The reduction of nitrogen fertilizer combined with humic acid application improves potato yield by modulating soil microbial community structure in arid and semi-arid regions.

INTRODUCTION: Humic acid is a biostimulant with considerable potential to reduce nitrogen (N) fertilizer input and improve fertilizer use efficiency. However, the mechanisms by which it regulates soil nutrient supply, N utilization, and yield formation in potato remain unclear. METHODS: A single-site, single-season field experiment was conducted in the northern Yinshan Mountains using a randomized block design with a 3 × 3 factorial arrangement of N rates and humic acid applications. Three N levels (300, 270, and 240 kg N ha-1) and three humic acid rates (0, 7.5, and 15 L ha-1) were established. The effects of reduced N input combined with liquid humic acid application on rhizosphere soil nutrient status, microbial biomass, potential enzyme activities, microbial community composition, N use efficiency, and potato yield were evaluated. RESULTS: Compared with the conventional N input without humic acid treatment (H0N1), a 20% reduction in N input combined with 15 L ha-1 liquid humic acid application (H2N1) increased tuber yield by 13.0%, plant N accumulation and N uptake efficiency by 60.1%, and partial factor productivity of applied N by 13.0%, whereas N utilization efficiency decreased by 29.4%. Humic acid application enhanced rhizosphere soil nutrient availability, microbial biomass carbon and nitrogen, and the potential activities of enzymes involved in carbon and nitrogen transformation. Furthermore, N rate and humic acid application jointly altered bacterial and fungal community composition and promoted treatment-specific enrichment of indicator taxa, with fungal communities showing a stronger response than bacterial communities. DISCUSSION: Partial least squares path modeling indicated that soil nutrient availability was the strongest direct factor associated with potato yield, whereas humic acid was indirectly associated with yield through changes in microbial community structure, microbial biomass and potential enzyme activities, and soil nutrient availability. Under the conditions of this experiment, reducing N input by 20% combined with 15 L ha-1 liquid humic acid application showed potential for reducing N fertilizer use, sustaining potato productivity, and improving rhizosphere soil ecological properties. Multi-year and multi-site field experiments are required to further verify the stability and broader applicability of this strategy.