Soil Organic Carbon Sequestration Rates Under Greater Crop Sequence Diversity, Bio-Covers, and No-Tillage.

Climate change may be mitigated through soil organic carbon (SOC) additions in no-till systems; however, crop management influences on the rate of SOC storage is not well defined. Our objective was to compare long-term carbon fluxes under two no-tillage sites at Research and Education Centers (REC) in Milan (RECM) on Oxyaquic Fragiudalfs and at the Middle Tennessee (MTREC) on a Typic Paleudalf, in a split-block design with four replications. Whole-block was cropping sequences of corn, soybeans, and cotton; with split-block bio-cover treatments of: winter wheat, hairy vetch, poultry litter, and a fallow control. Same sequences were carried out at the MTREC without cotton. Soil carbon flux was calculated at surfaces (0-5 cm) and sub-surfaces (6-15 cm) during yr-0, 2, 4, and 8. During the first 2-yrs, small annual losses occurred (1.40 and 1.20 Mg ha^-1 at RECM and MTREC, respectively). However, by yr-4, surface SOC began to stabilize. By yr-8, sequences with greater frequencies of soybean and temporal complexity generally gained greater SOC at 0-5 cm depths. Poultry litter bio-covers gained more surface SOC compared to wheat, vetch and fallow covers (P<0.05).  Across all sequences and bio-covers, surface SOC had recovered after 8-yrs and surpassed initial levels, with mean surface gains of 1.33 and 1.16 Mg C ha^-1 from pre-experimental levels of 9.20 and 8.79 Mg ha^-1 at RECM and MTREC, respectively. After 8-yrs, nominal losses were observed in subsoil layers at both MTREC and RECM (0.11 and 0.09 Mg ha^-1, respectively). Results suggest surface carbon storage may be enhanced by crop sequence diversity combined with poultry litter bio-covers, with minor reductions in sub-surface C in no-till systems.