Abstract:

In order to study the soil denitrification rates under different vegetations in urban riparian zone of Shanghai, three sampling sites were chosen for denitrification measurement in Changfeng No. 2 greenbelt of Suzhou River riparian. The three sampling sites were distributed in the upstream part, middle part and downstream part along a transect of the riparian, which were respectively covered with three common green plants, namely, Fatshedera lizei, Cynodon dactylon and Rosa bracteata. Three parallel soil samples were taken from each soil layer (2-5 cm, 12-15cm, 22-25 cm and 32-35 cm) at each sampling site in March (early spring), May (spring), July (summer) and November (autumn) of 2012. In the laboratory, the denitrification rates of soil samples were determined using the C2H2 inhibition technique. Meanwhile, the basic physicochemical properties of soil including particle size, soil moisture content, soil organic matter (SOC) and concentrations of ammonia (NH4+) and nitrate (NO3-) nitrogen were also analyzed. The results showed that soil physicochemical properties and the denitrification rates of soils under the three representative green plants were different to some extent. Higher moisture content, SOC and ammonia nitrogen concentrations were found in soil under Rosa bracteata, while higher nitrate concentrations were found under Cynodon dactylon. In general, concentrations of SOC, NH4+ and NO3- were higher in early spring or spring and decreased with the increase of depth except in summer. The range of soil denitrification rates were 002-425 ngN/(g·h) under Fatshedera lizei, 009-761 ngN/(g·h) under Rosa bracteata and 009-302 ngN/(g·h) under Cynodon dactylon. The mean denitrification rate of soil was the highest under Rosa bracteata (093 ngN/(g·h)), next under Fatshedera lizei (076 ngN/(g·h)), and smallest under Cynodon dactylon (054 ngN/(g·h)). The denitrification rates under different vegetations were all significantly higher at the 2-5 cm layer of soil than that at the other depths, and no significant difference was found between the denitrification rates at the soil layers of 12-15, 22-25 and 32-35 cm. As for the seasonal change, the denitrification rates in the 2-5 cm soil layer under Rosa bracteata and Fatshedera lizei were significantly higher in early spring than that in the other seasons, while soil at depth of 2-5 cm under Cynodon dactylon showed higher denitrification rates in spring and summer than early spring and autumn. There were no significant seasonal differences between the denitrification rates at the other three soil layers. The results of threeway analysis of variance (ANOVA) suggested that the denitrification rates in soil of urban riparian were significantly affected by single and combined factors of vegetation type, soil depth and season except the combined factor of vegetation type and depth. Results of Pearson correlation analysis showed that denitrification rates were extremely correlated (p<001) with SOC (r=049), soil NH4+ concentrations (r=069) and NO3- concentrations (r=0508) suggesting that the vegetation might affect denitrification rates by controlling soil physicochemical properties, but there were no significant correlations between denitrification rates and soil moisture content or temperature