Understanding the characteristics and driving factors of soil carbon, nitrogen, phosphorus, and enzyme stoichiometry during land use/cover change is of great significance for assessing microbial nutrient restriction and sustainable land development during the process. China, the world’s largest tea producer, is witnessing a significant expansion of tea plantations into previously forested areas. We performed field sampling in three forest types with the area partially converted to tea plantations in Wuyishan National Park. We examined the changes in soil carbon (TC), nitrogen (TN), phosphorus (TP), and three kinds of extracellular enzyme activities, β-glucosidase (BG), β-n-acetylglucosidase (NAG), and acid phosphatase (ACP). By analyzing the enzyme stoichiometric ratio, vector length (VL), and vector angle (VA), the relative nutrient limitations of soil microorganisms were explored. The results showed that soil TC and TN decreased significantly (p < 0.05), TP increased significantly, and soil carbon (C):nitrogen (N), carbon (C):phosphorus (P), and nitrogen (N):phosphorus (P) ratios decreased significantly after the conversion of forest land to tea plantation. Soil BG, NAG, and ACP contents decreased significantly (p < 0.05). There were no significant differences in enzyme carbon:nitrogen ratios (E_C/N), enzyme carbon:phosphorus ratios (E_C/P), enzyme nitrogen:phosphorus ratios (E_N/P), VL, or VA (p > 0.05). Through the analysis of soil enzyme stoichiometry, it was found that forest soil was generally limited by P, which was, to some extent, relieved after the conversion to tea plantation. Redundancy analysis showed that TC, TN, and the C:N ratio were the main factors influencing enzyme activity and stoichiometry. These results indicated that land use/cover change had significant effects on soil nutrient status, enzyme activity, and stoichiometry. Soil enzyme activity is very sensitive to the changes in soil nutrients and can reflect the restriction of soil nutrients more accurately.

Kandelia candel is the most widely distributed tree species on the southeast coast of China and is also the main afforestation tree species along the coastal wetland. In recent years, inorganic nitrogen pollution has become increasingly severe, and investigating the effects of nitrogen input on methane emissions in Kandelia candel–soil systems has become significant from a global change perspective. However, the effect of nitrogen input on methane emissions in coastal wetland systems is still uncertain. The field tidal environment is complex and varied, and thus it is difficult to accurately control the amount of nitrogen in the system. Therefore, in order to accurately assess the effects of different concentrations of foreign nitrogen input on methane emission fluxes in a Kandelia candel–soil system, we use indoor tidal simulation experimental devices and design two simulation systems with and without plant planting to explore the difference of methane emission flux in this system under five nitrogen input concentrations: N0 (0 g N·m⁻²·a⁻¹), N1 (5 g N·m⁻²·a⁻¹), N2 (10 g N·m⁻²·a⁻¹), N3 (20 g N·m⁻²·a⁻¹), and N4 (30 g N·m⁻²·a⁻¹). The results showed that: (1) The introduction of Kandelia candel promoted methane emissions in coastal wetland ecosystem. Under each nitrogen application concentration, the mean CH₄ emission flux in the planting group was 42.98%, 65.59%, 40.87%, 58.93% and 39.23% higher than that in the non-planting group, respectively. (2) Nitrogen input significantly promoted methane emissions in both planted and non-planted environments, and the promoting effect showed as follows: N4 > N3 > N2 > N1 > N0. (3) After the introduction of Kandelia candel, the contribution of Kandelia candel and soil microorganisms to methane emissions was different under different concentrations of nitrogen addition. The contribution rate of Kandelia candel to CH₄ emission flux of Kandelia candel–soil system ranged from 10.74% to 60.25%, with an average contribution rate of 37.30%. The changed soil microbes contributed 39.75% to 89.26% to the CH₄ emission flux in the Kandelia candel–soil system, with an average contribution rate of 62.60%. Under N3 nitrogen application concentration, the emission flux of plant was the largest, which was significantly higher than that of the soil microbial pathway; at other concentrations, the methane emissions from the soil microbial pathway were greater than that of the plant pathway, and the contribution rate to the plant–soil system reached 60.25%. The results of this study provide an important basis for improving the estimation accuracy of carbon emissions in coastal waters and formulating policies for the restoration and protection of coastal wetlands.