Disentangling immobilization of nitrate by fungi and bacteria in soil to plant residue amendment

Plant residue incorporation improves soil microbial nitrate immobilization, which in turn alleviates nitrate accumulation, thereby improving N retention capacity and reducing N losses. However, how and why the respective nitrate immobilization by fungi and bacteria, which are dominant microorganisms in soil, responds to residue addition remains unknown. By adopting a novel amino sugar-based stable isotope probing approach, we show that both fungal and bacterial nitrate immobilization increased after Paspalum notatum residue addition. The increased proportion of the former was higher than that of the latter, and their difference became much larger with the increased residue addition rate. Furthermore, 70% and 32% of the variations in fungal and bacterial nitrate immobilization activities were explained by their respective biomass. The relative importance of fungal and bacterial nitrate immobilization shifted in a consistent manner with the relative abundance of fungi and bacteria, and the magnitude of these shifts was related to the residue addition rate. Our work demonstrates strong links between increased residue inputs, alterations in microbial community composition, and enhanced ecosystem functioning of nitrate immobilization and retention in soil.