Recycling of Dry-Batch Digestate as Amendment: Soil C and N Dynamics and Ryegrass Nitrogen Utilization Efficiency

Abstract

In this work a residue of dry-batch anaerobic digestion (DB) from the organic fraction of municipal solid waste, its composted homologous (CDB), and a municipal solid waste compost (MSWC) were characterized for their main physico-chemical traits and biological stability [oxygen uptake rate (OUR)]. These were then compared in soil incubations at 200 mg N kg−1, to assess carbon (C) and nitrogen (N) mineralization. The products’ nitrogen apparent recovery fraction (ARF) was assessed in a pot trial on Italian ryegrass. DB showed the highest OUR, followed by CDB and the more stable MSWC: 161.7; 20.7 and 5.7 mmol O2 kg−1 VS h−1, partially in agreement with the potentially mineralizable C pools in soil: 58.4; 16.6 and 19.5 % and their kinetics (k, 0.1906; 0.1405 and 0.1377 day−1). Composting greatly reduced the total carbon dioxide (CO2) emissions from 7,950 to 3,449 mg kg−1 in DB and CDB, even higher than MSWC (1,936 mg kg−1). After intense N-immobilization in soil (−22.3 %), DB finally reduced the gap (−6.9 %), also having a positive ARF (5.0 %), while CDB had greater N-immobilization (−12.9 %) and a negative ARF (−3.4 %). MSWC showed 3.8 % N-mineralization, and an intermediate ARF (2.7 %). The management of DB for plant nutrition therefore seems difficult since DB can furnish nitrogen not easily synchronizable with plant growth due to its release pattern characterized by initial immobilization. Composting increased stability but amplified N-immobilization due to the high C:N ratio of the bulking agent. A prolonged and more balanced composting process can reduce this restriction, aligning CDB with MSWC.