Soil acidification and its influencing factors in the purple hilly area of southwest China from 1981 to 2012

Soil acidification can negatively impact the sustainability of agricultural systems. Understanding the relationships between soil pH change and its influencing factors is critical to the maintenance of soil quality and the sustainability of land use. Based on regional observations conducted in 1981 and 2012, this study used the kriging method and an unpaired t-test to evaluate soil pH change and its influencing factors at the landscape scale of the hilly area in the Sichuan basin, China. The results show that soils in the surface layer of the study area were significantly acidified from 1981 to 2012, with a decline of 0.30 pH units. Spatially, the soil pH changes were highly variable and significant acidification only occurred in farmland soils. The declines in soil pH of farmland varied with the parent materials and soil types, and the high magnitude of soil pH decrease was closely associated with the low initial pH values and small amount of soil cation exchange capacity (CEC), suggesting the different response of the degree of soil acidification to the external factors. Base cation removal by crop straw, precipitation and nitrogen fertilization had significant impacts on soil acidification in farmland areas, and this study identified critical levels of 130 kg ha−1 yr−1, 1000 mm yr−1 and 200 kg ha−1 yr−1 for base cation removal by crop straw, precipitation and nitrogen fertilization, respectively, above which soil acidification was promoted. Moreover, a significant interaction between precipitation and nitrogen fertilizer applications was also observed, since no significant soil pH changes occurred in areas with low rainfall and nitrogen fertilizer rates, while soil acidification was significantly promoted by the combined effect of high precipitation and nitrogen fertilization levels. These results highlight the degree to which soil acidification in farmland soils depends on the crop yield, the interactions of nitrogen addition and precipitation and the buffering capacity of the different soil types and parent materials.