Compost biochemical quality mediates nitrogen leaching loss in a greenhouse soil under vegetable cultivation

Compost application has been suggested to sustain yields as well as reduce reactive nitrogen (N) loss in greenhouse vegetable production. However, there remains significant gaps in understanding the potential and mechanisms for compost application to reduce N leaching. Here, a leaching column experiment was conducted to examine N leaching under chili pepper cultivation in response to the application of synthetic N fertilizer (SNF) alone and in combination with composted maize straw (STR), spent mushroom compost (MUS), composted herb residue (HER), composted cattle manure (CAM) or composted pig manure (PIM). The biochemical quality of the composts was assessed by C-13 nuclear magnetic resonance spectroscopy, and soil microbial ecoenzymatic stoichiometry (C:N acquisition activity) was evaluated by ratio of beta-glucosidase to N-acetylglucosaminidase. Total N (TN) leaching was mainly in the form of dissolved organic N and nitrate, accounting for 54-67% and 30-42%, respectively, over the whole growing period. The amount of TN leaching was 23.6 kg N ha(-1) without N application, and was increased with the application of urea and compost with the exception of the PIM treatment (18.6 kg N ha(-1)). The largest amount of TN leaching was observed in the MUS treatment (39.3 kg N ha(-1)), which was significantly higher than the other treatments except HER. Interestingly, all forms of N leaching showed significantly positive relationships with the N-alkyl C and carbonyl C of the composts, but negative relationship with alkyl C. This could partly explain the highest N leaching from the MUS treatment and lowest from the PIM treatment. Moreover, all forms of N leaching were significantly correlated with soil enzymatic C:N acquisition ratio. The highest enzymatic C:N ratio found in the MUS treatment indicated that soil microorganisms invest more in C acquisition than N assimilation, which might in turn lead to more N leaching. Overall, our study highlights the role of the biochemical quality of composts in mediating N leaching, and suggests that composts with low N-alkyl C and carbonyl C and high alkyl C were effective in reducing the N leaching from the greenhouse soils under vegetable cultivation.