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Tillage impacts on microbial biomass and soil carbon and nitrogen dynamics of corn and cotton rotations
Applied Soil Ecology 29 (2005) 85–92 www.elsevier.com/locate/apsoil
Tillage impacts on microbial biomass and soil carbon and nitrogen dynamics of corn and cotton rotations Alan L. Wrighta,*, Frank M. Honsa, John E. Matocha Jr.b a
b
Department of Soil and Crop Sciences, Texas A&M University, 2474 TAMU, College Station, TX 77843-2474, USA Texas A&M University Agricultural Research and Extension Center, 10345 Agnes Street, Corpus Christi, TX 78406-1412, USA Received 26 May 2004; accepted 13 September 2004
Abstract Long-term no tillage (NT) may enhance soil C sequestration and alter soil C and N dynamics. The objectives of this study were to investigate the impacts of tillage on soil C and N sequestration and microbial C and N dynamics of corn (Zea mays L.) and cotton (Gossypium hirsutum L.) cropping sequences after 20 years of management. Tillage regimes included conventional tillage (CT), moldboard plow (MP), minimum tillage (MT), and NT. No tillage increased soil organic carbon (SOC) and nitrogen (SON) concentrations in surface soil (0–2.5 cm) for cotton but not for corn. Few tillage effects on SOC and SON were observed in subsurface soils. For corn, SOC and SON were 11 and 21% higher under NT than other tillage regimes at 0–2.5 cm, but were 22 and 12% lower under NT from 2.5 to 20 cm. Averaged between depths, SOC and SON for cotton were 8 and 7% greater under NT than CT, while NT and MT had 24 and 43% greater SOC and SON than MP. Soil organic C and SON were significantly greater for corn than cotton, but this did not result in greater microbial biomass and mineralizable C and N than for cotton. Microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were often highest under NT and MT in surface soils, but few tillage impacts were observed at 2.5–20 cm. Mineralizable C and N were highest under NT and MT in surface soils for corn and cotton, and in subsurface soils for cotton. Even though SOC and SON were greater for corn than cotton, cotton exhibited greater soil mineralizable C and N under NT and MT than corn, especially in subsurface soils. These results indicate a greater potential supply of N for the cotton than corn crop during the growing season. Increased SOM content in surface soils under reduced tillage may increase N mineralization and the nutrient supply to crops, but the potential of these soils for C and N sequestration appeared limited. # 2004 Elsevier B.V. All rights reserved. Keywords: Carbon sequestration; Microbial biomass; No tillage; Soil organic matter
1. Introduction Abbreviations: NT, no tillage; CT, conventional tillage; MP, moldboard plow; MT, minimum tillage; SOC, soil organic carbon; SON, soil organic nitrogen; MBC, microbial biomass C; MBN, microbial biomass N; SOM, soil organic matter * Corresponding author. Tel.: +1 979 845 3814. E-mail address: [email protected] (A.L. Wright).
No tillage has potential to enhance soil C sequestration. Impacts of tillage on soil organic matter (SOM) have been well documented, but results vary due to soil type, cropping system, residue
0929-1393/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2004.09.006
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management, and climate (Paustian et al., 1997). Much research on C sequestration in agricultural soils has been conducted in cooler, northern climates in the Midwestern USA. Soil organic matter contents are often lower in humid, southern areas compared to the northern USA. In Texas, soil C sequestration potential increases as temperature decreases (Potter et al., 1998). Most impacts of no tillage (NT) on C sequestration have been observed in surface soils near the rooting zone and crop residues (Paustian et al., 1997). However, long-term increases in SOM have been observed in subsurface soils after 20 years of NT in the southern USA (Wright and Hons, 2004). The greatest increases in SOM are usually observed in intensive cropping systems, where multiple crops are grown yearly (Franzluebbers et al., 1995; Ortega et al., 2002; Wright and Hons, 2004). Types of crop residues play important roles in C sequestration and organic matter cycling due to differences in C/N ratios or quality of residues (Lynch and Bragg, 1985; Franzluebbers et al., 1995; Potter et al., 1998). Degradation of fresh crop residues is often governed by C/N ratios (Oades, 1988; Chesire and Chapman, 1996). Tillage promotes SOM decomposition through crop residue incorporation into soil, physical breakdown of residues, and disruption of SOM protected within aggregates (Paustian et al., 2000; Six et al., 2000). Residue management and tillage influence SOM stratification and microbial community dynamics in addition to C sequestration (Follett and Peterson, 1988; Salinas-Garcia et al., 1997a,b). Changes in microbial community dynamics occur from the interactions of tillage, soil moisture, temperature, aeration, and substrate availability (Feng et al., 2003). Microbial biomass responds quickly to changes in soil management and is often used as an indicator of soil quality (Powlson et al., 1987; Sparling, 1997). Notillage management increases SOM and improves soil fertility (Paustian et al., 1997), and has potential for increasing the nutrient supply to crops (Doran, 1987) through changes in the mineralization and immobilization of nutrients by microbial biomass (Jansson and Persson, 1982). Considerable research on the effects of tillage on soil C and N dynamics has been reported for temperate climates (Paustian et al., 1997), but data is generally sparse for subtropical agricultural ecosystems. The objectives of this study were to
determine the long-term impacts of various tillage regimes on soil C and N dynamics, microbial biomass, and mineralizable C and N of corn and cotton cropping sequences in a subhumid, subtropical agricultural soil.
