Organic Matter Degradation across Ecosystem Boundaries: The Need for a Unified Conceptualization
Authors: Dolly N. Kothawala, Anne M. Kellerman, Núria Catalán, Lars J. Tranvik
Journal: Trends in Ecology & Evolution (2020)

Conceptualizations of organic matter (OM) degradation are diverging within and among the soil, freshwater, and marine subdisciplines.There is a need to step back from conceptualizations specific to certain ecosystems to holistically understand what controls the degradation of OM, regardless of the ecosystem.Ecosystem properties are changing rapidly due to anthropogenic stressors, and we encourage the development of conceptual and earth system models to be more flexible and consider conditions that may currently seem atypical for a particular ecosystem.We emphasize that the importance of molecular composition in regulating OM degradation should not be discarded, but rather we should understand when and why it appears more or less relevant than ecosystem properties. The global carbon cycle connects organic matter (OM) pools in soil, freshwater, and marine ecosystems with the atmosphere, thereby regulating their size and reactivity. Due to the complexity of biogeochemical processes and historically compartmentalized disciplines, ecosystem-specific conceptualizations of OM degradation have emerged independently of developments in other ecosystems. Recent discussions regarding the relative importance of molecular composition and ecosystem properties on OM degradation have diverged in opposing directions across subdisciplines, leaving our understanding inconsistent. Ecosystem-dependent theories are problematic since properties unique to an ecosystem may change in response to anthropogenic stressors, including climate change. The next breakthrough in our understanding of OM degradation requires a shift in focus towards developing a unified theory of controls on OM across ecosystems. The global carbon cycle connects organic matter (OM) pools in soil, freshwater, and marine ecosystems with the atmosphere, thereby regulating their size and reactivity. Due to the complexity of biogeochemical processes and historically compartmentalized disciplines, ecosystem-specific conceptualizations of OM degradation have emerged independently of developments in other ecosystems. Recent discussions regarding the relative importance of molecular composition and ecosystem properties on OM degradation have diverged in opposing directions across subdisciplines, leaving our understanding inconsistent. Ecosystem-dependent theories are problematic since properties unique to an ecosystem may change in response to anthropogenic stressors, including climate change. The next breakthrough in our understanding of OM degradation requires a shift in focus towards developing a unified theory of controls on OM across ecosystems. The susceptibility of organic matter (OM) to either persist and accumulate as a long-term sink of carbon, or cycle rapidly and become mineralized (see Glossary) into atmospheric CO2, is a key feature of carbon cycling across the biogeosphere. Yet, a seemingly simple question remains unresolved across soil, freshwater, and marine biogeochemistry: what controls the degradation of OM? We know that OM can be highly reactive, and is degraded within minutes in some soil and freshwater environments. Alternatively, it can also be highly persistent, lasting for millennia in soils, sediments, and the open ocean. Decades ago, the consensus would have been that OM persistence is reflected by its molecular composition. In time, flaws in this thinking were revealed, with the recognition that OM persists far longer than can be explained by molecular composition alone [1.Hedges J.I. et al.The molecularly-uncharacterized component of nonliving organic matter in natural environments.Org. Geochem. 2000; 31: 945-958Crossref Scopus (527) Google Scholar]. With this insight and recent technological advances [2.Moran M.A. et al.Deciphering ocean carbon in a changing world.