Effects of Different Land Use Patterns on Soil Physicochemical Properties in the Loess Plateau Area

[Objective] In order to explore the effects of different land use patterns on soil physicochemical properties in the Loess Plateau area. [Methods] This study used bare land as a control (CK), and set four different use patterns as conventional fertilization farmland (H), unfertilization farmland (L), and alfalfa grassland (A) in Changwu, Shaanxi Province since 2005. The physical and chemical properties of 0—200 cm soil in the four plots were determined in July 2021. [Results] After 17 years, there were significant differences between farmland and grassland on soil physicochemical properties. (1) In 0—40 cm soil surface, the volume fraction of clay and powder in alfalfa field slightly increased compared with the other three plots, the volume fraction of sand slightly decreased, while the proportion of clay and fine powder in bare field decreased. (2) In the 0—20 cm of the soil, compared to bare land, soil pH significantly increased by 0.8% in alfalfa grassland, and significantly decreased by 1.7% in fertilized farmland. Except for 80—100 cm, soil pH in 0—200 cm section of fertilized farmland was significantly lower than that in the other three plots. Compared with bare land, in the 0—20 cm of the soil, the organic carbon storage, organic matter and nitrogen content of alfalfa grassland and fertilized farmland increased significantly and the contents of them from alfalfa grassland was higher than those of fertilized farmland. The content of available phosphorus and available potassium of alfalfa grassland and unfertilized farmland decreased significantly. The content of available potassium of fertilized farmland also decreased significantly. The content of alkaline dissolved nitrogen and nitrate-ammonium nitrogen of unfertilized farmland decreased significantly. (3) Under different land use patterns, soil organic matter, organic carbon and total nitrogen showed a decreasing trend from 0 to 60 cm, then an increasing trend showed until it reached 100 cm depth and after where a decreasing trend showed again with the deepening of the soil layer. The available phosphorus content of the soil in grassland gradually increased after 60 cm. The vertical distribution of other nutrient indexes in 0—200 cm space showed a downward trend. (4) There was a significant positive correlation between soil organic matter, total nitrogen, alkaline dissolved nitrogen and nitrate-ammonium nitrogen at 0—100 cm, and a significant negative correlation between available phosphorus and organic matter, total nitrogen, alkaline dissolved nitrogen and pH. There was a significant positive correlation between soil organic matter and available potassium at 100—200 cm, and a significant negative correlation between available phosphorus and pH. [Conclusion] Reasonable farmland management and continuous planting of alfalfa in the Loess Plateau area can keep the nutrient indexes of organic matter, total nitrogen and available nitrogen at a high level, which can guarantee soil quality to a certain extent. In order to promote the sustainability of soil in this region, it is recommended to increase the input of potassium fertilizer and organic fertilizer in farmland, while the grassland needs to supplement the appropriate amount of phosphate and potassium fertilizer.