Tagasaste (Cytisus proliferus Link.) reforestation as an option for carbon mitigation in dryland farming systems

The Agriculture, Forestry and Other Land Use Sector (AFOLU) plays a major role in national and international strategies to manage increasing global greenhouse gas emissions. This study investigated the option of increasing carbon storage in biomass and poorly productive soils in dryland agricultural systems, while avoiding competition with food production, using tagasaste (Cytisus proliferus Link.), a woody N-fixing perennial species. Perennial plants often have deeper and more extensive root systems than annual plants, and therefore may increase soil organic carbon (SOC) stocks deeper than the IPCC standard sampling depth of 0.3m. Above- and below-ground biomass carbon and SOC to a depth of 2 m were measured on a 22-yr-old replicated field experiment in Western Australia (mean annual rainfall, 498 mm yr−1) comparing unmanaged plantations of tagasaste with conventional annual crop and pasture rotations. Carbon sequestration was 2.5 Mg C ha−1 yr−1 over the 22-yr lifespan for the tagasaste treatments, with a change of 0.9 Mg C ha−1 yr−1 in SOC and 1.6 Mg C ha−1 yr−1 in biomass. Tagasaste plots contained significantly larger SOC stocks compared with control plots for soil to 0.9 m, however beyond this depth, treatment differences were not significant. It is recommended that soil sampling be extended to depths of 1 m under such perennial systems with no benefit from sampling to depths deeper than this. In contrast to its current use as a fodder supplement for livestock, this study clearly demonstrates the potential of tagasaste for carbon mitigation within dryland farming systems, especially on soils marginal for conventional agriculture.