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Start of funding 01.07.2016

Soil carbon sequestration and floodplain restoration as strategies for climate change mitigation

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One potentially effective strategy of soil carbon sequestration to mitigate climate change is the conversion of agricultural land in riverine floodplains back to near-natural conditions. At our test site in California, it was shown in a previous study that agricultural cultivation of the river floodplain resulted in a loss of soil and a limitation of carbon sequestration in these soils. Moreover, it has been shown that deep soils (down to 3 m) contain significantly more C than typical carbon stock calculations to a depth of 1 m. These substantial amounts of C sequestered have not been yet accounted for in global carbon models. Our project will deliver insight into (i) the origin C and historical dynamics in those alluvial soils of the proposed field site, (ii) the stability of C and its different fractions under current conditions, and (iii) the fate of C under floodplain restoration.

Final report:
Activities

Within the framework of the BaCaTeC project 22[2016-1] three meetings were organized and accordingly, the travels to these meetings were supported with the funding.

Prof. Dr. Peter Fiener (applicant) visited the researchers at the University of California (UC) in Davis between November, 10 and 19 in 2016. During his stay, field trips to the study site and the catchment area were conducted, data analyzed and discussed. Furthermore, P. Fiener gave a scientific talk for the Smart lab group.

Prof. Dr. David Smart and Dr. Kristin Steger travelled to the Augsburg University on April, 22 2017 and stayed until April, 30 and May, 03 2017, respectively. New results of additional analyses which were conducted in California in collaboration with the USGS (United States Geological Survey) were discussed and further proceedings were agreed upon. Additionally, a concept for a mutual scientific publication was prepared.

K. Steger visited again the Augsburg University between October, 31 and November, 08 2017 for finalizing the joint publication and for discussing open questions and future options of cooperation.

Results

The goal of the common project between Prof. Dr. Peter Fiener (Augsburg University) and Prof. Dr. David Smart as well as Dr. Kristin Steger (both University of California Davis) was to analyze soil carbon stocks under floodplain restoration and different land use management practices in the floodplain and the catchment of Cosumnes River. Furthermore, our study was intended to give evidence about carbon storage or CO2 emission under land use change. A previous study in the floodplain revealed that deep soils down to 7 m contain significantly more carbon than typical carbon stock calculations to a depth of 1 m. This deep soil carbon storage is a consequence of the lateral input of organic matter from the catchment on the one hand, and otherwise the result of wet conditions in the floodplains which decelerate the degradation of soil organic carbon. Additional soil analyses (mercury contamination in our soil samples) and data evaluation within the BaCaTeC project revealed that the onset of the Californian Gold Rush in the 1850s had a significant impact on carbon storage in the floodplain soils. This was partly due to the fact that the headwater catchments were affected by hydraulic mining and significant amounts of sediments were transported to the floodplains, and on the other hand the floodplains were affected by land use changes for agricultural purposes.

High amounts of carbon were stored during the Gold Rush (1850-1880) as sedimentation rates increased and deposited huge layers with low carbon contents, but simultaneously deposited over organically rich topsoil which promoted the stabilization of SOC. Those conclusions were possible after the analysis of total mercury concentrations which are associated with gold mining to amalgamate fine gold, and we used it as tracer for carbon origin of deposition.

In summary, this project gave evidence that human-induced changes of river sediment dynamics and land use within the floodplain have an enormous impact on SOC storage. The results of our mutual work were concluded in a manuscript which was submitted to the journal Global Change Biology in November 2017.

Outlook

Based on these study results other research projects between the Augsburg University and UC Davis and UC Merced are planned. Such projects will address questions like how widespread are alluvial soils in the floodplains of the Californian Central Valley and to what extent we can find the influence of gold mining and sediment transportation in the entire region.

This study gave evidence that human-induced changes of river sediment dynamics and land use within the floodplain have an enormous impact on SOC storage. As this phenomenon is so widespread in the entire Central Valley of California it has a substanial effect on soil organic carbon storage in the entire region and needs to be taken into account to improve our understanding of the human-induced interference with C cycling.