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

由于人类活动导致大气中CO2浓度增加，预计将增强森林生态系统的光合作用和碳储存。然而，目前还不清楚养分限制将如何限制预计的CO2施肥效应。因此，有必要评估养分限制将如何影响森林对CO2浓度上升的反应，以及它将如何反馈养分的可利用性，特别是氮（N），随着时间的推移，它可以成为限制。本研究的目的是评估N-有限的北方落叶温带森林和磷（P）-有限的桉树为主的森林中的N循环过程对CO2熏蒸的响应。该研究在两个自由空气二氧化碳富集（FACE）设施中进行：位于英国伯明翰附近的BIFOR FACE和澳大利亚新南威尔士州的EucFACE。在此基础上，采用15N池稀释法测定了不同磷水平下EucFACE（磷限制林）土壤总蛋白质解聚和总矿化量，并研究了磷对氮素循环的影响。硝化和反硝化产生的N2O排放将使用15N气体通量法进行评估，土壤中的自由生活固氮将通过15N同化法进行评估。此外，将通过定量N循环基因表达的宏基因组方法比较同位素测量。我们假设，在一个N有限的森林，树木将分配更大比例的碳地下，以促进土壤有机质（SOM）分解的N供应给树木。这一过程将提高树木的氮素利用率，但也会导致氮素通过淋溶和反硝化作用损失。相反，在磷有限的森林中，树木分泌物将无法有效地释放磷（P）的有效性，阻碍光合作用的增强和土壤中N循环的反馈。然而，这些森林中添加磷将改变生态系统从磷限制到氮限制，引发N供应过程的增强。此外，EucFACE（磷限制林）将部分施磷，以探讨突然缓解磷限制将如何影响氮循环。这项研究的结果旨在通过揭示养分限制在未来碳吸收中的作用，提高我们对森林对未来气候反应的理解。

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.