Interactive effects on organic matter processing from soils to the ocean: are priming effects relevant in aquatic ecosystems?

Organic matter (OM) is degraded during transport from soils to oceans. However, there are spatial and temporal variabilities along the aquatic continuum, which hamper the development of carbon cycling models. One concept that has been applied in this context is the priming effect (PE), describing non-additive effects on OM degradation after mixing sources of contrasting bioavailability. Studies on the aquatic PE report divergent results from positive (increased OM degradation rates) to neutral, to negative (decreased OM degradation rates) effects upon mixing. Here, we aim to condense the outcomes of these studies on aquatic PE. Based on a literature review, we discuss differences in the reported PEs across freshwater and marine ecosystems, identifying system-specific features that could favour non-additive effects on OM degradation. Using a quantitative meta-analysis approach, we evaluated the occurrence, direction (positive vs. negative) and magnitude of aquatic PE. The meta-analysis revealed a mean PE of 12.6%, which was not significantly different from zero across studies. Hence, mixing of contrasting OM sources in aquatic ecosystems does not necessarily result in a change in OM degradation rates. Therefore, we suggest to focus on molecular and microbial diversity and function, which could provide a better mechanistic understanding of processes driving OM interactions.