Soils worldwide contain twice as much carbon as the atmosphere, so to understand the global carbon cycle it is important to understand the factors influencing soil organic carbon accumulation and decomposition. Tropical soils contain potentially high concentrations of both organic carbon and iron minerals. In the laboratory these iron minerals have been demonstrated to react with oxygen in a way that decomposes organic compounds that are otherwise difficult for micro-organisms to degrade. The current study will test whether these reactions affect soil organic carbon in natural soils, a finding that would transform the understanding of soil carbon cycling because up to now micro-organisms were thought to be primarily responsible for most soil organic matter decomposition.
Insights from these experiments could improve conceptual and mathematical models that describe the influence of soils on the global carbon cycle, and thus improve prediction of the causes and consequences of climate change. Understanding whether and how organic matter decomposition processes driven by iron and oxygen are important in natural ecosystems could also contribute to remediation of soil organic pollutants. Scientific training of graduate and undergraduate students and community volunteers will be supported by this project, and findings will be disseminated in local schools and through online curricula.
Soils in natural ecosystems provide a host of services beneficial to society, including providing plants with nutrients for growth and improving water quality. Soil organic matter is a key factor influencing soil chemical and biological activity and provides an additional benefit by storing a large amount of carbon. Worldwide, soils store at least twice as much carbon as the atmosphere and likely help to reduce the impact of human-caused carbon dioxide emissions that contribute to climate change. Soil organic matter is the product of dead plant material that has been partially decomposed by microorganisms. As a consequence of decomposition, carbon in organic matter is ultimately transformed to carbon dioxide and lost from the soil. Understanding the environmental factors that regulate rates of soil organic matter decomposition represents a major scientific challenge with a wide range of applications in agriculture, forestry, and environmental management. Soils in tropical rainforests are especially important because they are highly productive and often contain high organic matter content. Tropical soils also frequently contain large quantities of iron minerals that support microbial growth under very wet conditions. Iron that has been transformed by microbial respiration can subsequently react with oxygen, potentially generating free radicals that can decompose organic matter. This process could accelerate the decomposition of components of organic matter that are otherwise difficult for most microbes to break down, such as lignin (a major chemical component of trees). We found strong evidence for the importance of these coupled biological and abiotic processes involving iron minerals, microbial respiration, and oxygen in stimulating the breakdown of lignin in Puerto Rican tropical rainforest soils, using a combination of field studies and laboratory experiments in collaboration with scientists from the US Forest Service. We found that soils undergoing fluctuations in oxygen availability, which can occur under wet conditions in natural forests, can stimulate the breakdown of lignin relative to other components of organic matter. This contradicts our previous understanding of lignin decomposition, where lignin was thought to accumulate in wet soils. Our findings suggest that fluctuations in moisture accompanying climate change could impact decomposition rates and the chemical composition of soil organic matter in wet, iron-rich soils, with potential implications for soil fertility, forest productivity, and the capacity of soils to store carbon. As a consequence of this research, several undergraduate and graduate students were trained in scientific and research methods at UC Berkeley, including two students from under-represented minority groups who have since pursued careers in science. In addition to publishing articles in scientific journals, we have disseminated our findings using informal presentations to youth in San Francisco Bay area schools and at summer camps. We have also participated in online forums devoted to answering public questions related to tropical rainforest science.
