and 35°C) may enhance decomposer activity and limit the potential for soil C sequestration.

The occlusion of particulate organic matter (partially decomposed plant residues) and more stable soil organic matter (SOM), such as humics within aggregates, stabilizes soil C through physical protection (Powlson et al., 1987; Gregorich and Bettany, 1995; Needelman et al., 1999; Six et al., 2001). Physical separation techniques have been used to identify various SOM pools that exhibit different levels of stability (Gaunt et al., 2001). Many researchers have shown that CT increases particulate organic matter or light fraction organic matter, an unstable, transitory C pool (Schwenke et al., 2002; Fabrizzi et al., 2003; Liebig et al., 2004). This pool of C forms a nucleus for aggregate formation and is central to aggregate stability (Tisdall and Oades, 1982; Cambardella and Elliott, 1993; Rees et al., 2001) because it promotes the formation of organo-mineral complexes that bind mineral particles together (Lal et al., 1998). Light fraction organic matter that is physically or chemically protected has a lower decomposition rate than unprotected light fraction, suggesting that it is protected from microbial attack (Gale and Cambardella, 2000; von Lutzow et al., 2002). The source of C inputs (root- or shoot-associated) may influence the potential to form aggregates. Root-associated C may promote aggregate formation more than shoot C (Gale and Cambardella, 2000; Wander and Yang, 2000; Puget and Drinkwater, 2001; Schwenke et al., 2002). Plant roots exert pressure on soil particles and exude polysaccharides, promoting soil aggregation (Kooistra and van Noordwijk, 1996). In CT systems where shoots primarily remain on the soil surface and roots are left intact, roots and hyphae are directly incorporated within aggregates as protected organic matter (Jastrow and Miller, 1998). For these reasons, CT may foster aggregate formation and thus stabilize more organic matter within aggregates than ST systems.

We examined the potential of CT, cover cropping, and their interactions to sequester C in intensively managed, irrigated California cropping systems. The accumulation and stabilization of soil C was determined by measuring total soil C (accumulation) and changes in the concentrations of C in the LF and two stable fractions of soil organic matter: iLF and mC (stabilization). To further understand organic matter stabilization and soil C dynamics, we used a ¹³C tracer to differentiate between root and shoot C inputs, expecting that more root C than shoot C would be stabilized within aggregates. Overall, we were particularly interested in whether CT and cover cropping are sustainable approaches to sequester C in arid, irrigated California cropping systems.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Site
A long-term CT and cover cropping experiment was established in 1999 at the University of California West Side Research and Extension Center in Five Points, CA (36°20'29'' N, 120°7'14'' W). The experiment was located on the west side of the San Joaquin Valley where the soil type is Panoche clay loam (fine-loamy, mixed, superactive, thermic Typic Haplocambid). The area receives 18 cm of precipitation annually and has a mean maximum air temperature of 24°C and minimum of 8°C. Four treatments were examined combining CT and cover cropping: CTNO, CTCC, STNO, and STCC. See Table 1 for a description of tillage passes for each treatment. Each treatment was replicated four times in a randomized split plot design. During the 2-yr rotation, cotton and tomato were each grown on half the area. The crops were switched annually. The winter cover crop was a cereal–legume mix of triticale (xTriticosecale spp.), rye (Secale cereale L.) and vetch (Vicia sativa L.). The treatments were fertilized and irrigated according to the established management practices of the region (see Baker et al. [2005] for further details).

