关键词: 黄土高原, 生态化学计量学, 土地利用方式, 土壤, 植被带

Abstract: The Loess Plateau in China is one of the most eroded areas in the world. Accordingly, the “Grain-for-Green Program” was implemented on a large scale by the central government from 1999; vegetation restoration has been implemented in this area to remedy the soil degradation problem. Vegetation type plays an essential role in ecosystem recovery and affects soil quality, especially soil carbon (C), nitrogen (N) and phosphorus (P) contents. Nutrient stoichiometry has been successfully used to indicate community succession and vegetation restoration in recent years. Studies on the effects of different vegetation types on soil C, N and P stoichiometry are helpful to understand the relationships between vegetation restoration and soil quality, and also beneficial to understand the processes and functions of the ecological system. The Yanhe River is a tributary of the Yellow River and the Yanhe watershed has fragile ecological environment, limited vegetation and serious soil erosion. We chose the Yanhe watershed as the subject of this study because there are different vegetation zones in the watershed, including trees, shrubs, grasses and different combinations of the three types of vegetation. The effects of vegetation (Forest, Forest-Grass, and Grass zones) on soil C, N, and P stoichiometry in Yanhe watershed were studied in this paper. Soil and vegetation were surveyed in 115 sample sites from 9 watersheds, with 24 sample sites in the Forest zones, 58 sample sites in the Forest-Grass zones and 33 sample sites in the Grass zones. We collected soils at the 0-10 and 10-20 cm soil layers, and analyzed total organic C, total N and total P of the soil. The results showed that type of vegetation had significant effects on soil properties. And soil total nutrients changed with the depth. Soil organic C and total N were higher in the 0-10 cm soil depth than in the 10-20 cm depth in Forest, Forest-Grass, and Grass zones. Soil organic C and total N decreased in the following order: Forest zones > Forest-Grass zones>Grass zones. There were no significant differences in soil organic C and total N content in Forest-Grass and Grass zones. Soil total P did not change significantly among different vegetation types or in the soil profile, ranging from 0.58 to 0.69 g/kg in the 0-10 cm soil layer and from 0.57 to 0.65 g/kg in the 10-20 cm soil layer. Soil C∶N ratios in the 0-10 and 10-20 cm soil layers remained stable, ranging from 9.57 to 12.68 and 9.40 to 11.42, respectively. Soil C∶P ratios varied substantially in both soil layers, with coefficients of variation of 90.95% and 79.41% for the 0-10 cm and 10-20 cm soil layers, respectively. Soil N∶P ratios varied similarly to the soil C∶P ratios. The soil C∶N ratio, which was positively related to soil organic C and total N, was stable in the three vegetation zones. Vegetation type had significant impacts on soil C∶P and N∶P ratios. Soil C∶P and N∶P ratios in the Forest zones were significantly higher than those in the Forest-Grass and Grass zones. Soil C∶N ratio was significantly correlated with the C∶P and N∶P ratios (P>0.01). Overall, vegetation restoration improved soil properties, and the Forest zones were better than the Grass zones for improving soil properties in the Loess Plateau. Vegetation restoration was a suitable and effective method to solve soil degradation problems in the Loess Plateau.

Key words: land use, Loess Plateau, soil ecological stoichiometry, vegetation zones