2. Materials and methods 2.1. Site description A long-term field experiment was initiated in 1980 at the Texas A&M University Agricultural Research and Extension Center near Corpus Christi, Texas (278460 N, 978300 W). The climate is classified as subhumid subtropical. Annual precipitation and temperature average 765 mm and 22 8C. The soil is an Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) having 68% sand, 5% silt, 26% clay, and 2% CaCO3, with pH 8.2. The experimental design is a split–split plot within a randomized complete block. Tillage treatment serves as the main plot, cropping sequence is the split plot, and N fertilization rate is the split–split plot. However, only one N application rate was tested in this study. The current cropping sequences have been imposed since 1980 and include continuous corn for 4 years followed by 4 years of cotton, with each crop represented every year. Soil samples were taken in 2000 after 4 years of continuous corn or cotton. Tillage treatments included NT, minimum tillage (MT), conventional tillage (CT), and moldboard plow (MP). Under NT, no soil disturbance occurred except for shredding of stalks after harvest, fertilizer application, and planting. The MT plots received three to five operations annually, including shredding of stalks after harvest, formation of low-profile beds, fertilization, and planting, with a tillage depth of 7.5 cm. Conventional-tillage plots underwent shredding and disking of stalks after harvest, disking for weed control, bedding, fertilization, and planting. Maximum tillage depth was 15 cm and up to 10 operations occurred annually. For MP, stalks were shredded after harvest, followed by moldboard plowing to a depth of 30 cm, disking for weed control, bedding, fertilization, and planting, with approximately seven operations annually. Fertilizers were banded preplant, with corn receiving 90 kg N ha1 and 10 kg P ha1 and cotton 67 kg N ha1 and 22 kg P ha1. Corn was planted in February
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and harvested in July, while cotton was planted in March and harvested in August. Field split–split plots measured 3.86 m 12.2 m with row spacing of 0.94 m, and were replicated four times. 2.2. Soil sampling and analysis Soil cores (10 cm diameter) were collected from duplicate corn plots in February 2000 and from duplicate cotton plots in March 2000 prior to planting. Samples from each plot consisted of two composited soil cores sectioned into 0–2.5 cm, 2.5–7.5 cm, 7.5– 13 cm, and 13–20 cm depths. Soil was air-dried, ground to pass a 5-mm sieve, and visible roots were removed. Soil used for SOC and SON determinations was ground to pass a 0.5 mm sieve. Soil organic carbon was measured using the modified Mebius method (Nelson and Sommers, 1982). Approximately 0.5 g soil was digested with 5 mL of 1N K2Cr2O7 and 10 mL of concentrated H2SO4 at 150 8C for 30 min, followed by titration of digests with FeSO4. Soil total N was quantified using a Kjeldahl digestion procedure (Gallaher et al., 1976) with NH4-N analyzed colorimetrically (Technicon Industrial Systems, 1977a). Soil organic nitrogen was the difference between Kjeldahl-N and residual NH4-N. Microbial biomass carbon (MBC) was determined by chloroform fumigation-incubation (Jenkinson and Powlson, 1976). Approximately 40 g of soil (at 55% of water-holding capacity) were placed into 50-mL glass beakers, fumigated for 1 d, evacuated, and incubated with 10 mL of 1N KOH in 1-L glass jars at 25 8C for 10 d. Carbon dioxide production was quantified after titration of KOH with 1N HCl (Anderson, 1982). Soil MBC was calculated by dividing the mg CO2-C produced per kg of fumigated soil by an efficiency factor of 0.41 (Voroney and Paul, 1984). Nonfumigated soils (7 g) taken prior to incubation and 7 g of chloroform-fumigated soil were shaken for 1 h with 28 mL of 2N KCl and filtered. Extracts were analyzed for NH4-N as previously described. Microbial biomass nitrogen (MBN) was determined by the difference in NH4-N concentrations between fumigated and non-fumigated samples, divided by an efficiency factor of 0.41 (Carter and Rennie, 1982). For mineralizable C determination, 40 g of soil (at 55% water-holding capacity) were placed in 50-mL beakers in 1-L glass jars and incubated with vials
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containing 10 mL of 1N KOH at 25 8C for 21 d. At 21 d, KOH traps were removed and absorbed CO2-C was quantified by titration as previously described. Mineralizable C was calculated from the quantity of CO2-C produced during 21 d. For mineralizable N, 7 g of incubated and non-incubated soil were extracted with 28 mL of 2N KCl for 1 h, filtered, and extracts analyzed for NH4-N as previously described and for NO3-N by the cadmium-reduction method (Technicon Industrial Systems, 1977b). Mineralizable N was calculated as the difference between NH4 + NO3-N mineralized during the 21 d incubation and initial soil NH4 + NO3-N. 2.3. Statistical analysis Data were analyzed using CoStat (CoHort Software, 1996). Correlation coefficients (r) were calculated and ANOVAs were used for individual tillage treatment comparisons at P < 0.05, with separation of means by LSD. A two-way ANOVA was used to determine overall differences between soil depths and cropping sequences.
3. Results and discussion 3.1. Inorganic N concentrations Soil NH4-N concentrations were not impacted by tillage and did not vary with soil depth (data not shown). However, NH4-N was significantly higher for cotton (29 mg NH4-N kg1) than corn (6 mg NH4N kg1). Nitrate concentrations were not impacted by tillage or cropping sequence, but were significantly higher at 0–2.5 cm (25 mg NO3-N kg1) than 2.5– 20 cm (13 mg NO3-N kg1). 3.2. Soil organic C and N concentrations After 4 years of continuous corn, there were no significant impacts of tillage on SOC at any soil depth (Fig. 1). However, SOC under NT tended to be highest at 0–2.5 cm and lowest at the three deepest depth intervals. Soil organic C under NT was 11% higher than other tillage regimes at 0–2.5 cm, but 22% lower at 2.5–20 cm. The increase in SOC under NT compared to CT in surface soils was substantially
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Fig. 1. Soil organic carbon (SOC) and soil organic nitrogen (SON) concentrations of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
less than observed under cotton (Zibilske et al., 2002) and corn rotations in similar south Texas soils (Salinas-Garcia et al., 1997a). Averaged across tillage regimes, SOC was significantly highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest at 7.5–20 cm. After 4 years of continuous cotton, NT had the highest and MP the lowest SOC at 0–2.5 cm. Few differences between tillage regimes were observed at other depths, but MP generally resulted in the lowest SOC. Similar to corn, SOC for cotton were highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 13 to 20 cm. Averaged across soil depths, NT for cotton had higher SOC than both CT and MP, and MT had higher SOC than MP. No tillage and MT treatments had an average of 8% higher SOC than CT, and 24% higher SOC than MP. In related studies, MP also resulted in the lowest SOC in surface and subsurface soils (Salinas-Garcia et al., 1997a,b). Averaged across tillage regimes and depths, SOC was significantly higher under corn than cotton, but only by 8%. Soil organic N for corn was higher under NT than MP at 0–2.5 cm, but there were no differences between tillage regimes at deeper depths, although NT tended to have the lowest SON (Fig. 1). Averaged across depths, SON was significantly higher under CT than MP. On average, SON for corn was 21% higher
