Different Responses of Dissolved Black Carbon and Dissolved Lignin to Seasonal Hydrological Changes and an Extreme Rain Event

Abstract

Hydrology, especially extreme hydrological events, has been recognized as an important driver of the land‐to‐ocean export of terrigenous dissolved organic matter (tDOM). Nevertheless, how various types of tDOM that differ in source and reactivity respond to changes in hydrology is not known. Seasonal and event exports of dissolved organic carbon (DOC), dissolved black carbon (DBC), and dissolved lignin were studied in a small subtropical river. We found that seasonal variations in DBC concentration were significantly related to hydrology, while DOC and dissolved lignin were not. In contrast, DOC, DBC, and dissolved lignin changed similarly during an extreme rain event. The variation magnitudes of DOC, DBC, and dissolved lignin concentrations were in the lower end compared to other rivers, which may be related to the limited coverage of wetlands and riparian vegetation and poor development of organic‐rich soil. Dilution effects were observed when the runoff exceeded 0.4 mm/hr, and the fluxes of both DBC and dissolved lignin decreased during the runoff peak, which was caused by surface flow and potentially by removal processes during peak discharge. Our results suggest that the influence of hydrology varies with tDOM source and reactivity and that high enough runoff (e.g., 0.7 mm/hr in the Jiulong River) may not enhance the export rate of tDOM. However, our study was carried out in a small watershed with limited wetlands and riparian vegetation, and more studies are needed to verify whether this trend is consistent among global rivers.

Environmental Controls on the Riverine Export of Dissolved Black Carbon

“…Overall, our results indicate that seasonal hydrology (represented by soil moisture) and hydrological setting (represented by catchment slope) are critical factors moderating the rate at which SBC stocks are solubilized and made available for export as DBC, while the dilution effect of preceding rainfall on riverine DBC concentrations may result from the flushing of DBC from soil pore space and a deficit in the period that follows. This interpretation is also consistent with the dilution of DBC concentrations observed during high-discharge events in temperate catchments (Bao et al, 2019;Wagner et al, 2015) and with the asymptotic limitation to increases in DBC concentration across seasonal discharge gradients that was previously observed in AF2 (Dittmar, de Rezende, et al, 2012).…”

“…Aerosol BC has traditionally been considered a negligible source of riverine DBC on the basis that it contributes less than 30% toward the total global production flux for BC (Bird et al, 2015). Further evidence is required in order to validate the contribution of BC aerosol to riverine DBC at regional and global scales (Bao et al, 2019;Wagner et al, 2018). Further evidence is required in order to validate the contribution of BC aerosol to riverine DBC at regional and global scales (Bao et al, 2019;Wagner et al, 2018).…”

“…In practical terms, hysteresis means that different fractions of SOC can respond discordantly and on different timescales to a pulse. On this basis it is plausible that for a period following rainfall, pore water DOC is deficient in DBC and the relative stock of DBC available for export to channels is reduced (Bao et al, 2019;Wagner et al, 2015). On this basis it is plausible that for a period following rainfall, pore water DOC is deficient in DBC and the relative stock of DBC available for export to channels is reduced (Bao et al, 2019;Wagner et al, 2015).…”

“…Nonetheless, a number of recent studies have challenged this view by demonstrating that aerosol BC makes contributions on the order of 5-25% of the total riverine DBC load in some high-latitude (Ding et al, 2015;, temperate (Wang et al, 2016), and tropical (Jones et al, 2017) catchments. Further evidence is required in order to validate the contribution of BC aerosol to riverine DBC at regional and global scales (Bao et al, 2019;Wagner et al, 2018). The deposition rate of aerosol BC relates to regional rates of emission from several sources (e.g., industry, deforestation, wildfires, and agriculture) combined with wind patterns and the efficiency of wet and dry deposition processes (Bao et al, 2017;Bond et al, 2013;Jurado et al, 2008).…”

“…SBC, as a subpool of bulk SOC stocks, has unique molecular properties that limit its rate of decomposition relative to other fractions of SOC, and this indicates that, following a pulse, the latent stock of DBC in soil pore space is likely to recover over longer timescales than the latent stock of bulk DOC (Hockaday et al, 2007;Kuzyakov et al, 2014). On this basis it is plausible that for a period following rainfall, pore water DOC is deficient in DBC and the relative stock of DBC available for export to channels is reduced (Bao et al, 2019;Wagner et al, 2015). These concepts may help to explain our observation of a negative relationship between riverine DBC concentrations and antecedent rainfall (which occurred predominantly in the 3-7 days prior to sampling), despite the apparent absence of any rainfall effect on riverine DOC concentrations.…”