Estimates of rhizosphere priming effects are affected by soil disturbance

Laboratory studies have shown that priming effects, caused by inputs of carbon into the rhizosphere, can change the rate of soil organic matter (SOM) decomposition and could have significant impacts on soil carbon cycling. However, there have been few studies in field conditions because of experimental constraints but field data are needed to improve models that forecast the effects of climate change on SOM decomposition rates and the impact of these changing rates on atmospheric CO 2 concentration. In 2009 a fire at a Eucalyptus forest site in Australia killed all standing vegetation. Trenched plots were installed in 2010, approximately 12 months after the fire, and were maintained plant-free for the subsequent year. In 2011, after forest re-growth outside the trenched plots, we compared SOM decomposition rates in the presence of plants (rooted plots) and in the absence of plants (trenched plots) using a natural abundance stable carbon isotope technique with minimal disturbance of the soil. We then compared our results to those obtained in another study conducted at the same time and the same plots using laboratory incubations of sieved soil samples. There was no difference in SOM decomposition rates between the trenched and the rooted plots estimated using our non-disruptive technique. In contrast, laboratory incubations of sieved soils highlighted a two-fold increase in SOM decomposition rates in the rooted plots compared with rates from the trenched plots. Our results suggest that rhizosphere priming may not actively influence soil carbon turnover in the undisturbed soil environment and question conclusions from laboratory incubation studies. We attribute the different findings from laboratory and field studies to the physical disturbance of the soil involved in laboratory incubations causing the release of previously protected substrates, making them available for decomposition