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 3143-3151Crossref PubMed Scopus (142) Google Scholar], the relative importance of OM composition has shifted in diverging directions over time (Box 1). Most notably, the soil and freshwater lines of thinking are developing in opposing directions. The soil community came from a history of considering that soil OM could progressively become 'refractory' due to humification and selective preservation [3.Kleber M. et al.Old and stable soil organic matter is not necessarily chemically recalcitrant: implications for modeling concepts and temperature sensitivity.Glob. Change Biol. 2011; 17: 1097-1107Crossref Scopus (269) Google Scholar, 4.von Lutzow M. et al.Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review.Eur. J. Soil Sci. 2006; 57: 426-445Crossref Scopus (1731) Google Scholar, 5.Hayes M.H.B. Swift R.S. Vindication of humic substances as a key component of organic matter in soil and water.Adv. Agron. 2020; : 1-37Google Scholar], with molecular composition (e.g., lignin:N) being important for predicting degradation rates [6.Parton W.J. et al.Analysis of factors controlling soil organic matter levels in great plains grasslands.Soil Sci. Soc. Am. J. 1987; : 1173-1179Crossref Scopus (2750) Google Scholar,7.Melillo J.M. et al.Nitrogen and lignin control of hardwood leaf litter decomposition dynamics.Ecology. 1982; 63: 621-626Crossref Scopus (1950) Google Scholar]. The soil community now increasingly recognizes the importance of ecosystem properties [8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar], with molecular composition being less relevant [9.Kleber M. What is recalcitrant soil organic matter?.Environ. Chem. 2010; 7: 320-332Crossref Scopus (241) Google Scholar]. By contrast, the freshwater community first proposed the river continuum concept during the 1980s [10.Vannote R.L. et al.River continuum concept.Can. J. Fish. Aquat. Sci. 1980; 37: 130-137Crossref Scopus (6418) Google Scholar], where shifts in the molecular composition of dissolved organic matter (DOM) with movement downstream were expected due to the preferential use of substrates. This theory continues to be validated, even with high-resolution methods [11.Mosher J.J. et al.Longitudinal shifts in dissolved organic matter chemogeography and chemodiversity within headwater streams: a river continuum reprise.Biogeochemistry. 2015; 124: 371-385Crossref Scopus (40) Google Scholar, 12.Sun L. et al.Use of elemental composition to predict bioavailability of dissolved organic matter in a Georgia river.Limnol. Oceanogr. 1997; 42: 714-721Crossref Scopus (197) Google Scholar, 13.Kellerman A.M. et al.Persistance of dissolved organic matter in lakes related to its molecular characteristics.Nat. Geosci. 2015; 8: 454-457Crossref Google Scholar]. In the marine community, there are debates about the relative importance of molecularly stable DOM [14.Hertkorn N. et al.Characterization of a major refractory component of marine dissolved organic matter.Geochim. Cosmochim. Acta. 2006; 70: 2990-3010Crossref Scopus (513) Google Scholar] and environmental constraints [15.Jannasch H.W. Growth of marine bacteria at limiting concentrations of organic carbon in seawater.Limnol. Oceanogr. 1967; 12: 264-272Crossref Scopus (110) Google Scholar, 16.Arrieta J.M. et al.Dilution limits dissolved organic carbon utilization in the deep ocean.Science. 2015; 348: 331-333Crossref PubMed Scopus (149) Google Scholar, 17.Sexton P.F. et al.Eocene global warming events driven by ventilation of oceanic dissolved organic carbon.Nature. 2011; 471: 349-352Crossref PubMed Scopus (195) Google Scholar] to understand why marine DOM has an average radiocarbon age of 6000 years [18.Hansell D.A. et al.Dissolved organic matter in the ocean: a controversy stimulates new insights.Oceanography. 