In the last decade, several studies questioned the becoming of Terrestrial Organic Matter, such as transported soils, in aquatic systems. Soil Organic Matter (SOM) is classically considered as more recalcitrant than autochtonous organic matter. However, SOM contributes largely to carbon budget of aquatic ecosystems, mainly via the incorporation of soil organic carbon by bacteria. In soil sciences, it is now well established that presence of Labile Organic Matter (LOM), such as glucose or roots exudates, can influence SOM mineralization, this phenomenon being called Priming Effect (PE). Despite the abundance of LOM (e.g. transparent exo-polymers or algal exudates) in aquatic ecosystems, occurrence of PE during SOM processing in these ecosystems has almost never been evoked. In this study, we experimentally tested if PE can occur in aquatic environments. Microcosms were amended with two levels of nitrogen and phosphorus inputs, and 50mgC from four soil types were introduced in each microcosm. Selected soils represent three major temperate agro-ecosystems (forest, meadow and agricultural soil) and a bare fallow soil since 1929 considered as a model of the recalcitrant SOM. To simulate the effect of LOM on SOM mineralization in aquatic media, half of the microcosms were amended with 13C-glucose. SOM and glucose mineralization were distinctly monitored during 45 days.
Results/Conclusions
The 13C-glucose addition led to a SOM over-mineralization compared to control without glucose excepted for forest SOM that was under-mineralized in presence of 13C-glucose. When comparing SOM C:N ratios, the forest SOM was largely depleted in N compared to the others. We hypothesize that the effect of LOM on SOM mineralization in lakes may depend on its C:N ratio. LOM may serve as energy to synthesize enzymes able to mineralize nutrient-rich SOM. However, when SOM is nutrient-poor, synthesis of SOM-degrading enzymes might be limited. Our results also showed that PE was more important in the nutrient-rich than in the nutrient-poor systems, suggesting that synthesis of SOM-degrading enzymes was certainly co-limited by energy and nutrients (N and/or P). Because no theoretical constraints limit PE application to terrestrial systems, we hypothesize that it could be an important process in aquatic systems that could explain the elevated rates of SOM mineralization frequently reported. Currently, human activities tend to increase soil erosion and input into aquatic systems. Aquatic PE could contribute for a large part to SOM mineralization, and could thus play an important role in the global carbon cycle.
