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 ¹⁵NH₄⁺-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.

草地转化为农田对氮进入微生物残留的影响尚不清楚。我们分析了氨基糖，作为微生物残留的生物标志物，并采用稳定同位素探测技术追踪¹⁵ NH ₄ ⁺ -N（以¹⁵  N 标记的硫酸铵形式添加）到氨基糖中的同化，有或没有65 天内天然草地和转化农田土壤中的有机碳源（葡萄糖和作物残留物）。共^{15 个} 与草地相比，无论碳源如何，农田中的 N-氨基糖均显着降低了 16-29%。这表明在转化后，微生物对微生物残留物的有效氮同化减少，并且草地土壤对底物输入的反应更强。添加可用的和复杂的 C 底物触发了^{15 个}不同的积累模式 N-氨基糖，表明 C 可用性是微生物 N 固定的主要驱动因素。同时，草地的转化导致真菌和细菌的固氮活性受到抑制。这些结果表明，长期的土地利用变化可能会显着影响真菌和细菌在合成代谢过程中对氮的利用，这可能对土壤有机氮储存产生长期影响。