Assessment and modeling of the impact of land use on soil properties, and carbon and nitrogen stocks in Irish peatlands

Peatlands are carbon and nitrogen stores of global importance. The vast majority of carbon and nitrogen in peatlands is stored within peat soil (~98%). However, globally around 12% of peatland carbon reservoirs are affected by anthropogenic disturbance with the proportion of peatlands drained for land use highest in Europe. This drainage leads to huge carbon losses and changes in the nitrogen balance of peatlands and is estimated to generate ~5% of global greenhouse gas emissions. The sequestration function of peatlands is believed to be a suitable mechanism for mitigation of climate change when appropriately managed. However, land use impacts often are detrimental for the physical and chemical conditions of peat and therefore affect carbon and nitrogen storage in peatlands. In the Republic of Ireland nearly 1.5 million hectares of ombrotrophic peatland cover more than 20% of the land area. However, the role of land use onto peat soil properties and carbon and nitrogen stocks has not yet been investigated in a representative manner for the entire country. The major objective of this thesis was to identify land use impacts on Irish ombrotrophic peat soils and to investigate how carbon and nitrogen stocks are affected by land use activity. It presents countrywide estimates of major peatland and peat soil properties, based on data from a large peatland survey. Within the present thesis significant effects of land use- induced drainage were found, influencing carbon storage throughout the country. Results confirmed that land use affects physical and chemical peat soil properties interchangeably. Interactions with a strong tendency of following a well-known pattern of primary and secondary subsidence were pointed out and compaction of peat and carbon losses through drainage, accompanied by increased ash contents and pH were identified. Topsoil carbon contents were reduced by 14-25%, while a more than three-fold increase of bulk density indicated strong compaction. Overall, carbon densities were largest in topsoils of intensive grassland, however; carbon mass loss could be shown with increasing carbon densities. This compaction with carbon loss was larger in systems of intensive grassland management than in extensive systems, highlighting the importance of adapted grassland management for climate change mitigation. Nitrogen was found to be enriched in topsoils of managed peatlands, but it was difficult to isolate a clear influence of land use due to the likely effects of atmospheric N fixation and deposition. Carbon stocks were found to be nearly double as large as previously estimated and 2032 Mt carbon were estimated for soils of Irish ombrotrophic peatlands. Nitrogen storage amounted to around 73 Mt. Half of this carbon and more than one third of nitrogen are currently stored in peatlands under domestic extraction, showing the importance of a shift away from energy peat. The densest carbon store was found in natural raised bog (~1917 t ha-1), mainly due to very deep peat deposits. Peat depth was shown to be the major determinant of peat  carbon and  nitrogen stocks and models with depth as the single predictor performed acceptably well to estimate carbon stocks across ROI. While the established datasets provide suitable baselines for future carbon and nitrogen monitoring, it is recommended that estimates of peat carbon stocks may be improved through countrywide peat depth measurements. The present study provides a suitable modeling framework and models for the prediction of carbon contents and stocks across the Republic of Ireland. However, no suitable model could be found for nitrogen contents and stocks, so that further research may be required to better understand nitrogen dynamics and storage in Irish peatlands.