Carbon mitigation from agroforestry in salinized low rainfall farmland landscapes

Efforts to reduce greenhouse gas emissions have become a global priority and the land sector can contribute significantly to achieving this via a range of mitigation strategies such as the biosequestration of carbon and substitution of fossil fuels through bioenergy. However, the implementation of land sector mitigation is constrained by several uncertainties and knowledge gaps particularly within low rainfall (300 to 400 mm yr-1) farmland environments. This thesis examines aspects of land sector mitigation through reforestation systems integrated into dryland (300 to 400 mm yr-1) farming systems in Western Australia. The uncertainties that are examined in this thesis include (a) estimation of tree root carbon storage, (b) exploring whether carbon mitigation can be achieved through new agroforestry systems that reduce competitive effects and (c) assessing the sustainability of these new systems in terms of nutrient removal. Estimates of below ground biomass pools are critical to establishing carbon fluxes on regional scales which can then be applied in global modeling of climate change mitigation strategies. A new methodology for tree below-ground biomass estimation was developed, including a purpose-designed coring machine. Monte Carlo simulation was used to assess the accuracy of a range of sampling regimes through estimates of uncertainty (precision) and bias (error) and these sampling methods were subsequently used to develop allometric relationships to estimate the carbon mitigation potential of tree phases integrated into agricultural systems. The implications of integrating tree phases into agricultural systems and the effects of this on the sustainability of existing farming systems were investigated. This included an assessment of potential land use synergies targeting abandoned or marginal land for multiple land use outcomes via landscape rehabilitation and carbon mitigation. The integration of short tree phases (3 years) into low rainfall salinized farmland for the purpose of soil salinity amelioration was shown to have additive environmental benefits as a potential source of biomass feedstock for renewable energy. Allometric relationships were developed for three candidate species (Eucalyptus globulus, E. occidentalis and Pinus radiata) and their carbon storage was assessed based on whole tree destructive sampling, including below ground sampling. The biomass production for different planting density and landscape placement strategies, and for different tree components was estimated to assist in future development of harvesting systems and management of nutrient removal. It was shown that tree phases inserted into farming systems for the purpose of ground water control could potentially serve as a biomass feedstock for renewable energy, either bioenergy for power generation or as feedstock for lignocellulosic (second generation) biofuel, thus offsetting the use of non-renewable fossil fuel. The sustainability of these systems was investigated to determine their impact on current farming systems and the potential removal of nutrients. Harvesting regimes that remove woody biomass while retaining leaves on site are likely to be more sustainable from a nutrient management perspective. A nutrient assimilation index was developed for these short rotation tree crop systems to aid the management of nutrient removal. The removal of nutrients via a short (3 year) tree phase was less than the cereal cropping systems currently in place and had potential to retrieve leached nutrients from deeper in the soil profile. Planting of tree and shrub species in severely salinized abandoned farmland was shown to be a potential avenue for carbon mitigation, and a resultant positive land use change. With species selection, management of stand density and landscape position, tree growth and carbon sequestration can be manipulated with rates of sequestration of 1.1 to 2.3 t ha-1 yr-1 following 8 years growth in the highly saline environment. A combination of shrub (Atriplex nummularia) and tree (Eucalyptus occidentalis) species were used to mimic natural saline wetland succession and were successful in rehabilitating degraded farmland while effectively sequestering carbon and mitigating atmospheric CO2. The challenge remains to integrate these mitigation initiatives and systems into existing economic and social environments and for them to be accepted as typical economic activities. This is not only a challenge from the scientific view point, but encompasses social and political aspects which often makes its application difficult.