Nitrogen addition alters soil microbial community structure and decrease nitrogen cycling gene abundance in acidic...

Nitrogen addition can alter soil microbial communities and regulate nitrogen cycling processes. However, the role of nitrogen-induced soil acidification in modulating nitrogen cycling in macadamia orchard soils remains poorly understood. Based on a 3-year field fertilization experiment, this study established three treatments: no-nitrogen (N0), medium-nitrogen (N1), and high-nitrogen (N2). We systematically investigate the effects of nitrogen addition on soil chemical properties, the abundance of nitrogen cycling genes, enzyme activities, microbial community structure, and co-occurrence networks. The results showed that nitrogen addition significantly increased the contents of alkaline hydrolyzable nitrogen (AHN) and nitrate nitrogen (NO₃ ^--N), altered soil stoichiometric ratios, and exacerbated soil acidification. Quantitative PCR (qPCR) analysis revealed that nitrogen addition significantly decreased the abundance of nitrification genes (AOA-amoA), ammonification gene (ureC), and denitrification genes (narG, nirK, nosZ), while no significant changes were observed for AOB-amoA or nirS. 16S rRNA gene sequencing demonstrated that nitrogen addition altered the succession of microbial communities towards a copiotrophic (R-strategy) state and altered the predicted carbon-metabolism functional profiles. Co-occurrence network analysis showed that the N2 treatment reduced network complexity but enhanced modularity, suggesting that microbial communities may respond to nitrogen addition through functional modular reorganization. The asynchronous changes between key enzyme activities and their corresponding gene abundances revealed the complex regulatory mechanisms of the nitrogen cycle. Furthermore, correlation analyses identified pH, electrical conductivity (EC), and NO₃ ^--N were important environmental factors affecting nitrogen cycle function gene groups. These findings provide new insights into the response mechanisms of the nitrogen cycle in the soil ecosystem of Macadamia orchards.