Dissolved Black Carbon in the Headwaters-to-Ocean Continuum of Paraíba Do Sul River, Brazil

Abstract

Rivers annually carry 25-28 Tg carbon in the form of pyrogenic dissolved organic matter (dissolved black carbon, DBC) into the ocean, which is equivalent to about 10% of the entire riverine land-ocean flux of dissolved organic carbon (DOC). The objective of this study was to identify the main processes behind the release and turnover of DBC on a riverine catchment scale. As a model system, we chose the headwater-to-ocean continuum of Paraíba do Sul River (Brazil), the only river system with long-term DBC flux data available. The catchment was originally covered by Atlantic rain forest (mainly C3 plants) which was almost completely destroyed over the past centuries by slash-and-burn. As a result, large amounts of wood-derived charcoal reside in the soils. Today, fire-managed pasture and sugar cane (both dominated by C4 plants) cover most of the catchment area. Water samples were collected along the river, at the main tributaries, and also along the salinity gradient in the estuary and up to 35 km offshore during three different seasons. DBC was determined on a molecular level as benzenepolycarboxylic acids (BPCAs). Stable carbon isotopes (δ 13 C) were determined in solid phase extractable DOC (SPE-DOC) to distinguish C4 and C3 sources. Our results clearly show a relationship between hydrology and DBC concentrations in the river, with highest DBC concentrations and fluxes in the wet season (flux of 770 moles s −1 in 2013 and 59 moles s −1 in 2014) and lowest in the dry season (flux of 27 moles s −1 ). This relationship indicates that DBC is mainly mobilized from the upper soil horizons during heavy rainfalls. The relationship between DBC concentrations and δ 13 C-SPE-DOC indicated that most of DBC in the river system originated from C3 plants, i.e., from the historic burning event of the Atlantic rain forest. A conservative mixing model could largely reproduce the observed DBC fluxes within the catchment and the land to ocean continuum. Comparably slight deviations from conservative mixing were accompanied by changes in the molecular composition of DBC (i.e., the ratio of benzenepenta-to benzenehexacarboxylic acid) that are indicative for photodegradation of DBC.

Seasonal dynamics of atmospheric and river inputs of black carbon, and impacts on biogeochemical cycles in Halong Bay, Vietnam

“…The spatial and seasonal distributions of DBC in Halong Bay suggest that riverine runoff that occurred during the wet season was efficient in mobilizing and transporting DBC to the ocean. Previous studies have reported a similar relation between salinity and the concentration of DBC (Mitra et al, 2002;Mannino and Harvey, 2004;Marques et al, 2017), with global estimates indicating that the riverine DOC pool, of which about 10% is DBC, provides an input pathway that brings about 26 Tg DBC yr -1 to the ocean (Jaffé et al, 2013). The high contribution of DBC to the pool of T-DOC in riverine flux reported in previous studies (Jaffé et al, 2013;Stubbins et al, 2015;Wang et al, 2016) is confirmed for the river system converging in Cua Luc Bay by the observed gradient of increasing DBC fraction toward the river and the high contribution of DBC to the pool of DOC (> 6% at station A) during the wet season, and by the high concentration of DBC at salinity = 0 estimated by regression analysis (about 8 µmol C L -1 ; Figure 16).…”

Impact of a Historical Fire Event on Pyrogenic Carbon Stocks and Dissolved Pyrogenic Carbon in Spodosols in Northern Michigan

“…This would be consistent with observations of a continuous, long-term release of DPyC resulting from the degradation of historically accumulated PyC (Dittmar et al, 2012;Marques et al, 2017).…”

“…This suggests that PyC is much more dynamic in soils than previously thought. For example, large fluxes of dissolved PyC (DPyC, <0.7 µm particle size) in a tropical river were reported to increase during heavy rain and derive from charcoal remaining in surface soils from historical fires rather than from recent burning (Marques et al, 2017). Thus, quantifying the relative impact of historical fires on PyC stocks and export from soils is essential to understand the impact of fires on PyC stabilization in and mobilization mechanisms from soils.…”

“…In aquatic systems, DPyC is typically reported as a measure of dissolved poly-condensed aromatic C (Dittmar, 2008) and has been shown to correlate with dissolved organic carbon (DOC) (Ding et al, , 2014Jaffé et al, 2013;Güereña et al, 2015). Few studies have examined the export pathways for the transport of DPyC from terrestrial to aquatic ecosystems (Güereña et al, 2015;Cotrufo et al, 2016;Marques et al, 2017). While a study conducted in mixed tropical highland forest and agricultural catchments dominated by Nitisols showed that subsurface transport was more important than erosion in driving the movement of DPyC from soils to streams (Güereña et al, 2015), a study conducted in a savanna Oxisol showed that leaching accounted for less than 1% of the annual DPyC losses (detected based on 13 C stable isotope approaches) one and two years after the addition of charcoal to tropical soil (Major et al, 2010).…”

Environmental Controls on the Riverine Export of Dissolved Black Carbon

“…The Cerrado study regions featured a typical degraded Cerrado landscape; agricultural land uses are interspersed with fragmented sections of natural dry forest, shrubland, and grassland. Sampling campaigns in AF2 were completed by Marques et al (2017) during three periods with differing hydrological conditions, specifically in January 2013 (wet season), August 2013 (dry season), and February 2014 (wet season). The samples collected in AF1, AF2, and AMZ1 represented headwater channels and tributaries to major rivers, with catchment areas ranging from 10 to 1,300 km 2 .…”