Rogers, Isabel and Stevens, Carly and Ostle, Nick (2016) Carbon storage in grasslands : the impact of atmospheric nitrogen pollution. PhD thesis, Lancaster University.
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Abstract
Atmospheric nitrogen deposition is a major threat to biodiversity and ecosystem service provision around the globe. It is known that nitrogen enrichment affects various chemical and biological processes involved in carbon cycling and storage in soil. This is especially significant as soil carbon storage is an essential form of climate change mitigation. However, there is a lot of uncertainty regarding the impact of nitrogen accumulation on terrestrial carbon storage. Determining the impacts of nitrogen addition on soil carbon is crucial to our understanding of how soil can be managed as a carbon sink. Evidence suggests that the chemical form of nitrogen may affect how grasslands respond to nitrogen enrichment. In addition, nitrogen has both direct and indirect (via plant community change) effects on carbon. In order to understand how nitrogen affects carbon storage, these different effects must be disentangled. By using two seven-year field nitrogen addition experiments, a microcosm incubation, and a two-year mesocosm study, this thesis aimed to investigate the effects of nitrogen addition on carbon cycling and storage in acid grasslands. Results show that reduced nitrogen is likely to have the strongest long-term effects on carbon storage, due to decreases in soil pH and potentially adverse effects of ammonium accumulation. Moreover, nitrogen addition was found to have a negative effect on soil respiration, possibly via nitrogen-enhanced carbon and phosphorus limitations, as well as possible effects of nitrogen-induced acidification. Results also suggest that nitrogen addition may have different direct and indirect effects on soil carbon. Indirect effects, driven by plant community change, strongly influenced inputs of fresh carbon to soil. However, direct effects of nitrogen could alter the storage of older, mineral-associated soil carbon. Finally, this thesis highlights the need for more long-term (over ten-year) studies in order to determine the true effects of nitrogen on soil carbon storage.