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

TN contents were relatively high in Phragmites australis wetland, Typha wetland, paddy field and maize field, which were significantly higher than those of other soil types. Soil TN content was significantly negatively correlated with pH, but significantly positively correlated with Eh. Paddy field had the highest TP content, followed by Phragmites australis wetland, and Ulmus woodland had the lowest TP content.

Different land‑use types also significantly affected the contents of dissolved organic carbon (DOC), nitrate‑nitrogen, ammonium‑nitrogen and sulfate. DOC contents in Phragmites australis wetland, Typha wetland and paddy field were significantly higher than those of other land‑use types. Nitrate‑nitrogen content in maize field was significantly higher than in other sites, whereas ammonium‑nitrogen content was the highest in paddy field (P < 0.05), which was mainly related to soil nitrification and denitrification.

Affected by tidal action, Suaeda wetland possessed the highest sulfate content, with no significant differences among other sites (P < 0.05). Soil C:N ratios were greater than 20 in Suaeda wetland, oil‑field reed wetland and paddy field, whereas values below 20 were observed in other sites. This indicates that nitrification in the former three soils is controlled by organic carbon availability, while nitrification in other sites is controlled by ammonium‑nitrogen availability.

Except for the oil‑field reed wetland, soil C:P ratios of all other sites were lower than 200, suggesting high soil phosphorus availability favorable for plant growth. The average N:P ratio in the study area was 3.5, much lower than the national average value (8.0), implying that nitrogen is the limiting nutrient element for soils in this region. The C:S ratio of oil‑field reed wetland exceeded 400, indicating net immobilization of mineral sulfur in this area.

The C:S ratio of paddy‑field soil ranged from 200 to 400, meaning that soil sulfur was neither used for organic‑sulfur synthesis nor released from organic sulfur. For other sites, soil C:S ratios were below 200, demonstrating that these areas are generally at the net‑release stage of soil organic‑sulfur mineralization, and sulfur is 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 such changes contribute to stable ecological‑environment development requires further in‑depth research.