Boles, J., G.P. Robertson, and L.O. Hedin
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Most studies on nitrogen loss from ecosystems measure only the inorganic forms of nitrogen, ignoring mobile organic forms such as amino acids, aminated sugars, and humic acids that dissolve into soil water and as such can be lost. Although long an important parameter in aquatic research, dissolved organic nitrogen(DON) in soil leachate has only been measured in a few forest studies. Some preliminary data from lysimeters in the LTER site, however, suggest that loss of nitrogen as DON can occasionally be as great as the amount lost as NO3-.Over the past year we have taken samples of soil leachate with quartz (Teflon) suction lysimeters from all LTER treatments except the never tilled grassland (T8), including the forested sites. Lysimeters are placed in the C-horizon of each sample site, and thus provide an integrated estimate of solution N loss to groundwater. Quartz/PTFE lysimeters are used because they are less likely to filter out DON than ceramic cup lysimeters.Samples were analyzed for NO3-, NH4+, and DON. Results discussed here are for samples taken between May and December 1995. Although absolute levels were rarely higher than 20 ppm for each form, the relative proportion of organic nitrogen to inorganic was often surprisingly high (50-100%). Differences in NO3- and DON between LTER treatments were inconclusive due to high variability. In general NO3- levels were higher under tilled, fertilized sites. The most unexpected result was the apparent correlation between NO3- and DON concentrations in samples with greater than 1 ppm DON (Figure 1). In Figure 1 note that there is no relation between DON and NO3- for all samples under 1 ppm DON. Above 1 ppm DON, however, there is always an almost equal amount of NO3. Separately, the samples with greater than 1 ppm DON showed a strong correlation with NO3- (r2 = 0.84; Figure 2). This has implications for interpreting nitrogen budget deficits, long assumed to be due to denitrification, as well as for our general understanding of hydrologic N losses from upland ecosystems.
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