Fires prime terrestrial organic carbon for riverine export to the global oceans

Black carbon (BC) is a recalcitrant form of organic carbon (OC) produced by landscape fires. BC is an important component of the global carbon cycle because, compared to unburned biogenic OC, it is selectively conserved in terrestrial and oceanic pools. Here we show that the dissolved BC (DBC) content of dissolved OC (DOC) is twice greater in major (sub)tropical and high-latitude rivers than in major temperate rivers, with further significant differences between biomes. We estimate that rivers export 18 ± 4 Tg DBC year−1 globally and that, including particulate BC fluxes, total riverine export amounts to 43 ± 15 Tg BC year−1 (12 ± 5% of the OC flux). While rivers export ~1% of the OC sequestered by terrestrial vegetation, our estimates suggest that 34 ± 26% of the BC produced by landscape fires has an oceanic fate. Biogeochemical models require modification to account for the unique dynamics of BC and to predict the response of recalcitrant OC export to changing environmental conditions.


Supplementary Note 2: Comparison of Different Flux Estimation Approaches
We evaluated whether global riverine DBC flux estimates are sensitive to the method of estimation where DBC is treated either as a globally uniform function of DOC concentration or as a spatially variable fraction of DOC concentration (Supplementary Table 2).
Supplementary Figure 2 shows how the relationship between DBC and DOC concentrations is represented by these two contrasting a priori perspectives. In total, we consider 5 methods of global DBC flux estimation (Supplementary Table 2 Table   3); • in method B, as an adjustment to method A, we incorporate more recent estimates for DOC fluxes from (sub)tropical (< 30° N/S; 128 ± 23 Tg C year -1 ), temperate (30-60° N/S; 38 ± 4 Tg C year -1 ) and high latitude (> 60° N/S; 39 ± 3 Tg C year -1 ) rivers according to the meta-analysis of Dai  • in method C, as a further adjustment to method B, we apply the predictive model that was re-fitted to the extended dataset presented herein (n = 409; Supplementary Table 5); • in method D, we apply the average (± standard deviation) DBC content of DOC observed in all major global rivers ( Supplementary Table 1). There were minimal differences in global or latitudinal export fluxes between methods D and C, which is due to these methods relying on the same extended dataset of 409 DOC and DBC concentrations and their global approach to flux estimation; the regression line fitted to this dataset intersects close to the origin and thus the slope of the regression line is close to the mean DBC content of DOC.
The spatially variable method E, which we use as our estimate of global DBC export fluxes (see main text), produced an estimate for the global DBC flux of 18.0 ± 3.9 Tg C year -1 composed of 12.4 ± 3.8 Tg C year -1 from (sub)tropical rivers, 1.8 ± 0.6 Tg C year -1 from temperate rivers, and 3.8 ± 0.6 Tg C year -1 from high latitude rivers. The global DBC flux estimate deriving from method E was near to the value achieved by applying methods C and D, which were the most directly comparable to the method E as they utilised the same dataset of DOC and DBC concentrations and the same DOC export flux estimates (Supplementary Table 2).
Nonetheless, substantially different estimates for the DBC flux from individual latitudinal ranges were produced by the spatially variable approach (method E) and the spatially continuous approaches (methods C and D; Supplementary Table 2). The greatest absolute difference was observed in (sub)tropical latitudes, where the central estimate produced by approach E was 2.8 Tg C year -1 (22%) greater than the estimate produced by approach C and 2.0 Tg C year (16%) greater than the estimate produced by approach D. The central estimate produced by approach E for high-latitude export of DBC was also 0.6 Tg C year -1 (18%) greater than approach D and 0.4 Tg C year -1 (11%) greater than approach C.
On the other hand, the central estimate for DBC export by temperate rivers produced by method E was 1.0 Tg C year -1 (35%) lower than in approach C and 1.3 Tg C year -1 (43%) lower than in approach D (Supplementary Table 2). These results indicate that DBC export fluxes are overestimated in temperate regions and underestimated in the (sub)tropics if a globally homogenous relationship between DBC and DOC concentrations is used to estimate these fluxes. The differences in DBC flux estimates between the two methods were smaller in the high latitudes due to the disproportionate influence that high-latitude samples on the regression line fitted to DBC and DOC concentrations (Supplementary Note 1). This preordained that the mean contribution of DBC to DOC load for this region is closer to the global mean DBC content of DOC (Supplementary Figure 3).

Supplementary Tables
Supplementary Table 1

Supplementary Figures
Supplementary Figure 1