Co-ensiling, co-composting and anaerobic co-digestion of vegetable crop residues: Product stability and effect on soil carbon and nitrogen dynamics

- Ensiling, composting & anaerobic digestion of vegetable residues may reduce N loss.
- Silage, compost & digestate: different characteristics and soil C and N dynamics.
- On-farm facilities and different soil effects determine the valorization option.

Nitrogen (N)-rich vegetable crop residues left in the field may result in a high risk for N losses during autumn. Removal and conservation of these residues followed by reuse in the field could contribute to improved recycling of nutrients, but some form of processing is required to allow storage before re-application. We have compared co-ensiling, co-composting and anaerobic co-digestion as conservation and valorization options for fresh crop residues. We studied (1) the product quality and stability and (2) the short-term effects of application of these silages, composts and digestates on soil C and N mineralization and N2O emissions. Ensiling resulted in highly biodegradable products with a low pH (4.2-5.2) and more NH4+-N compared to composts. Consequently, soil incorporation of silages resulted in higher net C mineralization (up to 47% after 82 days) and microbial biomass C (up to 93 μg C g−1 soil after six weeks), and temporary N immobilization (up to 42 mg kg−1 soil). Digestates and composts led to lower C mineralization rates (between 2 and 27%) and microbial biomass C (max. 51 μg C g−1 soil) and no net N immobilization nor mineralization. Application of digestates resulted in high mineral N contents (47-192 mg kg−1 soil) and a decrease of the soil pH. In all three treatments, short-term N2O losses after soil application were very small (<0.11 kg N ha−1 after 12 days). Growers can choose the most appropriate treatment option and application moment and location, depending on the local soil and crop requirements and the on-farm facilities. Furthermore, we conclude that the parameters biodegradation potential (based on the biochemical composition) and oxygen uptake rate have potential as less time-consuming proxies for C mineralization to assess the product stability.

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