Determination of Aggregate-Protected Carbon Dynamics
Carbon-13 Enrichment
In the summer of 2003, we established 16 cotton microplots (four replicates of each treatment from above). Plots were 76 cm wide (30 inches). Each plot included eight cotton plants, and because plant spacing varied, plots were between 0.8 and 1.2 m long. Every 2 wk during the growing season, ¹³CO₂ was applied to cotton plants. Polyvinyl chloride frames with adjustable widths and heights to accommodate plant growth were enclosed with 8-mil (0.2 mm) transparent polyethylene plastic sheeting. Before use in the field, we tested the polyethylene's CO₂ retention; after a 24-h period there was no detectable CO₂ loss through the plastic. Enclosures were sealed at the soil surface by placing sandbags around the perimeter. The ¹³CO₂ was injected into the polyethylene enclosures, four plots at a time, from 1 h after sunrise until as late as 1100 h, depending on the rate of plant uptake. Labeling was performed early in the morning to maximize plant uptake while keeping temperatures low inside the enclosures. Enclosures were removed if temperatures exceeded 45°C. After the injection of ¹³CO₂, CO₂ concentrations were measured inside the canopy, using a Qubit CO₂ Monitor (Qubit Systems, Kingston, ON). Enclosures were removed when the CO₂ concentration within the enclosure approached the compensation point (<60–80 mg L^–1 CO₂). The volume of ¹³CO₂ applied varied each week; we calculated the volume of ¹³CO₂ to apply by estimating the cotton plants' expected biomass and assuming 40% of the expected biomass was C (Hake et al., 1996). Using an uptake efficiency of 55%, the volume of ¹³CO₂ needed each week was estimated to achieve a desired ¹³C value of 360. The actual uptake efficiency was much lower, resulting in an enriched cotton residue ¹³C value of 31. This may be because C loss to nighttime respiration was higher than expected. An improved method would be to cover the plants with the plastic enclosures overnight to capture respired ¹³CO₂ and thus maximize ¹³C assimilation.

At the end of the growing season, labeled aboveground portions of cotton plants were removed, cotton bolls were harvested by hand, and the remaining shoot residue was cut into 5- to 15-cm pieces, combined, and distributed evenly among 16 new plots adjacent to those where the ¹³C-labeled cotton roots remained. This allowed us to differentiate between the decomposition of root and shoot residues. The shoot material was analyzed for initial ¹³C content and root ¹³C content was assumed to be the same as that of the shoot material (Gregorich et al., 1995; Wander and Yang, 2000). After the shoot and root plots were established, post-harvest tillage in the standard tillage plots was simulated using a rototiller in the STNO and STCC plots. Soil samples were collected, using a 2-cm-diameter soil corer, following ¹³C labeling in January, May, and November 2004. Each microplot was sampled to two depths: 0 to 5 and 5 to 15 cm. On each sampling date, 10 cores from each root or shoot treatment microplot pair were taken, mixed, and then the organic matter was fractionated from each replicate according to the following fractionation procedure.

Light Fraction Organic Matter Fractionation
We used a slightly modified version of the physical fractionation procedure proposed by Sohi et al. (2001). Soil from the root and shoot plots (35–40 g) was placed in 250-mL plastic centrifuge bottles filled with a 1.80 g cm^–3 solution of NaI. The bottles were vigorously shaken by hand for 30 s and then centrifuged at 8000 x g for 30 min. Floating LF was removed by suction from the surface of the soil–NaI mixture. The remaining solution was filtered through a 20-µm filter. The LF was rinsed off the filter into metal drying tins and dried overnight. The tins were weighed and then the LF material was ground to pass through a 53-µm sieve. The ground LF was weighed and analyzed for ¹³C (see below). To disperse aggregates and release iLF, more NaI solution was added to the remaining soil in the 250-mL centrifuge bottle and the soil–NaI solution was sonified for 6 to 8 min to apply 1500 kJ kg^–1 of soil. After sonification, the soil–NaI solultion was centrifuged (8000 x g, 25°C for 30 min) to separate the iLF from the remaining mineral material. Floating iLF was split into two subsamples by pouring the supernatant onto two separate 2-µm glass fiber filters (Fisherbrand G6 filter circles, Fischer Scientific, Hampton, NH). One glass fiber filter was ashed in a 550°C oven for 4 h to determine the ash content of the iLF. The other glass fiber filter was ground to pass through a 53-µm sieve. The remaining mineral soil was rinsed with 250 mL of deionized water and centrifuged (8000 x g, 25°C for 10 min) two more times or until the supernatant was clear. Then the mineral soil was dried at 40°C and ground to pass through a 53-µm sieve.

