Disproportionate Contribution of Riparian Inputs to Organic Carbon Pools in Freshwater Systems.

A lack of appropriate proxies has traditionally hampered our ability to distinguish riverine organic carbon (OC) sources at the landscape scale. However, the dissection of C ₄ grasslands by C ₃ -enriched riparian vegetation, and the distinct carbon stable isotope signature (δ ¹³ C) of these two photosynthetic pathways, provides a unique setting to assess the relative contribution of riparian and more distant sources to riverine C pools. Here, we compared δ ¹³ C signatures of bulk sub-basin vegetation (δ ¹³ C _VEG ) with those of riverine OC pools for a wide range of sites within two contrasting river basins in Madagascar. Although C ₃ -derived carbon dominated in the eastern Rianala catchment, consistent with the dominant vegetation, we found that in the C ₄ -dominated Betsiboka basin, riverine OC is disproportionately sourced from the C ₃ -enriched riparian fringe, irrespective of climatic season, even though δ ¹³ C _VEG estimates suggest as much as 96% of vegetation cover in some Betsiboka sub-basins may be accounted for by C ₄ biomass. For example, δ ¹³ C values for river bed OC were on average 6.9 ± 2.7‰ depleted in ¹³ C compared to paired estimates of δ ¹³ C _VEG . The disconnection of the wider C ₄ -dominated basin is considered the primary driver of the under-representation of C ₄ -derived C within riverine OC pools in the Betsiboka basin, although combustion of grassland biomass by fire is likely a subsidiary constraint on the quantity of terrestrial organic matter available for export to these streams and rivers. Our findings carry implications for the use of sedimentary δ ¹³ C signatures as proxies for past forest-grassland distribution and climate, as the C ₄ component may be considerably underestimated due to its disconnection from riverine OC pools.