Terrestrial pyrogenic carbon export to fluvial ecosystems: Lessons learned from the White Nile watershed of East Africa

Abstract

Pyrogenic carbon (PyC) is important because of its role in the global organic C (OC) cycle and in modifying soil properties. However, our understanding of PyC movement from terrestrial to fluvial ecosystems is not robust. This study examined (i) whether erosion or subsurface transport was more important for PyC export from headwaters, (ii) whether PyC was exported preferentially to total OC (TOC), and (iii) whether the movement of PyC from terrestrial to aquatic ecosystems provides an explanation for the coupling of PyC and non‐PyC observed in rivers at a global scale. In the Guineo‐Congolian highland forest region of western Kenya, duplicate catchments with sizes of 1–12 ha were equipped with stream gauges in primary forest and adjacent mixed agricultural landscapes that were cleared by fire 10, 16, or 62 years before. Stream water samples were taken weekly throughout 1 year and compared with runoff to assess PyC movement. Additional stream samples were taken from all major tributaries of the White Nile watershed of Lake Victoria. PyC was not found to be preferentially eroded relative to TOC or non‐PyC, as topsoil (0–0.15 m) PyC concentrations (6.3 ± 0.3% of TOC; means and standard errors) were greater than runoff sediment (1.9 ± 0.4%) and dissolved PyC concentrations (2.0 ± 0.4%, n = 252). In addition, PyC proportions in eroded sediment were lower than and uncorrelated (r2 = 0.04; P = 0.14) with topsoil PyC. An enrichment of PyC was found with depth in the soil, from 6.3 ± 0.3% of TOC in the topsoil (0–0.15 m) to 12.3 ± 0.3% of TOC at 1–2 m. Base flow PyC proportions of TOC correlated well with subsoil PyC (r2 = 0.57; P < 0.05) but not with topsoil PyC (r2 = 0.18; P > 0.05). Similar PyC proportions were found in the studied headwater streams (2.7 ± 0.2%), their downstream inflow into Lake Victoria (3.7%), the other nine major rivers into Lake Victoria (4.9 ± 0.8%), and its outflow into the White Nile (1.1%). A strong positive correlation between dissolved PyC and non‐PyC (r2 = 0.91; P < 0.0001) in the headwater streams reflect relationships previously seen for a range of globally important rivers, and contrasts with a negative relationship for suspended sediments (r2 = −0.5; P < 0.0001). The estimated PyC export from the Lake Victoria watershed of 11 Gg yr−1 may therefore originate to a large extent from subsoil pathways in dissolved form that appeared to be an important source of PyC in aquatic environments and may explain the coupling of PyC and non‐PyC at a global scale.

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

“…The leaching of PyC also depends on the particle size (dissolved vs. particulate form) and vegetation type. Slow leaching of PyC downward may be altered by the quality of PyC material left after the initial mass movement event and the post-fire vegetation recovery (Major et al, 2010;Kindler et al, 2011;Güereña et al, 2015). Leaching may be occurring in the depositional landform position, but we did not sample at depth into this landform position, as no pre-fire comparison samples exist for the deep soils in the depositional landform position.…”

“…However, gaps remain in our understanding of the mechanisms through which erosional redistribution of PyC across fire-impacted landscapes can affect its stock and persistence in soil. So far, the relative rates of lateral and vertical transport of PyC, with erosion, leaching, and bioturbation, across differing landscapes remains largely unknown (Güereña et al, 2015;Rumpel et al, 2015). The erosion of PyC is also significant, because this loss from eroding hillslopes post-fire, as illustrated after the Gondola fire, can lead to differences in its apparent environmental persistence based upon topographic features of a landscape.…”

A New Perspective on the Apparent Solubility of Dissolved Black Carbon

“…Although, recent wildfire activity does not seem to have a significant effect on in-stream DBC Güereña et al, 2015;Wagner et al, 2015a), this material is continually exported from fire-impacted watersheds for decades after a burn event (Dittmar et al, 2012). This suggests that the aging of BC in soils may be a prerequisite to its dissolution and export to aquatic systems as DBC.…”

Flux of Dissolved and Particulate Low-Temperature Pyrogenic Carbon from Two High-Latitude Rivers across the Spring Freshet Hydrograph

“…Soil regimes in these systems vary widely; for example, the Yenisei River watershed is underlain by every type of permafrost regime, causing the soil active layer depth to vary dramatically within high latitude watersheds (Kawahigashi et al, 2004). It has been suggested that subsoil transport is a primary mechanism for PyC export from terrestrial to aquatic systems (Güereña et al, 2015), which may be regulated by soilPyC associations. Czimczik and Masiello (2007) suggest that the ability of soil minerals to sorb PyC may be a main controller of PyC storage within soils, similar to bulk soil organic matter (SOM).…”