Long-term changes in soil chemical properties with cropland-to-orchard conversion on the Loess Plateau, China: Regulatory factors and relations with apple yield

Farmland-to-apple orchard conversion prevails on the Loess Plateau in China and in other developing countries, but simultaneously, apple orchards have been severely overfertilized in the past two decades. We quantified the changes in soil chemical properties between the 1980s and 2010s, and identified environmental factors that controlled spatial variability of soil properties and relations with apple yields. In addition, early warnings were provided by forecasting the changes in soil properties to 2100 under various fertilization scenarios. Geostatistics was performed using soil data obtained from the 1980s and 2010s to evaluate spatial-temporal variations in soil chemical properties. Drivers of the variation were identified by boosted regression tree analysis. Variance partitioning analysis and regression analysis were conducted to quantify the threshold values and then identify the potential factors limiting apple yield. Scenario analysis was conducted to forecast soil degradation, and thus, help to determine optimum fertilization. Soil properties, except pH, varied considerably in the 1980s and 2010s, with coefficients of variation between 22.55% and 60.62%. Compared with the 1980s, pH decreased in the 2010s, whereas concentrations of soil organic matter (SOM), total nitrogen (TN), available N (AN), available phosphorus (AP), and available potassium (AK) increased. Fertilization was the dominant factor affecting soil properties (except AP), followed by climate, terrain, and soil physical factors. Apple yield was significantly related to all properties, and critical thresholds were identified for pH, SOM, AN, AP, and AK to obtain maximum apple yield. In most apple orchards, pH, SOM, AP, and AK were much lower than corresponding thresholds, whereas AN exceeded the threshold. VPA identified soil as the most important factor explaining variation in apple yield, followed by anthropogenic, climatic, and topographic factors. Compared with an optimal fertilization scenario, continuing with conventional fertilization was predicted to lead to severe soil acidification and excessive accumulation of AN, AP, and AK in 2100. Thus, overfertilization must be controlled to improve apple orchard soil fertility and productivity. To summarize, soil chemical properties in apple orchards on the Loess Plateau changed substantially following long-term overfertilization, and soil acidification and low levels of SOM, AP, and AK were key variables limiting apple production. The results provide new insights into spatial variability of soil properties and relations of soil properties to apple yields at a regional scale, and thus, contribute to optimizing orchard management in China and other countries with similar dilemmas in apple production systems.