Soil respiration: implications of the plant-soil continuum and respiration chamber collar-insertion depth on measurement and modelling of soil CO2 efflux rates in three ecosystems

Key uncertainties remain in accurately measuring soil respiration, including how the commonly-used techniqueof collar insertion affects measured soil and root-derived CO2 fluxes. We hypothesized that total soil respirationis frequently under-estimated because soil collar insertions sever surface roots, which coupled with thepreferential practice of taking daytime measurements, leads to the autotrophic (root-derived) componentfrequently being missed. We measured root distribution and soil CO2 efflux in three contrasting ecosystems:a Lodgepole pine (Pinus contorta) plantation, an upland heather-dominated peatland and a lowland sheepgrazedgrassland, where we combined shallow surface collars with collars at different soil insertion depths foroccasional and continuous hourly flux measurements. Collar insertion by only a few centimetres reduced totalsoil CO2 efflux in all three ecosystems by an average of 15% but at times by up to 30–50%, and was directlyproportional to the quantity of cut fine roots. Most reduction occurred in the shallow-rooted peatland systemand least in the deep-rooted grassland. In the forest and grassland, soil temperatures explained most of thedeep-collar (largely heterotrophic) variation and did not relate to the root-derived (largely autotrophic) fluxcomponent, whilst the opposite was true for the peatland site. For the forest, the autotrophic flux componentpeaked at night during moist periods and was drought-limited. Mean flux estimates differed between samplingtime and insertion depth. Our results suggest strongly that accurate measurement and modelling of soilrespiration needs explicitly to consider collar insertion, and the root-derived flux component, with its owntemperature sensitivity and potential time-lag effects.