1. The effects of increased nitrogen or phosphorus supply on the productivity of Sphagnum-dominated ombrotrophic bogs in northern and southern Sweden were studied. Atmospheric nitrogen deposition in southern Sweden (high-N site) exceeds that in northern Sweden (low-N site) by about tenfold. 2. Vertical height growth of the Sphagnum carpet was measured by the cranked-wire method. Length growth of individuals was measured by autoradiography after labelling with (CO2)-C-14. The results of both methods were significantly correlated, but the cranked-wire data were systematically lower. 3. Productivity of Sphagnum at the low-N site increased almost fourfold after additional nitrogen supply (4 g N m-2 year-1), but no increase was found after additional phosphorus supply (0.4 g P m-2 year-1). At the high-N site, phosphorus supply caused an almost threefold productivity increase, but nitrogen supply did not result in any productivity increase. Thus, in an area with a high atmospheric nitrogen supply, plant productivity is P-limited instead of N-limited. 4. At an intermediate nitrogen supply (2 g N m-2 year-1) the recovery of the supplied nitrogen in the Sphagnum carpet was not different for both sites (60% and 69%, respectively). However, at a high nitrogen supply (4 g N m-2 year-1) nitrogen recovery at the low-N site significantly exceeded that at the high-N site (73% and 47%, respectively). At the low-N site, the supplied phosphorus was not recovered, but at the high-N site the phosphorus recovery was 85% (intermediate phosphorus supply) and 100% (high phosphorus supply), respectively. 5. It is suggested that a high atmospheric nitrogen supply may affect the carbon balance of ombrotrophic bogs, because productivity is under these circumstances not N-limited, but decomposition is probably increased by high loads of nitrogen. In the end, this may turn these C-accumulating systems into C-emitting systems.
