Drivers and soil water thresholds influencing soil carbon, nitrogen, and phosphorus loss during 40a anthropogenic shrub encroachment in desert steppes

Shrub encroachment has been widespread globally over the past century or two. However, changes in soil organic carbon (SOC), total nitrogen (STN), total phosphorus (STP), and their driving mechanisms during shrub encroachment remain unclear. This study explored the dynamics of these soil variables and their driving mechanisms during the 40a anthropogenic transition from desert grassland to shrubland in northwest China. The results showed that shrub encroachment increased shrub and litter biomass and plant total nitrogen and phosphorus content, while decreasing herbaceous species richness (P < 0.05). Furthermore, shrub encroachment reduced the contents and stocks of SOC, STN, and STP (P < 0.05). Notably, the SOC and STN were coupled in terms of their contents, stocks, and sequestration (P < 0.001). However, although the contents of SOC and STP, as well as those of STN and STP, were coupled (P < 0.001), their stocks and sequestration were decoupled (P > 0.05). Our results also indicated that shrub above-ground biomass exerted indirect negative effects on SOC, STN, and STP contents. Simultaneously, soil mineral-associated organic carbon and mineral-associated total nitrogen played direct positive roles in driving SOC and STN contents, respectively, while plant total phosphorus had a direct negative effect on STP contents. In addition, our results indicated that soil water played a central role in driving SOC, STN, and STP contents. As soil water content decreased, SOC and STN contents initially increased slightly, but declined sharply when soil water dropped below thresholds of 8.85% and 8.90%, respectively. In contrast, STP content exhibited a continuously decreasing trend, with the rate of decline slowing at the soil water threshold of 6.79%. This study provides novel insights into the cycling of soil carbon, nitrogen, and phosphorus in the context of global shrub encroachment.