Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Abstract

Soil CO2 efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO2 efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO2 efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO2 efflux, and determine their environmental correlates. Total soil CO2 efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha−1 yr−1 in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO2 efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha−1 yr−1. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO2 flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO2 efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO2 efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO2 efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO2 m−2 s−1 °C−1. For mean peak summer efflux rates (2.03 μmol CO2 m2 s−1), this is equivalent to 8% per °C, or a Q10 temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO2 efflux primarily through its positive relationship with SOM-decomposition rate.