Crop cover and manure compost: Their varied effects on nitrogen availability and nitrogen cycling functional gene abundances in sandy soils for organic farming

Cover crops and manure compost application are two common nutrient management practices in organic production. However, their interactive influences on the biogeochemical properties of sandy Coastal Plains soils are not well-documented. Here, we investigated their effects on nitrogen (N) availability and the abundance of selected N functional genes in organic vegetable production. An experiment was established in 2020 with a fully crossed treatment of manure compost application (2365 and 0 kg ha⁻¹) and cover crop inclusion (cereal rye, hairy vetch, mixtures of rye and vetch, and no cover crop control). Two years after establishment, the highest nitrate (NO₃⁻) concentrations were found in the vetch + manure compost plots, while manure compost plots increased ammonia (NH₄⁺) concentrations compared to non-manure compost plots. N-acetyl-β-D-glucosaminidase (NAG) activities were higher in vetch plots than in rye and control plots, while leucine aminopeptidase (LAP) activities were higher in manure compost plots than in the non-manure compost plots. Integrating cover crops introduced higher N mineralization potentials, which, however, was not observed for manure compost. Manure compost resulted in higher fungal abundance than no manure control plots. Manure compost increased the abundance of ammonia-oxidizing archaea (AOA) only in the mixture, and the vetch plots had a higher abundance of ammonia-oxidizing bacteria (AOB) than other cover crop plots. Both cover crops and manure compost increased N availability but posed distinct short-term impacts on soil microbial communities.