PS 40-99 - Soil microbial resource-uses and enzyme activities along a gradient of forest degradation in logged-over tropical rain forests, Borneo

Abstract: Soil microbial extracellular enzymes drive soil organic carbon (SOC) decomposition and nutrient dynamics, and their activities are a consequence of microbial metabolisms. Activity of a given soil enzyme is dependent on soil microbial community composition and/or microbial biomass per se., and both or either microbial composition and/or microbial biomass would shift along a gradient of a critical soil organic substrate for microbes. Therefore, the changes of substrates may affect enzyme activities and microbial composition and/or microbial biomass at the same time. We hypothesized that the ratio of enzyme activity to microbial biomass would increase when the amount of a most critical soil organic substrate for microbes declines, and tested this hypothesis along a forest degradation gradient in Bornean tropical rain forests, where soil organic matter (C and N) is known to decline linearly with the magnitude of forest degradation. We compared the ratios of extracellular enzyme activities to soil microbial biomass carbon (SMBC) with soil organic carbon (SOC) along a forest degradation gradient. Our soil samples were taken from the topsoil (0-10cm depth) of 35 plots in timber production forests of Deramakot and Tangkulap Forest Reserve in Malaysian Borneo. The 35 plots varied in above-ground biomass and tree-species composition, which formed a gradient of forest degradation reflecting past logging intensities. The concentration of SOC decreased with increasing magnitude of above-ground forest degradation. The activities of four extracellular enzymes, which were associated with C, N and P degradation, were analyzed. The acid phosphatase (ACP) was determined with pNPP as substrate at pH 6.0, while β-glucosidase (BG), leucine aminopeptidase (LAP) and N-acetyl-glucosaminidase (NAG) were analyzed by a high throughput fluorescent measurement. In addition, SMBC was determined by chloroform fumigation. Our results showed that the ratios of ACP activity/SMBC and LAP activity/SMBC increased with decreasing SOC, but BG and NAG did not show statistically significant patterns. ACP was considered as an important enzyme regulating P cycling, while LAP mainly related to N cycling. This study suggested that microbial communities probably shift to more P-use and N-use efficient taxa during the course of forest degradation where their critical organic-P and N substrate is limited in below-ground ecosystems. Soil P and N might become more strongly limiting soil microbial communities in more degraded forests.