No-tillage effects on the soil nitrogen-cycling prokaryotic community inhabiting soil aggregates in upland fields

In no-tillage field soils, where degradation and erosion are minimized, soil aggregates form more readily, supporting a stable physical structure. These aggregates contain pores that serve as microhabitats for microbes involved in key biogeochemical processes, such as organic matter decomposition and redox reactions. In this study, we evaluated the physical and chemical properties of soils under different management practices (tillage vs. no-tillage) and conducted DNA metagenomic analysis to investigate microbial functions related to the nitrogen cycle. Long-term no-tillage led to the accumulation of persistent organic matter, which promoted the formation of soil aggregates. These aggregates maintained anaerobic microenvironments that suppressed organic matter decomposition in macro-aggregates. In the no-till soil, N₂O emissions and nitrogen accumulation were observed. Metagenomic analysis of macro-aggregates revealed that the relative abundance of ammonia oxidation (amoA, amoB, and amoC) and nitric oxide reduction (norB and norC) genes were 0.53 and 0.66 times lower, respectively, in no-till soil aggregates than in tilled soil aggregates. Not only the relative abundance but also the player of nitrification differed between no-tilled and tilled soil aggregates. Additionally, the relative abundance of dissimilatory nitrate reduction genes (nrfA, nrfH, nirB, and nirD) was 1.4 times higher in the tilled soil aggregates than in the no-tilled soil aggregates. Microbial community analysis showed that no-tillage practices altered nitrogen-cycling microbes, inhibiting nitrification and denitrification while promoting dissimilatory nitrate reduction to ammonia. These findings suggest that nitrogen cycling regulation in aggregates to accumulate nitrogen in no-tillage soils due to reduced microbial consumption and enhanced ammonia production.