Publications

<div class="page" title="Page 1"> <div class="section"> <div class="layoutArea"> <div class="column"> At 6 A.M. on the first day of winter break, a van full of high school stu- dents and teachers set out for Fossil Creek in Arizona to conduct measurements for an ongoing environmental project (which monitors changes to the creek). The group had a long day ahead—hiking about 7 km down to Fossil Creek, wading in cold water to re- trieve samples, and carrying about 13 kg of wet sample bags back up the trail. Later that day, collected samples were analyzed in an aquatic biology lab at Northern Arizona Uni- versity (NAU). That cold, winter day ended with students saying that participating in the project was a great way to begin winter break. </div> </div> </div> </div>

Rising atmospheric carbon dioxide (<em>C</em>_a), a product of fossil fuel burning, land-use change, and cement manufacture, is expected to cause a large carbon sink in land ecosystems, partly mitigating human-driven climate change (<a id="xref-ref-1-1" class="xref-bibr" href="http://science.sciencemag.org/content/304/5675/1291#ref-1"><em>1</em></a>). Increasing biological nitrogen fixation with rising <em>C</em>_a has been invoked as a means to provide the N necessary to support C accumulation (<a id="xref-ref-2-1" class="xref-bibr" href="http://science.sciencemag.org/content/304/5675/1291#ref-2"><em>2</em></a>). As in many short-term experiments (<a id="xref-ref-3-1" class="xref-bibr" href="http://science.sciencemag.org/content/304/5675/1291#ref-3"><em>3</em></a>), we found that <em>C</em>_a enrichment increased N fixation during the first year of treatment in an oak woodland. However, the effect declined and disappeared by the third year. <em>C</em>_a enrichment consistently depressed N fixation during the 5th, 6th, and 7th years of treatment. Reduced availability of the micro-nutrient molybdenum, a key constituent of nitrogenase, best explains this reduction in N fixation. Our results demonstrate how multiple element interactions can influence ecosystem responses to atmospheric change and caution against expecting increased biological N fixation to fuel terrestrial C accumulation.

A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.

Rising atmospheric CO₂ and temperatures are probably altering ecosystem carbon cycling, causing both positive and negative feedbacks to climate. Below-ground processes play a key role in the global carbon (C) cycle because they regulate storage of large quantities of C, and are potentially very sensitive to direct and indirect effects of elevated CO₂ and temperature. Soil organic matter pools, roots and associated rhizosphere organisms all have distinct responses to environmental change drivers, although availability of C substrates will regulate all the responses. Elevated CO₂ increases C supply below-ground, whereas warming is likely to increase respiration and decomposition rates, leading to speculation that these effects will moderate one another. However, indirect effects on soil moisture availability and nutrient supply may alter processes in unexpected directions. Detailed, mechanistic understanding and modelling of below-ground flux components, pool sizes and turnover rates is needed to adequately predict long-term, net C storage in ecosystems. In this synthesis, we discuss the current status of below-ground responses to elevated CO₂ and temperature and potential feedback effects, methodological challenges, and approaches to integrating models and measurements.