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...
Main Authors: | , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-07-01
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Series: | Biogeosciences |
Online Access: | http://www.biogeosciences.net/13/3807/2016/bg-13-3807-2016.pdf |
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. |
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ISSN: | 1726-4170 1726-4189 |