The effect of soil P on N2O emission and N-cycling in Irish grasslands

This research aims to address nutrient use efficiency by investigating the key nutrients; carbon, nitrogen and phosphorus relative to one another the effect that imbalanced nutrient supply has on N2O emissions. To first address if there was in fact any effect to be observed, two short-term incubation experiments were set up using soils of high and low phosphorus concentrations were taken from a long-term cut field trial. The soils were incubated in a climate controlled chamber and treated with a typical field rate of N fertiliser, further a split treatment of carbon was added such that the microcosms in the first incubation did not receive carbon and those in the second incubation received both carbon and nitrogen. The results of these incubations clearly demonstrated that carbon deficiency inhibited all activity with negligible CO2 and N2O emissions being recorded from the carbon-omitted microcosms at both phosphorus levels, whereas P deficiency had an impact on nitrogen cycling in particular. When both carbon and nitrogen were added to soils of low phosphorus there was a 70-fold increase in N2O emissions compared to the high phosphorus microcosms with no significant difference between CO2 levels. This was a very important result as it suggested a separation between activity and process-based inhibition, both of which are essential to balance for a healthy soil system. Expanding on this research led to a third incubation experiment, carried out on soils taken from the same field trial, incubated under the same climate conditions and treated with the same rates of C and N fertiliser. The nitrogen fertiliser applied however, carried an isotopic 15N label to identify the applied nitrogen from the more abundant 14N present in the atmosphere, such that the mineral nitrogen and N2O results could be used to trace the nitrogen transformation processes occurring in these soils in order to establish the dominant pathways contributing to N2O production. This experiment identified a previously unreported pathway of oxidation of labile nitrogen via heterotrophic nitrification being the dominant transformation rate occurring in both phosphorus levels but at a significantly greater rate in the low phosphorus soils. Alongside all of these incubations, subsamples from the microcosms were collected and stored for molecular analysis. The genetic abundance of key nitrogen cycling genes as well as bacterial, archaeal and fungal community identifying genes were quantified by extracting DNA from the soil samples and quantifying genetic abundance using qPCR analysis. This revealed varying abundances of different genes between phosphorus levels with nitrifier genes found to be most abundant in low phosphorus soils and denitrifier genes to be more abundant at high phosphorus soils. Furthermore, this analysis revealed a positive correlation between the fungal community and N2O emissions from the low phosphorus soils, which was the only correlation observed in the low phosphorus soils. The final experiment up-scaled this research to the field level and carried out N2O measurements from static chambers which were installed in another long-term phosphorus trial under a grazed management regime. The results of this trial confirmed those of the laboratory in that low phosphorus results in greater N2O emissions and lower nutrient use efficiency. The results of this thesis offer an immediately implementable area of nutrient management that can be used to maximise efficiency, and reduce wastage which benefits both environmental mitigation goals as well as financial savings. These results also highlight further areas to direct future research to, broadening the knowledge base on nutrient interactions can offer further mitigation and efficiency-based land management which contribute to national and international environmental improvement goals and emission-reducing targets