2009; 22: 202-211Crossref Scopus (529) Google Scholar]. In marine sediments, the wide variability in OM preservation efficiencies continues to be debated, with controlling factors ranging from molecular composition and age, to environmental factors, such as association with mineral particles [19.Hedges J.I. Keil R.G. Sedimentary organic matter preservation - an assessment and speculative synthesis.Mar. Chem. 1995; 49: 81-115Crossref Scopus (1656) Google Scholar, 20.Middelburg J.J. A simple rate model for organic matter decomposition in marine sediments.Geochim. Cosmochim. Acta. 1989; 53: 1577-1581Crossref Scopus (440) Google Scholar, 21.Blair N.E. Aller R.C. The fate of terrestrial organic carbon in the marine environment.Annu. Rev. Mar. Sci. 2012; 4: 401-423Crossref PubMed Scopus (362) Google Scholar, 22.Burdige D.J. Preservation of organic matter in marine sediments: Controls, mechanisms, and an imbalance in sediment organic carbon budgets?.Chem. Rev. 2007; 107: 467-485Crossref PubMed Scopus (619) Google Scholar, 23.Arndt S. et al.Quantifying the degradation of organic matter in marine sediments: A review and synthesis.Earth-Sci. Rev. 2013; 123: 53-86Crossref Scopus (448) Google Scholar]. Thus, our fundamental conceptualization of controls on OM degradation is fragmented and developing in an inconsistent manner within and across ecosystems.Box 1Diverging Historical Conceptualizations of OM Degradation in Soil, Freshwater, and Marine Ecosystems over TimeThe Soil Perspective: OM Persistence as a Function of Ecosystem PropertiesHistorically, the molecular composition of OM (e.g., lignin:N, aromatic content, solubility, molecular weight, or presence of humic substances) was thought to make soil OM persist [65.Aber J.D. et al.Predicting long-term patterns of mass-loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems.Can. J. Bot.-Rev. Can. Bot. 1990; 68: 2201-2208Crossref Google Scholar, 66.Alexander M. Biodegradation of chemicals of environmental concern.Science. 1981; 211: 132-138Crossref PubMed Scopus (503) Google Scholar, 67.Schulten H.R. Schnitzer M. Chemical model structures for soil organic matter and soils.Soil Sci. 1997; 162: 115-130Crossref Scopus (231) Google Scholar, 68.Piccolo A. The supramolecular structure of humic substances.Soil Sci. 2001; 166: 810-832Crossref Scopus (690) Google Scholar]. Accordingly, soil OM models assumed that decay rates were linked to molecular structure [6.Parton W.J. et al.Analysis of factors controlling soil organic matter levels in great plains grasslands.Soil Sci. Soc. Am. J. 1987; : 1173-1179Crossref Scopus (2750) Google Scholar,69.Jenkinson D.S. The turnover of organic-carbon and nitrogen in soil.Philos. Trans. R. Soc. B-Biol. Sci. 1990; 329: 361-368Crossref Google Scholar]. These traditionally held viewpoints are now actively challenged [4.von Lutzow M. et al.Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review.Eur. J. Soil Sci. 2006; 57: 426-445Crossref Scopus (1731) Google Scholar,8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar,70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar], as reviewed elsewhere [8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar] (Figure I). The long-term preservation of soil OM is increasingly recognized to be a function of its ecosystem properties, such as temperature [71.Ågren G.I. Bosatta E. Reconcilling differences in predictions of temperature response of soil organic matter.Soil Biol. Biochem. 2002; 34: 129-132Crossref Scopus (94) Google Scholar,72.Davidson E. Janssens I. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change.Nature. 2006; 440: 165-173Crossref PubMed Scopus (4060) Google Scholar], biological inaccessibility due to adsorption to mineral surfaces [8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar,60.Hemingway J.D. et al.Mineral protection regulates long-term global preservation of natural organic carbon.Nature. 2019; 570: 228-231Crossref PubMed Scopus (149) Google Scholar,73.Kalbitz K. et al.Stabilization of dissolved organic matter by sorption to the mineral soil.Soil Biol. Biochem. 2005; 37: 1319-1331Crossref Scopus (310) Google Scholar, 74.Kaiser K. Guggenberger G. The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils.Org. Geochem. 2000; 31: 711-725Crossref Scopus (720) Google Scholar, 75.Torn M.S. et al.Mineral control of soil organic carbon storage and turnover.Nature. 