Soil pH changes in a small catchment on the Chinese Loess Plateau after long-term vegetation rehabilitation

Soil pH is a fundamental parameter of soil chemical properties, and changes in soil pH (CIPs) alter hydrogeochemical processes in the Earth's critical zone. Afforestation has long been reported to change surface soil pH at the site scale or regional scale, while few studies have discussed the effects of afforestation and microtopography on soil pH at the catchment scale. We hypothesize that both afforestation and microtopography affect surface soil pH in a catchment. To test this hypothesis, two neighboring catchments in which one catchment was restored with natural vegetation and the other was afforested in 1954 were selected and surface soils were sampled. The soil pH, soil water content (SWC), total nitrogen (TN), soil organic and inorganic carbon (SOC and SIC) were analyzed for those soil samples. Moreover, trees coverage, LAI and grass cover were investigated in the two catchments. The results showed that over 60 years of afforestation in the catchment reduced the soil pH values at the depth of 0–80 cm in only the topography of the downhill gully, and the CIPs ranged from −0.02 to −0.15. In the topography of the upper hillslope, afforestation showed no significant impacts on soil pH. The reduction in soil pH in the gullies of the forestland catchment had a significant linear and negative relationship with TN and SOC. Moreover, microtopography showed significant impacts on soil pH. In the forestland catchment, the downhill gully showed lower soil pH values than those in the upper hillslope, while the opposite difference was detected in the grassland catchment. The high coverage of vegetation in the forestland gully and frequent gravity erosion in the grassland gully could explain the contrasting trends of soil pH differences between the two landforms in the catchment. The results of this study deepen our understanding of the effects of afforestation and catchment topography on soil pH and the related critical zone processes.