Carbon-13 Analysis
The LF, iLF, and mineral fractions were weighed into Ag capsules and fumigated with HCl (12 M) for 4 h according to the method outlined by Harris et al. (2001) to remove residual carbonates. The HCl-fumigated samples were analyzed for ¹³C on an isotope ratio mass spectrometer (Stable Isotope Laboratory at the University of California, Davis).

¹³C calculation:

[1]

where (¹³C/¹²C)_sample is the ratio of ¹³C to ¹²C in the sample, and (¹³C/¹²C)_standard is the ratio of ¹³C to ¹²C in the Vienna Pee Dee Belemnite standard.

Fraction of ¹³C ( f) calculation:

[2]

where ¹³C_sample is the ¹³C value for each sample, ¹³C_control is the ¹³C value for the corresponding unlabeled-soil control (an unlabeled-soil control value was obtained for unlabeled plots from each treatment type for each sampling date), and ¹³C_{added residue} is the ¹³C value for the added ¹³C-labeled residue—the average value used for all treatments was 31.

Recovery percentage calculation:

[3]

where m_{13C added} is the mass of ¹³C applied to the plot as labeled cotton residue (the biomass of shoot residue was known, while the biomass of root residue was estimated from values obtained from adjacent unlabeled plots, see Table 2), f_{organic matter fraction} is the fraction of ¹³C in the organic matter fraction (LF, iLF, or mC), D_b is the bulk density of the soil, which is an average value for each treatment, and V_plot is the volume of the plot.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Bulk Density
Soil bulk density is an important factor that affects the interpretation of changes in total soil C. Often total soil C is measured on a mass-per-mass basis (%); however, using this method, soils with similar C percentages but different bulk densities would have different total soil C contents on a mass-per-volume basis. In this study, soil bulk density was much lower in the CTNO treatment (1.05 g cm^–3) than the other treatments (=1.24 g cm^–3); STCC had the highest bulk density (1.28 g cm^–3; Table 3). On a mass-percentage basis, the CTNO treatment had a higher total C concentration than the STNO plots, but considering the changes in bulk density, the two treatments had similar total C contents on a volume basis.

View larger version (33K): [in this window] [in a new window]   Fig. 1. Total C in the 0- to 15- and 15- to 30-cm depths for conservation (Cons.) and standard (Std.) tillage with and without cover cropping. Background lines represent the 1999 mean total C from across the field for each depth. Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05.

In this experiment, the reduced soil disturbance under CT contributed to C redistribution, not overall C accumulation compared with ST. The rate and degree of organic matter accumulation associated with surface residues varies widely because of climate, soil type, and residue quality (Schomberg et al., 1994). Temperature and soil water content strongly affect decomposition rates (Scharpenseel and Pfeiffer, 1997). The optimal conditions for increasing soil C storage would be in soils originally high in C levels, such as those found in temperate moist regions (Karlen and Cambardella, 1996; Liebig et al., 2004). Therefore, California's warm climate combined with frequent irrigation may limit C accumulation in CT systems.

This study was conducted on the west side of the San Joaquin Valley, a semiarid Mediterranean environment. Few studies have looked at the potential for CT to store C under semiarid conditions, and those that have reveal inconsistent results. Some studies of CT and no-till systems in arid regions found small C increases (Hernanz et al., 2002; Mrabet, 2002). Other CT or no-till studies, however, have shown that under hot, semiarid conditions no soil organic C accumulation occurs (Buschiazzo et al., 1999; Chan et al., 2001). In semiarid conditions, with low precipitation and high temperature, crop residue decomposition is limited by water content (Hajabbasi and Hemmat, 2000; Mrabet, 2002; Bronson et al., 2004). Conservation tillage may actually increase mineralization potential in warm climates by reducing surface evaporation (Unger et al., 1997). In California, furrow irrigation maintains soil moisture, and in combination with CT, this may further increase mineralization rates.

