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Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2.

ABSTRACT: Forest ecosystems are important sinks for rising concentrations of atmospheric CO(2). In previous research, we showed that net primary production (NPP) increased by 23 +/- 2% when four experimental forests were grown under atmospheric concentrations of CO(2) predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO(2) enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO(2) at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO(2) at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO(2). Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO(2) result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO(2).

SUBMITTER: Finzi AC

PROVIDER: S-EPMC1955801 | biostudies-other | 2007 Aug

REPOSITORIES: biostudies-other

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Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2.

Finzi Adrien C AC Norby Richard J RJ Calfapietra Carlo C Gallet-Budynek Anne A Gielen Birgit B Holmes William E WE Hoosbeek Marcel R MR Iversen Colleen M CM Jackson Robert B RB Kubiske Mark E ME Ledford Joanne J Liberloo Marion M Oren Ram R Polle Andrea A Pritchard Seth S Zak Donald R DR Schlesinger William H WH Ceulemans Reinhart R

Proceedings of the National Academy of Sciences of the United States of America 20070820 35

Forest ecosystems are important sinks for rising concentrations of atmospheric CO(2). In previous research, we showed that net primary production (NPP) increased by 23 +/- 2% when four experimental forests were grown under atmospheric concentrations of CO(2) predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necess ...[more]

PMID: 17709743

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Project description:Understanding the relationships between biodiversity and ecosystem productivity has become a central issue in ecology and conservation biology studies, particularly when these relationships are connected with global climate change and species extinction. However, which facets of biodiversity (i.e. taxonomic, functional, and phylogenetic diversity) account most for variations in productivity are still not understood very well. This is especially true with regard to temperate forest ecosystems. In this study, we used a dataset from a stem-mapped permanent forest plot in northeastern China exploring the relationships between biodiversity and productivity at different spatial scales (20 × 20 m; 40 × 40 m; and 60 × 60 m). The influence of specific environmental conditions (topographic conditions) and stand maturity (expressed by initial stand volume and biomass) were taken into account using the multivariate approach known as structural equation models. The variable "Biodiversity" includes taxonomic (Shannon), functional (FDis), and phylogenetic diversity (PD). Biodiversity-productivity relationships varied with the spatial scales. At the scale of 20 × 20 m, PD and FDis significantly affected forest biomass productivity, while Shannon had only indirect effects. At the 40 × 40 m and 60 × 60 m scales, biodiversity and productivity were weakly correlated. The initial stand volume and biomass were the most important drivers of forest productivity. The local environmental conditions significantly influenced the stand volume, biomass, biodiversity, and productivity. The results highlight the scale dependency of the relationships between forest biodiversity and productivity. The positive role of biodiversity in facilitating forest productivity was confirmed at the smaller scales. Our findings emphasize the fundamental role of environmental conditions in determining forest ecosystem performances. The results of this study provide a better understanding of the underlying ecological processes that influence specific forest biodiversity and productivity relationships.

Project description:Anaerobic ammonium oxidation with nitrite reduction to dinitrogen (termed anammox) has been reported to be an important process for removing fixed nitrogen (N) in marine ecosystems and in some agricultural and wetland soils. However, its importance in upland forest soils has never been quantified. In this study, we evaluated the occurrence of anammox activity in two temperate forest soils collected from northeastern China. With (15)N-labeled NO3 (-) incubation, we found that the combined potential of the N2 production rates of anammox and codenitrification ranged from 0.01 ± 0.01 to 1.2 ± 0.18 nmol N per gram of soil per hour, contributing 0.5% to 14.4% of the total N2 production along the soil profile. Denitrification was the main pathway of N2 production and accounted for 85.6% to 99.5% of the total N2 production. Further labeling experiments with (15)NH4 (+) and (15)NO2 (-) indicated that codenitrification was present in the mixed forest soil. Codenitrification and anammox accounted for 2% to 12% and 1% to 7% of the total N2 production, respectively. Two anammox species, "Candidatus Brocadia fulgida" and "Candidatus Jettenia asiatica," were detected in this study but in very low abundance (as indicated by the hzsB gene). Our results demonstrated that the anammox process occurs in forest soils, but the contribution to N2 loss might be low in these ecosystems. More research is necessary to determine the activities of different N2 releasing pathways in different forest soils.In this study, we examined the anammox activity in temperate upland forest soils using the (15)N isotope technique. We found that the anammox process contributed little to the N2 production rate in the studied forest soil. Two anammox organisms, "Candidatus Brocadia fulgida" and "Candidatus Jettenia asiatica," were detected. In addition, we found that codenitrification was another N2 production pathway in forest soils. Our results could contribute to the understanding of soil gaseous N losses and microbial controls in forest soils.

Dataset Information

Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2.

Publications

Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2.

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