Abstract:

Soil acidity or alkalinity is one of the most important physico-chemical features of the soil， and affects the ecological stoichiometry of soil carbon， nitrogen， and phosphorus through its impact on physical， chemical and biological processes of an ecosystem. Accordingly， the soil pH in grassland ecosystems has changed as a result of climate change and human activities for decades or even centuries. Existing studies have generally focused on the changes in soil nutrients in response to grazing， fertilization， and altered temperature or precipitation. It remains unclear how ecological stoichiometry of soil carbon， nitrogen and phosphorus in various categories of steppe responds to changing soil pH. In this study， three types of steppe （i.e. desert， typical and meadow steppe） with a range of pH values were selected and their soil acidity or alkalinity was altered through manipulative experiments by adding sulphuric acid or sodium hydroxide. We found that： 1） The soil pH values of the 0-10 cm layer in both desert and typical steppes were significantly higher than in meadow steppe. However， the soil pH values of the 10-30 cm layer in all three types of steppe were not significantly different. The values of soil organic carbon， total nitrogen and phosphorus， as well as the ratios of soil carbon to nitrogen， nitrogen to phosphorus and carbon to phosphorus in desert steppe were lower than in typical and meadow steppes. The soil total nitrogen in typical steppe was significantly higher than that in meadow steppe for the 10-30 cm layer. The ratio of carbon to nitrogen in the 0-10 cm layer in meadow steppe was significantly higher than in typical steppe. 2） For the 0-10 cm layer， the addition of acid significantly increased the soil organic carbon， the ratios of carbon to nitrogen and carbon to phosphorus in desert steppe. Soil organic carbon and the ratio of nitrogen to phosphorus in desert steppe， and soil total nitrogen and phosphorus in typical steppe were also significantly reduced by the addition of alkali. In contrast， the addition of alkali enhanced the ratio of soil carbon to phosphorus in desert steppe. For the 10-30 cm layer， the addition of alkali increased the ratio of soil carbon to nitrogen in desert steppe， but it decreased total nitrogen and the ratio of nitrogen to phosphorus in typical steppe. The ratio of carbon to phosphorus also declined in meadow steppe after adding alkali. 3） Soil organic carbon， total nitrogen， and the ratios of carbon to phosphorus and nitrogen to phosphorus at the 0-10 cm soil layer in desert steppe were negatively correlated with soil pH values. The correlations between soil pH and ecological stoichiometry of soil carbon， nitrogen and phosphorus did not differ significantly in typical steppe or in meadow steppe. These results indicate that the ecological stoichiometric characteristics of soil carbon， nitrogen and phosphorus in the three studied steppe types were affected by changing soil pH in the order： desert steppe>typical steppe>meadow steppe. The surface soil was more sensitive to the change in acidity or alkalinity than the subsoil. The similarities and differences of the results between variance and correlation analyses reflected the different effects of soil pH on the three types of steppe in short and long-term adaptation to a changing acidic or alkaline environment. Therefore， in the context of changes in soil pH caused by climate change and anthropogenic activities， grassland ecosystems are likely to adjust their soil pH during the restoration and reconstruction of soil degradation， in order to ensure a nutrient balance and cycling.

Key words: soil pH, ecological stoichiometry of soil carbon， nitrogen and phosphorus, desert steppe, typical steppe, meadow steppe