Hotspots and hot moments of DNRA in the Vadose Zone of Agricultural Croplands

High nitrogen (N) input in intensive cropping systems has resulted in significant nitrate (NO₃⁻) accumulation in agricultural soils of China. However, despite substantial N input (500-600 kg ha-1 y-1) in the Taihu Lake region, NO₃⁻ accumulation in soils and groundwater therein remains minimal with the mechanisms behind are unclear. Here, we investigated the spatiotemporal distribution and activity of dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonium oxidation (anammox), and denitrification, and the associated microbial communities—in the vadose zones of rice-wheat, vegetable, and orchard fields of the Taihu Lake region. Results revealed NO₃⁻ content decreased progressively with soil depth, while NH₄⁺ levels increased, particularly in deeper soil layers. DNRA emerged as the primary pathway for NO₃⁻ reduction, contributing to over 50% of NO₃⁻ removal, especially in the 50–190 cm depth range. Seasonal variations indicated that DNRA activity was highest during spring and autumn, with lower rates observed in winter and summer. DNRA significantly contributed to NH₄⁺ accumulation, with rates strongly positively correlated with NH₄⁺ content, especially in rice-wheat rotation fields characterized by high OC/ NO₃⁻ ratios. Interestingly, DNRA rates were significantly negatively correlated with groundwater N₂O concentrations and the N₂O/(N₂ + N₂O) ratios. Microbial community analysis revealed that the nrfA gene, a marker for DNRA, exhibited higher diversity compared to genes related to denitrification. Additionally, the abundance of DNRA-specialist microbes was positively associated with DNRA rates, particularly in deep layer soils, emphasizing the role of microbial community composition in shaping DNRA activity. These findings demonstrate that DNRA plays a crucial role in facilitating NH₄⁺ accumulation, attenuating NO₃⁻ accumulation, and mitigating N₂O emission in the vadose zone of agricultural croplands in the Taihu Lake region.