Spatiotemporal variation characteristics of nitrogen content and nitrogen conversion rates in estuary wetlands in eastern China

To systematically elucidate the spatio-temporal variations of nitrogen contents in both soil and water body, as well as soil nitrogen transformation rates in estuarine wetlands of eastern China, this study compiled and analyzed relevant data collected from 18 major estuarine wetlands spanning latitudes from 19°07′12″N to 41°08′46″N. The investigation focused on the concentrations of different forms of nitrogen, including total nitrogen, ammonium nitrogen, and nitrate nitrogen in both soil and aqueous environments, alongside key nitrogen transformation processes such as denitrification, nitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA). The primary objective was to elucidate the spatial patterns and seasonal dynamics of these variables. The results showed that the contents of different forms of nitrogen in soils were significantly higher than those in water bodies, and showed a spatial distribution pattern of higher in the south and lower in the north (p<0.05), and the soil nitrogen content in winter was significantly higher than that in other seasons. There was no obvious spatial pattern of different forms of nitrogen in water bodies, but it had higher seasonal variation characteristics in spring and autumn (p<0.05). The soil nitrogen conversion rates did not exhibit significant spatial distribution pattern (p>0.05). Specifically, soil transformation rates were significantly higher during spring compared to other seasons. The random forest model showed that several soil properties, including total organic carbon, ammonium nitrogen, nitrate nitrogen and salinity in estuarine wetlands significantly affected the spatiotemporal distribution of nitrogen conversion rates, suggesting that carbon and nitrogen content and salinity were the main driving factors of nitrogen conversion rates in estuarine wetlands in China. However, compared with other ecosystems, the nitrogen contents of different forms in soils of estuarine wetlands were low, and the nitrogen content of different forms in water bodies were lower than those in river ecosystems. Nevertheless, the nitrogen transformation rates in estuarine wetlands exceeded those measured in farmland, forest, and grassland ecosystems in different climate zones in China. Moreover, the rates were significantly higher than the global average level of wetlands. Collectively, the above results underscore the high potential of estuarine wetlands in nitrogen removal and provide an important scientific basis for evaluating the nitrogen reduction capacity of estuary wetlands in China.