Studies that have shown soil C accumulation are often in no-till systems, which eliminate most soil disturbance. Conservation tillage, in this experiment, was a reduction in tillage practices, not the complete elimination of tillage (Table 1). Cotton was planted directly into the tomato residue, but after cotton harvest the soil surface was disturbed when cotton roots were undercut to meet county requirements for pink bollworm outbreak prevention. In addition, soil bed and furrow maintenance was required for the following tomato crop. Therefore, our CT system greatly reduced tillage and soil disturbance, but did not completely eliminate it. Some soil disturbance combined with warm temperatures and furrow irrigation may limit C accumulation in this environment compared with what would be expected under cooler or drier conditions and even less soil disturbance.

In spite of the lack of C accumulation in our CT systems, treatments that included cover crops showed an overall increase in total C regardless of tillage intensity. The CTCC treatment shows a larger increase (4504 kg C ha^–1) than STCC (3834 kg C ha^–1); this is probably a result of the CTCC having a larger average yearly cover crop yield than STCC (Table 4).

Free Light Fraction and Aggregate-Associated Light Fraction
Mass of Carbon in Organic Matter Fractions
Often when total C increases in CT systems, those differences only begin to appear after as long as 10 to 20 yr (Mrabet, 2002; Liebig et al., 2004). Our study examined changes in soil C after 5 yr, so over the longer term, our systems may show different results. Six et al. (2001) suggested that the accumulation of C in aggregates and mineral-associated C are indicators of future soil C increases. To determine if there is a trend toward soil C accumulation, we examined C distribution among three pools: LF, iLF, and mC. In the 0- to 5-cm depth, CT treatments had significantly more LF-C and mC than the control (Fig. 2 ). There were no treatment differences for any fraction in the 5- to 15-cm depth. Conservation tillage, by reducing soil disturbance, tends to increase LF in the 0- to 5-cm portion of the soil profile, redistributing C toward the soil surface (Needelman et al., 1999; Mrabet, 2002; Fabrizzi et al., 2003).

View larger version (30K): [in this window] [in a new window]   Fig. 2. Total mass of C in each organic matter fraction after 5 yr of conservation (Cons.) or standard (Std.) tillage with or without cover cropping (LF = light fraction). Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05.

The Short-term Fate of Cotton Carbon
Effects of Conservation Tillage.
Three months after the addition of ¹³C-labeled cotton residue (January sampling date), more new cotton-derived C was found in the LF and mC fractions in the 0- to 5-cm depth in the CT treatments than in the ST treatments (Fig. 3 ). There was also more ¹³C recovered in the CT treatments in the mC fraction in the 5- to 15-cm depth and in the iLF fraction in both depths, but the differences were not statistically significant. One year after treatment (November sampling date), although the differences were not statistically different, CT had more cotton-residue-derived C in LF and mC in the 0- to 5-cm depth than did ST, but CT and ST had the same levels of cotton-derived C in the remaining fractions and depths.

View larger version (37K): [in this window] [in a new window]   Fig. 3. Percentage of added cotton C (13C) recovered in the organic matter fractions light fraction (LF), intraaggregate light fraction (iLF), and mineral-associated C (mC), by tillage treatment (Cons. = conservation, Std. = standard) and depth. Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05. There were no significant differences in iLF among the treatments.

View larger version (35K): [in this window] [in a new window]   Fig. 4. The amount of added 13C derived from new cotton residue as a fraction of total C in the organic matter fractions light fraction (LF), intraaggregate light fraction (iLF), and mineral-associated C (mC), with respect to tillage treatment (Cons. = conservation, Std. = standard) and soil depth. Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05. There were no significant differences in mC among the treatments.