under NT than other tillage regimes at 0–2.5 cm, but was 12% lower under NT than other tillage regimes from 2.5 to 20 cm. Soil organic N for cotton was higher under NT than MP at 0 to 2.5 cm, but few differences between tillage regimes were observed at other depths. For cotton, SON was highest at 0–2.5 cm and lowest at 13–20 cm. Averaged across depths, MP exhibited the lowest SON. The NT and MT treatments for cotton had 7% higher SON than CT, and 43% higher SON than MP. Similar to SOC, corn exhibited significantly higher SON than cotton, but only by 10%. Soil organic N was significantly related to SOC in corn and cotton (r = 0.92). Soil C/N ratios did not vary with depth, but were higher under MP (C/N = 12.3) than other tillage regimes (C/N = 11.2). No tillage generally produced the greatest SOC and SON at 0–2.5 cm, and often the lowest at deeper depth intervals. Soil organic C and SON decreased 36 and 38%, respectively, from 0–2.5 to 13–20 cm. Similar trends for SOC were observed in related studies after 16 years of treatment imposition, as SOC storage was higher under NT than other tillage regimes at 0–5 cm, but few impacts of tillage were observed from 5 to 20 cm (Salinas-Garcia et al., 1997a,b). The quantity of surface crop residues was likely greater under NT and MT than CT, while lowest for MP (Salinas-Garcia et al., 1997b). Increases in SOM at 0–2.5 cm under NT may have been a result of reduced contact of crop residues with soil. Residues at the soil surface often experience wide fluctuations in moisture content in contrast to environmental conditions of buried residues, which often decompose faster than surface residues (Beare et al., 1993; Schomberg et al., 1994). Buried residues and roots undergo more extensive and rapid decomposition than surface residues, and the more slowly decomposed residues may have potential for nutrient supply to crops over the long-term (Ghidey and Alberts, 1993). The lack of tillage and deposition of crop residues in subsurface soils was likely responsible for less SOM at depth under NT than other tillage regimes. Under MP, crop residues may have been returned to depths lower than those sampled, and this may also account for low SOM under MP. Excluding the 0–2.5 cm depth, NT for corn tended to have the lowest SOC. However, for cotton, MP had the lowest SOC for the same depth intervals. No tillage
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was expected to increase SOM in subsurface soils as was observed in other studies after 20 years of NT (Wright and Hons, 2004). Soil organic C and SON in the 5–15 cm depth interval were greater under NT than CT for various cropping sequences (Wright and Hons, 2004). However, this study was conducted in a temperate region of Texas having a greater average annual temperature. Temperature differences may explain the failure of NT to increase SOM below the 0–2.5 cm depth, as crop residues are more rapidly decomposed at higher temperatures. These results are supported by evidence of greater enhancement of soil C sequestration in cooler compared to warmer regions of Texas (Potter et al., 1998). 3.3. Microbial biomass C and MBN For corn, MBC was higher under NT than other tillage regimes at 0–2.5 cm, but there were no differences between tillage regimes at 2.5–20 cm (Fig. 2). Microbial biomass C for corn tended to be lowest under NT at the two deepest depth intervals,
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and significantly higher at 0–2.5 cm than at other depths. For cotton at 0–2.5 cm, MBC was lowest under MP. At 2.5–7.5 cm, MBC was highest under MT, and at 13–20 cm was lowest under NT. Averaged across soil depths, MT for cotton had greater MBC than CT and MP, and CT had greater MBC than MP. Similar to corn, MBC for cotton was highest at 0– 2.5 cm and decreased with depth. No differences in MBC between corn and cotton were observed. Microbial biomass C was significantly related to both SOC (r = 0.63) and SON (r = 0.63). Tillage had no significant impacts on MBN at any soil depth for corn (Fig. 2). Microbial biomass N was highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 7.5 to 20 cm. Tillage had greater impacts on MBN for cotton than corn. At 0–2.5 cm under cotton, MBN was highest under NT and lowest under MP, while few tillage effects were observed from 2.5 to 20 cm. Microbial biomass N was highest at 0– 2.5 cm, but no differences were observed between other soil depths. Averaged across soil depths, NT for cotton had greater MBN than both CT and MP, while
Fig. 2. Soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and MBC/MBN ratios of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
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MT had greater MBN than MP. Microbial biomass N exhibited a more dramatic decline with depth than MBC, which also corresponded to decreases in NO3 concentrations. In contrast to MBC, MBN was significantly greater for corn than cotton by an average of 100%. Microbial biomass N was significantly related to SOC (r = 0.88), SON (r = 0.81), and MBC (r = 0.52). For corn, the MBC/MBN ratio was generally not affected by tillage regime, but was highest at 13– 20 cm and lowest at 0–2.5 cm (Fig. 2). For cotton, MBC/MBN was greater under MT than other tillage regimes at 2.5–7.5 cm, but at deeper depth intervals, was greater under CT and MP than NT and MT. Similar to corn, MBC/MBN for cotton was lowest at 0–2.5 cm. The MBC/MBN ratios were significantly greater for cotton than corn by an average of 153%, likely due to lower N content in cotton than corn residues. Microbial biomass N was more variable with depth than MBC, as MBC and MBN decreased 127 and 400%, respectively, from 0–2.5 cm to 13–20 cm. The MBC/MBN ratios increased with depth for cotton and corn, indicating that MBN may be limiting in subsurface soils. Feng et al. (2003) reported that impacts of tillage on microbial biomass were most evident during winter fallow or the early growth stages of cotton when root growth was minimal. Thus, changes in microbial biomass in this study may be more directly related to tillage operations and resulting changes in the soil physicochemical environment rather than crop-mediated responses. In related studies, MBC of corn rotations was higher at planting than at flowering or harvest (Salinas-Garcia et al., 1997a), which was likely due to decomposition of crop residues during winter fallow, which increased microbial biomass by the planting date. 3.4. Mineralizable C and N For corn, mineralizable C was greater under MP than NT at 0–2.5 cm, and greater under CT than NT at 2.5–7.5 cm (Fig. 3). However, no impacts of tillage were observed at the two deepest depth intervals. Averaged across tillage treatments, mineralizable C was highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 7.5 to 20 cm. For cotton, mineralizable C was greater under NT than all other tillage regimes at
