Spatial and seasonal changes in soil nitrogen contents and stocks along a water and salinity gradient in the Yellow River Delta , China

Soil samples were collected to the depth of 50 cm in three sampling belts in the Yellow River Delta, including Calamagrostis pseudophragmites wetlands (S1), Typha orientalis wetlands (S2), Phragmites australis wetlands (S3), Tamarix chinensis and Suaeda salsa wetlands (S4) and Suaeda salsa wetlands (S5) along a water and salt gradient in each sampling belt in four sampling seasons in 2014 and 2015. Total nitrogen contents and other selected soil properties were analyzed to examine the effects of water and salinity changes on soil total nitrogen contents and stocks along the water and salt gradient. Total nitrogen in the top 10 cm soils showed a ‘decreasingthenincreasing’ tendency along the water and salinity gradient, with the highest level at S2 and the lowest at S4. For the top 50 cm soils, no significant differences in average total nitrogen were observed in five sampling sites (P>005). Soil nitrogen contents and stocks generally decreased in both lowand highsalinity wetlands along the soil profile. However, soil nitrogen in mediumsalinity wetland increased after an initial decrease. Soil total nitrogen in wetlands with lowand highsalinity mainly accumulated in the top 20 cm soil, accounting for about 50％ of the nitrogen stocks in the top 50 cm soil. However, soil nitrogen stocks of >40～50 cm soil in mediumsalinity wetlands ranked only second to the 0～10 cm soil and were higher than other three soil increments. Correlation analysis showed that, soil nitrogen contents and stocks were positively correlated with soil moisture, clay, silt and soil organic carbon (P<005), but a negative correlation was observed between soil nitrogen and Na+, Mg2+, Cl-,Cl-／SO2-4 ratio, SAR, pH, bulk density, sand and C／N ratio (P<005). 〖WT5HZ〗Keywords〓〖WT〗the Yellow River Delta; coastal wetlands; water and salt gradient; soil total nitrogen; spatial and seasonal changes