Vertical distribution and regulation of Olsen-phosphorus in 6-m soil profiles after farmland-to-apple orchard conversion on the Chinese Loess Plateau

Abstract Data on the dynamics of Olsen-P in deep soil profiles are crucial for improving P use efficiency but remain scarce worldwide. Hence, we sampled sixty 6-m-deep soil profiles from farmland and in 8-yr, 17-yr, and 25-yr-old apple orchards on China’s Loess Plateau, and aimed to investigate the vertical distribution of soil Olsen-P following the farmland-to apple orchard conversion. Land-use change from farmland to apple orchard significantly affected Olsen-P at 0–40 cm soil, with the concentration generally increasing with age of apple orchard. By contrast, Olsen-P at 40–600 cm was not significantly different among the cropping systems. In both farmland and apple orchards, the concentration of Olsen-P decreased at 40–100 cm, compared with that in the 0–40 cm layer, increased with depth from 100 to 400 cm and then stabilized from 400 to 600 cm. The highest accumulation of residual soil Olsen-P in the 6-m profile (RSP0−600) was in the 25-yr orchard, followed by the 17-yr and 8-yr orchards, with the lowest accumulation in farmland. The differences in RSP0−600 cm were primarily associated with RSP0−100 cm, which was significantly positively related to the accumulative application of P fertilizer. The change point in soil Olsen-P against P leaching was 71.2 mg kg−1, but in farmland and 8-yr, 17-yr, and 25-yr apple orchards, the averaged-depth contents of Olsen-P were all below the critical value, indicating no P leaching risk. Regression analysis revealed that the vertical distribution of soil Olsen-P was regulated by P fertilization at 0–100 cm and by crop roots at 100–600 cm, irrespective of cropping system. The insights gained on Olsen-P accumulation and migration within deep soil profiles after the farmland-to-apple orchard conversion can help to optimize land use and P fertilization management in major apple-producing countries with burgeoning growth in apple production.