Fig. 3. Soil mineralizable C and N of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
0–2.5 cm, and greater than CT and MP at 2.5–7.5 cm. No differences between tillage regimes were observed at the two deepest depth intervals. Mineralizable C for cotton decreased with depth for NT due to lack of return of crop residues to subsurface soils, but did not decrease for other tillage regimes. For both corn and cotton, mineralizable C was significantly related to SOC (r = 0.73) and SON (r = 0.66), in addition to MBC (r = 0.40) and MBN (r = 0.71). After 16 years of management, NT and MT had 34% greater mineralizable C than CT, MP, and chisel plowing, but exhibited considerable seasonal variation for corn cropping sequences (Salinas-Garcia et al., 1997a). However, in this study, samples were taken approximately 6 months after harvest, so considerable decomposition of corn residues may have occurred during winter fallow before soil samples were taken at planting, thus lower mineralizable C was observed under NT corn in surface soils than under MP. Mineralizable N was greater for NT and MT than other tillage regimes for corn at 0–2.5 cm (Fig. 3). However, no significant impacts of tillage were observed at other depths, although mineralizable N tended to be lower under NT from 2.5 to 20 cm. For cotton, mineralizable N was highest for NT at 0– 2.5 cm, and higher for NT and MT than CT and MP at 2.5–7.5 cm. Even at the two deepest depth intervals,
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mineralizable N for cotton was often significantly greater under NT and MT than CT and MP. Mineralizable N for cotton decreased with depth for all tillage treatments. For both crops, mineralizable N was significantly related to SOC (r = 0.78) and SON (r = 0.80), in addition to MBC (r = 0.65), MBN (r = 0.58), and mineralizable C (r = 0.58). Greater mineralizable C and N under NT and MT were observed in subsurface soils under cotton. This was especially evident for mineralizable N for cotton. Mineralizable C for cotton did not decrease with soil depth for CT, MP, or MT, but significantly decreased with depth under NT. Likewise, decreases in mineralizable N with depth were more pronounced for NT than other tillage regimes. The less pronounced decreases with depth of mineralizable C and N of CT and MP compared to NT may be related to the return of crop residues to subsurface soils by tillage, which tended increase SOC and SON for CT and MP to levels comparable to NT and MT (Fig. 1). The lack of differences between tillage treatments for mineralizable C and N in subsurface soils of corn may be due to lower C/N ratios of corn compared to cotton roots and residues (Ghidey and Alberts, 1993), which tended to promote more rapid decomposition of corn than cotton residues. Cotton roots generally decompose more slowly than corn due to their greater diameter and higher C/N ratio (Ghidey and Alberts, 1993). No tillage enhanced the potential N supply for cotton more so than corn, as greater mineralizable N was observed for cotton in subsurface soils. This was likely a result of crop residues with high C/N ratios, such as cotton, decomposing at slower rates than residues having lower C/N ratios, such as corn (Ghidey and Alberts, 1993; Hulugalle, 2000). Incorporation of crop residues with high C/N ratios may cause immobilization in the short-term, but long-term potential for N mineralization is enhanced (Powlson et al., 1987; Saffigna et al., 1982), thus increasing the potential N supply to cotton.
4. Conclusions After 20 years of management in a subhumid, subtropical south Texas, NT increased SOC and SON only in surface soils cropped to cotton but not for soils
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cropped to corn. In subsurface soils under corn, NT resulted in lower SOM than other tillage regimes. Overall, SOC and SON in the top 20 cm of soil were not increased by NT, and were only slightly higher for corn than cotton. The potential of soils and cropping sequences in this region for C and N sequestration appeared limited. The depth distribution of microbial biomass was impacted by long-term tillage, which may influence the potential nutrient supply to crops. No tillage and MT exhibited greater mineralizable C and N for cotton than corn, especially in subsurface soils, indicating that SOM and residue decomposition during the growing season may be important sources of nutrients to cotton. Tillage regimes that promoted the maintenance of crop residues at the soil surface also had beneficial impacts on soil fertility through enhancement of soil microbial biomass and supply of mineralizable nutrients.
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C in long-term management systems in Texas. Soil Tillage Res. 47, 309–321. Powlson, D.S., Brooke, P.C., Christensen, B.T., 1987. Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation. Soil Biol. Biochem. 19, 159–164. Saffigna, P.G., Cogle, A.L., Strong, W.M., Waring, S.A., 1982. The effect of carbonaceous residue on 15N fertilizer nitrogen transformations in the field. In: Galbally, I.E., Freney, J.R. (Eds.), The Cycling of Carbon, Nitrogen, Sulfur and Phosphorus in Terrestrial and Aquatic Ecosystems. CAB International, NY, pp. 83–87. Salinas-Garcia, J.R., Hons, F.M., Matocha, J.E., 1997a. Long-term effects of tillage and fertilization on soil organic matter dynamics. Soil Sci. Soc. Am. J. 61, 152–159. Salinas-Garcia, J.R., Matocha, J.E., Hons, F.M., 1997b. Long-term tillage and nitrogen fertilization effects on soil properties of an Alfisol under dryland corn/cotton production. Soil Tillage Res. 42, 79–93. Schomberg, H.H., Steiner, J.L., Unger, P.W., 1994. Decomposition and nitrogen dynamics of crop residues: residue quality and water effects. Soil Sci. Soc. Am. J. 58, 372–381. Six, J., Elliott, E.T., Paustian, K., 2000. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem. 32, 2013–2099. Sparling, G.P., 1997. Soil microbial biomass, activity, and nutrient cycling as indicators of soil health. In: Pankhurst, C., Doube, B.M., Gupta, V.V.S.R. (Eds.), Biological Indicators of Soil Health. CAB International, NY, pp. 97–119. Technicon Industrial Systems, 1977a. Determination of nitrogen in BD acid digests. Technicon industrial method 334-74 W/B. Technicon Industrial Systems, Tarrytown, NY. Technicon Industrial Systems, 1977b. Nitrate and nitrite in soil extracts. Technicon industrial method 487-77A. Technicon Industrial Systems, Tarrytown, NY. Voroney, R.P., Paul, E.A., 1984. Determination of Kc and Kn in situ for calibration of the chloroform-incubation method. Soil Biol. Biochem. 16, 9–14. Wright, A.L., Hons, F.M., 2004. Soil aggregation and carbon and nitrogen storage under soybean cropping sequences. Soil Sci. Soc. Am. J. 68, 507–513. Zibilske, L.M., Bradford, J.M., Smart, J.R., 2002. Conservation tillage induced changes in organic carbon, total nitrogen and available phosphorus in a semi-arid alkaline subtropical soil. Soil Tillage Res. 66, 153–163.
