This research will investigate how sensitively carbon dioxide loss from soil responds to increasing temperature and why this varies among different soils. Soils from a variety of ecosystems across the United States will be collected and assayed under controlled laboratory conditions in order to determine how fast soil carbon is respired by microbes at different temperatures and how increases in biologically available nitrogen affect this temperature sensitivity. The research will be testing predictions as to whether recalcitrant soil carbon loss will be more sensitive to increases in temperature than less recalcitrant soil carbon. In addition, the research will test whether increases in nitrogen availability decrease temperature sensitivity.
Globally, soil carbon is one of the largest repositories of carbon, yet its fate in a warmer world will be impossible to predict without mechanistic understanding of what controls how soil microbes utilize it. For global change biologists, this research will test a critical set of hypotheses about soil functioning and will provide data that are essential to improving models of how ecosystems respond to climate change and deposition of biologically available nitrogen. At the heart of the research is determining whether warming soils are likely to provide a positive feedback to global temperatures while also identifying those soils and components of soils that are most susceptible to carbon loss due to warming.
Globally, soils contain about twice as much carbon as found in the atmosphere and three times as much found in vegetation. The fate of organic carbon stored in the terrestrial biosphere depends in large part on the temperature sensitivity of microbial decomposition. Increased temperatures have the potential to drastically increase the amount of carbon dioxide in the atmosphere as soil microbes decompose organic matter in the soil at a faster rate. Yet, we know too little about the controls over the temperature sensitivity of soil organic matter decomposition to accurately forecast the amount of carbon that soils will store in a warmer future. Currently, there is still debate over the relative sensitivity of different forms of carbon in the soil to increases in temperature. Microbial decomposition is complex, but proximally decomposition is an enzymatic process. As such, at least short-term responses to temperature changes should be governed by chemical laws. The degree to which they actually do is still up in the air. Current models used to predict the responses of the Earth’s ecosystems to warming assume that respiration by microbes doubles with every 10°C increase in temperature, but there is a lot of room to improve these formulations. Our research examined the responses of soil organic matter from 28 soils from across the United States to short-term and long-term increases in temperature. Soils ranged from the Arctic of Alaska to the deserts of New Mexico to the rainforests of Puerto Rico. Soils were mailed to Kansas State University and incubated in the laboratory for one year. Our first results showed that the temperature sensitivity of microbial decomposition of leaves, roots, and soils to short-term increases in temperature follows chemical laws. Because of the generality of the results, we should be able to more accurately model temperature responses geographically to short-range temperature changes. Despite this generality, we also showed that there is almost as much variability within a landscape in how much soils respond to increased temperature as we observed at the continental scale. Beyond short-term increases in temperature, microbial populations can adjust to long-term increases in temperature. Generally referred to as "acclimation", there are multiple mechanisms by which microbial populations can reduce their activity with warming. Our second results showed that in general, microbes reduced their decomposition of soil organic matter with sustained increases in temperature across almost all soils. More importantly, although we could predict the responses of microbial decomposition of soil organic matter to short-term increases in temperature, the long-term responses of decomposition to increases in temperature were effectively decoupled from the short-term responses. As such, we will need novel approaches to understand how soils will respond to changes in temperature that are long enough for soil microbes to acclimate. Lastly, we also examined how the temperature sensitivity of soils to increased temperature was influenced by changes in water and nitrogen availability. Responses to increased temperature were different in wet and dry soils, yet nitrogen had little influence on temperature sensitivity. In all, our research showed promise in generating more accurate predictions of the responses of soils to future warming, but at the same time showed how complex soils are at the local and continental scale. There is great likelihood that we are underestimating the amount of carbon dioxide that soils could release in the future as they warm.
