Bechtold, Scott. 2000. Seasonal and succesional controls on nitrate leaching from a floodplain forest. M.S.
Strong seasonal increases in aquatic nitrate have been observed in a variety of ecosystems. In most cases, seasonal hydrological changes occur contemporaneously with changes in vegetative activity, making it difficult to determine whether soil leaching is being driven by increases in the availability of leachable N or is simply due to flushing of N that has accumulated over longer periods. Three studies were conducted to better determine controls on soil nitrate leaching in a near-pristine temperate floodplain ecosystem receiving large N inputs via N-fixation by red alder: 1) an artificial rainfall experiment was conducted to estimate N-leaching potential during the summer, when uptake is high and new inputs of organic matter are low; 2) soil solution, groundwater and surface water were sampled during a major autumn storm to document exchanges at the seasonal transition, when uptake is low and inputs of senescent organic matter are high; and 3) monthly samples of soil and aquatic nitrogen were collected in 1997 and 1998 to document seasonal patterns of N exchanges. Collectively, these studies demonstrate the importance of hydrologic factors in controlling N flux. Nitrate was rapidly leached from soils during actual and simulated rainstorms. Two pathways of nitrate leaching were identified. Surface runoff and direct leaching of streamside soils into surface waters contributed to high solute concentrations in peak flows. Nitrate that leached into interstital waters was subject to various factors that could delay or reduce its delivery to surface waters. Greater residence time may increase the influence of this component of stormflow on ecosystem productivity . While soil nitrate pools were rapidly depleted during rainstorms, accumulation of soil nitrate occurred over long dry periods. Large differences in soil and aquatic nitrate concentrations between two years with contrasting rainfall illustrate patterns in which inter-annual hydrologic variability affects ecosystem nutrient cycling.
Pervasive sorting of sediments during fluvial transport creates strong longitudinal, lateral, and vertical patterns in the size distribution of riparian soil particles. In order to consider implications of this for ecosystem function, the influence of soil texture on nitrogen cycling was studied in a chronosequence of early successional forest patches on the Queets River floodplain, a near-pristine system on Washington's Olympic Peninsula. Soil organic nitrogen was strongly positively correlated to concentrations of fine (silt plus clay) sediments, indicating that soil texture was an important control on nitrogen accumulation. Soil nitrate concentrations in samples collected on five dates during the summer and fall of 1998 were also correlated with soil texture, although the relationship was more complex. At high fines concentrations, nitrate was a low proportion of the total nitrogen pool, as would be expected if association with sediment particles were inhibiting decomposition. However, at low concentrations of fines, the proportion of the total nitrogen occurring as nitrate increased with the concentration of fines. Moisture limitation of decomposition in the coarsest soils may limit nitrate accumulation during dry periods.
Concentrations of fines among sites were highly variable, and were only weakly correlated with site age. Circumstances of the depositional environment early in patch development may set a physical template with long-term implications for nutrient dynamics. This may be predictable at the reach and watershed scales, and may be an important component of the heterogeneity that contributes to the high biodiversity of natural floodplains.