Tillage, Cropping Systems, and Nitrogen Fertilizer Source Effects on Soil Carbon Sequestration and Fractions

Quantification of soil carbon (C) cycling as influenced by management practices is needed for C sequestration and soil quality improvement. We evaluated the 10‐yr effects of tillage, cropping system, and N source on crop residue and soil C fractions at 0‐ to 20‐cm depth in Decatur silt loam (clayey, kaolinitic, thermic, Typic Paleudults) in northern Alabama, USA. Treatments were incomplete factorial combinations of three tillage practices (no‐till [NT], mulch till [MT], and conventional till [CT]), two cropping systems (cotton [ Gossypium hirsutum L.]‐cotton‐corn [ Zea mays L.] and rye [ Secale cereale L.]/cotton‐rye/cotton‐corn), and two N fertilization sources and rates (0 and 100 kg N ha −1 from NH 4 NO 3 and 100 and 200 kg N ha −1 from poultry litter). Carbon fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Crop residue varied among treatments and years and total residue from 1997 to 2005 was greater in rye/cotton‐rye/cotton‐corn than in cotton‐cotton‐corn and greater with NH 4 NO 3 than with poultry litter at 100 kg N ha −1 The SOC content at 0 to 20 cm after 10 yr was greater with poultry litter than with NH 4 NO 3 in NT and CT, resulting in a C sequestration rate of 510 kg C ha −1 yr −1 with poultry litter compared with −120 to 147 kg C ha −1 yr −1 with NH 4 NO 3 Poultry litter also increased PCM and MBC compared with NH 4 NO 3 Cropping increased SOC, POC, and PCM compared with fallow in NT. Long‐term poultry litter application or continuous cropping increased soil C storage and microbial biomass and activity compared with inorganic N fertilization or fallow, indicating that these management practices can sequester C, offset atmospheric CO 2 levels, and improve soil and environmental quality.