Dissolved black carbon is not likely a significant refractory organic carbon pool in rivers and oceans

For the samples we studied, we found a very good linear correlation (R² = 0.99, p < 0.001) between the concentrations of SPE-DOC with DOC in the river, coastal and ocean waters (Fig. 3a). This strong linear relationship could suggest an interesting phenomenon that, regardless the differences in DOC concentrations and the sample locations, the SPE method using the PPL cartridges likely extracted the same organic materials that were proportionally dissolved in DOC in natural waters. This fraction of SPE-DOC could have similar chemical properties such as molecular weight, composition and polarity^9,24,26. In contrast, the DBC concentrations isolated in SPE-DOC also had a linear but relatively weak correlation (R² = 0.45, p < 0.001) with DOC in the samples we studied (except for the Heilongjiang River). We think that this could be reasonable because the content of DBC, unlike the SPE-DOC fraction, is source-specific and should be largely controlled by the input of DBC source, while DOC could be derived from multiple sources such soil OC, biomass degradation and in situ production. The Heilongjiang River, for example, had the highest DOC concentrations (857 ± 337 μM) but much lower DBC contents (0.9 ± 0.5%) compared to other rivers. This could reflect relatively lower input of DBC in the less populated and cold northern climate with fewer landscape fires than the populated warm southern region with more biomass burning¹³. The lack of correlation between DBC and DOC concentrations has also been reported recently for the Amazon River and attributed to the divergent effects of soil properties, temperature, rainfall, and aerosol deposition on DOC and DBC mobilization from the catchments of tropical rivers⁹. Despite the different DBC isolation methods used, however, the DBC contents reported in the different rivers are in the comparable range of 4–10% in general. For example, based on the BPCA method, DBC has been reported to account for 7.0–9.9% of riverine DOC in the Paraíba do Sul²⁵ and Amazon⁹ rivers, 7.3% in the Mississippi River and 6.9–8.4% in the Congo River²⁴, comparable to the values of 3.7–7.6% for the six rivers we determined using the CTO method. For the four large continental rivers in China that we studied, based on the average discharge rates, measured DOC concentration and DBC/DOC ratio of each river, we calculated an annual flux of DBC of 5.5 × 10¹⁰, 3.5 × 10⁹, 2.2 × 10¹⁰, and 1.1 × 10¹⁰ g carried by the Yangtze, Yellow, Pearl, and Heilongjiang rivers, respectively. Together, 9.2 × 10¹⁰ g of DBC was transported by these rivers. This flux accounts for only a very small fraction (0.34%) of the 27 Tg of global riverine DBC export estimated by¹² and is only 3.4–4.8% of the annual DBC flux (1.9–2.7 Tg) estimated for the Amazon River, the largest river in the world, largely due to the high DOC and DBC concentrations in the river⁹.

The variable DBC contents we determined in the rivers could also reflect the seasonal variations because we collected our samples during different seasons and years. This could be demonstrated by the monthly samples of the Yellow River. In general, lower DBC/DOC rations were measured in the winter months (January, November, and December) and summer months (June and July), with more rain precipitation and a relatively high flow rate²⁶. This could suggest that the cold and frozen temperature in the winter in the lower Yellow River drainage region can reduce the release and dissolution of DBC from charcoal preserved in soils. On the other hand, more precipitation could dilute the DBC concentrations in the river. For most of the year, the DBC content was within the range of the other world rivers^9,24,25.

For DBC in the ocean, a relatively large range of DBC/DOC has been reported in different studies. Using the BPCA method, Wagner et al.²⁴ reported that very low DBC abundance accounted for 0.8–1.8% of the DOC in the Pacific and Atlantic oceans, while using the same method, Coppola and Druffel¹⁰ and Lewis et al.²⁷ reported relatively higher DBC contents, accounting for 4.2–8.6% of the DOC in the Atlantic, Pacific, and Arctic oceans, which are comparable to the values we obtained in the ECS (2.9 ± 1.2%) and the Mariana Trench site in the western NP (2.6–6.8%).

