Below Ground Control of Ecosystem Carbon Sequestration under Elevated CO2

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As atmospheric CO2 concentrations rise, plants can photosynthesise more and take more CO2 out of the atmosphere. This carbon (C) can then be stored in plants and soils, reducing atmospheric CO2 and global warming. However, our ability to predict global increases in C uptake are severely constrained because there are so few studies on the effects of elevated CO2 (eCO2) on ecosystems where the availability of the nutrient phosphorus (P) restricts plant growth (P-limited ecosystems) - yet more than 40% of the world's ecosystems are P-limited. Furthermore, it is below ground processes, including interactions between plants, soil microbes and soil chemistry, that will control the ability of these ecosystem to store more C, yet we have very little understanding of how these processes respond to eCO2 in P-limited ecosystems. Understanding below ground responses is essential because soils are the largest store of C in terrestrial ecosystems so we must understand how elevated CO2 changes stores of C in soils as well as in plants. Furthermore, soil C inputs influence the cycling of P and so can determine whether plants can gain the extra P needed for increased growth and C gain, while the fate of C in soil will determine whether it contributes to long-term C stocks or is released. This project will address these knowledge needs using our unique experiment that exposes two contrasting grassland ecosystems (a limestone and an acidic grassland) to elevated CO2 in a natural outdoor setting using Free Air CO2 Enrichment technology (FACE). The grasslands are both P-limited but, critically, plants have responded to eCO2 in directly opposing ways with the limestone grassland increasing biomass, and the acidic grassland reducing biomass. Significantly, the grasslands have contrasting soils linked to developmental age that makes them well suited to gaining mechanistic insights that can be applied broadly across P-limited soils, and may explain the opposing responses. The CO2 enrichment is in combination with P and N nutrient manipulations that allow us to investigate the importance of P-limitation and the influence of globally important atmospheric N deposition that can also influence P cycling. There is a strong theoretical basis for hypothesising that it is the differences in soil biogeochemistry that explains the contrasting plant biomass responses to eCO2: in developmentally young soils (such as our limestone soil), increased C inputs may enhance weathering of primary P-minerals like calcium phosphates and stimulate enzymatic mineralization of organic P, leading to greater plant P availability for growth and C sequestration. In contrast, in soils at later stages of pedogenesis (such as our acidic soil), P is locked up in secondary minerals that are difficult for soil microbes and plants to access. While elevated CO2 may provide the C for enzymes to increase mineralisation of organic P, this is predominantly done by soil microbes which will have reduced efficacy in acidic soil, leading to strong competition for P between plants and microbes, limiting the plant growth benefit of elevated CO2. Critically, these theories remain untested.Using a combination of stable- and radioisotope C tracers, we will trace the short and long-term fate of C and its pathway through plants, microbes and soil. We will determine how the fate of new plant C inputs controls P dynamics by using P isotope tracers to determine how CO2 influences P availability, plant uptake, and competition for P with soil microbes and the soil matrix. Finally, we will use advanced molecular approaches to understand how soil microbes respond to the C inputs and how this influences their cycling of P and its availability to plants. This work aims to determine mechanistically the reasons for the contrasting plant productivity responses in the two grasslands and, in so doing, develop the broad understanding needed to predict future rates of C uptake in P-limited ecosystems.