Floodplain biogeochemical mosaics: A multidimensional view of alluvial soils

Abstract

The alluvial floodplains of large rivers are exceptionally productive and dynamic ecosystems, characterized by a complex mosaic of vegetation at different successional stages overlying soils sorted by historic floods. Natural floodplains are widely credited with efficiently removing nitrogen from surface waters and accumulating carbon in biomass, yet very little floodplain research has examined carbon and nitrogen cycling below surficial soils. We evaluated the extent to which vegetation cover could be used to predict subsurface carbon and nitrogen dynamics and to estimate whole-floodplain carbon storage and denitrification rates. We dug soil pits under three dominant vegetation communities on a gravel-bedded floodplain in northwest Montana to the depth of the permanent water table (1-3 m). We compared depth profiles of total and dissolved carbon (C) and nitrogen (N), denitrification potentials (DEAs), organic particulates, moisture, and pH across vegetation types. Near-surface soils (0-10 cm) of forests had larger C and N pools and DEAs than grasslands or gravel bars, but such vegetation effects dissipated within the upper~50 cm of soil. At depth, spatial heterogeneity in carbon and nitrogen pools and fluxes depended instead on soil texture, and relatively high rates of DEA and carbon storage were measured in zones of buried organic debris. Although C storage and denitrification potential are generally low in subsurface soils, these deep soils might nonetheless contribute substantially to whole-floodplain C storage and denitrification because of their large volume, high hydrologic connectivity, and heterogeneous biogeochemistry.

Geomorphic Controls on Floodplain Soil Organic Carbon in the Yukon Flats, Interior Alaska, From Reach to River Basin Scales

“…Our results demonstrate that Preacher Creek generally has coarser sediment compared to the other study rivers, although the significant interaction between river and geomorphic unit makes these results somewhat difficult to generalize (Table ). Difference in grain size has been an influencing factor for other studies of floodplain OC, with coarser sediment containing lower OC concentrations (Appling et al, ; Pinay et al, ; Sutfin & Wohl, ). The Dall River samples also have higher soil moisture than Preacher Creek and the Teedrinjik River, which could also influence OC concentration (Figure ).…”

Dissolved organic carbon lability increases with water residence time in the alluvial aquifer of a river floodplain ecosystem

“…Although the soil, vadose zone, and stream sediments act as strong filters for POM, large pools of POM are often observed in the subsurface of alluvial river floodplains [ Gurwick et al ., ; Appling et al ., ]. Invertebrate abundance tends to increase with organic matter content in interstitial habitats [ Williams and Hynes , ; Strayer et al ., ; Crenshaw et al ., ], and the breakdown of stored POM can increase labile DOC within stream and floodplain subsurface environments [ Crocker and Meyer , ; Schindler and Krabbenhoft , ; Gurwick et al ., ].…”

“…Invertebrate abundance tends to increase with organic matter content in interstitial habitats [ Williams and Hynes , ; Strayer et al ., ; Crenshaw et al ., ], and the breakdown of stored POM can increase labile DOC within stream and floodplain subsurface environments [ Crocker and Meyer , ; Schindler and Krabbenhoft , ; Gurwick et al ., ]. Numerous organic deposits have been identified within the Nyack aquifer, directly through the excavation of soil pits [ Appling et al ., ] and indirectly through measurements of distinct areas of low dissolved oxygen that may be indicative of areas of high microbial activity caused by coarse POM buried in the aquifer [ Reid , ; Valett et al ., ]. In fact, Appling et al .…”

Evaluating carbon storage on subalpine lake deltas

“…The magnitude of OC storage in a landform can be controlled by a variety of environmental conditions. On the scale of particles of mineral soil storing OC, soil moisture and texture (Pinay et al, 1992;Howard and Howard, 1993;Jobbágy and Jackson, 2000;Appling et al, 2014) control the decomposition rate of and storage capacity for OC. Wetter soils prevent microbial respiration of OC, and finer textured soils provide more surface area for OC to bond to the mineral grains in the soil.…”

“…OC storage in soil is controlled by a balance of stabilizing processes that reduce microbial respiration and increase adsorption of OC to mineral soil and destabilizing processes that prevent adsorption and increase respiration (Pinay et al, 1992;Jobbágy and Jackson, 2000;Doetterl et al, 2015). OC storage is generally maximized in wetter, colder environments with higher net primary production and resulting higher total ecosystem carbon stocks (Howard and Howard, 1993;Yuste et al, 2007;Appling et al, 2014;Sutfin et al, 2015). We hypothesize that the differences in environmental conditions between the Cascades and the Front Range will be reflected in the OC content of their subalpine lake deltas: deltas in the Cascades are expected to have higher concentrations of OC than those in the Front Range due to a wetter climate, higher primary productivity, and higher total ecosystem carbon stocks (Smithwick et al, 2002;Bradford et al, 2008;Sutfin et al, 2015).…”