Organic Carbon Indirectly Alters Soil Structure in Highly Weathered Tropical Soils

The Science

Soil pore space constrains soil capacity to store carbon and water. Total pore space increases with increasing organic matter content, but mechanisms leading to soil structure changes are unclear. A team of researchers quantified and compared soil characteristics across 2 contrasting soils to clarify the effects of organic matter content on soil porosity. They found that high organic matter content fosters higher biological activity, including root growth and animal burrowing, which increases soil porosity and decreases bulk density.

The Impact

Soil organic matter is an important ecosystem component, providing habitats and food for soil organisms, supplying nutrients for plants, and increasing soil water storage. This study demonstrates a biological mechanism for increases in soil porosity and decreases in soil bulk density often observed with increasing organic matter. Organic matter provides additional resources for roots, microbes, and soil fauna, which in turn alter soil physical structure. This research clarifies biology’s importance in modifying hydrologic and gaseous transport in soils and calls for improved representation of bioturbation in soil models.

Summary

The team sampled and compared 2 contrasting highly weathered tropical soils from Brazil to 1 m depth: one with high carbon content and one with low carbon content. Researchers developed soils from similar parent materials, with similar soil texture, and in areas currently under savanna vegetation.

The team first verified that differences in porosity and bulk density attributed to soil carbon differences could not be explained by variation in soil texture, mineral composition, or dilution of soil minerals by lower-density organic matter. Researchers also determined that differences in total porosity could not be explained by variation in pore space inside soil aggregates using X-ray tomography. Instead, they found that high-carbon soils had nearly twice as many roots and burrows as low-carbon soils and soil bulk density decreased with increasing carbon content, carbon: nitrogen ratio, black carbon content, and Δ¹⁴C.

Results suggest that in high-carbon soils, increased plant growth, bioturbation, and vertical transport facilitated by high soil porosity bring fresh plant inputs and charcoal down the soil profile from the surface. The team presents a conceptual model detailing organic matter’s indirect effects on soil structure.

Principal Investigator

Karis McFarlane
Lawrence Livermore National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

The Brazilian National Council for Scientific and Technological Development supported sample collection and most analyses under Grant Numbers 165394/2020-0, 301818/2017-1, and 203749/2014-6. The U.S. Department of Energy’s Office of Science Early Career Program Award supported 14C analyses and interpretation under Grant Number SCW1572. X-ray tomography analyses were supported by the Major Research Instrumentation Program of the National Science Foundation’s Earth Sciences Grant Number 1531316.

References