Peter Fiener Mark C. Marvin-DiPasquale Joshua H. Viers David R. Smart Kristin Steger 2019 <p><span>Active floodplains can putatively store large amounts of&nbsp;organic carbon&nbsp;(SOC) in&nbsp;subsoils&nbsp;originating from&nbsp;catchment&nbsp;erosion processes with subsequent floodplain deposition. Our study focussed on the assessment of SOC pools associated with alluvial floodplain soils that are affected by human-induced changes in floodplain deposition and in situ SOC&nbsp;mineralisation&nbsp;due to&nbsp;land use change&nbsp;and drainage. We evaluated depth-dependent SOC contents based on 23 soil cores down to 3 m and 10 drillings down to 7 m in a floodplain area of the lower Cosumnes River. An estimate of 266 Mg C ha</span>⁻¹<span>&nbsp;or about 59% of the entire SOC stored within the 7 m profiles was found in the upper 2 m. Most profiles (n = 25) contained discrete buried A horizons at depths of approximately 0.8 m. These profiles had up to 130% higher SOC stocks. The mean δ</span>¹³<span>C of all deep&nbsp;soil profiles&nbsp;clearly indicated that&nbsp;arable land&nbsp;use has already altered the stable isotopic signature in the first meter of the profile.&nbsp;Radiocarbon dating&nbsp;showed that the&nbsp;</span>¹⁴<span>C age in the buried horizon was younger than in overlaying soils indicating a substantial&nbsp;sedimentation&nbsp;phase for the overlaying soils. An additional analysis of total mercury contents in the soil profiles indicated that this sedimentation was associated with upstream hydraulic gold mining after the 1850s. In summary, deep&nbsp;alluvial soils&nbsp;in floodplains store large amounts of SOC not yet accounted for in global carbon models. Historic data give evidence that large amounts of sediment were transported into the floodplains of most rivers of the Central Valley and deposited over organically rich topsoil, which promoted the stabilization of SOC, and needs to be considered to improve our understanding of the human-induced interference with C cycling.</span></p> application/pdf 10.1016/j.scitotenv.2018.09.205 en Elsevier Human-induced and natural carbon storage in floodplains of the Central Valley of California article