Tillage impacts on microbial biomass and soil carbon and nitrogen dynamics of corn and cotton rotations Alan L. Wrighta,*, Frank M. Honsa, John E. Matocha Jr.b a
b
Department of Soil and Crop Sciences, Texas A&M University, 2474 TAMU, College Station, TX 77843-2474, USA Texas A&M University Agricultural Research and Extension Center, 10345 Agnes Street, Corpus Christi, TX 78406-1412, USA Received 26 May 2004; accepted 13 September 2004
Abstract Long-term no tillage (NT) may enhance soil C sequestration and alter soil C and N dynamics. The objectives of this study were to investigate the impacts of tillage on soil C and N sequestration and microbial C and N dynamics of corn (Zea mays L.) and cotton (Gossypium hirsutum L.) cropping sequences after 20 years of management. Tillage regimes included conventional tillage (CT), moldboard plow (MP), minimum tillage (MT), and NT. No tillage increased soil organic carbon (SOC) and nitrogen (SON) concentrations in surface soil (0–2.5 cm) for cotton but not for corn. Few tillage effects on SOC and SON were observed in subsurface soils. For corn, SOC and SON were 11 and 21% higher under NT than other tillage regimes at 0–2.5 cm, but were 22 and 12% lower under NT from 2.5 to 20 cm. Averaged between depths, SOC and SON for cotton were 8 and 7% greater under NT than CT, while NT and MT had 24 and 43% greater SOC and SON than MP. Soil organic C and SON were significantly greater for corn than cotton, but this did not result in greater microbial biomass and mineralizable C and N than for cotton. Microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were often highest under NT and MT in surface soils, but few tillage impacts were observed at 2.5–20 cm. Mineralizable C and N were highest under NT and MT in surface soils for corn and cotton, and in subsurface soils for cotton. Even though SOC and SON were greater for corn than cotton, cotton exhibited greater soil mineralizable C and N under NT and MT than corn, especially in subsurface soils. These results indicate a greater potential supply of N for the cotton than corn crop during the growing season. Increased SOM content in surface soils under reduced tillage may increase N mineralization and the nutrient supply to crops, but the potential of these soils for C and N sequestration appeared limited. # 2004 Elsevier B.V. All rights reserved. Keywords: Carbon sequestration; Microbial biomass; No tillage; Soil organic matter
1. Introduction Abbreviations: NT, no tillage; CT, conventional tillage; MP, moldboard plow; MT, minimum tillage; SOC, soil organic carbon; SON, soil organic nitrogen; MBC, microbial biomass C; MBN, microbial biomass N; SOM, soil organic matter * Corresponding author. Tel.: +1 979 845 3814. E-mail address: [email protected] (A.L. Wright).
No tillage has potential to enhance soil C sequestration. Impacts of tillage on soil organic matter (SOM) have been well documented, but results vary due to soil type, cropping system, residue
0929-1393/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2004.09.006
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management, and climate (Paustian et al., 1997). Much research on C sequestration in agricultural soils has been conducted in cooler, northern climates in the Midwestern USA. Soil organic matter contents are often lower in humid, southern areas compared to the northern USA. In Texas, soil C sequestration potential increases as temperature decreases (Potter et al., 1998). Most impacts of no tillage (NT) on C sequestration have been observed in surface soils near the rooting zone and crop residues (Paustian et al., 1997). However, long-term increases in SOM have been observed in subsurface soils after 20 years of NT in the southern USA (Wright and Hons, 2004). The greatest increases in SOM are usually observed in intensive cropping systems, where multiple crops are grown yearly (Franzluebbers et al., 1995; Ortega et al., 2002; Wright and Hons, 2004). Types of crop residues play important roles in C sequestration and organic matter cycling due to differences in C/N ratios or quality of residues (Lynch and Bragg, 1985; Franzluebbers et al., 1995; Potter et al., 1998). Degradation of fresh crop residues is often governed by C/N ratios (Oades, 1988; Chesire and Chapman, 1996). Tillage promotes SOM decomposition through crop residue incorporation into soil, physical breakdown of residues, and disruption of SOM protected within aggregates (Paustian et al., 2000; Six et al., 2000). Residue management and tillage influence SOM stratification and microbial community dynamics in addition to C sequestration (Follett and Peterson, 1988; Salinas-Garcia et al., 1997a,b). Changes in microbial community dynamics occur from the interactions of tillage, soil moisture, temperature, aeration, and substrate availability (Feng et al., 2003). Microbial biomass responds quickly to changes in soil management and is often used as an indicator of soil quality (Powlson et al., 1987; Sparling, 1997). Notillage management increases SOM and improves soil fertility (Paustian et al., 1997), and has potential for increasing the nutrient supply to crops (Doran, 1987) through changes in the mineralization and immobilization of nutrients by microbial biomass (Jansson and Persson, 1982). Considerable research on the effects of tillage on soil C and N dynamics has been reported for temperate climates (Paustian et al., 1997), but data is generally sparse for subtropical agricultural ecosystems. The objectives of this study were to
determine the long-term impacts of various tillage regimes on soil C and N dynamics, microbial biomass, and mineralizable C and N of corn and cotton cropping sequences in a subhumid, subtropical agricultural soil.
