Potassium and phosphorus additions modify the response of Sphagnum capillifolium growing on a Scottish ombrotrophic bog to enhanced nitrogen deposition

This study reports the impacts of wet reduced versus oxidised N on in situ responses of Sphagnum capillifolium: architecture, chemistry and Fv/Fm. Nitrogen inputs were enhanced under realistic treatment scenarios in a unique experiment on an ombrotrophic bog ecosystem in the Scottish Borders, UK, receiving, historically and currently, low pollutant deposition. Ammonium (NH4Cl) and nitrate (NaNO3) treatments were applied from 2002 (June) at 16 kg and 64 kg N ha−1 a−1 with or without K and P (K2HPO4), provided in a 1:14 ratio of P:N. These wet treatments were applied to three replicates in precipitation collected at the site, as ‘real time’ applications to large, 13 m2 plots. A high application frequency >100 events was achieved and the maximum spray concentration was 4 mM. Over the study period June 2003–May 2004 rainfall was low over several months enabling the effects of water stress to be looked at. Branching and the length of photosynthetically active material were reduced by the high N input and the effect was more pronounced when the water-table was low, but the form of N was not important. Where the N form was important the effects of oxidised N were less favourable than those of reduced N. The findings are discussed in relation to current knowledge along with implications for critical N loads and management options for bogs to counteract the detrimental impact of elevated N deposition. These results provide further evidence that additions of K and P to N-polluted bogs can ameliorate the direct, negative impacts of N deposition on Sphagnum at least in the short-term.