Resource stoichiometry, vegetation type and enzymatic activity control wetlands soil organic carbon in the Herbert River catchment, North-east Queensland

Wetlands are highly productive ecosystem with great potential to store carbon (C) and retain nitrogen (N) and phosphorus (P) in their soil. Changes in vegetation type and land use can affect organic matter inputs and soil properties. This work aimed to examine how these changes affected elemental stoichiometry and C-, N-, and P- associated enzyme activities and wetland soil organic C stock. We quantified organic C concentrations, and stoichiometric ratios of C, N, and P in total and microbial biomass pools, along with the activities and ratios of C-, N-, and P-associated enzymes for soils of natural coastal wetlands with different vegetation types, namely Melaleuca wetland (Melaleuca spp), mangrove forests (Bruguiera spp), and saline marsh (Eleocharis spp). We also compared these natural wetlands to an adjacent sugarcane plantation to understand the effects of vegetation types. Hypothesis-oriented path analysis was used to explore links between these variables and soil organic C stocks. Tidal forested soils (0–30 cm) had the highest organic C, N, and P contents and potential activities of C-, N-, P- acquiring enzymes, compared with other vegetation types. Mangroves soils had the highest total soil C:N and microbial biomass C:P ratios. Microbial biomass C:P ratios were significantly and positively related to total C:P, while microbial biomass N:P ratios were positively associated with total soil C:P and N:P ratios. Path analysis suggested that soil organic C stock was largely explained by total C:P ratio, microbial biomass N:P ratios, total P content, and the ratio of C- and P-associated enzymes. Different types of wetlands have different soil properties and enzymatic activities, implying their different capacity to store and process C and N. The resource quality and stoichiometry direct influence the organic C stock.