Nutrient response to soil and litter metabolic activity in a transect across a seasonal wetland

Abstract

Many wetlands in the interdunal depression of coastal sand dunes of the Swan Coastal Plain are surrounded by partially vegetated catchments. Although litter can be an important source of nutrients, little is known about litter related microbial activity in these catchments, especially its role in regulating nutrient concentrations in soils during the wet season. Heterotrophic microbial activity and phosphorus (P) dynamics were monitored before and after the onset of the wet season along transects from a vegetated catchment to the lakebed in Thomsons Lake, south-western Australia. Microbial activity was measured in the field as CO₂ efflux (range from 47 to 176 mg CO₂ m^–2 h^–1) and in the laboratory as substrate-induced-respiration (SIR; range from 11 to 133 μg g^–1 h^–1). Substrate-induced-respiration was positively correlated with soil organic content, and was concentrated in surface soils. In contrast, in the exposed lakebed most microbial biomass was below the surface, in the 10–30 cm depth zone. There were significant changes in nutrient dynamics in response to soil microbial activity. Before rain, P extracted by anion exchange membrane (P_AEM) was well correlated with site litter and plant debris (r² = 0.90, P < 0.001), suggesting that P_AEM in soils was litter-sourced. This relationship was modified during the wet season: there was an overall increase in microbial biomass P (P_MB; from average 7.5 μg g^–1 to 21.6 μg g^–1), and a decrease in P_AEM : P_MB in surface soils. Along the study transect, the assimilation index P_AEM : P_MB declined towards the wetland, where soils were more silty and organic, and CO₂ production was significantly higher. Our data suggest that heterotrophic microbial activity has a significant role in regulating P flux from catchment litter during the wet season, which would affect the mobility of litter-sourced P from catchment to the wetland.