2. Materials and methods 2.1. Site description A long-term field experiment was initiated in 1980 at the Texas A&M University Agricultural Research and Extension Center near Corpus Christi, Texas (278460 N, 978300 W). The climate is classified as subhumid subtropical. Annual precipitation and temperature average 765 mm and 22 8C. The soil is an Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) having 68% sand, 5% silt, 26% clay, and 2% CaCO3, with pH 8.2. The experimental design is a split–split plot within a randomized complete block. Tillage treatment serves as the main plot, cropping sequence is the split plot, and N fertilization rate is the split–split plot. However, only one N application rate was tested in this study. The current cropping sequences have been imposed since 1980 and include continuous corn for 4 years followed by 4 years of cotton, with each crop represented every year. Soil samples were taken in 2000 after 4 years of continuous corn or cotton. Tillage treatments included NT, minimum tillage (MT), conventional tillage (CT), and moldboard plow (MP). Under NT, no soil disturbance occurred except for shredding of stalks after harvest, fertilizer application, and planting. The MT plots received three to five operations annually, including shredding of stalks after harvest, formation of low-profile beds, fertilization, and planting, with a tillage depth of 7.5 cm. Conventional-tillage plots underwent shredding and disking of stalks after harvest, disking for weed control, bedding, fertilization, and planting. Maximum tillage depth was 15 cm and up to 10 operations occurred annually. For MP, stalks were shredded after harvest, followed by moldboard plowing to a depth of 30 cm, disking for weed control, bedding, fertilization, and planting, with approximately seven operations annually. Fertilizers were banded preplant, with corn receiving 90 kg N ha1 and 10 kg P ha1 and cotton 67 kg N ha1 and 22 kg P ha1. Corn was planted in February
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and harvested in July, while cotton was planted in March and harvested in August. Field split–split plots measured 3.86 m 12.2 m with row spacing of 0.94 m, and were replicated four times. 2.2. Soil sampling and analysis Soil cores (10 cm diameter) were collected from duplicate corn plots in February 2000 and from duplicate cotton plots in March 2000 prior to planting. Samples from each plot consisted of two composited soil cores sectioned into 0–2.5 cm, 2.5–7.5 cm, 7.5– 13 cm, and 13–20 cm depths. Soil was air-dried, ground to pass a 5-mm sieve, and visible roots were removed. Soil used for SOC and SON determinations was ground to pass a 0.5 mm sieve. Soil organic carbon was measured using the modified Mebius method (Nelson and Sommers, 1982). Approximately 0.5 g soil was digested with 5 mL of 1N K2Cr2O7 and 10 mL of concentrated H2SO4 at 150 8C for 30 min, followed by titration of digests with FeSO4. Soil total N was quantified using a Kjeldahl digestion procedure (Gallaher et al., 1976) with NH4-N analyzed colorimetrically (Technicon Industrial Systems, 1977a). Soil organic nitrogen was the difference between Kjeldahl-N and residual NH4-N. Microbial biomass carbon (MBC) was determined by chloroform fumigation-incubation (Jenkinson and Powlson, 1976). Approximately 40 g of soil (at 55% of water-holding capacity) were placed into 50-mL glass beakers, fumigated for 1 d, evacuated, and incubated with 10 mL of 1N KOH in 1-L glass jars at 25 8C for 10 d. Carbon dioxide production was quantified after titration of KOH with 1N HCl (Anderson, 1982). Soil MBC was calculated by dividing the mg CO2-C produced per kg of fumigated soil by an efficiency factor of 0.41 (Voroney and Paul, 1984). Nonfumigated soils (7 g) taken prior to incubation and 7 g of chloroform-fumigated soil were shaken for 1 h with 28 mL of 2N KCl and filtered. Extracts were analyzed for NH4-N as previously described. Microbial biomass nitrogen (MBN) was determined by the difference in NH4-N concentrations between fumigated and non-fumigated samples, divided by an efficiency factor of 0.41 (Carter and Rennie, 1982). For mineralizable C determination, 40 g of soil (at 55% water-holding capacity) were placed in 50-mL beakers in 1-L glass jars and incubated with vials
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containing 10 mL of 1N KOH at 25 8C for 21 d. At 21 d, KOH traps were removed and absorbed CO2-C was quantified by titration as previously described. Mineralizable C was calculated from the quantity of CO2-C produced during 21 d. For mineralizable N, 7 g of incubated and non-incubated soil were extracted with 28 mL of 2N KCl for 1 h, filtered, and extracts analyzed for NH4-N as previously described and for NO3-N by the cadmium-reduction method (Technicon Industrial Systems, 1977b). Mineralizable N was calculated as the difference between NH4 + NO3-N mineralized during the 21 d incubation and initial soil NH4 + NO3-N. 2.3. Statistical analysis Data were analyzed using CoStat (CoHort Software, 1996). Correlation coefficients (r) were calculated and ANOVAs were used for individual tillage treatment comparisons at P < 0.05, with separation of means by LSD. A two-way ANOVA was used to determine overall differences between soil depths and cropping sequences.
