Seasonal and Successional Controls on Nitrate Leaching from a Floodplain Forest
Scott Bechtold, Master's Project

Soil Dynamics

The objectives of this study were to determine what major variables control leaching of nitrogen from floodplain soils into hyporheic flowpaths, and to understand how leaching patterns vary with overlying forest patch structure.

Event-based and seasonal patterns in nitrogen exchanges

Strong seasonal pulses in aquatic nitrate have been observed in many ecosystems. Typically, seasonal changes in biotic activity occur contemporaneously with seasonal changes in rainfall, making it difficult to determine whether the observed patterns result from rapid increases in inorganic nitrogen due to changes biologically activity or from mobilization of soil nitrate that has accumulated over longer periods of time.

During the summer and fall of 1998, we conducted two studies designed to isolate the relative importance of biotic versus hydrologic processes in controlling this important flux of nitrogen. We experimentally irrigated a mature alder stand with sprinklers to simulate a typical rain storm. This experiment was conducted in late summer, when new soil inputs of organic matter were low and biologic uptake should have been high. Nevertheless, we were able to stimulate massive nitrate leaching. Nitrate concentrations remained very high for the three days that we irrigated, indicating a substantial pool of leachable N. A similarly rapid transfer of soil nitrate to the hyporheic zone was measured during intensive sampling of a major storm in November 1998. In comparison to the summer rainfall experiment, the soil nitrate pool was much smaller, and was depleted to low concentrations within the first two days of sampling.

Together with these two studies, patterns observed in our monthly sampling of soil and hyporheic water demonstrate a controlling role for hydrologic factors in controlling terrestrial-aquatic nitrogen exchanges. In 1997, which had an unusually wet summer, aquatic nitrate levels were elevated throughout the growing season. Although small increases in soil nitrate were measured at about the time of leaf abscission, no autumn increase in aquatic nitrate was measured. 1998, in contrast, was very dry. By late summer, soil nitrate concentrations were 5 to 10 times higher than they were in 1997.

Long-term patterns in soil nitrogen accumulation and leaching

Comparison of total nitrogen, total carbon, and soil particle size distribution across a chronosequence of 12 sites indicated an important role for fine mineral sediments in retaining organic nitrogen. Soil nitrogen and carbon accumulated rapidly early in the period of alder domination. Carbon accretion was strongly related to the concentration of fine sediments (silt plus clay). Although this relationship was partially due to co-deposition of organic matter and fine sediments in overbank flows, the correlation was strong throughout the 250 year chronosequence. With the rapid turnover in soil organic matter that is characteristic of these forests, the temporal stability of this correlation suggests that organic matter was being retained by association with fine sediments. These findings were reinforced by strong correlations between soil nitrate and soil texture. Soil nitrate, which was used as an indicator of the leachable soil N pool, was measured in eight of the sites on five dates during the summer through fall 1998. The combined effects of total soil nitrogen and soil texture explained over 90% of the variation in soil nitrate, although high correlation between total soil nitrogen and soil texture made it impossible to isolate the relative contribution of each to explaining soil nitrate concentrations.

We unexpectedly discovered that approximately one half of the nitrogen measured in soil samples is contained within mineral sediments. This nitrogen was apparently deposited during diagenesis of the sedimentary parent material. We assume that, given the large fluxes of nitrogen through the system, release of nitrogen from weathering of the parent material is slow enough that it does not contribute significant amounts of biologically available nitrogen. However, we know almost nothing about the behavior of this pool. At a minimum, uncritical inclusion of this geologic nitrogen in total nitrogen estimates may greatly overestimate the size of the active soil nitrogen pool. This work formed the basis of a Master's thesis by J. Scott Bechtold, which was completed in March, 2000.