Soil moisture decline and residual nitrate accumulation after converting cropland to apple orchard in a semiarid region: Evidence from the Loess Plateau

The cropland-to-apple orchard conversion is expanding in the semiarid Loess Plateau, yet its impacts on deep soil moisture and nitrate are poorly documented. This study was conducted in Luochuan, typically the best area for apple planting in China, where fifteen 6-m-deep soil profiles were sampled from each cropland (as the control), as well as 8-, 17- and 25-yr apple orchards, to investigate the vertical distributions of soil moisture and nitrate and their storage. Results showed that the 8-yr apple orchards were comparable to cropland in soil moisture content (SMC), soil water storage (SWS), soil nitrate concentration (SNC) and residual soil nitrate (RSN). In contrast, 17- and 25-yr apple orchards, caused serious SWS deficits, of 140 mm and 150 mm, and substantially increased RSN, by 4270 and 7250 kg N ha(-1), respectively, when compared with cropland. Notably, RSN in deep soils was strongly enhanced by the stand age of apple orchard, which was primarily associated with accumulated N fertilizer inputs. Similar SNCs in the 540-600 cm layer among land use types implied a 6-m-deep soil profile might be robust for evaluating NO3- dynamics following cropland-to-orchard conversion in the study region. Regression relationships between RSN and SWS were significantly positive in the 100-300 cm soil layer under cropland but significantly negative in the 0-100 cm and 100-300 cm layers under 8-yr apple orchards, indicating distinct hydrological processes of NO3- under differing land uses. In conclusion, converting cropland to apple orchards could result in soil moisture decline and nitrate accumulation in the 6-m profile that may threaten the Loess Plateau's sustainability. Therefore, integrated water and N management should be practiced to mitigate the unfavorable impacts and develop sustainable apple production in China and other semiarid areas of the world.