Intellectual Merit: When organic matter becomes associated with iron (Fe) minerals in soils it is generally thought to be quite persistent and locked-up against microbial decomposition. Our work has explored how these iron-organic matter (Fe-OM) associations form and how they behave in soils of wet tropical forests. We find that while Fe-OM associations form rapidly in wet soils, microorganisms that breath iron instead of oxygen can dissolve these newly formed iron-organic matter solids and release the organic matter. The liberated organic material then becomes accessible to microbial decomposition and can contribute to production and emission of the greenhouse gases carbon dioxide and methane. Changes in the oxygen concentrations in soils, related to both rainfall patterns and biological activity, stimulate Fe reduction (as soil oxygen declines) and Fe oxidation (as oxygen increases). Oxygen fluctuations are an index of redox dynamics and occur on the scale of hours to days driving rapid changes in Fe-OM interactions.

Phosphorus is an essential nutrient for plants and microbes in soils and tropical forests on old, deeply weathered soils may lack sufficient phosphorus availability for some biological activity. This problem is compounded by the presence of oxidized forms of Fe that can strongly bind with phosphorus, making it inaccessible to most plant roots and microbes. The binding strength of Fe declines when it is reduced, in turn affecting the form and accessibility of phosphorus in soils.

Below are the key intellectual merit findings from the research:

1. We quantified patterns and drivers of redox fluctuations in humid tropical forest soils and found that rainfall events, drought, and topography were important predictors across space and time.

2. We quantified the rates and products of Fe-OM formation via iron oxidation under laboratory and field conditions.

3. We have shown that Fe stimulates C degradation unless soils are kept fully oxic; that Fe-OM associations are more susceptible to Fe reduction than older Fe phases suggesting that if OM is to reside in association with Fe minerals over long time scales in humid soils, it will need to be protected against Fe reduction by other minerals (or even aged Fe minerals), or within aggregates.

4. Rates of Fe reduction in the field are correlated with rainfall a couple days prior, with important influences of the amount of dissolved organic carbon in the soil solution. Using ecohydrological modeling we can show that Fe reduction rates are likely very high in wet tropical forests like Luquillo because of frequent wetting/drying events throughout the year; similar Fe-C dynamics also occur in intermittently wet cropland soils, whereas in comparable soils in the southeastern US, Fe reduction rates are likely only regionally significant in the winter and early spring. This same analysis could be extended to globally using climatic and soil data.

5. We found that low redox conditions did not increase the amount of phosphorus released into the soil solution, likely because of the increase in surface area available for binding phosphorus to reduced Fe; we also found that phosphorus was more easily extracted from soils using salt solutions when Fe was reduced under low redox conditions.

6. We showed that microbes were more limited by oxygen than phosphorus under low redox conditions and that the degree of limitation varied along a rainfall gradient.

These findings have been communicated in 41 scientific publications among many public presentations.

Broader Impacts: This NSF project has enhanced the learning of 3 PhD students, 6 post-docs, as well as 13 undergrads and four high- school interns. The demographics of these trainees include 12 people from underrepresented groups and 15 women. This project has contributed to the Luquillo CZO-LTER Schoolyard program that includes a research component in K-12 curricula delivered in Puerto Rico. In addition, we have been involved in considerable training and outreach activities. Activities included twice speaking about soils, climate change, and climate change mitigation and adaptation in public lectures, on radio, and on television. We contributed to numerous print and internet media sources, such as the New York Times, the San Francisco Chronicle, Slate, Science Magazine, and Grist, among others. We also briefed policy makers on the potential role of soil in climate change mitigation.

Last Modified: 12/08/2021
Modified by: Aaron Thompson