3. Results and discussion 3.1. Inorganic N concentrations Soil NH4-N concentrations were not impacted by tillage and did not vary with soil depth (data not shown). However, NH4-N was significantly higher for cotton (29 mg NH4-N kg1) than corn (6 mg NH4N kg1). Nitrate concentrations were not impacted by tillage or cropping sequence, but were significantly higher at 0–2.5 cm (25 mg NO3-N kg1) than 2.5– 20 cm (13 mg NO3-N kg1). 3.2. Soil organic C and N concentrations After 4 years of continuous corn, there were no significant impacts of tillage on SOC at any soil depth (Fig. 1). However, SOC under NT tended to be highest at 0–2.5 cm and lowest at the three deepest depth intervals. Soil organic C under NT was 11% higher than other tillage regimes at 0–2.5 cm, but 22% lower at 2.5–20 cm. The increase in SOC under NT compared to CT in surface soils was substantially
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Fig. 1. Soil organic carbon (SOC) and soil organic nitrogen (SON) concentrations of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
less than observed under cotton (Zibilske et al., 2002) and corn rotations in similar south Texas soils (Salinas-Garcia et al., 1997a). Averaged across tillage regimes, SOC was significantly highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest at 7.5–20 cm. After 4 years of continuous cotton, NT had the highest and MP the lowest SOC at 0–2.5 cm. Few differences between tillage regimes were observed at other depths, but MP generally resulted in the lowest SOC. Similar to corn, SOC for cotton were highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 13 to 20 cm. Averaged across soil depths, NT for cotton had higher SOC than both CT and MP, and MT had higher SOC than MP. No tillage and MT treatments had an average of 8% higher SOC than CT, and 24% higher SOC than MP. In related studies, MP also resulted in the lowest SOC in surface and subsurface soils (Salinas-Garcia et al., 1997a,b). Averaged across tillage regimes and depths, SOC was significantly higher under corn than cotton, but only by 8%. Soil organic N for corn was higher under NT than MP at 0–2.5 cm, but there were no differences between tillage regimes at deeper depths, although NT tended to have the lowest SON (Fig. 1). Averaged across depths, SON was significantly higher under CT than MP. On average, SON for corn was 21% higher
under NT than other tillage regimes at 0–2.5 cm, but was 12% lower under NT than other tillage regimes from 2.5 to 20 cm. Soil organic N for cotton was higher under NT than MP at 0 to 2.5 cm, but few differences between tillage regimes were observed at other depths. For cotton, SON was highest at 0–2.5 cm and lowest at 13–20 cm. Averaged across depths, MP exhibited the lowest SON. The NT and MT treatments for cotton had 7% higher SON than CT, and 43% higher SON than MP. Similar to SOC, corn exhibited significantly higher SON than cotton, but only by 10%. Soil organic N was significantly related to SOC in corn and cotton (r = 0.92). Soil C/N ratios did not vary with depth, but were higher under MP (C/N = 12.3) than other tillage regimes (C/N = 11.2). No tillage generally produced the greatest SOC and SON at 0–2.5 cm, and often the lowest at deeper depth intervals. Soil organic C and SON decreased 36 and 38%, respectively, from 0–2.5 to 13–20 cm. Similar trends for SOC were observed in related studies after 16 years of treatment imposition, as SOC storage was higher under NT than other tillage regimes at 0–5 cm, but few impacts of tillage were observed from 5 to 20 cm (Salinas-Garcia et al., 1997a,b). The quantity of surface crop residues was likely greater under NT and MT than CT, while lowest for MP (Salinas-Garcia et al., 1997b). Increases in SOM at 0–2.5 cm under NT may have been a result of reduced contact of crop residues with soil. Residues at the soil surface often experience wide fluctuations in moisture content in contrast to environmental conditions of buried residues, which often decompose faster than surface residues (Beare et al., 1993; Schomberg et al., 1994). Buried residues and roots undergo more extensive and rapid decomposition than surface residues, and the more slowly decomposed residues may have potential for nutrient supply to crops over the long-term (Ghidey and Alberts, 1993). The lack of tillage and deposition of crop residues in subsurface soils was likely responsible for less SOM at depth under NT than other tillage regimes. Under MP, crop residues may have been returned to depths lower than those sampled, and this may also account for low SOM under MP. Excluding the 0–2.5 cm depth, NT for corn tended to have the lowest SOC. However, for cotton, MP had the lowest SOC for the same depth intervals. No tillage
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was expected to increase SOM in subsurface soils as was observed in other studies after 20 years of NT (Wright and Hons, 2004). Soil organic C and SON in the 5–15 cm depth interval were greater under NT than CT for various cropping sequences (Wright and Hons, 2004). However, this study was conducted in a temperate region of Texas having a greater average annual temperature. Temperature differences may explain the failure of NT to increase SOM below the 0–2.5 cm depth, as crop residues are more rapidly decomposed at higher temperatures. These results are supported by evidence of greater enhancement of soil C sequestration in cooler compared to warmer regions of Texas (Potter et al., 1998). 3.3. Microbial biomass C and MBN For corn, MBC was higher under NT than other tillage regimes at 0–2.5 cm, but there were no differences between tillage regimes at 2.5–20 cm (Fig. 2). Microbial biomass C for corn tended to be lowest under NT at the two deepest depth intervals,
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and significantly higher at 0–2.5 cm than at other depths. For cotton at 0–2.5 cm, MBC was lowest under MP. At 2.5–7.5 cm, MBC was highest under MT, and at 13–20 cm was lowest under NT. Averaged across soil depths, MT for cotton had greater MBC than CT and MP, and CT had greater MBC than MP. Similar to corn, MBC for cotton was highest at 0– 2.5 cm and decreased with depth. No differences in MBC between corn and cotton were observed. Microbial biomass C was significantly related to both SOC (r = 0.63) and SON (r = 0.63). Tillage had no significant impacts on MBN at any soil depth for corn (Fig. 2). Microbial biomass N was highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 7.5 to 20 cm. Tillage had greater impacts on MBN for cotton than corn. At 0–2.5 cm under cotton, MBN was highest under NT and lowest under MP, while few tillage effects were observed from 2.5 to 20 cm. Microbial biomass N was highest at 0– 2.5 cm, but no differences were observed between other soil depths. Averaged across soil depths, NT for cotton had greater MBN than both CT and MP, while
Fig. 2. Soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and MBC/MBN ratios of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
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MT had greater MBN than MP. Microbial biomass N exhibited a more dramatic decline with depth than MBC, which also corresponded to decreases in NO3 concentrations. In contrast to MBC, MBN was significantly greater for corn than cotton by an average of 100%. Microbial biomass N was significantly related to SOC (r = 0.88), SON (r = 0.81), and MBC (r = 0.52). For corn, the MBC/MBN ratio was generally not affected by tillage regime, but was highest at 13– 20 cm and lowest at 0–2.5 cm (Fig. 2). For cotton, MBC/MBN was greater under MT than other tillage regimes at 2.5–7.5 cm, but at deeper depth intervals, was greater under CT and MP than NT and MT. Similar to corn, MBC/MBN for cotton was lowest at 0–2.5 cm. The MBC/MBN ratios were significantly greater for cotton than corn by an average of 153%, likely due to lower N content in cotton than corn residues. Microbial biomass N was more variable with depth than MBC, as MBC and MBN decreased 127 and 400%, respectively, from 0–2.5 cm to 13–20 cm. The MBC/MBN ratios increased with depth for cotton and corn, indicating that MBN may be limiting in subsurface soils. Feng et al. (2003) reported that impacts of tillage on microbial biomass were most evident during winter fallow or the early growth stages of cotton when root growth was minimal. Thus, changes in microbial biomass in this study may be more directly related to tillage operations and resulting changes in the soil physicochemical environment rather than crop-mediated responses. In related studies, MBC of corn rotations was higher at planting than at flowering or harvest (Salinas-Garcia et al., 1997a), which was likely due to decomposition of crop residues during winter fallow, which increased microbial biomass by the planting date. 3.4. Mineralizable C and N For corn, mineralizable C was greater under MP than NT at 0–2.5 cm, and greater under CT than NT at 2.5–7.5 cm (Fig. 3). However, no impacts of tillage were observed at the two deepest depth intervals. Averaged across tillage treatments, mineralizable C was highest at 0–2.5 cm, followed by 2.5–7.5 cm, and lowest from 7.5 to 20 cm. For cotton, mineralizable C was greater under NT than all other tillage regimes at
Fig. 3. Soil mineralizable C and N of corn and cotton cropping sequences for various tillage regimes and soil depth intervals. Columns followed by the same letter for each depth interval were not significantly different at P < 0.05.
