Is there a linear relationship between priming effect intensity and the amount of organic matter input

Inputs of fresh organic matter (FOM) are known to affect the rate of soil organic matter (SOM) mineralization. SOM mineralization can be accelerated or decelerated by FOM inputs. This phenomenon, known as the Priming effect (PE), may largely influence the carbon (C) storage capacity of soils. However, the link between PE intensity and FOM inputs is not clearly understood. Indeed, almost all the studies about PE used only one FOM amount which is generally largely below the amount of FOM observed in field conditions. In our study, we incubated soil amended with three levels of ¹³C-labeled straw as FOM and a control without FOM amendment for 80 days. The three levels used were in the same range as the natural FOM inputs observed on our sampling site. Various levels of mineral nitrogen were added within each level of straw supply so that the final input C:N ratios ranged among 44, 30 and 20. CO₂ and δ ¹³C-CO₂ were measured during the experiment allowing us to distinguish the FOM respired CO₂ from the SOM respired CO₂. We observed that PE intensity did not increase linearly with increasing FOM additions. Moreover, decreasing the input C:N ratios did not systematically affect PE intensity probably because of shifts in the microbial characteristics such as their C:N ratio or their assimilation yields. These results suggest that PE is a saturating function of FOM inputs that is only weakly influenced by initial N availability. Our results may be explained (i) by the existence of a limited SOM pool subject to PE (ii) or by the occurrence of two simultaneous and antagonistic mechanisms: an increase of the total active microbial biomass accelerating SOM mineralization (i.e. a positive PE) and a preferential substrate utilization of FOM over SOM decreasing SOM mineralization (i.e. a negative PE). Finally, irrespective of the mechanisms implied, our results suggest that the importance of positive PE relatively to the amount of FOM may decrease when FOM inputs increase, which is favorable to carbon sequestration in soils. Indeed, in the case of the lower amount of FOM, the PE corresponded to 6.25% of the total amount of CO₂ mineralized at the end of the experiment while, for the higher amount of FOM, the PE corresponded to 5% of the total amount of CO₂ mineralized at the end of the experiment.