Changes in nitrogen functional genes in soil profiles of grassland under long-term grazing prohibition in a semiarid...

Grazing prohibition has been used to restore degraded grassland ecosystems in semiarid areas; however, the impact of this measure on soil nitrogen (N) cycling is poorly understood. Furthermore, recent studies have tended to focus on the topsoil and ignored a steep gradient of nutrient accumulation with soil depth. Here, we investigated changes in N functional genes (NFGs) involved in organic N decomposition (chiA), archaeal and bacterial ammonia oxidation (amoA-AOA and amoA-AOB), respectively, denitrification (nirK and nirS), and N fixation (nifH) in soil profiles from a chronosequence of grazing prohibition (0, 10, 15, 25, and 35 years) in the semiarid grasslands of the Loess Plateau, China. The abundance of all the investigated NFGs in grassland soils after 35 years' grazing prohibition was higher than in grazed grassland. This result suggests that microbial N turnover potential is facilitated by grazing prohibition, probably through enhanced biomass production via increases in nutrient input into the soil. The higher ratio of (chiA + nifH)/(amoA-AOA + amoA-AOB) and values of (nirK + nirS) in grazing-prohibited grasslands than in grazed grassland suggest that prohibition of grazing not only improved microbial N storage potential but also increased N gas emission potential. The abundances of NFGs varied along the soil profiles and responded differently to environmental factors. The chiA and nifH abundances decreased with soil depth and were associated with variation in aboveground biomass, NH₄⁺-N, and organic carbon, while amoA-AOA, nirK, and nirS genes increased with depth and were more affected by soil organic carbon, moisture, and bulk density. Multivariate regression tree analysis demonstrated that aboveground biomass was the best explanatory variable for the changes in NFGs in grazed grassland, while soil organic carbon was the best in the grazing-prohibited grasslands. Our results provide new insight into the soil N cycling potential of degraded and restored semiarid grassland ecosystems.