Effects of Conservation Tillage and Cover Cropping.
The addition of a cover crop slowed the mineralization of the cotton-residue-derived LF-C. In both tillage treatments, the amount of cotton-residue-derived C in LF remained steady through the January and May sampling dates, whereas in the treatments without cover crops, there was less cotton-derived LF found in samples collected in May (Fig. 5 ). This is probably a substitution effect. Because the cover crop systems had more C inputs into the system overall, less of the cotton-derived C was decomposed initially, but this effect diminished by the November sampling date.

View larger version (39K): [in this window] [in a new window]   Fig. 5. Amount of new cotton C (13C) as a fraction of total C found in the organic matter fractions light fraction (LF), intraaggregate light fraction (iLF), and mineral-associated C (mC), and the percentage of added 13C that was recovered, by date, tillage treatment, and cover crop treatment. Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05. There were no significant differences in the recovery percentage of 13C in iLF among the treatments.

Root vs. Shoot Dynamics
We recovered significantly more shoot C than root C (Fig. 6 ). We may have underestimated root ¹³C input initially because we assumed roots were enriched similarly to shoots. We made this assumption because we did not want to disturb our treatments belowground, especially in the CT system. Wander and Yang (2000) assumed corn (Zea mays L.) roots were ¹³C enriched similarly to shoots for the same reason. Previous studies where corn was labeled with ¹³CO₂ found shoots and roots were similarly enriched (Gregorich et al., 1994). Puget and Drinkwater (2001) observed that shoots of vetch were more enriched than roots following ¹³CO₂ labeling. Our cotton roots may have had a lower ¹³C enrichment than we estimated, leading us to underestimate root input. Because so little ¹³C was found belowground in subsequent sampling, however, we feel the data are representative of above- and belowground inputs.

View larger version (37K): [in this window] [in a new window]   Fig. 6. Root and shoot C dynamics. Amount of new cotton C (13C) as a fraction of total C found and percentage of added 13C recovered in the organic matter fractions light fraction (LF), intraaggregate light fraction (iLF), and mineral-associated C (mC), by date, tillage treatment (Cons. = conservation, Std. = standard), and root or shoot treatment. Error bars represent standard error of the mean (n = 8). Bars not associated with the same letter are significantly different at P = 0.05. There were no significant differences in recovery percentage of 13C in iLF among the treatments.

Because we found no significant differences in the cotton-root-derived C dynamics, much of the differences discussed above can be attributed to the differences we saw within the cotton-shoot-derived C dynamics.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Aggregate-Protected Organic Matter Dynamics in California Conservation Tillage Systems
After 5 yr of CT and cover cropping, overall aggregate-protected organic matter dynamics were largely unchanged, regardless of tillage treatment. Both CT and cover cropping merely slowed the rate of LF incorporation and turnover, but neither stabilized more C within aggregates than ST treatments. Recently added cotton-residue C was cycled rapidly through aggregates and mineralized in all treatments. This C turnover occurred in less than a year's time, which was much more quickly than expected and can be explained by the warm, irrigated environment. Although we expected that more root C would be incorporated into aggregates, very little root C was recovered. Instead, much more shoot C was recovered in all three organic matter fractions. This lack of root C is probably related to cotton root architecture, because most of the C is concentrated in a 1- to 2-m-long woody taproot.

Conservation Tillage, Cover Cropping, and Carbon Sequestration
Conservation tillage is often purported to be a potentially effective method to sequester elevated atmospheric CO₂ by accumulating and stabilizing it as soil C. In this experiment after 5 yr, however, CT neither accumulated nor stabilized soil C. The only treatments that exhibited overall C increases were the systems with cover crop additions. This additional C found in the cover crop systems was primarily in the unstable LF form. The dearth of C in the mC fraction may be attributable to the warm climate and frequent soil drying on the surface between irrigation events. Although it appears that reduced tillage and cover cropping in a cotton–tomato rotation in California's Mediterranean climate does not effectively sequester more C than ST systems, the additional organic matter found in the cover-cropped systems has the potential to improve water infiltration, mitigate dust production, and improve overall soil quality and fertility.

Received for publication June 14, 2006.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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