Aims Although the effects of N addition on plant biomass are well understood, we know a lot less about the importance of N form even though some studies have shown different impacts from reduced and oxidized forms of N. Furthermore, responses to grazing are likely to interact with the response to N addition. This experiment investigates the interactive effects of N addition and form with clipping on competition between three grassland species.

Methods The three species (Anthoxanthum odoratum L., Plantago lanceolata L. and Prunella vulgaris L.) were grown alone and in combination with factorial additions of deionized water, sodium nitrate and ammonium chloride, and a clipping treatment. Above- and belowground biomass was harvested after 4 months.

Important Findings In monocultures, the results show increases in biomass with N addition, but clipping resulted in fewer changes with species displaying varying degrees of growth compensation. A. odoratum was the strongest competitor when grown with other species. In monocultures without clipping, N form was not important, but in the presence of clipping and in different species combinations, N form became important. Significant two- and three-way interactive effects were observed showing that complex interactions exist between N addition, clipping and species identity. The results have important implications when considering the effects of N deposition.