Long-term effects of film mulching and fertilization regimes on gross N transformations in calcareous dryland soils

Plastic film mulching (PM) and nitrogen (N) fertilization regimes significantly affect crop yield, N supply capacity, and N losses. However, the long-term effects and the underlying mechanisms, like the belowground N transformations, call for in-depth investigation. Here, a ¹⁵N tracing study was conducted to quantify the gross N transformation rates of the calcareous soil subjected to 12 years of PM and various fertilization regimes. We found that autotrophic nitrification (O_NH4) and mineralization (M) were the predominant soil N conversion processes, while dissimilatory nitrate reduction to ammonium (DNRA) and nitrate immobilization (I_NO3) were negligible in the calcareous soil, leading to the accumulation of nitrate. Long-term PM significantly decreased the rates of M, recalcitrant organic-N mineralization (M_Nrec), O_NH4, and NH₄⁺ immobilization to labile organic-N (I_{NH4_Nlab}) due to the negative effect on the abundances of fungi and ammonia-oxidizing bacteria (AOB) amoA gene compared to control soil. Relative to no N control, different fertilization regimes significantly increased the AOB amoA gene abundance, decreased fungal abundance and ITS:16S ratio, thus increasing O_NH4 and M, decreasing NH₄⁺ immobilization rates to labile and recalcitrant organic-N. Compared to normal N rate (F₂₂₅), high N rate (F₃₈₀) and normal N plus manure (F_225+M) markedly increased O_NH4 and I_NO3. Regression analyses revealed that M and AOB amoA gene abundance affected O_NH4, and in turn N₂O production. The findings provide an improved understanding of the long-term effects of PM and N managements on soil N supply capacity and potential N losses based on internal N cycling and molecular biology.