The isotopic signatures of DOC, SPE-DOC, and DBC we studied for the various samples provide more insight for the transport and cycling of these different OC materials in rivers and oceans. As shown in Fig. 4, we see strong linear correlations for the plots of Δ¹⁴C values among DOC, SPE-DOC, and DBC. Good positive correlations exist between the Δ¹⁴C values of DOC vs. SPE-DOC (Fig. 4a, R² = 0.90, p < 0.0001), DOC vs. DBC (Fig. 4b, R² = 0.92, p < 0.0001), and SPE-DOC vs. DBC (Fig. 4c, R² = 0.97, p < 0.0001). These strong correlations suggest that DOC, SPE-DOC, and DBC have systematic covariances in terms of their Δ¹⁴C values, and DBC was coupled with DOC and aged on the same time scales during transport from river to coast and open oceans. In the four rivers we studied, the ¹⁴C ages of riverine DBC are all younger (modern to 1510 years, except in the two small mountainous rivers in Taiwan) than the ¹⁴C ages of DOC (modern to 1720) especially in the Yangtze and Pearl rivers, consistent with the results reported for the Amazon River⁹. The relatively younger ¹⁴C ages of DBC in the rivers suggest that riverine DBC is mainly sourced from biomass burning rather than fossil fuel combustion. In our previous study, we calculated based on a two end-member isotope mass balance model that 78–85% of the DBC transported in the Yangtze and Yellow rivers was derived from biomass burning²⁶. Regardless of the monthly variations of DBC contents in the Yellow River, their ¹⁴C ages (1143 ± 180) had less variation, supporting the source-specific input of DBC in the river. The same calculation reveals that 83 and 100% of the riverine DBC transported in the Pearl and Heilongjiang rivers and 65% of DBC in the Taiwan mountainous rivers are derived from biomass burning. The DBC was dissolved over time from the charcoal preserved in soils and entered waters in streams and rivers^12,25,28. The older DBC in the Taiwan mountain rivers could suggest that some charcoals were preserved in soil for longer amounts of time. In contrast, the riverine DBC ¹⁴C ages are distinctively different than the PBC transported in rivers. Our previous study showed that PBC transported in the Yangtze and Yellow rivers had ¹⁴C ages of 4550 and 5830 years, respectively, as derived mainly from fossil fuel combustion²⁶. For 18 global rivers, it was reported that the average PBC ¹⁴C age was 3700 ± 400 years, much older than the ¹⁴C age of DBC in the rivers⁶.

Very few ¹⁴C measurements of oceanic DBC have been reported in the literature. Ziolkowski and Druffel¹¹ reported the first ¹⁴C measurement of DBC isolated by ultrafiltration in the Atlantic and Pacific oceans. The ¹⁴C ages of DBC were 15,680–18,300 years old in the Atlantic and 17,000–20,100 years old in the Pacific, much older than the bulk DOC ¹⁴C ages of 4000–6000 years in the 2 oceans^29,30. More recently, using SPE-DOC isolation, Coppola and Druffel¹⁰ reported that the ¹⁴C ages of DBC were 4800 ± 620 years in the surface water and 23,000 ± 3000 years in the deep water of the Arctic, Atlantic, and Pacific oceans. They concluded that DBC in the ocean is not homogeneous; one younger DBC pool is cycling on centennial time scales, and one ancient DBC pool is cycling on >10⁵-year time scales¹⁰. The isotopic signatures of DBC in the ocean determined in our study, however, are sometimes inconsistent with these previous findings. For DBC in the oceans, we found that the average ¹⁴C ages were 1835 ± 616 years in the ECS; 2570 ± 184 years in the surface water; and 5900 ± 14 years and 5820 ± 28 years in the deep waters (3000–6000 m) and hadal depth (8000–10,000 m) at the Mariana Trench site in the NP, respectively. The DBC ¹⁴C ages were all younger than the bulk DOC ¹⁴C ages in the Yangtze River and Yellow River estuaries, and the ECS, and approximately the same as the DOC ¹⁴C ages in the surface water and slightly younger in the deep waters at the Mariana Trench site. Based on these results, we speculate that DBC in the ocean was transported and aged during its cycling through rivers and estuaries to coastal and open oceans in a continuum process coupled with DOC. DBC is likely cycled and aged on the same time scales as the bulk DOC pool in the ocean. Meanwhile, we remain puzzled about the DBC age differences found between our study and previous studies. It is possible that the age difference could be due to the CTO and BPCA methods used, which could have determined different organic compounds with very different ¹⁴C signatures. The relatively consistent Δ¹⁴C values of DBC we determined for the monthly samples in the Yellow River did suggest that we isolated the same materials. This age inconsistency is certainly an important question that needs to be further investigated.

