Nitrogen immobilization caused by chemical formation of black- and amide-N in soil

Nitrogen (N) immobilization controls the N availability in soil, however, mechanisms involved in the chemical N fixation into soil organic N (SON) through reactions of reactive N compounds with soil organic matter (SOM) is not clear. Knowledge about the composition and stability of chemically produced SON is limited, which impedes understanding of the interplay of N and carbon (C) cycles at both the local and global scale. Here, we studied the chemical N immobilization of nitrite in soils from grassland, cropland, and forest with 15N labelling technique. And solid state 15N- and 13C NMR spectroscopies were applied to further explore the structure of chemically immobilized SON. We found that the chemical retention rate of nitrite did not differ significantly between land-uses, while the fulvic acid fraction was the SOM component most reactive to nitrite. In contrast to the common assumption that amides are mainly of biological origin and that black N compounds are formed from organic N compounds at high temperature during fires, our study revealed that amides and black N in the form of pyrroles were the main products of chemical reactions of nitrite with SOM. These findings indicate that chemical processes play a key role in biogeochemical N cycling, and provide new insight into the mechanisms of Csingle bondN interactions in soil