Long-term enhanced nitrogen deposition increases ecosystem respiration and carbon loss from a Sphagnum bog in the Scottish Borders

Nitrogen (N) deposition has increased in the last few decades with implications for the functioning of Sphagnum mosses, the main peat forming genus in peatlands. However, there are few in situ measurements of the carbon balance, especially where the N additions have been realistically manipulated in the field, and none with respect to the effect of N form. The aim of this study was to look at the effects of experimental N additions as oxidized or reduced N, with and without phosphorus (P) and potassium (K), on CO2 fluxes from Sphagnum capillifolium hummocks in a long-term N addition experiment, Whim bog, in the Scottish Borders. In situ static chamber measurements were made during 2008 on 20 plots (control, and N treatments receiving 56 kg N ha−1 y−1 of either nitrate (NO3−) or ammonium (NH4+) added, with and without PK) to assess N effects on CO2 exchange. Almost all the measured fluxes were negative, i.e. Sphagnum hummocks lost CO2 to the atmosphere, irrespective of the treatments applied. N treatment did not have a significant effect on ecosystem respiration (ER) or net ecosystem productivity (NEP) but adding PK with N increased gross photosynthesis (PG) significantly, compared to the other treatments. Summed monthly averages of NEP for each treatment indicated that increasing N deposition increased CO2 loss from the system. The form of N affected the response: compared to the control, adding nitrate increased the CO2 loss more than ammonium, both with and without PK. Nitrogen (both forms) increased the ecosystem respiration fluxes at a certain temperature, adding PK with N further enhanced the response. The positive (increasing) temperature response of ecosystem respiration with N suggests that in high N deposition areas the potential increase in ecosystem respiration, CO2 loss, will be exacerbated with climate change.