Coupled Dynamics of Soil Water and Nitrate in the Conversion of Wild Grassland to Farmland and Apple Orchard in the Loess Drylands

Understanding the dynamics of soil water and nitrate in response to typical agricultural crops in dryland ecosystems are crucial for assessing ecological consequences and informing land use planning. This study was conducted in the Changwu tableland, a representative area for agricultural crop cultivation in the Loess Plateau of China. Fifteen soil profiles, including grassland, farmland, and young, mature, and old apple orchards, were sampled to investigate the effects of different land uses on soil water and nitrate dynamics using a "space-for-time" substitution approach. The results showed that the soil water content and nitrate content in farmlands were comparable to those in wild grassland. However, significant differences in soil water were observed below a depth of 2 m in apple orchards, with mature and old orchards experiencing water deficits compared to grassland of 624.9 mm and 690.0 mm, respectively. Moreover, a dried soil layer formed below a depth of 5 m in these orchards. In terms of soil nitrate, the concentration in the 0-5 m depth of apple orchards was significantly higher than that in agricultural land and grassland, and it increased with the age of the orchards. However, below 5 m, the residual nitrate stock per unit depth in apple orchards decreased to levels comparable to grassland and farmland, primarily due to the inhibitory effect of the dried soil layer on downward migration and leaching processes. Furthermore, the relationship between nitrate and soil water at 0-5 m soil depths differed during the conversion from grassland to farmland and apple orchard, with positive and negative correlations observed, respectively. This indicates that water plays a key role in influencing nitrate movement, and distinct hydrological processes occur for soil water and nitrate nitrogen under different land use change conditions. In conclusion, converting grassland and farmland to apple orchards can lead to soil water decline and nitrate accumulation in the vadose zone, posing potential threats to ecosystem sustainability and security in dryland regions. Therefore, implementing appropriate water-fertilizer management practices is crucial for promoting sustainable land use in loess drylands, with potential implications for similar areas worldwide.