Removing macromolecular organic matter from biogas slurry prior to its application reduces organic N leaching and soil NO emissions.

Biogas slurry is an organic fertilizer with a low C/N ratio and a high water content. It can promote soil nitrous oxide (NO) emissions and the leaching of small-molecule organic N compounds. In this study, membrane micro- and ultrafiltration were performed to separate organic matter from biogas slurry. The effects of the application of biogas slurry with organic matter compounds of different sizes on soil NO emissions and N leaching were observed via simulating leaching through a 30-cm-high soil column over 62 days. In the treatment with filtrated biogas slurry (BS-F), the total NO emissions and dissolved organic N leaching were significantly increased by 63.74 % and 153.85 %, respectively, over the experimental period compared to the mineral fertilizer treatment (MF). In contrast, the removal of particulate organic matter and large-molecular-weight humic substance from the original biogas slurry (BS-U) resulted in a 30.87 % reduction in soil NO emissions and a 30.30 % reduction in dissolved organic N leaching, along with a 90.75 % increase in the soil microbial biomass nitrogen (MBN) content. The relative abundances of the genera Methyloversatilis (26.05 %) and Thauera (2.41 %) were higher in the BS-U treatment, whereas those of the functional genes narG/H and nirK/S, involved in denitrification, were lower. The relative abundance of the functional gene nifD, which plays a role in N fixation, was higher. The removal of large-molecular-weight (>100 kDa) organic substances from biogas slurry can reduce soil NO emissions by impeding denitrification and facilitating other N reduction pathways. Our study provides a theoretical foundation for enhancing the environmental sustainability of biogas slurry in agricultural practices.