Microbial immobilization of ammonium and nitrate fertilizers induced by starch and cellulose in an agricultural soil

Abstract

Combined applications of plant residues and nitrogen (N) fertilizer in agroecosystems may increase N immobilization and reduce N loss through microbial propagation using fresh organic matter (FOM). However, the effects of different carbon (C) availabilities in FOM to microbes on the immobilization of two forms of fertilizer-N (NH4+-N and NO3−-N) are still not clear. In this study, starch (high availability) and cellulose (low availability) were used to represent different polysaccharides in crop residues, and their effects on the immobilization of two forms of fertilizer-N (NH4+-N and NO3−-N) were investigated. Following the additions of polysaccharides and 15N (99.9 atom %) (2.1 g C kg−1 soil and 100 mg N kg−1 soil), we measured soil respiration and traced immobilized fertilizer-N over 48 days. The starch addition quickly depleted mineral N (on day 3) while cellulose caused a slow but continuous immobilization for 4 weeks. Despite the same amount of C and N input, the maximum fertilizer-N immobilization in the cellulose treatment was lower than in the starch treatment and accounted for 56% and 40% of added N for NH4+-N and NO3−-N forms, respectively. NH4+-N was preferentially immobilized over NO3−-N in the cellulose treatment during incubation, but it also occurred in the starch treatment when mineral N was sufficient. Our results suggest that it was not the abundance of C, but rather the availability of C to microbes, that strongly determined the rate and capacity of fertilizer-N immobilization. Further, the preferential immobilization of NH4+-N highlighted that the use of ammonium-based fertilizer can increase the level of fertilizer-N immobilization.