The Nitrogen Dynamics of Multi-species Grasslands when Subject to Drought and Re-wetting.

Abstract

<p>It is predicted that climate change will result in more extreme and frequent weather events including flooding and drought. Nitrous oxide (N<sub>2</sub>O) is a potent greenhouse gas having 298 times the global warming potential of CO<sub>2</sub>. The ‘Birch effect’, the term given to high  N<sub>2</sub>O fluxes following the drying and re-wetting of soils, is an accelerator of this process. Multi species grasslands have been shown have higher nitrogen use efficiency and potential for drought resilience and recovery. This experiment analysed the nitrogen dynamics of multi-species grasslands by means of quantifying the responses of soil mineral nitrogen (NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>-) </sup>and N<sub>2</sub>O fluxes during an eight week simulated drought, re-wetting and fertiliser application two weeks after the re-wetting event. A simplex experimental design was used to determine species and functional group effects which could potentially influence responses. The hypothesis of this study was therefore that multi species grasslands would mitigate the ‘Birch effect’ resulting in less erratic transformations of soil mineral nitrogen and lower N<sub>2</sub>O fluxes compared to monocultures. This study also predicted a lasting legacy effect of drought on soil systems resulting in prolonged heightened N<sub>2</sub>O fluxes. Drought resulted in a depletion of soil NO<sub>3-</sub>, increased  levels of NH<sub>4</sub><sup>+ </sup>and background level N<sub>2</sub>O emissions. Following re-wetting soil mineral N underwent transformations from NH<sub>4</sub><sup>+</sup> to NO3- indicating nitrification. Four times more N<sub>2</sub>O emissions were recorded during re-wetting period compared to fertilizer application. There was no lasting legacy effect of drought and re-wetting on N<sub>2</sub>O fluxes observed during fertilizer application two weeks after re-wetting bar T. repens which has implications for grassland management strategies.</p>