Grassland conversion to cropland decreased microbial assimilation of mineral N into their residues in a Chernozem soil

The effect of grassland conversion to cropland on the incorporation of N into microbial residues remains unclear. We analyzed for amino sugars, as a microbial residue biomarker, and adopted stable-isotope-probing technique to trace the assimilation of (NH4+)-N-15-N (added in the form of (15) N-labelled ammonium sulphate) into amino sugars with or without organic C sources (glucose and crop residue) in native grassland and converted cropland soils over a 65-day period. Total (15) N-amino sugars in the cropland were significantly decreased by 16-29% compared to grassland regardless of C source. This suggests that the microbial assimilation of available N into microbial residues was decreased, after conversion, and that stronger responses to substrate inputs occurred in the grassland soils. The addition of available and complex C substrates triggered divergent accumulation patterns of (15) N-amino sugars, indicating that C availability was a major driver for microbial N immobilization. Meanwhile, the conversion of grassland led to suppressed N immobilization activity of both fungi and bacteria. These results suggest that long-term land-use change could considerably affect the N utilization by fungi and bacteria during their anabolic processes, which may exert long-lasting impacts on soil organic N storage.

Automated summary

The conversion of grassland into cropland decreased the assimilation rate of nitrogen into microbial residues, while enhancing responses depending on carbon sources. The addition of available and complex carbon substrates in cropland triggered different accumulation patterns of (15) N-amino sugars. Conversion of grassland suppressed nitrogen immobilization activities of both fungi and bacteria.