Mineralogical Vs. Biological Drivers of Macroaggregate Structure in Costa Rican Forest Soil with Vegetation Treatments.

Soil aggregation is a characteristic resulting from a variety of biological, chemical, and mineralogical interactions. It has applications in organic matter stabilization, and great capability to sequester carbon. Similarly, soils high in iron oxides have a great sorption potentials of organic components, and are powerful cementing agents of aggregation. While each driver of aggregation is individually well-known, the degree to which each driver influences aggregation is unclear and highly variable. Our study aims to compare two dominant aggregation drivers (biological and mineralogical) by measuring macroaggregate structure in soils under six vegetation types and characterizing the mineralogy of three soils.

Vegetation treatments were implemented in 1988 at La Selva Biological Station in Costa Rica: one abandoned pasture, one mature forest, and four monodominant tree species (Hieronyma alchorneoides, Pentaclethra macroloba, Virola koschnyi, and Vochysia guatemalensis).

Macroaggregate structure was measured in the 0-15 cm layer of soils under these six vegetation types, as well as total carbon and nitrogen content of water-stable aggregates. We found that macroaggregate structure significantly differed in the smallest aggregate sizes, and that in four of the treatments, total carbon increased as aggregate size decreased. Mineralogical drivers are thought to dominate in smaller aggregate sizes, which gives rise to the question: what minerals are interacting with organic material?

Mineralogical analyses showed significantly high iron content, which consisted of well-ordered iron oxides (hematite and goethite) as well as poorly crystalline minerals. Prevalence of gibbsite and kaolinite was expected. However 0-15 cm depths contained small amounts of halloysite. While it supports the findings of Markus Kleber, the dichotomy of the two minerals co-existing indicates different stages of weathering. This may be a biological function in which trees relocate silica from root zones into plant biomass, then redistribute that silica on the soil surface after plant death, where it precipitates as halloysite.