A topographic template for estimating soil carbon pools in forested catchments

There is a growing need to describe and quantify the heterogeneity in soil carbon pools in forests. In landscapes where topography is an important control on geomorphological, hydrological and/or biogeochemical processes, topographic features can be useful for partitioning the landscape into homogeneous units of soil carbon. Of particular interest are topographic features that are rare or cover a small proportion of the landscape but are disproportionately important in terms of soil carbon pools and/or fluxes. We developed an automated method for classifying topographic features by combining expert knowledge and a probabilistic approach. The topographic features were then used as a template from which to collect samples to estimate carbon of canopy leaves, freshly fallen litter, the forest floor, and the organic-rich A horizon or peat to a 5 cm depth within a small catchment of the Turkey Lakes Watershed of central Ontario. While the carbon in canopy foliage was homogeneous, there was significant heterogeneity in soil carbon pools among the topographic features, reflecting the importance of physical processes in shaping the distribution of soil carbon pools within this forested landscape. The catchment-aggregated estimates were under-estimated by 17% when only the dominant topographic feature was considered (i.e., 242 Mg C if backslope only used compared to 291 Mg C if all topographic features used). A Monte Carlo simulation method used to bound the uncertainty in soil carbon pools within each topographic feature resulted in catchment-aggregated estimates of 288 ± 56.0 (maximum probability) and 290 ± 51.3 Mg C (weighted probability) in the combined freshly fallen litter, forest floor, and the organic-rich A horizon or peat pool. The creation and application of this topographic template was useful for detecting, strategically sampling, and then mapping and scaling heterogeneity in soil carbon pools on the landscape.

Research Highlights
► Improvements in carbon accounting of forests urgently needed.
► Challenges lie in the heterogeneity of soil carbon pools.
► Soil sampling based on topographic features improves estimates of size and error of these pools.
► Failure to consider these features leads to substantial under-estimation of soil carbon pools.