Humans have impacted atmospheric C-exchange since the introduction of agriculture by changing the geomorphic cascade

Abstract

Early human impact on the global C cycle through deforestation has been demonstrated and it is estimated that between 50 and 357 Pg C have been released from vegetation and soils to the atmosphere in the pre-Industrial era. However, the contribution of erosion and sediment storage on C exchange between terrestrial ecosystems and the atmosphere has not been accounted for, although long-term sediment budget studies show that large volumes of soil material have been mobilized since the introduction of agriculture. Furthermore, through human impact also the geomorphic setting itself has changed thus altering ecosystems such that the C exchange with the atmosphere changed as well. For the 780 km² Dijle River catchment in the western European loess belt, the impact of human induced soil erosion on C exchange with the atmosphere was quantified by combining a sediment budget with detailed inventories of C in soils and sediments. For the period 4000 BC to AD 2000 it was estimated that anthropogenic erosion induced a net C sink, offsetting 39% of the C emissions due to land cover change since the advent of agriculture. However, this sink is limited by a significant loss of buried C in colluvial settings lagging the burial: ca. half of the original C buried in the colluvial stores remains after 500 years. Contrary to colluvial settings, C burial in alluvial settings appears to be more conservative, which is most probably related to higher autochthonous production and preservation rates in wet floodplain soils. Data from the floodplain sediments also indicate that human induced high rates of minerogenic sedimentation since the Middle Ages were able to capture more C in the floodplain than the Mid-Holocene natural wetlands through peat formation. The result for the Dijle River catchment, with a long history of human impact, can be used to estimate the longer term impact of the major agricultural expansion of the 19th and 20th century worldwide on global C budgets