Soil microbial responses to anthropogenic impacts in Southeast Asian tropical forests

Background/Question/Methods

Human land use alters soil microbial composition and function in a variety of systems, although few comparable studies have been done in tropical forests and tropical agricultural production areas. Logging and the expansion of oil palm agriculture are two of the most significant drivers of tropical deforestation and the latter is most prevalent in Southeast Asia. We tested the hypothesis that major anthropogenic disturbance causes shifts in soil microbial communities and that these changes are associated with altered soil nutrient pools and fluxes. We compared bacterial and fungal communities from three sites in Malaysia that represent the dominant ecosystem types in the Southeast Asia tropics: an old-growth forest, a regenerating forest that had been selectively logged 50 years prior, and a 25-year old oil palm plantation. Soil cores were collected from three replicate plots within each land-use type and fungal and bacterial communities were sequenced using the Illumina platform. Soil pH and a suite of micro and macro nutrients were analyzed for each sample and microbial biomass was measured with phospholipid fatty acid analysis. Microbial function was assessed with extracellular enzyme assays and quantitative PCR.

Results/Conclusions

Both fungal and bacterial communities were significantly clustered across all land-use types (P < 0.001), although differences between forest and oil palm soils were much stronger than for comparisons across forests. Individual functional groups of bacteria and fungi showed varying responses to forest conversion to oil palm versus historical logging that were often correlated with specific changes in soil physicochemical properties. Ectomycorrhizal fungi were virtually absent from oil palm soils and were compositionally distinct across forest sites; these findings have implications for regeneration potentials of the dominant Dipterocarpaceae trees that rely upon these fungi. Microbial biomass, extracellular enzyme activity, and quantitative PCR indicated that oil palm plantations experience more significant alterations in C, N, and P dynamics than do historically logged forests. Together, these results show that human land use in tropical forests alters microbial community and function, although selective logging appears to be less detrimental than conversion to oil palm agriculture in maintaining forest-associated microbial taxa and soil function. Since oil palm agriculture is currently the mostly rapidly expanding equatorial crop and logging is pervasive across tropical ecosystems, these findings may have broad applicability.