Depth-dependent effects of cover crops in citrus orchards on soil carbon and nitrogen cycling, greenhouse gas emissions, and soil microbial communities

Fruit orchards are a major part of agricultural production, and planting cover crops (CCs) in the row middles of these agroecosystems can increase soil nutrient availability. However, it is largely unknown how different mixtures of CCs may impact soil carbon (C) and nitrogen (N) concentrations, greenhouse gas emissions (GHG), and the soil microbiome at different soil depths, which is important for understanding the contribution of CCs to these agroecosystems. We examined how CC composition (legumes and non-legumes vs. non-legumes only compared to a grower-standard treatment) affects soil properties, nitrification rates and gene abundances, N and C mineralization rates, denitrification gene abundances, and emissions of nitrous oxide (N2O) and dinitrogen (N2) at different soil depths (0-to 10-cm, 10-to 20-cm, and 20-to 30-cm soil layers) in the row middles of a Florida (USA) commercial citrus orchard after 30 months of CC treatment. We found that CC impacts on soil N and C concentrations and mineralization rates, emissions of N2O and N2, and the abundance of N-cycling genes mainly occurred in the top 0-to 10-cm soil layer whereas differences in these variables in deeper layers were not statistically significant. The combined use of legumes and non-legumes led to significantly greater soil NH4+ concentrations, net nitrification and N mineralization rates, and lower N2O emissions. Lower N2O emissions with legumes were linked to a greater abundance of N2O reducers vs. N2O producers compared to non-legumes alone and the grower-standard treatment in the topsoil layer. This study shows that CC impacts on soil nutrient cycling and microbial communities occur primarily in the topsoil of citrus orchards and that planting diverse CC mix-tures is important to enhance soil-microbe interactions to diminish GHGs.