Atmospheric N deposition causes carbon balance gains in a seven year field experiment in subalpine grassland

Abstract

Air pollution agents interact when affecting biological sinks for atmospheric CO 2, 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 seven year experiment in subalpine grassland under N- and O 3 -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 O 3 dose was 49, 65 and 89 mmol m −2 projected leaf area. We hypothesized that SOC of this mature ecosystem would not change in control treatments, but that effects of air pollutants on plant yield, net ecosystem productivity (NEP) and SOC content would develop in parallel, leading to SOC content to increase with N deposition. In the control treatment SOC increased significantly by 9 % in seven years. Cumulative plant yield showed a highly significant N effect (+38 % in N54), but no O 3 -effect. Cumulative NEP did show a strong, yet statistically insignificant, hump shaped response pattern to N deposition with a +62 % increase in N14, and only +39 % increase in N54. SOC had a similar response to N, with highest gains at intermediate N deposition rates (9 and 14 kg N ha −1 yr −1), suggesting an unimodal response, too. 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.