Predicting nitrogen mineralization from organic amendments: beyond C/N ratio by 13C-CPMAS NMR approach

Background and aim The processes of nitrogen (N) mineralization from organic amendments (OA) is important to provide mineral forms (NH4+ and NO3-) of N for plants. Assessment of N mineralization is usually done by using the C/N ratio of OAs but limits to the predicting capability of such indicator have been reported. Here, we propose a new definition of organic carbon (C) quality based on C-13-CPMAS NMR that provide a more accurate prediction of N mineralization. Methods A mesocosm incubation experiment was carried out to assess the effects of ten OA types on soil N mineralization in three different soils. OAs were chemically characterized by C/N ratio and in solid state spectroscopy by C-13-CPMAS NMR. Nitrogen mineralization in terms of NH4+ and NO3- was monitored after 3, 10, 30, 100, and 300 days of incubation. Results N mineralization was fast for OAs with high C quality coupled with high N content (e.g., meat powder, fish meal and alfalfa litter), while the same process was slow for amendment with low C quality even with high N content (e.g. humus, alfalfa biochar). On the contrary, a rapid but short-term N immobilization was found for OAs with high C quality but low N content (e.g., glucose). OAs with low C quality and low N content (e.g., sawdust, cellulose, wood biochar and grass litter) showed a slow, but long-lasting N immobilization. We found that C/N ratio was unable to predict the N mineralization for OAs with low C quality. Considering C-13-CPMAS NMR spectral regions, the carbonyl C, and N-alkyl and methoxyl C regions had the most significant positive correlation with N mineralization, while the di-O-alkyl C and O-alkyl C were strongly associated with N immobilization. Conclusions This study demonstrates that the biochemical quality of organic C defined by C-13-CPMAS NMR is capable of predicting N dynamic pattern better than C/N ratio.