Unlike their Δ¹⁴C signatures, the correlations between the δ¹³C values of DOC and SPE-DOC (Fig. 4d, R² = 0.23, p < 0.01), DOC and DBC (Fig. 4e, R² = 0.31, p < 0.001) are weak. In comparison, the correlation between SPE-DOC and DBC δ¹³C values is much better (Fig. 4, R² = 0.75, p < 0.0001). This was due to the variable ranges of DOC δ¹³C values (−21.0 to −29.0‰) in the rivers we studied. Since δ¹³C in DOC is mainly an OC source indicator, the variable DOC δ¹³C values in the rivers indicate that DOC was derived from different sources. For DOC in the Heilongjiang River, for example, it was mainly derived from the leaching and degradation of plant biomass with modern ¹⁴C ages. Its DOC δ¹³C values (−28.2 to −29.0‰) were consistent and significantly depleted relative to the DOC δ¹³C values in the three large continental rivers, Yangtze, Yellow, and Pearl, where the sources of DOC were more complicated, including both natural and anthropogenic inputs^31,32. As discussed above, the large variations of DOC δ¹³C values (−21.0 to −27.0‰) measured in the Yellow River could reflect seasonal source variations of DOC, consistent with the weak correlation between DBC and DOC concentrations (Fig. 3b). In contrast to bulk DOC, the good correlations between SPE-DOC and DBC δ¹³C values (Fig. 4f) again suggests that these organic fractions are more source-specific than other organic fractions.

In our study, we also found that the DBC δ¹³C values are all enriched (by 2–4‰ in general) relative to the δ¹³C values of bulk DOC and SPE-DOC in rivers, coastal waters and the open ocean (Fig. 1c). We examined the correlations between the ¹⁴C ages and δ¹³C values of DBC for the samples (Fig. 5). A linear correlation (R² = 0.52, p < 0.0001) appears between DBC ¹⁴C ages and its δ¹³C values (except for the Taiwan river and deep ocean). When the DBC ages increase from the rivers to the ocean, its δ¹³C values become enriched. Riverine DBC has more depleted δ¹³C values than oceanic DBC. This phenomenon is consistent with the results reported recently by Wagner et al.²⁴, who found that oceanic DBC was approximately 6–8‰ more enriched in δ¹³C than riverine DBC. Based on the δ¹³C differences, they concluded that oceanic DBC was not derived from rivers, so the DBC in rivers and oceans originates from different sources. As discussed above for the ¹⁴C ages, we believe that DBC in the ocean is likely transported mainly by rivers through estuaries and coasts into the open ocean as a continuum, considering that the major pathways mobilizing and transporting DBC (27 Pg per year) from the land to the ocean are rivers^12,25,27,28, compared to the rather smaller source of DBC (0.002–0.006 Gt per year) derived from atmospheric deposition in the Northern Hemisphere³³. Clearly, what happens to the riverine DBC in the ocean and the causes of the different DBC δ¹³C in rivers and ocean remain interesting questions, which need to be further studied.

Our long-team charcoal leaching experiments clearly demonstrated two important processes: (1) DBC was leached out from charcoal and dissolved in DOC with time, and (2) the released DBC was degraded by bacteria (Fig. 2). If we assume that 322 μM (as measured in the bacteria-inhibited case) was also leached out from the charcoal in the bacteria-active case, then this amount of DBC was completely degraded by bacteria. The measured DOC Δ¹⁴C values provided good evidence that no modern DBC was added to the DOC (−170‰) at the end of the experiment based on the initial DOC Δ¹⁴C value (−166‰). In contrast to the bacteria-active case, when bacterial activities were inhibited, the DOC Δ¹⁴C value (−90‰) was significantly increased at the end of the experiment. Based on the ¹⁴C mass balance and assuming that the DBC released from the wood charcoal had a modern Δ¹⁴C value of 20‰, 41% of the modern DBC was added to the DOC pool at the end of the bacteria-inhibited experiment, which was somehow lower than the 48% calculated from the increased DOC concentration (440–228 μM). This discrepancy could be because at the end of the bacteria-inhibited experiment, we also measured some bacteria (2.8 × 10⁴ cell mL⁻¹) present in the water that consumed some DBC. For DOC δ¹³C, the value in the bacteria-active water (−25.1‰) was close to the original value (−24.6‰) but lower (−27.8‰) in the bacteria-inhibited water, which was close to the δ¹³C value of the locust tree wood charcoal (−27.0‰) used. These findings support the Δ¹⁴C results, indicating that DBC was leached from the charcoal and dissolved in DOC in the bacteria-inhibited case.

Our results provide direct evidence supporting the speculation that a large fraction of riverine DBC could be degraded in the estuaries and ocean^9,24. Even though the dissolution of DBC is a slow process that is likely dynamically controlled by its water-solubility, this process could be significant in the natural environment because C-rich charcoal, a common residue of incomplete biomass combustion, is widely preserved in soils^34,35, and a large fraction of this charcoal could be removed as DBC with time and transported by streams and rivers to the ocean^{28,36,37,38,39}. The dominant DBC transported by the rivers is derived from biomass burning, and the DBC is labile and biodegradable. This could be the reason why we did not measure elevated DBC contents in the oceans. Our study suggests that DBC is unlikely to be a significant refractory DOC pool cycling in rivers and oceans, and DBC is likely aged on the same time scales as DOC cycling in the ocean.

Source: Ecology - nature.com