1997; 389: 170-173Crossref Scopus (1038) Google Scholar], and nutrient limitation [76.Vitousek P.M. Farrington H. Nutrient limitation and soil development: experimental test of a biogeochemical theory.Biogeochemistry. 1997; 37: 63-75Crossref Scopus (519) Google Scholar], while molecular composition is considered less relevant [70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar]. A defining feature of soils is the presence of mineral surfaces, whereby adsorption and physical encapsulation within micropore spaces can retain OM [75.Torn M.S. et al.Mineral control of soil organic carbon storage and turnover.Nature. 1997; 389: 170-173Crossref Scopus (1038) Google Scholar,77.Kaiser K. Guggenberger G. Mineral surfaces and soil organic matter.Eur. J. Soil Sci. 2003; 54: 219-236Crossref Scopus (547) Google Scholar,78.Kothawala D.N. et al.Soil properties controlling the adsorption of dissolved organic carbon to mineral soils.Soil Sci. Soc. Am. J. 2009; 73: 1831-1842Crossref Scopus (67) Google Scholar], and protect it from biological degradation [3.Kleber M. et al.Old and stable soil organic matter is not necessarily chemically recalcitrant: implications for modeling concepts and temperature sensitivity.Glob. Change Biol. 2011; 17: 1097-1107Crossref Scopus (269) Google Scholar,4.von Lutzow M. et al.Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review.Eur. J. Soil Sci. 2006; 57: 426-445Crossref Scopus (1731) Google Scholar,70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar,79.Sollins P. et al.Organic C and N stabilization in a forest soil: evidence from sequential density fractionation.Soil Biol. Biochem. 2006; 38: 3313-3324Crossref Scopus (305) Google Scholar]. When stabilized, even 'labile' OM becomes protected from enzymatic degradation. In fact, soil OM with a radiocarbon age of several thousand years can be highly labile once destabilized back into solution phase [48.Caraco N. et al.Millennial-aged organic carbon subsidies to a modern river food web.Ecology. 2010; 91: 2385-2393Crossref PubMed Scopus (99) Google Scholar], and likewise for thawing permafrost leachates [80.Wang Y.H. et al.Selective leaching of dissolved organic matter from alpine permafrost soils on the Qinghai-Tibetan Plateau.J. Geophys. Res.-Biogeosci. 2018; 123: 1005-1016Crossref Scopus (13) Google Scholar,81.Drake T.W. et al.The ephemeral signature of permafrost carbon in an Arctic fluvial network.J. Geophys. Res.-Biogeosci. 2018; 123: 1475-1485Crossref Scopus (33) Google Scholar]. Accordingly, it has been noted that radiocarbon age per se is not a good predictor of long-term persistence of soil OM [50.Marin-Spiotta E. et al.Paradigm shifts in soil organic matter research affect interpretations of aquatic carbon cycling: transcending disciplinary and ecosystem boundaries.Biogeochemistry. 2014; 117: 279-297Crossref Scopus (138) Google Scholar].The Freshwater Perspective: DOM Persistence due to Molecular Composition and TimeIn stark contrast to soil ecosystems, emerging freshwater studies are reinforcing the long-held notion that molecular composition is relevant to DOM degradation. DOM in freshwaters tends to be highly reactive (mean half-life of 2.5 years) with water residence time emerging as a strong predictor of degradation rates across a range of water bodies [39.Catalán N. et al.Organic carbon decomposition rates controlled by water retention time across inland waters.Nat. Geosci. 2016; 9: 501-504Crossref Scopus (190) Google Scholar]. Thus, there is an inherent assumption that labile compounds are selectively consumed first, and the remaining OM is increasingly difficult to degrade [11.Mosher J.J. et al.Longitudinal shifts in dissolved organic matter chemogeography and chemodiversity within headwater streams: a river continuum reprise.Biogeochemistry. 2015; 124: 371-385Crossref Scopus (40) Google Scholar,82.Creed I.F. et al.The river as a chemostat: fresh perspectives on dissolved organic matter flowing down the river continuum.Can. J. Fish. Aquat. Sci. 2015; 72: 1272-1285Crossref Scopus (160) Google Scholar, 83.Curtis P.J. Schindler D.W. Hydrologic control of dissolved organic matter in low-order Precambrian Shield Lakes.Biogeochemistry. 