Effects of land-use on soil C, N, and P stocks and stoichiometry in coastal wetlands dependent on soil depth and latitude

Coastal land-use is increasingly affected by Spartina alterniflora invasion, coastal reclamation, and climate change, which have been suggested to exert vital controls on soil carbon (C), nitrogen (N), and phosphorus (P) cycles, however, the effects on the geographical distributions of these element stocks and stoichiometry remain unclear. We analyzed the depth (0–100 cm) and latitudinal (19–41°N) patterns of soil C, N, and P contents, stocks, and ratios in 120 Chinese coastal wetlands differing in land-use (reclaimed wetlands, natural wetlands, invasive wetlands, and bare mudflats) and climate (tropics, subtropics, and temperate). We observed stable depth patterns of soil contents, stocks, and ratios, owing to the stable depth patterns of soil conditions in each land-use. Top soils (0–30 cm) generally had higher C, N, and P contents and N:P ratio but lower C:N and C:P ratios than deep soils (30–100 cm). We found that soil C content and P stock remained constant, soil C stock, N and P contents and N:P ratio increased significantly, but soil N stock and C:N and C:P ratios decreased significantly with decreasing latitude; however, such correlations tended to be weaker in reclaimed wetlands and invasive wetlands. Land-use changes also significantly altered the magnitude of soil element stoichiometry. The effects of S. alterniflora invasion on soil C, N, and P storages can be shifted from negative in tropics to positive in temperate regions, whereas the negative effects of coastal reclamation in tropics disappeared in temperate regions. Overall, compared with natural wetlands, S. alterniflora invasion slightly decreased, whereas coastal reclamation largely decreased soil C, N, and P contents and stocks. Compared with bare mudflats, S. alterniflora invasion significantly increased C and N contents and N:P ratio but decreased C:N ratio, whereas coastal reclamation significantly decreased P content and stock and C:N ratio but increased N:P ratio, especially in top soils. Our findings will be helpful for understanding and assessing soil C sequestration and nutrient cycling in globally disturbed coastal wetlands.