Distribution of Soil Organic Matter Pools among Land Uses across a Diversity of Tropical Soils

Our understanding of the response of soil organic matter (SOM) to land-use change in the tropics is strongly biased towards volcanic and highly-weathered soils, despite the diversity of tropical soil environments. SOM plays an important role in many chemical and biological functions of the biosphere, including soil fertility and carbon cycling. In this study, we measured the distribution of SOM pools across a diversity of soil orders under different land cover types, including forests, pastures, croplands, and rangelands to gain a better picture of potential mechanisms influencing soil C cycling in tropical soils. The fractionation, or isolation, of SOM into different pools with different turnover times can reveal insights into C cycling beyond those offered by measurements of bulk C stocks. Sampling sites were selected from across the Caribbean islands of Puerto Rico and St. Croix to provide a comprehensive look at distinct microenvironments and factors influencing SOM. We used a density fractionation approach to separate the particulate organic matter in the free light fraction (FLF), particulate organic matter released after the disruption of soil aggregates, or the occluded light fraction (oLF), and the heavy or dense residual mineral-associated fraction (HF). We report differences in the amount of C and nitrogen associated with different SOM pools as well as C:N ratios, which provide information about organic matter source and stage of decomposition. This study will provide an understanding of how soil organic carbon cycling varies between land uses and soil orders among tropical soils.