Microbial Community Structure Varies Across Soil Organic Matter Aggregate Pools During Tropical Land Cover Change

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Soil microorganisms regulate multiple input and loss pathways of soil carbon (C); hence, changes in microbial communities are expected to affect soil organic matter (SOM) cycling and storage. Despite this, very little is known about how microbes respond to changes in soil structure and vegetation with land use and land cover change. This study aimed to identify relationships between microbial community composition and the distribution of SOM among soil aggregate fractions to answer the following research 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 succession? 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 regrowing on abandoned pastures, and reference or primary forests in Puerto Rico. We found different microbial communities associated with different soil aggregate fractions. Fungal to bacterial ratios decreased and gram-positive to gram-negative bacterial ratios increased with decreasing physical fraction size (from the macroaggregates to the silt and clay fractions). Microbial composition also varied with land cover type and forest successional stage, with consistent trends among soil fractions. These results show that the soil matrix and soil microsite properties play an important role in the spatial distribution of fungal and bacterial-dominated communities. The similarities in land cover effects on microbial communities at different spatial scales suggest similar controls may be influencing microbial composition with potential implications for SOM storage and turnover. In addition, the majority of C and N (relative to total soil C and fraction mass) was isolated in the macroaggregate-occluded silt and clay-sized fractions, suggesting that association with mineral surfaces, and not occlusion of particulate organic matter within aggregates, is the dominant stabilization mechanism for SOM in these highly-weathered, fine-textured soils. These results highlight the importance of soil aggregation in C storage but through mechanisms different than those reported for temperate grassland soils.