Soil respiration on an aging managed heathland: identifying an appropriate empirical model for predictive purposes

Heathlands are cultural landscapes which are managed through cyclical cutting, burning or grazing practices. Understanding the carbon (C) fluxes from these ecosystems provides information on the optimal management cycle time to maximise C uptake and minimise C output. The interpretation of field data into annual C loss values requires the use of soil respiration models. These generally include model variables related to the underlying drivers of soil respiration, such as soil temperature, soil moisture and plant activity. Very few studies have used selection procedures in which structurally different models are calibrated, then validated on separate observation datasets and the outcomes critically compared. We present thorough model selection procedures to determine soil heterotrophic (microbial) and autotrophic (root) respiration for a heathland chronosequence and show that soil respiration models are required to correct the effect of experimental design on soil temperature. Measures of photosynthesis, plant biomass, photosynthetically active radiation, root biomass, and microbial biomass did not significantly improve model fit when included with soil temperature. This contradicts many current studies in which these plant variables are used (but not often tested for parameter significance). We critically discuss a number of alternative ecosystem variables associated with soil respiration processes in order to inform future experimental planning and model variable selection at other heathland field sites. The best predictive model used a generalized linear multi-level model with soil temperature as the only variable. Total annual soil C loss from the young, middle and old communities was calculated to be 650, 462 and 435 g Cm-2 yr(-1), respectively.