Seasonal and inter-annual variations in carbon fluxes in a tropical river system (Tana River, Kenya)

The hydrological status of river systems is expected to change due to dam operations and climate change. This will affect the riverine fluxes of sediment and carbon (C). In rivers with strong seasonal and inter-annual variability, quantification and extrapolation of sediment and C fluxes can be a challenge as measurement periods are often too short to cover all hydrological conditions. We studied the dynamics of the Tana River (Kenya) from 2012 to 2014 through daily monitoring of sediment concentrations at three sites (Garissa, Tana River Primate Reserve and Garsen) and daily monitoring of C concentrations in Garissa and Garsen during three distinct seasons. A bootstrap method was applied to calculate the range of sediment and C fluxes as a function of annual discharge by using daily discharge data (1942–2014). Overall, we estimated that on average, sediment and carbon were retained in this 600 km long river section between Garissa to Garsen over the 73 years (i.e., fluxes were higher at the upstream site than downstream): integration over all simulations resulted in an average net retention of sediment (~ 2.9 Mt year− 1), POC (~ 18,000 tC year− 1), DOC (~ 920 tC year− 1) and DIC (~ 1200 tC year− 1). To assess the impact of hydrological variations, we constructed four different hydrological scenarios over the same period. Although there was significant non-linearity and difference between the C species, our estimates generally predicted a net increase of C retention between the upstream and downstream site when the annual discharge would decrease, for example caused by an increase of irrigation with reservoir water. When simulating an increase in the annual discharge, e.g. as a potential effect of climate change, we predicted a decrease in C retention.AFRIVA

Human-induced erosion has offset one-third of carbon emissions from land cover change

Anthropogenic land cover change (ALCC) is an important carbon (C) loss mechanism, but current methods do not consider the role of accelerated soil organic C erosion and its burial in sediments in their assessments of net soil–atmosphere C exchange. Using a comprehensive global database and parsimonious modelling, we evaluate the impact of anthropogenic soil erosion on C fluxes between the Earth’s surface and atmosphere from the onset of agriculture to the present day. We find that agricultural erosion represents a very large and transient perturbation to the C cycle and has induced a cumulative net uptake of 78 ± 22 Pg C in terrestrial ecosystems during the period 6000 BC to AD 2015. This erosion-induced soil organic C sink is estimated to have offset 37 ± 10% of previously recognized C emissions resulting from ALCC. We estimate that rates of C burial have increased by a factor of since AD 1850. Thus, current assessments may significantly overestimate both past and future anthropogenic emissions from the land. Given that ALCC is the most uncertain component of the global C budget and that there is a strong connection between ALCC and erosion, an explicit representation of erosion and burial processes is essential to fully understand the impact of human activities on the net soil–atmosphere C exchange