Long-term manure and mineral fertilisation drive distinct pathways of soil organic nitrogen decomposition: Insights from a 180-year-old study

Soil organic nitrogen (SON) decomposition is a fundamental process in the nitrogen (N) cycle that influences N availability for plant uptake and soil health. However, the long-term effects of nutrient fertilisation on SON decomposition and its microbial drivers remain poorly understood. Here, we used the 180-year-old Broadbalk Winter Wheat Experiment to investigate how farmyard manure (FYM), mineral fertiliser (NPK), and no fertilisation input (NIL) affect crop yield, SON turnover, microbial community composition, and functional genes. Our findings showed that distinct and complementary microbial mechanisms regulate SON decomposition under different nutrient fertilisation treatments. FYM application increased gross N mineralisation to 43.1 mg N kg− 1 soil d− 1, by doubling microbial biomass and promoting bacterial-dominated protein and peptide decomposition. During the early stage of decomposition, CO2 release from protein and peptide turnover under FYM increased by 96 % and 44 %, respectively, compared to NIL. NPK fertilisation enhanced the decomposition of complex N compounds and promoted the turnover of high-molecular-weight N to support microbial growth by upregulating N-cycling genes and extracellular enzyme production. The carbon use efficiency of protein was increased to 0.68. NPK fertilisation also stimulated fungal and Actinobacteria populations, accelerating the turnover rate of peptides and amino acids to 22.7 and 2.4 mg N kg− 1 soil d− 1, respectively. These results provide new insights into how nutrient fertilisation practices affect microbialy-mediated N dynamics and crop productivity, emphasising the importance of microbial functional diversity in supporting soil N cycling and fertility.