Subalpine grassland carbon balance during 7 years of increased atmospheric N deposition

Air pollution agents interact when affecting biological sinks for atmospheric CO<sub>2</sub>, e.g., the soil organic carbon (SOC) content of grassland ecosystems. Factors favoring plant productivity, like atmospheric N deposition, are usually considered to favor SOC storage. In a 7-year...

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Bibliographic Details
Main Authors: M. Volk, J. Enderle, S. Bassin
Format: Article
Language:English
Published: Copernicus Publications 2016-07-01
Series:Biogeosciences
Online Access:http://www.biogeosciences.net/13/3807/2016/bg-13-3807-2016.pdf
Description
Summary:Air pollution agents interact when affecting biological sinks for atmospheric CO<sub>2</sub>, e.g., the soil organic carbon (SOC) content of grassland ecosystems. Factors favoring plant productivity, like atmospheric N deposition, are usually considered to favor SOC storage. In a 7-year experiment in subalpine grassland under N- and O<sub>3</sub>-deposition treatment, we examined C fluxes and pools. Total N deposition was 4, 9, 14, 29 and 54 kg N ha<sup>−1</sup> yr<sup>−1</sup> (N4, N9, etc.); annual mean phytotoxic O<sub>3</sub> dose was 49, 65 and 89 mmol m<sup>−2</sup> projected leaf area. We hypothesized that between years SOC of this mature ecosystem would not change in control treatments and that effects of air pollutants are similar for plant yield, net ecosystem productivity (NEP) and SOC content, leading to SOC content increasing with N deposition. Cumulative plant yield showed a significant N and N  ×  N effect (+38 % in N54) but no O<sub>3</sub> effect. In the control treatment SOC increased significantly by 9 % in 7 years. Cumulative NEP did show a strong, hump-shaped response pattern to N deposition with a +62 % increase in N14 and only +39 % increase in N54 (N effect statistically not significant, N  ×  N interaction not testable). SOC had a similar but not significant response to N, with highest C gains at intermediate N deposition rates, suggesting a unimodal response with a marginal (<i>P</i> = 0.09) N  ×  N interaction. We assume the strong, pollutant-independent soil C sink developed as a consequence of the management change from grazing to cutting. The non-parallel response of SOC and NEP compared to plant yield under N deposition is likely the result of increased respiratory SOC losses, following mitigated microbial N-limitation or priming effects, and a shift in plant C allocation leading to smaller C input from roots.
ISSN:1726-4170
1726-4189