Response of soil quality to artificial vegetation restoration patterns in hilly area of Southern Ningxia

Abstract: Soil, as a vital natural resource, performs key environmental, economic, and social functions. High quality of soil not only produces better food and fiber, but also helps establish natural ecosystems and enhances air and water quality. Soil quality improvement is important for sustaining a global biosphere. Understanding the variability of soil quality is important in improving an ecological environment. Vegetation restoration played an important role in the ecological environment construction, and it could reduce the soil erosion and improve the soil quality. The hilly area of southern Ningxia, one of the soil erosion areas, was the key region of the conversion of cropland to forest and grassland and ecological construction. According to the complexity of the ecological environment in the hilly area of southern Ningxia, lots of vegetation restoration patterns had developed from the 1990s. Medicago sativa and Caragana microphylla were the main plant species in the vegetation restoration of this area. Some studies had also shown the variability of soil aggregate content and soil microorganisms under different artificial vegetation restoration patterns in the hilly area of southern Ningxia. However, little attention had been paid to the effects of vegetation restoration patterns on soil quality using a mathematical model. The objective of this research is to explore the response of soil quality to artificial vegetation restoration patterns in the hilly area of southern Ningxia. In this paper, soil samples were taken in 0-10 and ＞10-20 cm soil layers under seven artificial patterns. Soil samples were air-dried, divided, and passed through a 2-mm nylon sieve for measuring soil available phosphorus, available potassium, and soil pH value. Part of the air-dried and sieved samples were grounded and passed through either a 1-mm or a 0.25-mm nylon sieve for determining soil total nitrogen, total phosphorus, and organic matter. The soil organic matter and total nitrogen were determined using the potassium dichromate hydrometer method and the semi-micro Kjeldahl method, respectively. Soil total phosphorus was digested by H2SO4-HClO4 and followed by molybdate colorimetric measurement. Soil available phosphorus and available potassium was attained by extracting soil with 0.5 mol/L sodium bicarbonate (NaCO3) and 1 mol/L ammonium acetate (NH4Ac) then measured by molybdate colorimetric measurement and an atomic absorption spectrometer, respectively. A pH meter determined soil pH after extraction from a soil water ratio of 1:1. Soil water content was measured using a drying method. Considering the natural conditions of the hilly area of southern Ningxia, soil total nitrogen, total phosphorus, available phosphorus, available potassium, organic matter, pH, and soil water content in 0-10 and ＞10-20 cm soil layers were selected as the index. Using the grey correlation model and cluster analysis, seven artificial patterns were selected to study the effect of vegetation restoration on the soil quality and determine the optimum pattern. The results showed that different artificial vegetation restoration patterns had different effects on soil quality. Compared to farmland, both the Medicago sativa pattern with 5, 10, 15, 20 years and Caragana microphylla pattern with 30 years could improve soil quality. The best pattern was the Caragana microphylla pattern with 30 years. It indicated that with the development of the Conversion of Cropland to Forest and Grassland Project, planting both Medicago sativa and Caragana microphylla should be the optional choices to restore soil quality, since they could decrease the soil erosion and improve soil condition, especially in the hilly area of Southern Ningxia.