0–2.5 cm, and greater than CT and MP at 2.5–7.5 cm. No differences between tillage regimes were observed at the two deepest depth intervals. Mineralizable C for cotton decreased with depth for NT due to lack of return of crop residues to subsurface soils, but did not decrease for other tillage regimes. For both corn and cotton, mineralizable C was significantly related to SOC (r = 0.73) and SON (r = 0.66), in addition to MBC (r = 0.40) and MBN (r = 0.71). After 16 years of management, NT and MT had 34% greater mineralizable C than CT, MP, and chisel plowing, but exhibited considerable seasonal variation for corn cropping sequences (Salinas-Garcia et al., 1997a). However, in this study, samples were taken approximately 6 months after harvest, so considerable decomposition of corn residues may have occurred during winter fallow before soil samples were taken at planting, thus lower mineralizable C was observed under NT corn in surface soils than under MP. Mineralizable N was greater for NT and MT than other tillage regimes for corn at 0–2.5 cm (Fig. 3). However, no significant impacts of tillage were observed at other depths, although mineralizable N tended to be lower under NT from 2.5 to 20 cm. For cotton, mineralizable N was highest for NT at 0– 2.5 cm, and higher for NT and MT than CT and MP at 2.5–7.5 cm. Even at the two deepest depth intervals,
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mineralizable N for cotton was often significantly greater under NT and MT than CT and MP. Mineralizable N for cotton decreased with depth for all tillage treatments. For both crops, mineralizable N was significantly related to SOC (r = 0.78) and SON (r = 0.80), in addition to MBC (r = 0.65), MBN (r = 0.58), and mineralizable C (r = 0.58). Greater mineralizable C and N under NT and MT were observed in subsurface soils under cotton. This was especially evident for mineralizable N for cotton. Mineralizable C for cotton did not decrease with soil depth for CT, MP, or MT, but significantly decreased with depth under NT. Likewise, decreases in mineralizable N with depth were more pronounced for NT than other tillage regimes. The less pronounced decreases with depth of mineralizable C and N of CT and MP compared to NT may be related to the return of crop residues to subsurface soils by tillage, which tended increase SOC and SON for CT and MP to levels comparable to NT and MT (Fig. 1). The lack of differences between tillage treatments for mineralizable C and N in subsurface soils of corn may be due to lower C/N ratios of corn compared to cotton roots and residues (Ghidey and Alberts, 1993), which tended to promote more rapid decomposition of corn than cotton residues. Cotton roots generally decompose more slowly than corn due to their greater diameter and higher C/N ratio (Ghidey and Alberts, 1993). No tillage enhanced the potential N supply for cotton more so than corn, as greater mineralizable N was observed for cotton in subsurface soils. This was likely a result of crop residues with high C/N ratios, such as cotton, decomposing at slower rates than residues having lower C/N ratios, such as corn (Ghidey and Alberts, 1993; Hulugalle, 2000). Incorporation of crop residues with high C/N ratios may cause immobilization in the short-term, but long-term potential for N mineralization is enhanced (Powlson et al., 1987; Saffigna et al., 1982), thus increasing the potential N supply to cotton.
4. Conclusions After 20 years of management in a subhumid, subtropical south Texas, NT increased SOC and SON only in surface soils cropped to cotton but not for soils
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cropped to corn. In subsurface soils under corn, NT resulted in lower SOM than other tillage regimes. Overall, SOC and SON in the top 20 cm of soil were not increased by NT, and were only slightly higher for corn than cotton. The potential of soils and cropping sequences in this region for C and N sequestration appeared limited. The depth distribution of microbial biomass was impacted by long-term tillage, which may influence the potential nutrient supply to crops. No tillage and MT exhibited greater mineralizable C and N for cotton than corn, especially in subsurface soils, indicating that SOM and residue decomposition during the growing season may be important sources of nutrients to cotton. Tillage regimes that promoted the maintenance of crop residues at the soil surface also had beneficial impacts on soil fertility through enhancement of soil microbial biomass and supply of mineralizable nutrients.
Acknowledgment This research was partially funded by the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS). References Anderson, J.P.E., 1982. Soil respiration. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 2. second ed. ASA and SSSA, Madison, WI, (Agronomy monograph 9), pp. 837–871. Beare, M.H., Pohland, B.R., Wright, D.H., Coleman, D.C., 1993. Residue placement and fungicide effects on fungal communities in conventional and no-tillage soils. Soil Sci. Soc. Am. J. 57, 392–399. Carter, M.R., Rennie, D.A., 1982. Changes in soil quality under zero tillage farming systems: distribution of microbial biomass and mineralizable C and N potentials. Can. J. Soil Sci. 62, 587–597. Chesire, M.V., Chapman, S.J., 1996. Influence of N and P status of plant material and of added N and P on the mineralization of C from 14C-labelled ryegrass in soil. Biol. Fertil. Soils 21, 166– 170. CoHort Software, 1996. CoStat v. 5, Monterey, CA. Doran, J.W., 1987. Microbial biomass and mineralizable nitrogen distribution in no-tillage and plowed soils. Biol. Fertil. Soils 5, 68–75. Feng, Y., Motta, A.C., Reeves, D.W., Burmester, C.H., van Santen, E., Osborne, J.A., 2003. Soil microbial communities under conventional-till and no-till continuous cotton systems. Soil Biol. Biochem. 35, 1693–1703.
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