Structural composition of immobilized fertilizer N associated with decomposed wheat straw residues using advanced nuclear magnetic resonance spectroscopy combined with 13C and 15N labeling

When crop residues are returned to soil, nitrogen (N) from applied chemical fertilizers can be immobilized by microbial growth and incorporated back into the residues themselves to form organic N compounds with poor bioavailability. However, the chemical composition of these compounds has remained poorly understood. Knowledge of the chemical composition of immobilized fertilizer N is crucial for understanding its fate in the field and its associated impacts on fertilizer applications and management. Here, we characterized immobilized fertilizer N by coupling isotopic labeling of wheat straw (13C) and fertilizer N (urea-15N) with two advanced nuclear magnetic resonance (NMR) techniques, (13C{15N} rotational-echo double resonance, REDOR) and two-dimensional (2D) 15N−13C heteronuclear single quantum coherence (HSQC) spectroscopy. We found that the immobilization of N under aerobic conditions was greater than that under anaerobic conditions (52% of fertilizer 15N vs 38% of fertilizer 15N). Under anaerobic conditions, most of the immobilized fertilizer 15N was in the form of protonated amide N. Compared to anaerobic conditions, 15N spectra of aerobic samples showed more diversity amongst N containing functional groups, including peptide-amides (54%), methylated amides (22%), amines (7%), anilides (5%) and heterocyclics (3%). Additionally, from the perspective of its structural composition, 55-80% of the immobilized fertilizer 15N was estimated to be a potentially labile N fraction. Collectively, these results indicate that the immobilization of fertiziler N during aerobic decomposition of straw residues produces organic N functional groups with a strong potential for remineralization available for use by crops when they are released into soil.