Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/11/2004
Publication Date: 2/1/2005
Citation: Hu, S., Wu, J., Burkey, K.O., Firestone, M.K. 2005. Plant and microbial n acquisition under elevated atmosphereic CO2 in two mesocosm experiments with annual grasses. Global Change Biology. 11(2):213-223. Interpretive Summary: The impact of rising atmospheric carbon dioxide at the ecosystem level is uncertain because utilization of the additional carbon depends upon the capacity of plants to extract additional nitrogen and other nutrients from the soil. Soil microbial processes convert soil nitrogen into forms that can be metabolized by plants and microbes, possibly resulting in competition for available nitrogen. Thus, there is a need to understand nitrogen utilization by plants and soil microbes under elevated carbon dioxide conditions. In this study, two species of wild oats were grown in controlled environments under twice ambient carbon dioxide. Plant biomass nitrogen increased under elevated carbon dioxide and was associated with increased soil microbial activity, including increased colonization of roots by mycorrhizal fungi. However, microbial nitrogen content did not increase, suggesting that plants and microbes did not compete for nitrogen in this system. The results suggest that soil microbes may facilitate enhanced nitrogen acquisition by plants under elevated carbon dioxide conditions.
Technical Abstract: The impact of elevated CO2 on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO2 on N availability to plants and microbes. We examined the effects of atmospheric CO2 enrichment (ambient + 370 micromol mol-1) on plant and microbial N acquisition in two different experiments, using model ecosystems of annual grasses of Avena barbata and Avena fatua, respectively. The A. barbata experiment was conducted in a N-poor sandy loam and the A. fatua experiment was on a N-rich clayey loam. Plant-microbial N partitioning was examined through determining the distribution of a 15N tracer. In the A. barbata experiment, 15N tracer was introduced to a field labeling experiment in the previous year so that 15N predominantly existed in non-extractable soil pools. In the A. fatua experiment, 15N was introduced in a mineral solution [(15NH4)2SO4 solution] during the growing season of A. fatua. Results of both N budget and 15N tracer analyses indicated that elevated CO2 increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO2 increased plant biomass N by ca. 10% but there was no corresponding decrease in extractable N, suggesting that plants might have obtained N from the non-extractable organic N pool due to enhanced microbial activity. In the A. fatua experiment, however, the CO2-led increase in plant biomass N was statistically equivalent to the reduction in the extractable N. Although atmospheric CO2 enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO2 increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO2 may tighten N cycling through facilitating plant N acquisition. However, it is unknown that to what degree, results from these short-term microcosm experiments can be extrapolated to field conditions. Long-term studies in less disturbed soils are needed to determine whether CO2-enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO2 environment.