Both priming and temperature sensitivity of soil organic matter decomposition depend on microbial biomass – An incubation study

The effect of temperature and the influence of fresh substrate addition on soil organic matter decomposition are two key factors we need to understand to forecast soil carbon dynamics under climate change and rising CO2 levels. Here we perform a laboratory incubation experiment to address the following questions: 1) Does the temperature sensitivity differ between freshly added organic matter and bulk soil carbon? 2) Does the addition of fresh organic matter stimulate the decomposition of soil organic matter (“priming effect”)? 3) If so, does this priming effect depend on temperature? In our study, we incubated sieved soil samples without and with two labelled plant litters with different 13C signals for 199 days. The incubations were performed with two diurnal temperature treatments (5–15 °C, 15–25 °C) in a flow-through soil incubation system. Soil CO2 production was continuously monitored with an infrared gas analyser, while the 13C signal was determined from gas samples. Phospholipid fatty acids (PLFA) were used to quantify microbial biomass. We observed that the instantaneous temperature sensitivity initially did not differ between the original and the amended soil. However in the amended treatment the temperature sensitivity slightly but significantly increased during the incubation time, as did the PLFA amount from microbial biomass. Further, we found that addition of fresh plant material increased the rate of decomposition of the original soil organic matter. On a relative basis, this stimulation was similar in the warm and cold treatments (46% and 52%, respectively). Overall our study contrasts the view of a simple physico-chemically derived substrate–temperature sensitivity relationship of decomposition. Our results rather request an explicit consideration of microbial processes such as growth and priming effects.