1997; 36: 125-138Crossref Scopus (116) Google Scholar, 84.del Giorgio P.A. Pace M.L. Relative independence of dissolved organic carbon transport and processing in a large temperate river: the Hudson River as both pipe and reactor.Limnol. Oceanogr. 2008; 53: 185-197Crossref Scopus (78) Google Scholar, 85.Kothawala D.N. et al.Controls of dissolved organic matter quality: evidence from a large-scale boreal lake survey.Glob. Chang. Biol. 2014; 20: 1101-1114Crossref PubMed Scopus (209) Google Scholar, 86.Minor E.C. et al.Structural characterization of dissolved organic matter: a review of current techniques for isolation and analysis.Environ. Sci.-Process Impacts. 2014; 16: 2064-2079Crossref PubMed Google Scholar]. Recent high-resolution approaches have confirmed that, with time, there is a preferential loss of oxidized, aromatic compounds of high molecular weight, whereas reduced aliphatic and N-containing compounds are resistant to degradation or tightly cycled, and, thus, persistent [13.Kellerman A.M. et al.Persistance of dissolved organic matter in lakes related to its molecular characteristics.Nat. Geosci. 2015; 8: 454-457Crossref Google Scholar]. As with soil studies, radiocarbon age is a poor predictor of long-term persistence in lakes and streams [87.McCallister S.L. del Giorgio P.A. Evidence for the respiration of ancient terrestrial organic C in northern temperate lakes and streams.Proc. Natl. Acad. Sci. U. S. A. 2012; 109: 16963-16968Crossref PubMed Scopus (93) Google Scholar,88.Casas-Ruiz J.P. et al.Drought-induced discontinuities in the source and degradation of dissolved organic matter in a Mediterranean river.Biogeochemistry. 2016; 127: 125-139Crossref Scopus (27) Google Scholar].Emerging Marine Perspective: Molecular Composition and Ecosystem DependenceTwo main lines of thinking, which are not exclusive of each other, prevail in the marine community, as reviewed elsewhere [42.Dittmar T. Reasons behind the long-term stability of dissolved organic matter.in: Hansell D.A. Carlson C.A. Biogeochemistry of Marine Dissolved Organic Matter. Elsevier, 2015: 369-388Crossref Scopus (62) Google Scholar]. The formation of refractory DOM during microbial and phtolytic degradation, as well as thermogenesis, is supported by detailed molecular methods [14.Hertkorn N. et al.Characterization of a major refractory component of marine dissolved organic matter.Geochim. Cosmochim. Acta. 2006; 70: 2990-3010Crossref Scopus (513) Google Scholar,89.Lechtenfeld O.J. et al.Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean.Geochim. Cosmochim. Ac. 2014; 126: 321-337Crossref Scopus (155) Google Scholar] (Marine 1; Figure I). Another theory suggests that environmental conditions are relevant [17.Sexton P.F. et al.Eocene global warming events driven by ventilation of oceanic dissolved organic carbon.Nature. 2011; 471: 349-352Crossref PubMed Scopus (195) Google Scholar,42.Dittmar T. Reasons behind the long-term stability of dissolved organic matter.in: Hansell D.A. Carlson C.A. Biogeochemistry of Marine Dissolved Organic Matter. Elsevier, 2015: 369-388Crossref Scopus (62) Google Scholar]. One example is the dilution theory [15.Jannasch H.W. Growth of marine bacteria at limiting concentrations of organic carbon in seawater.Limnol. Oceanogr. 1967; 12: 264-272Crossref Scopus (110) Google Scholar,16.Arrieta J.M. et al.Dilution limits dissolved organic carbon utilization in the deep ocean.Science. 2015; 348: 331-333Crossref PubMed Scopus (149) Google Scholar], which suggests that highly dilute conditions of the deep sea (DOC: <42 μM C) restrict decomposition, rather than compositional recalcitrance [16.Arrieta J.M. et al.Dilution limits dissolved organic carbon utilization in the deep ocean.Science. 