Separating the temperature response of soil respiration derived from soil organic matter and added labile carbon compounds

Abstract

Broadly, soil organic C can be characterised as either labile (highly available and soluble) or stable (physiochemically protected) carbon. Previous studies have attributed differences in the temperature response of these C pools to overall availability or chemical complexity. Here, we estimated the decomposition of the labile pool as the difference of CO2 from soils with added highly available C substrates (arginine, glucose, glutamine, lysine, and maltose) and yeast extract, and CO2 from soil without added C. We also measured the temperature response of the decomposition of soil organic matter (SOM), which encompasses both labile and stable C (no added C). Soil was incubated for five hours using a temperature gradient block consisting of 18 discrete temperatures (~8–52 °C). We also tested to see if temperature responses differed between three contrasting soils (allophanic, gley, and organic/degraded peat). We applied macromolecular rate theory (MMRT) to the RS data to calculate the temperature optimum (Topt) and the inflection temperature (Tinf).The decomposition of SOM had an exponential, Arrhenius-like behaviour. In contrast, respiration of the five labile C compounds had a clear and similar Topt of around 37 °C and Tinf of 22 °C. The temperature response of the more labile C and of the SOM pools did not significantly vary between the three very different soils, suggesting that these responses might be independent of soil properties.Our findings provide support for the hypothesis that the overall temperature dependence of soil respiration is reliant on sorption/desorption and diffusion to microbes – a process that exhibits Arrhenius behaviour, and the biological decomposition of C by these soil microbes that exhibits MMRT behaviour. The overall response to temperature is governed by substrate availability as biological processes dominate when substrate is non-limiting, and physiochemical processes dominate when substrate availability is limiting.