CARBON SUPPLY AND STORAGE IN TILLED AND NON-TILLED SOILS AS INFLUENCED BY COVER CROPS AND NITROGEN FERTILIZATION

Authors: Sainju, Upendra; SINGH, BHARAT - FORT VALLEY STATE UNIV; WHITEHEAD, WAYNE - FORT VALLEY STATE UNIV; WANG, SHIRLEY - FORT VALLEY STATE UNIV

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2006
Publication Date: 6/6/2006
Citation: Sainju, U.M., Singh, B.D., Whitehead, W.F., Wang, S. 2006. Carbon supply and storage in tilled and non-tilled soils as influenced by cover crops and nitrogen fertilization. Journal of Environmental Quality. 35:1507-1517.

Interpretive Summary: Concerns for global warming have led to increased interests in sequestering atmospheric greenhouse gases, such as CO2, in the terrestrial ecosystem. Some of the ways to sequester atmospheric CO2 under croplands are to use improved soil and crop management practices, such as conservation tillage, cover cropping, crop rotation, and N fertilization. With these practices, C sequestered in the residue of above- and belowground biomass of crops after grain harvest is returned to the soil where minimum amount of residue will be incorporated due to less soil disturbance. As a result, C storage in the soil will increase due to increased C input and reduced mineralization. Agricultural soils, being depleted of large amount of organic C due to cultivation, have significant potentials to sequester atmospheric CO2. Increasing C sequestration can also enhance soil structure and improve soil water-nutrient-crop productivity relationships. Cover cropping provides additional residues that not only reduce soil erosion but also improve soil quality and productivity by increasing soil organic C. In subhumid regions, such as in southeast USA, cover crops are planted in the fall after summer crop harvest and grown during winter to provide vegetative cover. Besides providing many benefits in improving soil physical, chemical, and biological properties, some cover crops are also grown to supply N needs of the succeeding crops and to reduce N leaching. Similarly, N fertilization can increase SOC by increasing crop biomass production and amount of residue returned to the soil. Such management practices can provide opportunities to conserve SOC in the southeast USA where organic matter level is generally lower than in the northern regions because of rapid mineralization. Little is known about the long-term influence of cover crops and N fertilization in tilled and non-tilled soil. An experiment was conducted from 1999 to 2002 in central GA to study the effects of legume and nonlegume cover crops and N fertilization rates to cotton and sorghum on soil organic C in tilled and no-tilled soil. Total amount of C input returned from above- and belowground biomass of cover crops, cotton, and sorghum increased with cover cropping and N fertilization and the inputs were similar in tilled and no-tilled soils. Cover crops increased soil organic C at 0- to 10-cm depth in no-tilled soil and reduced its rate of mineralization at 10- to 30-cm in no-tilled soil and at 0- to 30-cm in tilled soil. As a result, bicuture of hairy vetch and rye cover crops sequestered the highest amount of C at a rate of 267 kg C ha-1 yr-1 in no-tilled soil due to its greater biomass yield and C input compared with monocultures or no cover crop. Cover crops with N fertilization also increased soil organic C compared with no cover crop and N fertilization. Because of increased biomass residue returned to the soil, cover crops and N fertilization can increase soil organic C in no-tilled soil or reduced its rate of mineralization in tilled soil.

Technical Abstract: Carbon sequestration, as a process to reduce atmospheric CO2 level, can be influenced by crop management practices in tilled and non-tilled soils due to differences in crop residue C returned to the soil and rate of mineralization. We examined the influence of four cover crops {legume [hairy vetch (vicia villosa Roth)], nonlegume [rye (Secale cereale L.)], biculture of legume and nonlegume (vetch and rye), and no cover crops (or winter weeds)}and three N fertilization rates (0, 60 to 65, and 120 to 130 kg N ha-1) on the quantity of residue C returned to the soil from above- and belowground biomass of cover crops, cotton (Gossypium hitsutum L.), and sorghum [Sorghum bicolor (L.) Moench)] and soil organic C (SOC) at 0- to 120-cm depth. A field experiment was conducted on no-tilled, strip-tilled, and chisel-tilled Dothan sandy loam (fine-loamy siliceous thermic Plinthic Paleudults) from 1999 to 2002 in central GA. As C supplied by above- and belowground biomass of cover crops, cotton, and sorghum varied by treatments and years, total residue C from 2000 to 2002 increased in cover crops compared to weeds and ranged from 6.8 to 22.8 Mg ha-1 in tilled and non-tilled soils. The SOC at 0- to 10-cm fluctuated with residue C addition from Oct. 1999 to Nov. 2002 and was greater in cover crops than in weeds in no-tilled soil. In contrast, SOC at 10- to 30-cm in no-tilled soil and at 0- to 60-cm in chisel-tilled soil declined gradually and was greater in biculture than in weeds. As a result, the biculture sequestered 267 compared with 33, -133, and -967 kg C ha-1 yr-1 at 0- to 30-cm in rye, vetch, and weeds, respectively, in no-tilled soil. In strip-tilled and chisel-tilled soils, SOC was lost from 233 to 1233 kg C ha-1 yr-1. The SOC at 0- to 10- and 10- to 30-cm also increased in cover crops with 120 to 130 kg N ha-1 (9.1 to 11.8 and 14.0 to 17.1 kg C ha-1) than in weeds with 0 kg N ha-1 (8.0 to 10.6 and 12.4 to 14.0 kg C ha-1), regardless of tillage. In the subhumid region, cover crops and N fertilization can increase the amount of crop residue C returned to the soil and C storage in tilled and non-tilled soils compared with no cover crop and N fertilization. Because of greater residue C, hairy vetch/rye biculture was more effective in sequestering C in no-tilled soil or reducing its decline in tilled soil than monocultures or no cover crop.