Effects of long-term cultivation on spatial-temporal variation of soil nitrogen and phosphorus: a case study in Shaanxi Province, China

Investigating the spatial-temporal variation of soil nitrogen (N) and phosphorus (P) is essential to determine the balance between increased food production and environmental protection. In this study, a total of 705 soil samples were collected at depths of 0-20 cm in 2017 and analyzed for laboratory tests of soil N and P. The results showed that from the 1980s to 2017, the total nitrogen (TN), available nitrogen (AN), and available phosphorus (AP) contents of farmland soils in Shaanxi Province increased by 33%, 17%, and 199%, respectively, while the total phosphorus (TP) content decreased by 40%. The best-fit model for spatial interpolation of soil TP and AP in Shaanxi Province was the exponential model (R-2 = 0.92 and 0.95); the Gaussian model was the best-fit model for spatial interpolation of soil TN and AN (R-2 = 0.98 and 0.96). The spatial distribution characteristics of soil TN, AN, TP, and AP were consistent, all being higher in southern Shaanxi than in northern Shaanxi. The value of N:P* ratio (molar ratio) of cultivated soils in Shaanxi Province is 2.9, which is lower than the Chinese average (N:P* = 5.0). Based on the spatial-temporal variations of soil N and P contents between regions, it is recommended that fertilization should be strictly controlled in central and southern Shaanxi and optimized in northern Shaanxi to improve ground strength.