Carbon Supply and Storage in Tilled and Nontilled Soils as Influenced by Cover Crops and Nitrogen Fertilization Upendra M. Sainju,* Bharat P. Singh, Wayne F. Whitehead, and Shirley Wang ABSTRACT Soil carbon (C) sequestration in tilled and nontilled areas can be influenced by crop management practices due to differences in plant C inputs and their rate of mineralization. We examined the influence of four cover crops {legume [hairy vetch (Vicia villosa Roth)], non- legume [rye (Secale cereale L.)], biculture of legume and nonlegume (vetch and rye), and no cover crops (or winter weeds)} and three ni- trogen (N) fertilization rates (0, 60 to 65, and 120 to 130 kg N ha 21 ) on C inputs from cover crops, cotton (Gossypium hirsutum L.), and sorghum [Sorghum bicolor (L.) Moench)], and soil organic carbon (SOC) at the 0- to 120-cm depth in tilled and nontilled areas. A field experiment was conducted on Dothan sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudults) from 1999 to 2002 in central Georgia. Total C inputs to the soil from cover crops, cotton, and sorghum from 2000 to 2002 ranged from 6.8 to 22.8 Mg ha 21 . The SOC at 0 to 10 cm fluctuated with C input from October 1999 to November 2002 and was greater from cover crops than from weeds in no-tilled plots. In contrast, SOC values at 10 to 30 cm in no-tilled and at 0 to 60 cm in chisel-tilled plots were greater for biculture than for weeds. As a result, C at 0 to 30 cm was sequestered at rates of 267, 33, 2133, and 2967 kg C ha 21 yr 21 for biculture, rye, vetch, and weeds, re- spectively, in the no-tilled plot. In strip-tilled and chisel-tilled plots, SOC at 0 to 30 cm decreased at rates of 233 to 1233 kg C ha 21 yr 21 . The SOC at 0 to 30 cm increased more in cover crops with 120 to 130 kg N ha 21 yr 21 than in weeds with 0 kg N ha 21 yr 21 , regardless of tillage. In the subtropical humid region of the southeastern United States, cover crops and N fertilization can increase the amount of C input and stor- age in tilled and nontilled soils, and hairy vetch and rye biculture was more effective in sequestering C than monocultures or no cover crop. C ONCERNS for global warming have led to increased interests in sequestering atmospheric greenhouse gases, such as CO 2 , in the terrestrial ecosystem (Dolman et al., 2003). Some of the ways to sequester atmospheric CO 2 in croplands are to use improved soil and crop man- agement practices, such as conservation tillage, cover cropping, crop rotation, and N fertilization (Jastrow, 1996; Kuo et al., 1997a; Allmaras et al., 2000; Sainju et al., 2003). With these practices, C accumulated in the residue of above- and belowground biomass of crops after grain harvest is returned to the soil where a minimum amount of residue will be incorporated due to less soil distur- bance. As a result, C storage in the soil increases due to increased C input and reduced mineralization compared to the conventional practices. Agricultural soils, being depleted of large amount of organic C due to cultiva- tion, have significant potentials to sequester atmospheric CO 2 (Lal and Kimble, 1997; Paustian et al., 1997). In- creased C sequestration can also enhance soil structure and improve soil water–nutrient–crop productivity rela- tionships (Bauer and Black, 1994). Cover cropping provides additional residue that not only reduces soil erosion but also improves soil produc- tivity by increasing soil organic carbon (SOC) (McVay et al., 1989; Kuo et al., 1997a; Sainju et al., 2003). In humid subtropical regions, such as in the southeastern United States, cover crops are planted in the fall after summer crop harvest and grown during winter to pro- vide vegetative cover. Besides providing many benefits in improving soil physical, chemical, and biological prop- erties (Doran, 1987; Smith et al., 1987; McVay et al., 1989; Roberson et al., 1991), some cover crops are also grown to supply N needs of the succeeding crops (Hargrove, 1986; Clark et al., 1994; Kuo et al., 1997b) and to reduce N leaching (Meisinger et al., 1990; McCracken et al., 1994). Similarly, N fertilization can increase SOC by increasing crop biomass production and the amount of residue returned to the soil (Liang and Mackenzie, 1992; Gregorich et al., 1996; Omay et al., 1997). Such management practices can provide oppor- tunities to conserve SOC in the southeastern United States where organic matter level is generally lower than in the northern regions because of rapid mineralization (Doran, 1987; Doran and Smith, 1987). Cover cropping and N fertilization can have variable effects in storing SOC in tilled and nontilled areas due to differences in mineralization rates of crop residues and soil organic matter. Conventional tillage enhances min- eralization of SOC by incorporating crop residue, dis- rupting soil aggregates, and increasing aeration (Dalal and Mayer, 1986; Balesdent et al., 1990; Cambardella and Elliott, 1993), thereby reducing its level. In contrast, conservation tillage can increase C storage in the surface soil (Jastrow, 1996; Allmaras et al., 2000; Sainju et al., 2002). Studies suggest that conversion of conventional-till to no-till can sequester atmospheric CO 2 by 0.1% at the 0- to 5-cm soil depth every year, a total of 10 Mg in 25 to 30 yr (Lal and Kimble, 1997; Paustian et al., 1997). However, SOC below the 7.5-cm depth can be higher in tilled areas, depending on the soil texture, due to residue incorpora- tion at greater depths (Jastrow, 1996; Clapp et al., 2000). The impact of tillage on SOC can interact with cover cropping and N fertilization rate (Gregorich et al., 1996; Wanniarachchi et al., 1999; Sainju et al., 2002), soil texture and sampling depth (Ellert and Bettany, 1995), and time since treatments were initiated (Liang et al., 1998). Conservation tillage is getting more popular because of U.M. Sainju, USDA-ARS-NPARL, 1500 North Central Avenue, Sidney, MT 59270. B.P. Singh, W.F. Whitehead, and S. Wang, Agri- cultural Research Station, Fort Valley State University, Fort Valley, GA 31030. Received 12 May 2005. *Corresponding author (usainju@ sidney.ars.usda.gov). Published in J. Environ. Qual. 35:1507–1517 (2006). Special Submissions doi:10.2134/jeq2005.0189 ª ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: SOC, soil organic carbon. Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved. 1507 Published online July 6, 2006