2015; 348: 331-333Crossref PubMed Scopus (149) Google Scholar] (Marine 2; Figure I). In fact, the free energy released from highly labile substrates such as glucose can be outweighed by the high energetic costs of concentrating the substrate within the cell before metabolism [90.Williams P. Heterotrophic bacteria and the dynamics of dissolved organic material.in: Kirchman D.L. Microbial Ecology of the Oceans. John Wiley & Sons, 2000: 542-XXXGoogle Scholar]. Given that DOM comprises several thousand individual compounds [91.Hertkorn N. et al.High-field NMR spectroscopy and FTICR mass spectrometry: powerful discovery tools for the molecular level characterization of marine dissolved organic matter.Biogeosciences. 2013; 10: 1583-1624Crossref Scopus (208) Google Scholar], the low abundance of any individual compound under highly dilute conditions may limit specific catabolic pathways. Consequently, some DOM compounds may persist almost indefinitely in the deep ocean. The Soil Perspective: OM Persistence as a Function of Ecosystem Properties Historically, the molecular composition of OM (e.g., lignin:N, aromatic content, solubility, molecular weight, or presence of humic substances) was thought to make soil OM persist [65.Aber J.D. et al.Predicting long-term patterns of mass-loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems.Can. J. Bot.-Rev. Can. Bot. 1990; 68: 2201-2208Crossref Google Scholar, 66.Alexander M. Biodegradation of chemicals of environmental concern.Science. 1981; 211: 132-138Crossref PubMed Scopus (503) Google Scholar, 67.Schulten H.R. Schnitzer M. Chemical model structures for soil organic matter and soils.Soil Sci. 1997; 162: 115-130Crossref Scopus (231) Google Scholar, 68.Piccolo A. The supramolecular structure of humic substances.Soil Sci. 2001; 166: 810-832Crossref Scopus (690) Google Scholar]. Accordingly, soil OM models assumed that decay rates were linked to molecular structure [6.Parton W.J. et al.Analysis of factors controlling soil organic matter levels in great plains grasslands.Soil Sci. Soc. Am. J. 1987; : 1173-1179Crossref Scopus (2750) Google Scholar,69.Jenkinson D.S. The turnover of organic-carbon and nitrogen in soil.Philos. Trans. R. Soc. B-Biol. Sci. 1990; 329: 361-368Crossref Google Scholar]. These traditionally held viewpoints are now actively challenged [4.von Lutzow M. et al.Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review.Eur. J. Soil Sci. 2006; 57: 426-445Crossref Scopus (1731) Google Scholar,8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar,70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar], as reviewed elsewhere [8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar] (Figure I). The long-term preservation of soil OM is increasingly recognized to be a function of its ecosystem properties, such as temperature [71.Ågren G.I. Bosatta E. Reconcilling differences in predictions of temperature response of soil organic matter.Soil Biol. Biochem. 2002; 34: 129-132Crossref Scopus (94) Google Scholar,72.Davidson E. Janssens I. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change.Nature. 2006; 440: 165-173Crossref PubMed Scopus (4060) Google Scholar], biological inaccessibility due to adsorption to mineral surfaces [8.Schmidt M.W.I. et al.Persistence of soil organic matter as an ecosystem property.Nature. 2011; 478: 49-56Crossref PubMed Scopus (3149) Google Scholar,60.Hemingway J.D. et al.Mineral protection regulates long-term global preservation of natural organic carbon.Nature. 2019; 570: 228-231Crossref PubMed Scopus (149) Google Scholar,73.Kalbitz K. et al.Stabilization of dissolved organic matter by sorption to the mineral soil.Soil Biol. Biochem. 2005; 37: 1319-1331Crossref Scopus (310) Google Scholar, 74.Kaiser K. Guggenberger G. The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils.Org. Geochem. 2000; 31: 711-725Crossref Scopus (720) Google Scholar, 75.Torn M.S. et al.Mineral control of soil organic carbon storage and turnover.Nature. 1997; 389: 170-173Crossref Scopus (1038) Google Scholar], and nutrient limitation [76.Vitousek P.M. Farrington H. Nutrient limitation and soil development: experimental test of a biogeochemical theory.Biogeochemistry. 