Abstract: Recently, grassland degradation and ecosystem structural disorders caused by inappropriate use of grassland resources have become more common. Studies on the physical and chemical properties of soil are great significance for controlling grassland degradation, maintaining the balance of grassland ecosystems, and promoting the sustainable development of grassland animal husbandry. Soil nitrogen and phosphorus are important components of soil nutrients in grassland. The purpose of this study was to analyze the spatial distribution of soil total nitrogen and total phosphorus on the Gannan plateau. Using classical statistical and geostatistical methods, we focused on the vertical distribution of soil total nitrogen and total phosphorus, and variations in the nitrogen and phosphorus contents in soil among sampling areas on the basis of ground-measured data. After determining the characteristics of total nitrogen and total phosphorus contents at different elevations and different soil depths, we analyzed the correlations among total nitrogen, total phosphorus, N/P, C/N, soil organic matter, and soil moisture. The results showed that: 1) Total nitrogen content in the surface of grassland gradually decreased from southwest to northeast and from west to east of the study area. Similar to the distribution of total nitrogen content, total phosphorus content changed from southwest to northeast and from west to southwest. Total nitrogen and total phosphorus contents showed medium variability at the 0-40 cm soil depth. The variation in total nitrogen content was higher at 20-40 cm depth than at 0-20 cm depth. The variation in total phosphorus content increased with soil depth. The total nitrogen and total phosphorus contents gradually decreased with soil depth, but the total nitrogen and total phosphorus contents tended to be higher in the surface layer than in lower layers. The phenomenon of surface aggregation was common. 2) The total phosphorus content was significantly (P<0.01) correlated with total nitrogen content in the 0-10 cm, 10-20 cm, and 20-30 cm soil depths. The correlation coefficient progressively decreased with soil depth, and became insignificant in the deeper soil layers (30-40 cm). The N/P ratio was highly significantly correlated with total nitrogen content, but showed a weaker correlation with total phosphorus content. There were significant (P<0.05) positive correlations between soil organic matter content and total phosphorus content and the N/P ratio. The factors affecting soil organic matter content could be ranked, from strongest correlation to weakest, as follows: total nitrogen>N/P ratio>total phosphorus. Soil organic matter and soil moisture content were very significantly positively correlated in all soil layers. 3) The total nitrogen and total phosphorus contents in soil increased with increasing elevation. However, the coefficients of variation of total nitrogen and total phosphorus increased with soil depth at the same altitude.

Key words: grassland, total nitrogen, total phosphorus, organic matter, spatial distribution, spatial variability