Long-term fertilizer-derived chloride inputs suppress nitrification and alter nitrogen supply patterns in a temperate agricultural brown soil

Chloride-containing fertilizers, especially those used as accompanying components in nitrogen fertilizers, are widely applied into the soil for agricultural production. However, the long-term impacts of this chloride input on nitrification and inorganic nitrogen supply remain poorly understood. Here, in order to determine whether long-term chloride-containing fertilization is associated with soil acidification, suppressed potential nitrification rate, altered ammonia-oxidizing microbial communities, and a shift in inorganic nitrogen supply patterns, we used a 35-year field experiment in Northeast China to compare chloride-free, low chloride, and high chloride fertilization regimes under equal total N, P, and K inputs. The results showed that High chloride treatments were associated with lower soil pH, although the direct effect of chloride on pH was not statistically significant in the path model, suggesting that ammonium-based fertilization itself is a major contributor to acidification in this system. Linear mixed models (LMM) further verified the significant effects of chloride addition and soil pH on PNR and the abundance of ammonia-oxidizing microorganisms. Our results suggest chloride may suppress soil nitrification via two hypothesized pathways: putative direct inhibitory effects of chloride, and an indirect pathway mediated by chloride-associated shifts in soil pH that may suppress ammonia-oxidizing microorganisms. Together, these two pathways facilitate the accumulation of ammonium-N relative to nitrate-N. High chloride treatments suppressed ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), and the abundance of AOA decreased by up to 72%. Amplicon sequencing of harvest-stage soil samples demonstrated that high chloride input altered the diversity and community composition of ammonia-oxidizing microorganisms, and reduced maize grain yield. Our findings reveal that combined application of potassium chloride and chloride-containing nitrogen fertilizers exacerbates soil acidification and substantially disturbs soil nitrification as well as ammonia-oxidizing microbial communities. Such disturbances reshape soil inorganic nitrogen supply patterns within this long-term brown-soil agroecosystem.