1997; 37: 63-75Crossref Scopus (519) Google Scholar], while molecular composition is considered less relevant [70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar]. A defining feature of soils is the presence of mineral surfaces, whereby adsorption and physical encapsulation within micropore spaces can retain OM [75.Torn M.S. et al.Mineral control of soil organic carbon storage and turnover.Nature. 1997; 389: 170-173Crossref Scopus (1038) Google Scholar,77.Kaiser K. Guggenberger G. Mineral surfaces and soil organic matter.Eur. J. Soil Sci. 2003; 54: 219-236Crossref Scopus (547) Google Scholar,78.Kothawala D.N. et al.Soil properties controlling the adsorption of dissolved organic carbon to mineral soils.Soil Sci. Soc. Am. J. 2009; 73: 1831-1842Crossref Scopus (67) Google Scholar], and protect it from biological degradation [3.Kleber M. et al.Old and stable soil organic matter is not necessarily chemically recalcitrant: implications for modeling concepts and temperature sensitivity.Glob. Change Biol. 2011; 17: 1097-1107Crossref Scopus (269) Google Scholar,4.von Lutzow M. et al.Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review.Eur. J. Soil Sci. 2006; 57: 426-445Crossref Scopus (1731) Google Scholar,70.Lehmann J. Kleber M. The contentious nature of soil organic matter.Nature. 2015; 528: 60-68Crossref PubMed Scopus (12) Google Scholar,79.Sollins P. et al.Organic C and N stabilization in a forest soil: evidence from sequential density fractionation.Soil Biol. Biochem. 2006; 38: 3313-3324Crossref Scopus (305) Google Scholar]. When stabilized, even 'labile' OM becomes protected from enzymatic degradation. In fact, soil OM with a radiocarbon age of several thousand years can be highly labile once destabilized back into solution phase [48.Caraco N. et al.Millennial-aged organic carbon subsidies to a modern river food web.Ecology. 2010; 91: 2385-2393Crossref PubMed Scopus (99) Google Scholar], and likewise for thawing permafrost leachates [80.Wang Y.H. et al.Selective leaching of dissolved organic matter from alpine permafrost soils on the Qinghai-Tibetan Plateau.J. Geophys. Res.-Biogeosci. 2018; 123: 1005-1016Crossref Scopus (13) Google Scholar,81.Drake T.W. et al.The ephemeral signature of permafrost carbon in an Arctic fluvial network.J. Geophys. Res.-Biogeosci. 2018; 123: 1475-1485Crossref Scopus (33) Google Scholar]. Accordingly, it has been noted that radiocarbon age per se is not a good predictor of long-term persistence of soil OM [50.Marin-Spiotta E. et al.Paradigm shifts in soil organic matter research affect interpretations of aquatic carbon cycling: transcending disciplinary and ecosystem boundaries.Biogeochemistry. 2014; 117: 279-297Crossref Scopus (138) Google Scholar]. The Freshwater Perspective: DOM Persistence due to Molecular Composition and Time In stark contrast to soil ecosystems, emerging freshwater studies are reinforcing the long-held notion that molecular composition is relevant to DOM degradation. DOM in freshwaters tends to be highly reactive (mean half-life of 2.5 years) with water residence time emerging as a strong predictor of degradation rates across a range of water bodies [39.Catalán N. et al.Organic carbon decomposition rates controlled by water retention time across inland waters.Nat. Geosci. 2016; 9: 501-504Crossref Scopus (190) Google Scholar]. Thus, there is an inherent assumption that labile compounds are selectively consumed first, and the remaining OM is increasingly difficult to degrade [11.Mosher J.J. et al.Longitudinal shifts in dissolved organic matter chemogeography and chemodiversity within headwater streams: a river continuum reprise.Biogeochemistry. 2015; 124: 371-385Crossref Scopus (40) Google Scholar,82.Creed I.F. et al.The river as a chemostat: fresh perspectives on dissolved organic matter flowing down the river continuum.Can. J. Fish. Aquat. Sci. 2015; 72: 1272-1285Crossref Scopus (160) Google Scholar, 83.Curtis P.J. Schindler D.W. Hydrologic control of dissolved organic matter in low-order Precambrian Shield Lakes.Biogeochemistry. 1997; 36: 125-138Crossref Scopus (116) Google Scholar, 84.del Giorgio P.A. Pace M.L. Relative independence of dissolved organic car