Assessing the magnitude of soil carbon enhancement under canopy in an arboretum environment

Soil organic carbon (C) is a larger and more stable C storage pool than terrestrial biomass. Some trees are very effective in transferring C from the atmosphere to the soil, particularly soil under canopy. As a consequence, in environments containing a variety of trees, the distribution of soil organic C is patchy at the 1-10m scale making it challenging to quantify at the larger scales relevant for C sequestration accounting. The aim of this study was to develop and test a process for assessing soil organic C in such environments. An initial set of 53 soil cores was collected in open spaces across the Waite Arboretum in Adelaide, Australia, which contains over 800 different tree species across 27ha of parkland. Conventional soil organic C analysis indicated variation in organic C concentration of the 0-10cm soil layer through the 2-5% range with an average soil organic C concentration of 3.19%. A reliable model to predict soil organic C concentration from mid-infrared (MIR) spectra from further soils collected in the Waite Arboretum was developed using partial least squares regression (PLSR). Follow-up sampling targeted 144 soils collected in a radial pattern under and close to tree canopies and the soil organic C values obtained from MIR-PLSR prediction of these soils confirmed the open-space average (3.19%) is a substantial underestimate, with higher average values at half canopy (7.47%) canopy edge (5.10%) and twice-canopy distance (3.84%) for twelve selected trees of different species. Importantly, the elevation in under-canopy organic C concentration varied substantially from tree to tree. This study highlights the challenge posed in assessing soil organic C in urban forest environments, confirms soil organic C storage varies with tree species and demonstrates the value of using mid-infrared prediction in tandem with traditional chemical analyses for assessing soil organic C.