Soil Carbon, Nitrogen, Phosphorus and Sulfur Contents and Their Ecological Stoichiometric Characteristics under Different Land Use Types

Reed wetland, cattail wetland, paddy field and maize field had relatively high TN contents, which were significantly higher than other soil types. Soil TN content was significantly negatively correlated with pH, and significantly positively correlated with Eh. Paddy field had the highest TP content, followed by reed wetland, and elm woodland had the lowest TP content.

Different land use types also imposed significant impacts on soil dissolved organic carbon (DOC), nitrate nitrogen, ammonium nitrogen and sulfate contents. The DOC contents in reed wetland, cattail wetland and paddy field were significantly higher than those in other land use types; the nitrate nitrogen content of maize field was significantly higher than that of other sites, while the ammonium nitrogen content of paddy field was significantly higher than that of other sites (P < 0.05), which was mainly related to soil nitrification and denitrification.

Affected by tidal action, Suaeda salsa wetland had the highest sulfate content, and no significant differences were observed among other sites (P < 0.05). The soil C:N ratios of Suaeda salsa wetland, oilfield reed wetland and paddy field were greater than 20, whereas those of other sites were less than 20, indicating that nitrification in the first three soils was controlled by organic carbon availability, while nitrification in other sites was controlled by ammonium nitrogen availability.

Except for the oilfield reed wetland, the soil C:P ratios of all other sites were less than 200, suggesting high soil phosphorus activity, which is beneficial to plant growth. The average N:P ratio in the study area was 3.5, much lower than the national average N:P ratio (8.0). Therefore, N was the limiting nutrient element in the soils of the study area.

The C:S ratio of oilfield reed wetland was greater than 400, indicating net immobilization of mineral sulfur in this area. The soil C:S ratio of paddy field ranged from 200 to 400, implying that soil S was neither used for organic sulfur synthesis nor released from organic sulfur. The soil C:S ratios of other sites were all less than 200, showing that these areas were basically in the net release stage during soil organic sulfur mineralization, and S was not a soil nutrient-limiting factor.

Overall, changes in land use types can alter soil C, N, P, S contents and their ecological stoichiometric characteristics. Whether these changes are conducive to the stable development of the ecological environment requires further in-depth research.