Cumulative effects of a 17-year chemical fertilization on the soil quality of cropping system in the Loess Hilly Region, China

Soil-quality assessment provides a tool for evaluating the sustainability of alternative soil-management practices. Understanding the effects of the long-term use of chemical fertilizers on soil quality is essential for sustaining soil productivity. The cumulative effect of a 17-y-old chemical-fertilizer application on integrated soil quality was investigated in the surface (015cm) and subsurface (1530cm) soils of a soybroomcornmillet cropping system with an experiment design of two factors with three levels for each factor. The treatments were N0P0, N0P1, N0P2, N1P0, N1P1 (local farmer' fertilization strategy), N1P2, N2P0, N2P1, N2P2, and a control NF. The objectives were to describe and quantify the effect of continuous applications of chemical fertilizer through soil-quality index (SQI) and attempt to offer an efficient and appropriate nutrient-supply strategy for the local region. Following 17 y of chemical fertilization, the SQI increased markedly in the studied soil layers, and soil chemical indicators changed more significantly than physical properties. The soil-quality indicators in the surface soil layer were more variable and sensitive to fertilizer application than the subsurface layer. The indicators that sensitively responded to long-term fertilization could be classified into three types: soil-fertility indicators (soil organic matter [SOM], total P, and available N), soil-texture indicators (clay, silt), and soil-structure indicators (bulk density, mean weight diameter [MWD]). The simplified indicators (SOM, sand, available N, bulk density, and total P in the surface soil and SOM, MWD, and silt content in the subsurface soil) preferably reflected the status of the integrated soil quality as influenced by continuous fertilization. Among the fertilized treatments, the combined-fertilizer treatments maintained a higher SQI than the single-fertilizer treatments in the surface and subsurface soils. The N1P2 treatment maintained the highest SQI in both soil layers, followed by N1P1 treatment. However, for the slope cropland, given the relative costs, soil resource, and environmental protection, the traditional treatment of N1P1 may be the optimal fertilizer treatment in the local region.