Spatial and temporal distribution patterns of nitrogen in marsh soils from an inland alkaline wetland – A case study of Fulaowenpao wetland, China

Samples of surface (0–10 cm) and subsurface soils (10–20 cm) were collected using a grid sampling method in July and September in order to study the spatial and temporal distribution patterns of all forms of nitrogen and total nitrogen (TN) and the relationships between nitrogen concentrations and selected soil properties in Fulaowenpao wetland, a typical inland alkaline wetland. Results showed that there existed obvious heterogeneity at spatial and temporal scales. Generally, higher spatial variability for nitrate nitrogen (NO₃^-–N), ammonium nitrogen (NH₄⁺–N) and available nitrogen (AN) were observed compared to organic nitrogen (Org-N) and TN. At the spatial scale, concentrations of NO₃^-–N, NH₄⁺–N and AN in surface soils were higher than those in subsurface soils, but no significant differences were observed between both soil layers (p < 0.05). However, concentrations of Org-N and TN were significantly higher in surface soils compared to subsurface soils (p < 0.05), and both of them had similar spatial distribution patterns. At the temporal scale, with the exception of NH₄⁺–N in both soil layers and NO₃^-–N in subsurface soils, concentrations of all the other forms of nitrogen and TN were generally higher in September than them in July, while there were no significant differences between both sampling periods (p < 0.05) except for AN (p < 0.01) in both soil layers. Correlation analysis showed that AN, Org-N and TN were significantly and positively correlated with soil organic matter, total phosphorous, and clay contents, while they were significantly negatively correlated with soil pH values; NO₃^-–N was also correlated with soil organic matter and total phosphorous, however, NH₄⁺–N was only closely lined to water contents.