This Topic has been realized in collaboration with Dr Elise Morrison, Postdoctoral Research Associate at University of Florida.

The disciplines of terrestrial and aquatic biogeochemistry have previously developed along separate paths of strengths/weaknesses with different research foci. Although this continues today, several focus areas have recently brought more cross-fertilization between these disciplines, including investigations into the controls on biogeochemical processes across “critical zones” in the Anthropocene, and the role of priming processes in terrestrial and aquatic organic matter (OM) cycling. While much of this work began in terrestrial systems, aquatic researchers have recently followed suit, and the process of priming has gained more attention in aquatic systems. Priming can be defined as the enhancement of recalcitrant (stable) OM breakdown via microbial decay with the addition of a more labile (less-stable) OM sources. Priming can involve dissolved and/or particulate OM, in some cases accompanied by nutrients, and results in more efficient decay/consumption of stable OM material than would have occurred otherwise in the absence of the less-stable OM. This general concept of priming was first explored in soil humus mineralization about 100 years ago but has now been expanded to encompass numerous systems.

Numerous analyses of soil carbon dynamics, as well as emerging research on aquatic systems, have established that positive priming results from bacterial production of hydrolytic enzymes in response to uptake of low molecular mass substrates or substrates derived from readily degradable polymeric organics. Hydrolytic enzyme production in turn stimulates the formation of additional low molecular mass substrates through the degradation of existing polymer pools that might be compositionally unrelated to the inducing substrates. For example, monomeric sugars and amino acids may stimulate synthesis and excretion of lignin-degrading enzymes, producing simple aromatics and degradation by-products. The net effect is that priming enhances the mineralization of resistant or recalcitrant polymers along with those that are more labile. While the magnitude of priming remains uncertain, it is clear that it can affect soil carbon sequestration and respond positively or negatively to a variety of disturbances.

Observations of priming in aquatic environments are becoming more widespread, and recognition of this process may partially explain observations of greater consumption rates of terrestrial OM in inland and coastal waters than reported previously from human influences on nutrient cycles. Initial efforts have been made in the literature to address the microbial mechanisms resulting in priming effects, but we still know very little about the microbial functions related to priming and how they respond in different environments. While numerous studies have suggested that aquatic priming is important across the aquatic continuum, the implications of this effect on globally-relevant CO2 fluxes remains unclear. There is currently no clear consensus on the role of aquatic priming, as recent studies have observed positive priming, no priming at all, and negative priming effects in various settings.

This Research Topic will bring together research on priming effects from both terrestrial and aquatic biospheres, and we welcome submissions of original research articles that investigate priming in either aquatic or terrestrial systems to set a common path forward for understanding global implications of priming effects and coupled terrestrial-aquatic biogeochemical cycling more broadly.

The image shows the confluence of the Amazon and Tapajós rivers, where priming effects have been previously observed. Copyright by Topic Editor Dr. Nicholas Ward.

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