The role of terrestrially derived organic carbon in the coastal ocean: A changing paradigm and the priming effect

Abstract

One of the major conundrums in oceanography for the past 20 y has been that, although the total flux of dissolved organic carbon (OC; DOC) discharged annually to the global ocean can account for the turnover time of all oceanic DOC (ca. 4,000–6,000 y), chemical biomarker and stable isotopic data indicate that there is very little terrestrially derived OC (TerrOC) in the global ocean. Similarly, it has been estimated that only 30% of the TerrOC buried in marine sediments is of terrestrial origin in muddy deltaic regions with high sedimentation rates. If vascular plant material—assumed to be highly resistant to decay—makes up much of the DOC and particulate OC of riverine OC (along with soil OC), why do we not see more TerrOC in coastal and oceanic waters and sediments? An explanation for this “missing” TerrOC in the ocean is critical in our understanding of the global carbon cycle. Here, I consider the origin of vascular plants, the major component of TerrOC, and how their appearance affected the overall cycling of OC on land. I also examine the role vascular plant material plays in soil OC, inland aquatic ecosystems, and the ocean, and how our understanding of TerrOC and “priming” processes in these natural systems has gained considerable interests in the terrestrial literature, but has largely been ignored in the aquatic sciences. Finally, I close by postulating that priming is in fact an important process that needs to be incorporated into global carbon models in the context of climate change.

The role of waterborne carbon in the greenhouse gas balance of drained and re-wetted peatlands

“…Combining this estimate with a global estimate by Meybeck (1982) that 40% of terrestrial organic matter enters the ocean as POC, and assuming that DOC is preferentially oxidised, yields a range for ocean POC oxidation of 44  19%. This is consistent with the estimate of Bianchi (2011) that 50% of terrestrial POC inputs are remineralised. Assigning arbitrary 25% ranges on values where uncertainties were not specified (reflecting also our reliance on data that were not specific to peat-derived POC in many cases), the combination of these yields an overall FracPOC-CO2 of 0.70, with a range of 0.49 to 0.91 (Figure 4).…”

“…For terrestrial DOC that does reach the ocean, it appears that (although specific values for peat-derived DOC are not available) most DOC is microbially processed, on a timescale of years to decades (e.g. Bianchi, 2011;Opsahl and Benner, 1997). Burdige (2005) estimated that sedimentation accounted for only around 15-30% of all (dissolved and particulate) terrestrial organic matter inputs to the ocean, with the remainder mineralised to CO2.…”

Metabolic interdependencies between phylogenetically novel fermenters and respiratory organisms in an unconfined aquifer

“…A similar phenomenon, where the addition of labile carbon-stimulated decomposition of more recalcitrant carbon, described as 'priming', has been well documented in soils and marine systems, but has yet to be defined in the terrestrial subsurface (Bianchi, 2011). Priming can be caused by direct or indirect mechanisms (Kuzyakov et al, 2000); here we suggest the latter is more likely.…”

River organic matter shapes microbial communities in the sediment of the Rhône prodelta

“…However, sediment organic matter (OM) is a complex and heterogeneous mixture of high-and low-molecular-weight organic carbon compounds derived from multiple sources (for example, marine or terrestrial) at different states of degradation. Generally, land-derived OM are thought to be more recalcitrant to degradation than plankton-derived OM (Hedges et al, 1997;Baldock et al, 2004), but in environments with a mixture of both kinds, different microbial groups can allow refractory OM to be metabolized through mutualistic relationships (Bianchi, 2011). Such relationships are, however, poorly understood as microbial communities are rarely investigated in the same context as OM source or OM quality, defined here as composition and lability.…”