Nitrogen cycling under elevated CO2: response of a key soil nutrient to global change

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Increasing atmospheric CO2 concentrations in the atmosphere due to human activities, is projected to enhance photosynthesis and carbon storage of forest ecosystem. However, it is unclear how nutrient limitation will constrain the projected CO2 fertilization effect. Therefore, it is essential to evaluate how nutrient limitation will affect the response of forests to rising CO2 concentration and how it will feedback on nutrient availabilities and more especially nitrogen (N) which can become limiting with time. The purpose of this research is to evaluate the response of N cycling processes to elevated CO2 fumigation in a N-limited northern deciduous temperate forest and a phosphorus (P)-limited Eucalyptus dominated forest. The research was conducted in two Free Air Carbon Dioxide Enrichment (FACE) facilities: BIFOR FACE located near Birmingham, UK and EucFACE in New South Wales, Australia. Furthermore, EucFACE (P-limited forest) received partial phosphorus fertilization to study its effect on N cycling.A 15N pool dilution method will be used to quantify gross protein depolymerization and gross mineralization and estimate the microbial nitrogen use efficiency. N2O emissions from nitrification and denitrification will be assessed using a 15N Gas Flux method and free-living N fixation in soils will be assessed by the 15N assimilation method. Additionally, the isotopic measurements will be compared by a metagenomic approach quantifying the expression of N cycling genes. We hypothesize that in a N limited forest, trees will allocate a greater proportion of carbon belowground to boost soil organic matter (SOM) decomposition for N supply to trees. This process will enhance N availability for trees but will also lead to nitrogen losses via leaching and denitrification. Conversely, in the phosphorus-limited forest, tree exudates will fail to effectively unlock phosphorus (P) availability, hindering the enhancement of photosynthesis and the feedback on N cycling in the soil. However, fertilizing these forests with phosphorus will shift the ecosystem from being P-limited to N-limited, triggering an enhancement of N supply processes. In addition, EucFACE (P-limited forest) will be partially fertilized with phosphorus in order to explore how a sudden P limitation alleviation will affect N cycling. Results from this research aim at improving our understanding of forest response to future climate by unveiling the role of nutrient limitation in future C uptake.