Articles | Volume 13, issue 12
Research article
01 Jul 2016
Research article |  | 01 Jul 2016

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

Matthias Volk, Jan Enderle, and Seraina Bassin

Abstract. Air pollution agents interact when affecting biological sinks for atmospheric CO2, 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 O3-deposition treatment, we examined C fluxes and pools. Total N deposition was 4, 9, 14, 29 and 54 kg N ha−1 yr−1 (N4, N9, etc.); annual mean phytotoxic O3 dose was 49, 65 and 89 mmol m−2 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 O3 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 (P = 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.

Short summary
Grasslands hold 28 % of global soil organic carbon (SOC). We studied carbon-fluxes and -pools in subalpine grassland under atmospheric nitrogen (N) deposition (air pollution) treatment. Unlike plant yield, SOC and CO2 gains were largest at intermediate, not maximum N deposition. Thus, N deposition driven plant yield increases may not be considered as a valid proxy for ecosystem C-pool increases and the biological sink for CO2 greenhouse gas may be smaller under higher N deposition.
Final-revised paper