Collaborative Research: Effects of Elevated CO2 on Forest N Cycling: Assessment with Large-Scale 15N Tracers and Modeling

摘要。森林生态系统每年与大气交换大量的碳(C)，这种交换受到养分循环的强烈限制。预计森林生产力将随着大气中二氧化碳浓度在下个世纪增加一倍而增加。研究和合成的一个关键领域涉及二氧化碳升高对土壤氮循环的反馈效应。升高的CO2显著提高了植物生产力、土壤对N的吸收和土壤微生物对C的通量，但没有改变凋落物化学、分解、生态系统库中N的质量平衡、总氮矿化和净氮矿化或植物氮有效性。尽管在二氧化碳浓度升高的情况下，植物氮素有效性没有增加，但没有迹象表明氮素正在逐渐限制该生态系统在二氧化碳浓度升高下的第六个生长期的生产力提高。这一结果令人惊讶，因为(i)该生态系统的净初级生产力明显受到氮的限制，（ii）土壤中对可用的纳蒙植物、微生物和非生物汇存在激烈的竞争，以及（iii）该研究地点的单个FACE原型地块的长期生产力只有在添加氮肥后才会受到二氧化碳升高的刺激。这些结果表明，我们对森林生态系统C和N循环耦合性质的理解存在根本性差距，并有机会扩大我们对生态系统生物地球化学的认识。这项研究将通过解决覆盖美国东南部超过2400万公顷林地的生态系统中碳封存的主要潜在限制，为社会带来直接利益；有可能根据《京都议定书》对其碳汇潜力进行管理的生态系统。更广泛的影响：考虑到杜克森林FACE实验的跨学科和多机构性质，这项研究的影响将远远超出这个合作团队。这笔赠款将用于培训未来的科学家和征聘和留住女研究生，并将通过积极参加SEEDS方案（欧空局和基金的联合方案）努力扩大少数民族的参与。该项目将通过向NCEAS和一个由15个N示踪剂研究地点组成的国际网络提供数据集和增强型模型，加强研究基础设施。我们预计这项工作的影响将继续很高，无论是在基础生态系统科学方面的进展，还是在生态系统科学对全球变化问题的可论证应用方面；这些问题将在下个世纪对社会产生越来越大的影响。

Abstract. Forest ecosystems exchange large amounts of carbon (C) with the atmosphere each year, an exchange that is strongly constrained by nutrient cycling. Forest productivity is anticipated to increase with the projected doubling in the concentration of atmospheric CO2 over the next century. A critical area for research and synthesis concerns the feedback effects of elevated CO2 on soil N cycling. Elevated CO2 significantly increased plant productivity, the uptake of N from soils and the flux of C to soil microbes but did not change litter chemistry, decomposition, the mass balance of N among ecosystem pools, gross and net N mineralization or plant N availability. Even though plant N availability did not increase under elevated CO2, there is no indication that N is progressively limiting the enhanced productivity of this ecosystem now in it sixth growing season under elevated CO2. This result is surprising given that (i) Net Primary Productivity is demonstrably N limited in this ecosystem, (ii) that there is intense competition for available Namong plants, microbes and abiotic sinks in soils, and (iii) that the long-term productivity of asingle prototype FACE plot at this research site was stimulated by elevated CO2 only after the addition of N fertilizer. These results suggest a fundamental gap in our understanding of the coupled nature of C and N cycles in forest ecosystems, and an opportunity to expand our knowledge of ecosystem biogeochemistry. This research will have direct benefits to society by addressing a major potential limitation to C sequestration in ecosystems that cover over 24 million hectares of forest land in the southeastern US; ecosystems that are likely to be managed for their C-sink potential under the Kyoto Protocol.Broader Impacts: Given the interdisciplinary and multi-institutional nature of the Duke Forest FACE experiment, this research will have impacts that extend well beyond this collaborative team. This grant will train future scientists and recruit and retain women graduate students, and will make an effort to broaden the participation of minorities by actively participating in the SEEDS program (a joint program of ESA and UNCF). This project will enhance infrastructure for research by contributing data sets and an enhanced model to current synthesis activities at NCEAS and an international network of 15 N tracer study sites. We expect the impact of this work to continue to be high, both in advances to fundamental ecosystem science, and in the demonstrable application of ecosystem science to issues of global change; issues that will increasingly impact society over the next century.