UC Davis

Effective cover crop termination by roller crimping remains a challenge in organic no-till vegetable systems. In addition, slow decomposition of surface residues can delay planting and lead to competition for soil resources. Application of microbial inoculants to promote residue decomposition could potentially address these constraints. Two field studies were conducted in Davis, California to evaluate the effect of microbial tea treatments on the rate of cover crop surface residue decomposition, labile carbon and inorganic N pools, and sweet potato yield under different environmental conditions. To assess fall termination of a summer cover crop, four treatments were applied to sorghum sudangrass following roller crimping: thermophilic compost tea, vermicompost tea, a commercially available microbial inoculant (EM-1), and a roller crimped control. To assess spring termination of a winter cover crop, four treatments were applied to a multispecies cover crop mix (bell beans, peas, vetch, wheat) following roller crimping: vermicompost tea, EM-1, a roller crimped control, and a tilled treatment. Litter bags were used to measure residue decomposition and soil samples were obtained from 0-10 cm to determine inorganic N and POXC concentrations. In contrast to our hypothesis, we found no effect of the microbial tea treatments on the rate of residue decomposition in either study. Decomposition following roller crimping was much greater in the spring than the fall (41.6 vs 16.4%), likely due to the presence of legumes in the winter cover crop mix and higher soil surface moisture due to increased rainfall throughout the litter bag retrieval period in the spring. The microbial tea treatments had no effect on soil ammonium-N, nitrate-N, or POXC concentrations. However, soil ammonium-N and nitrate-N concentrations significantly increased by 39 and 51% in the tilled vs. roller crimped treatments, respectively (ammonium-N: 1.2 mg N kg-1 and 0.73 mg N kg-1; nitrate-N: 5.5 mg N kg-1 and 2.7 mg N kg-1). Meanwhile, POXC increased significantly over time in the fall termination study, but decreased over time in the spring termination study. There were no significant differences in sweet potato yield between the roller crimped treatments and the tilled treatment, indicating the presence of surface residues combined with the difference in soil N levels had no effect on sweet potato yield.