Oxygen Levels Affect Carbon and Iron Chemistry in Tropical Soils

The Science

In humid tropical forests, the activity of tiny microbes in fine-textured wet soils can temporarily deplete soil oxygen. The switch from having enough oxygen to having too little oxygen can happen relatively rapidly, causing minerals and organic matter to interact differently. Scientists knew that these changes in oxygen levels affect both carbon and iron forms in the soil but needed more information to correlate the changes in microbe–mineral–organic matter interactions. Studying tropical rainforest soils at the molecular level, with samples collected right after a switch in oxygen availability, provided important insights into the fate of these elements.

The Impact

Around the world, scientists are working to understand how carbon, a key contributor to climate change, cycles through the environment. Soils contain the largest terrestrial reservoir of organic carbon, and global-scale models simulate dynamics in this large pool. One area of uncertainty in these models is what happens to carbon in humid tropical soils as different amounts of rainfall and plant productivity cause oxygen levels to oscillate from high to low and back again. Scientists suspected that changes in iron and organic carbon were key. This study marks the first time that scientists have looked closely at changes in iron and carbon chemistry within a very short time after switching from ambient conditions to an oxygen-free environment. A proper understanding in this area is critical to developing more accurate coupled iron–carbon models.

Summary

Scientists analyzed soils from the U.S. Forest Service Luquillo Experimental Forest, a Critical Zone Observatory and Long-Term Ecological Research site in Puerto Rico. In a laboratory manipulation experiment, they sampled the iron- and carbon-rich surface soils as quickly as 30 minutes after a switch in soil oxygen level. Using a combination of traditional methods like chemical extraction and X-ray diffraction together with advanced molecular-scale Mössbauer spectroscopy, they then examined changes in carbon chemistry and iron mineralogy under changing oxygen levels. The Mössbauer spectroscopy, conducted at EMSL, the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy (DOE) User Facility, proved particularly valuable in determining molecular-level changes in iron mineral forms as a result of a depletion of soil oxygen levels. The scientists discovered that changes in iron and carbon in these tropical soils are intricately coupled and can happen very fast as a result of changes in iron mineral forms. Their findings can be used to improve predictions of carbon cycling in tropical ecosystems, providing important information for regional and global climate change models.

Contacts

Amrita Bhattacharyya, University of San Francisco,
abhattacharyya@usfca.edu

Ravi Kukkadapu, Environmental Molecular Sciences Laboratory, ravi.kukkadapu@pnnl.gov

Jennifer Pett-Ridge, Lawrence Livermore National Laboratory, pettridge2@llnl.gov

Funding

The DOE Biological and Environmental Research program funded this work, in part through a DOE Early Career Research Program. Mössbauer spectroscopy was performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility.

Publication

A. Bhattacharyya, et al., “Fast redox switches lead to rapid transformation of goethite in humid tropical soils: A Mössbauer spectroscopy study.” Soil Science Society of America Journal 86(2), 264-274 (2022). [DOI: 10.1002/saj2.20382]