Testing the significance of microbial ecology for soil carbon stabilization with tropical land cover change

Background/Question/Methods

Soil microorganisms control multiple input and loss pathways of carbon in soils; hence, understanding how microbial communities respond to disturbance and ecosystem recovery is important for predicting effects on soil carbon pools. We know that physical location in the soil matrix is an important control on carbon bioavailability, and that different microbial groups preferentially utilize different sources of soil carbon. However, we don’t know how microbial composition mediates soil organic matter (SOM) distribution and stabilization. This study aims to identify direct links between microbial community composition and the distribution of SOM to answer the questions: (1) Are different microbial groups associated with different SOM pools? and (2) How do these relationships differ with changes in vegetation during tropical forest recovery?We measured microbial composition via phospholipid fatty acid (PLFA) analysis, along with carbon and nitrogen concentrations on physically separated aggregate fractions of soils from pastures, secondary forests naturally regenerated on abandoned pastures (40 & 90 years old), and primary forests in Puerto Rico. While aggregate fractionation has been broadly used to infer SOM dynamics, few studies have examined microbial properties within particle size fractions. This study adds valuable information on the relationship between SOM dynamics and microbial community composition.

Results/Conclusions

Microbial community structure varies by aggregate fraction and land cover type with the greatest differences among versus within fractions. Shifts in the abundance of gram-positive bacteria (15:0iso), saprotrophic fungi (18:1w9c) and actinobacteria (16:0 10Me) account for the majority of variation of microbial composition among fractions. While total biomass (per g soil) and bacterial biomass does not differ between fractions, fungal abundance is greater in macroaggregates and is less abundant in the silt and clay fraction. At the same time, biomass (per mass of the aggregate fraction) is also greatest in the macroaggregate fraction. Across all land cover types, %C_org and %N remains constant among and within aggregate fractions. Yet, the majority of SOM-C and N is located in the intra-aggregate silt and clay fraction. This indicates that mineral interactions are the dominant stabilization mechanism for SOM in both our pastures and forests. However, shifts in the microbial composition and biomass may alter carbon mineralization activities and thus, contribute to stabilization. This study suggests that microbial community composition may play an important role in the distribution and stabilization of SOM.