Changes in soil organic carbon and total nitrogen in apple orchards in different climate regions on the Loess Plateau

Abstract

The Loess Plateau is the largest high-quality apple-producing area in China. However, the responses of soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks to long-term cultivation in apple orchards under different climate regions remain unclear. We conducted soil sampling in apple orchards distributed throughout the main apple-production area on the Loess Plateau. A total of 28 soil samples (max depth 500 cm) from stands of different ages at 7 soil sampling sites were collected to evaluate the changes in SOC and TN concentrations and stocks; furthermore, C:N ratios across apple orchards in different climate regions were evaluated. Compared with those in the semihumid region, SOC concentration and stock in the 0–500 cm soil profile were 6.6% and 32.1% lower, respectively, in the semiarid region. Furthermore, over the entire soil profile, TN concentration and stock were 12.1% and 38.4% lower, respectively, in the semiarid region than in the semihumid region. However, the C:N ratios in the semiarid region were higher than those in the semihumid region. SOC and TN concentrations generally decreased with increasing soil depth, and those in the 0–120 cm soil layer were significantly higher than those at the other soil depths in both climate regions. The most important factors, which control the variability of SOC and TN, were precipitation, plantation age, topographical properties (altitude and landform of apple orchards) and soil type. These results not only provide information on the effects of changes in SOC and TN concentrations and stocks and C:N ratios but also provide regional-level information that can aid the sustainable development of apple orchards on the Loess Plateau.