(1) Atmospheric particles act as cloud-condensing nuclei, promoting cloud formation (Kerminen et al., 2000; Riipinen et al., 2011). (2) Raindrops absorb organic and inorganic carbon through particle scavenging and adsorption of organic vapors while falling toward earth (Waterloo et al., 2006; Neu et al., 2016). (3) Burning and volcanic eruptions produce highly condensed polycyclic aromatic molecules (i.e., black carbon) that is returned to the atmosphere along with greenhouse gases such as CO2 (Baldock et al., 2004; Myers-Pigg et al., 2016). (4) Terrestrial plants fix atmospheric CO2 through photosynthesis, returning a fraction back to the atmosphere through respiration (Field et al., 1998). Lignin and celluloses represent as much as 80% of the OC in forests and 60% in pastures (Martens et al., 2004; Bose et al., 2009). (5) Litterfall and root OC mix with sedimentary material to form organic soils where plant-derived and petrogenic OC is both stored and transformed by microbial and fungal activity (Schlesinger and Andrews, 2000; Schmidt et al., 2011; Lehmann and Kleber, 2015). (6) Water absorbs plant and settled aerosol-derived DOC and DIC as it passes over forest canopies (i.e., throughfall) and along plant trunks/stems (i.e., stemflow) (Qualls and Haines, 1992). Biogeochemical transformations take place as water soaks into soil solution and groundwater reservoirs (Grøn et al., 1992; Pabich et al., 2001) and overland flow occurs when soils are completely saturated (Linsley et al., 1975) or rainfall occurs more rapidly than saturation into soils (Horton, 1933). (7) Organic carbon derived from the terrestrial biosphere and in situ primary production is decomposed by microbial communities in rivers and streams along with physical decomposition (i.e., photo-oxidation), resulting in a flux of CO2 from rivers to the atmosphere that are the same order of magnitude as the amount of carbon sequestered annually by the terrestrial biosphere (Richey et al., 2002; Cole et al., 2007; Raymond et al., 2013). Terrestrially-derived macromolecules such as lignin (Ward et al., 2013) and black carbon (Myers-Pigg et al., 2015) are decomposed into smaller components and monomers, ultimately being converted to CO2, metabolic intermediates, or biomass. (8) Lakes, reservoirs, and floodplains typically store large amounts of OC and sediments, but also experience net heterotrophy in the water column, resulting in a net flux of CO2 to the atmosphere that is roughly one order of magnitude less than rivers (Tranvik et al., 2009; Raymond et al., 2013). Methane production is also typically high in the anoxic sediments of floodplains, lakes, and reservoirs (Bastviken et al., 2004). (9) Primary production is typically enhanced in river plumes due to the export of fluvial nutrients (Cooley et al., 2007; Subramaniam et al., 2008). Nevertheless, estuarine waters are a source of CO2 to the atmosphere, globally (Cai, 2011). (10) Coastal marshes both store and export “blue carbon” (Odum et al., 1979; Dittmar et al., 2001; Moore et al., 2011). Marshes and wetlands are suggested to have an equivalent flux of CO2 to the atmosphere as rivers, globally (Wehrli, 2013). (11) Continental shelves and the open ocean typically absorb CO2 from the atmosphere (Cai, 2011), sequestering a small fraction of the fixed CO2 as organic carbon in (12) marine sediments due to the “biological pump” (Moran et al., 2016).

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