[Effects of Typical Amendments on Nitrogen Forms and Microbial Community Structure in Polygonal Solonchak]

Polygonal Solonchaks critically impede regional ecological security and agricultural sustainability due to compaction， infertility， weak nutrient retention， and low microbial activity. While amendments offer a key strategy for improving soil structure and fertility， their impact on crucial nitrogen （N） transformation processes and the underlying microbial communities remains unclear. To elucidate the response of N transformation and microbial communities to biochar （C） and desulfurized gypsum （T） amendments and identify key influencing factors， a field trial was conducted in Qianjin Farm， Pingluo County， Ningxia. Four treatments were evaluated： control （CK， no amendment）， biochar alone （C）， desulfurized gypsum alone （T）， and combined biochar and gypsum （C+T）. Soil N content and microbial community characteristics were measured. Key results show that： ① Treatment C significantly increased soil total nitrogen （TN）， ammonium nitrogen （NH₄⁺-N）， and nitrate nitrogen （NO₃^--N） by 96.1%， 21.5%， and 170%， respectively， compared to those under CK （P<0.05）. Treatment T increased TN by 42.1% but decreased NO₃^--N by 21.4% （P<0.05）. ② Amendments reduced microbial alpha diversity. Dominant taxa varied significantly among treatments： C increased the relative abundance of Actinomycetota by 9.8%， while T and C+T decreased it by 13.1% and 20.1%， respectively. C+T increased Acidobacteriota abundance by 10.6%. ③ Redundancy analysis （RDA） identified NO₃^--N as the primary factor influencing bacterial and archaeal community composition. Structural equation modeling revealed that amendments enhanced N transformation， optimized microbial community structure， and stimulated N metabolism by improving soil physicochemical properties， but inhibited nitrification-thereby reducing nitrogen leaching risk. Biochar exerted the most significant effect. Caution is warranted regarding gypsum application rates to prevent salt accumulation. This study provides a theoretical basis for efficient N utilization and non-point source pollution control in Polygonal Solonchak.