Fifteen years of crop rotation combined with straw management alters the nitrogen supply capacity of upland-paddy soil

Straw return affects the soil N supply; however, the mechanisms underlying the effects of different straw types and substance additions on soil N availability remain elusive. This study compared net N mineralization (NNM) in soils with various straw management histories under rotations rice-fallow (RF), rice-wheat (RW) and rice-potato with straw mulch (RP) during 2003–2017. The soils were then subjected to four substrate additions, including the no-substrate control, +N [(NH4)2SO4, 0.2 mg N kg−1 dry soil], +G (glucose, 0.6 mg C kg−1 dry soil) and +G+N (both). The responses of the soil NNM to substrate additions varied depending on the soils’ different straw histories. The direction of NNM was affected by substrate addition, but the magnitude depended on crop rotation and straw management. The NNM in the +G treatment ranged from 15.5 to 71.5 mg kg−1 in the soils with various straw histories, with RW and RP having 299% and 366% greater NNM than RF, respectively. Conversely, the addition of +N and +G+N decreased the soil NNM significantly in the soil with RW and RP straw histories, being 70~75% and 28~39% lower in the +N and +G+N treatments, respectively, than in the soil without straw history (RF). In addition, long-term straw return (RP and RW) significantly increased the soil ammonium N, amino acid N, and microbial biomass C and extracellular enzyme activity, while the addition of N and C substrates altered the decomposition of soil organic N. The long-term straw history also affected the soil bacterial community and formed a significant grouping compared to RF. In general, the soil from RF was dominated by heterotrophic bacteria such as Actinobacteria, Bacteroidetes and Proteobacteria, while the soils of RP and RW were mainly Firmicutes involved in straw glycolysis and part of Proteobacteria involved in N metabolism. Adding inorganic N altered bacteria in Proteobacteria, and the relative abundance of Proteobacteria had a negative relationship with the concentration of available N, indicating that bacteria induced by exogenous N were not involved in inherent N mineralization in soil. The addition of glucose enhanced the bacterial communities involved in N mineralization. The results imply that the soil N supply capability was enhanced with straw history under N deficiency and that an excess supply of N fertilizers was immobilized to minimize leaching losses.