How do sediment characteristics affect organic matter (OM) and nutrient dynamics in floodplains where the soils are young and fluvial sorting creates pronounced patterns in sediment size distribution? Texture is a primary influence on soil biogeochemistry, and fluvial redistribution of sediments is the defining characteristic of alluvial environments. Fluvial sediments are also often rich in unweathered surface area, and undergo rapid alteration once exposed to soils acids. We thus hypothesized that sediment size and mineralogy would be particularly important influences on the biogeochemistry of floodplain soils. Amounts and biological availability soil carbon, nitrogen and phosphorus were examined in 1,000 year alluvial soil chronosequence on the Queets River, Washington using a combination of measurement and modeling techniques. 
Results/Conclusions

Rapid increases in soil carbon to near-plateau concentrations were driven by overbank deposition of silt and clay, with subsequent accumulation of adsorbed OM. Clay and silt particles deposited by flood waters were substantially enriched in adsorbed carbon and nitrogen, and became further enriched to near-plateau concentrations within the first 100 years of site establishment. Overall, 69% of soil carbon and 76% of soil nitrogen was adsorbed to mineral sediments. Direct fluvial carbon deposition was small relative to autochthonous sources, and inconsequential for the balance of soil carbon. Simulations using soil equations from the CENTURY model similarly identified changes in soil texture as the rate limiting factor in Queets soil carbon accumulation. In the modeling exercise and in laboratory incubations, the influence of sediments on carbon and nitrogen dynamics was primarily attributed to increased cycling efficiency of labile OM rather than accumulation of decay resistant OM. Nitrogen retention, in turn, was limited by soil carbon.  Concentration of labile nitrogen-rich decomposition products on fine sediments suggested an important role for short to intermediate OM-retention on sediments  Phosphorus was steadily weathered from  sediments, with ~80% of potentially weatherable phosphorus released from parent material by 1,000 years. This rapid weathering of phosphorus sediments suggests that storage in floodplain soils is a major influence on the supply of reactive phosphorus to the river, which is re-input to the river as alluvial soils are reclaimed by channel migration. In these soils, fluvial sediment deposition established a physical template exerting far-reaching influence over OM accumulation and cycling of nitrogen and phosphorus. Spatial and temporal variation in alluvial sediment distribution across a range of scales is a source of ecological heterogeneity and is highly sensitive to human alteration of sedimentary regimes.