Abstract:

Shrub encroachment is an important ecological process that affects soil material cycling in grasslands. This study investigated the effect of different shrub encroachment gradients on soil enzyme activity， compared to grassland with no shrubs present， under three common shrub species on the eastern margin of the Qinghai-Tibetan Plateau： Spiraea alpina， Caragana microphylla and Potentilla fruticosa. Soil physical and chemical properties and sucrase， urease， polyphenol oxidase， glucosidase， protease and cellulase activities were analysed in 0-10 cm， 10-20 cm， 20-40 cm， 40-60 cm and 60-80 cm soil horizons with different shrub encroachment gradients （no shrubs， lightly and heavily shrub covered）. It was found that the pH and soil organic carbon （SOC） contents were significantly higher and the soil water content （SWC） and total nitrogen （TN） were significantly lower （P<0.05） in the 0-20 cm soil layer of the shrub-covered grassland than when no shrubs were present. Similarly， the activities of protease， glucosidase， cellulase， urease and sucrase were significantly lower in shrub-covered than in grassland with no shrubs （P<0.05）. The activity of polyphenol oxidase was not significantly different between shrub encroachment and no shrubs， and the activity of soil enzymes decreased with deepening of the soil layer at different shrub encroachment gradients. The main explanatory factor for the variation in soil enzyme activity across the different shrub encroachment gradients and soil depths was SWC， which explained 41.2%， 50.5%， 37.1%， 41.5% and 26.0%， respectively， of the variation in activities of the five above-mentioned soil enzymes. Structural equation modeling （SEM） showed that different shrub encroachment gradients mainly affected SOC， pH and SOC∶TN， but not SWC and TN， and that different shrub encroachment gradients could indirectly affect sucrase， urease， glucosidase， protease， polyphenol oxidase and cellulase activities by affecting pH and SOC content. SWC is still the main factor affecting the activities of the six enzymes. SWC directly and positively influenced the activities of the six enzymes， and also negatively influenced soil pH and positively influenced SOC to further influence TN and thus enzyme activities. Therefore， reduction of soil enzyme activity due to the invasion of grassland shrubs in the study area is beneficial for soil organic carbon sequestration.

Key words: Tibetan Plateau alpine grassland, shrub encroachment, shrub encroachment gradients, soil enzyme activity, structural equation model