Seasonal dynamics of soil microbial biomass in coastal sand dune forest

Sand dunes are a typical landscape in the coast of western Taiwan, where Casuarina forests were established decades ago to stabilize sand dunes and protect the inland vegetation. Study of microbial biomass in such an ecosystem may give insights into the role of microbes in soil fertility and nutrient cycling. We established our study sites in two topographic units based on elevation and drainage types: upland and lowland. The study lasted for 2 years, and soil samples were collected every 3 months. Microbial biomass C (Cmic) and N (Nmic) were high in a shallow humic layer that rested on top of the soil (1222-1319 mg kg−1 for Cmic and 245-276 mg kg−1 for Nmic) and declined sharply to only one-tenth of the above values in the underlying surface soil (0-10 cm depth). Microbial biomass Cmic and Nmic in humic and surface soil were not significantly different between upland and lowland sites. In the upland soils, the mean Cmic was highest in autumn for both the humic and surface soil, and lowest in spring and summer for the humic layer and summer for the surface soil layer. In the lowland soils, the Cmic was highest in winter for both humic and surface soil, and lowest in spring and autumn for the humic layer and spring and summer for surface soil. Strong fluctuations of Cmic and Nmic were associated with the soil moisture prior to sampling, which appeared to control the size of microbial biomass in this environment. Temperature had little effect on the dynamics of soil microbial biomass in the sand dune forest ecosystem.