Temperature and soil management effects on carbon fluxes and priming effect intensity

Any change in the intensity and sign of CO₂ flux between soil and atmosphere is expected to have a significant impact on climate. The net emission of CO₂ by soils depends on antagonistic processes: the persistence of dead plant matter and the mineralization of soil organic matter. These two processes are partly interdependent: their interaction is known as the “priming effect” (PE), i.e. the stimulation of the mineralization of stable soil organic matter by more labile fresh organic matter. Documenting the response of PE to global change is needed for predicting long term dynamics of ecosystems and climate change. We have tested the effects on PE of temperature, nutrient availability, biodegradibility of added organic matter (fresh vs. decomposed), soil cover (agricultural vs. forest soil) and interactions. Our results suggest that the biodegradability of plant debris (wheat straw, fresh or pre-decomposed) is the first determinant of the intensity of PE, far ahead of temperature and nutrients: fresh wheat straw addition induced up to 800% more CO₂ emission than pre-decomposed one. The raise of temperature from 15 to 20 °C, increased basal soil organic matter mineralization by 38%, but had little effect on PE. Interactions between biodegradability of straw and the other factors showed that the agricultural soil was more responsive to all factors than the forest soil. We have shown in our study that the intensity of PE could be more dependent on soil cover and plant residue management than on other drivers of global change, particularly temperature and nutrients. There is an urgent need to assess the genericity of our results by testing other soil types and plant debris for a better integration of PE in models, and for identifying alternative land carbon management strategies for climate change mitigation.