Forest soil carbon stocks and life cycle assessment of short rotation forestry

Inventories of forest soil carbon (C) stocks are necessary to determine spatial and temporal C stock changes and support climate change mitigation policy development. Afforested podzols and peaty podzols were sampled to measure their soil organic carbon (SOC) concentration and bulk density (BD), with the aim of improving baseline soil organic carbon density (SOCD) estimates for Irish forests. Podzols are not always distinguished from peaty podzols and both qualify as mineral soil types. The estimated SOCD for the podzol sites ranged from 129–139 Mg C ha−1, while the peaty podzols had 229–385 Mg C ha−1. The major disparity in their SOCD implies the need to disaggregate podzols and peaty podzols in conducting soil C inventories, with the need for development of C emission factors for peaty podzols to reduce uncertainty in SOCD estimates. Soil bulk density (BD) is a principal component in estimating the stock of any soil nutrient or other substances, such as organic carbon. There are several well-known methods of estimating soil BD such as coring, clod, and pit excavation, with variants of each method covered in the literature. In many older soil surveys, for reasons related to cost or the time and resources available to complete the work, the soil BD was not measured, often due to a high volume of rock fragments (RFs) in the soil. This study used the core and pit excavation methods to sample short rotation forestry (SRF) soils and determine their SOC concentration, BD, and RF content, and used those to estimate the SOCD to a depth of 40 cm. Novel methods of sampling and calculating BD for soils with high RF content were also devised, with results that compared more favourably to the pit excavation results than provided by the core method. Using SRF biomass for bioenergy has the potential to contribute to the Irelands increasing commitments under the EU Renewable Energy Directive to use more renewable energy and reduce greenhouse gas (GHG) emissions by 2020 and beyond. When sustainable forest management practices are employed SRF can also help offset Ireland’s GHG emissions from the combustion of decreasing reserves of peat and fossil fuels through their displacement in industrial power plants. This study investigated the GHG balance of a eucalyptus (E. nitens) SRF plantation over three 10 year rotations through the use of Life Cycle Assessment (LCA) methodology, combined with specialised LCA software tools and databases. The mean GHG balance of the SRF scenarios (i.e. 2441 t CO2-eq ha−1 and 1135 t CO2-eq ha−1, at the 30 % and 50 % co-firing rates, respectively) are both approximately seven times greater than the GHG balance of the Sitka spruce scenarios (i.e. 339 t CO2-eq ha−1 and 168 t CO2-eq ha−1, at the same respective co-firing rates). Hence, the mean GHG emission reductions from the SRF biomass scenarios outperform those from the Sitka spruce scenarios at both the per-MWhe and per-hectare levels.