OOS 47-4
Variations in microbial structure and function among soil organic matter aggregates
Microorganisms largely determine the fate of carbon (C) in soils, through their control over soil organic matter (SOM) decomposition processes. Soils provide a heterogeneous environment for microorganisms, with a non-uniform distribution of C substrates, nutrients and oxygen, and this heterogeneity is influenced by soil aggregation processes. Understanding the distribution of soil microorganisms in the soil aggregate matrix provides insight into SOM cycling and stabilization dynamics. This is especially important in the context of tropical land use change as it significantly contributes to global C emissions. 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? (2) How do these relationships differ with changes in vegetation during tropical forest recovery? and 3) What is the relative importance of seasonal, spatial and forest successional effects on microbial community composition in bulk soils? We measured microbial composition, via phospholipid fatty acid (PLFA) analysis, and C and nitrogen (N) concentrations on physically separated aggregate fractions of soils from pastures, secondary forests (40 and 90 years old) naturally regenerated on abandoned pastures, and primary forests in Puerto Rico.
Results/Conclusions
Our study yielded several main results: (1) Microbial community composition varied by aggregate fraction, with a smaller fungal-to-bacterial ratio in smaller-sized aggregates and a greater gram-positive to gram-negative bacterial ratio in the silt and clay fractions compared with the macroaggregate and microaggregate fractions; and (2) Land cover, or stage of forest recovery, was the strongest predictor of soil microbial community composition on bulk soils. Seasonal variation also played an important role in shaping bulk soil microbial community structure and function. In the wet seasons, the indicator for anaerobic, gram-negative bacteria explained most of the variability in the successional patterns indicating temporal and quite possibly spatial variations in anaerobic soil environments. Further, our soils show visible evidence of redoximorphic properties at the microsite scale, likely facilitated by the high clay content, well-developed aggregate structure, and high iron content. Our results indicate that association with mineral surfaces in the clay and silt-sized fractions contained within macroaggregates is the dominant stabilization mechanism for SOM in these highly-weathered, fine-textured soils. This study also shows that the soil matrix plays an important role in the spatial distribution of fungal and bacterial dominated communities, and that this distribution is sensitive to changes in vegetation, with potential implications for SOM storage and turnover.