Subsurface nutrient dynamics of the Cosumnes Floodplain: A study of deep soil carbon and nitrogen stocks and groundwater quality

Subsurface nutrient dynamics occurring in the soil and groundwater serve important ecosystem functions. Soils account for the largest terrestrial pool of carbon and have the potential for even greater amounts of carbon sequestration. Groundwater is an essential water resource for California, as well as worldwide, used for agricultural, industrial, urban and municipal purpose. In this study we assessed the presence of carbon and nitrogen within the soil subsurface to determine past nutrient dynamics, and we assessed nitrogen in the groundwater to assess current nutrient movement. Both were studied in the context of restoration and how hydrologic and ecologic restoration of a floodplain may impact soil and groundwater nutrient cycling. The study site covered approximately 350 ha of riparian and floodplain ecosystem along the Cosumnes River in the Central Valley of California. The first part of the study assessed deep carbon pools. Surface soils (0-15cm) and subsurface soils (0.15-3m) were collected and analyzed for C and N. Subsurface soils (0-3 m) were found to have significantly more C than typical soil C stocks (0-1 m), which would have represented less than half of the total C stock from 0 3 m. Burial of sediments resulting from flooding is most likely the key process for the observed storage of large amounts of C in these soils as is seen in apparent buried horizons found at approximately 1m in 14 of 25 subsurface boreholes. In addition past marsh and riparian habitat would have promoted high C inputs prior to burial indicating the importance of native riparian and marsh habitat. In the second part of the study, we addressed groundwater quality evaluating the role of riparian areas and biogeochemical processes in the mitigation of -2- groundwater nitrates and the spatial, temporal and restoration related changes seen in groundwater quality. Groundwater samples from 12 monitoring wells and river samples were taken bimonthly to monthly with concurrent and ongoing measurement of groundwater levels. These samples were measured for nitrate and ammonium, ∂ 15N and ∂18O as well as other common groundwater quality analyses. We found there was great spatial variability in the floodplain, with some temporal variability. Given the ongoing drought it was difficult to assess any changes related to restoration efforts. Nitrate concentrations in riparian areas were significantly reduced compared to those elsewhere on the floodplain. Isotopic data provided evidence of denitrification occurring in at least one well also in a riparian area. Overall, we found restoration of the floodplain could provide significant impacts to both soil and groundwater nutrient cycles by increasing the potential for soils to sequester carbon and for attenuation of groundwater nitrates.