Effects of tree species and nitrogen fertilization on cation cycling in single-species stands in the northern hardwood forest

Background/Question/Methods

Nitrogen (N) deposition has been linked to increased leaching losses of soil nutrient cations, with potential consequences for plant nutrition and vigor.  However, tree species differ in various nutrient cycling attributes, including rates of nitrification, which could be expected to influence the impacts of N deposition on other nutrient cycles.  Hence, the extent to which acidic deposition leads to changes in cation cycling might be expected to vary depending on the tree species present.   We measured calcium (Ca), magnesium (Mg) and potassium (K) in fresh foliage, litter, organic and mineral soils, and soil solution in single-species plots dominated by red oak (Quercus rubra), sugar maple (Acer saccharum), eastern hemlock (Tsuga canadensis), American beech (Fagus grandifolia), and yellow birch (Betula alleghaniensis) in the Catskill Mountains, NY.  Half the plots were fertilized with nitrogen by adding NH₄NO₃ (50 kg ha^-1 yr^-1) annually, starting in 1997.  Sampling and analysis of plant and soil samples was conducted in 1997 and 2003; soil solutions were sampled from 2002-2003.  We used mixed-model ANOVA, with species and fertilization as main effects, to test for differences in cation concentrations due to species and N fertilization.  
Results/Conclusions
Nitrate in soil solution was positively correlated with Ca in soil solution (R² = 0.89; p<0.0001), both of which increased with added N.  Among species, fertilization increased leaching of Ca and Mg in beech, hemlock, yellow birch and sugar maple, but not in oak plots.  We found no significant effects of fertilization on nutrient cation concentrations in any plant or soil material.  As expected, there were significant differences among species in Ca, Mg and K concentrations for most plant and soil materials.  American beech had the lowest concentrations of Ca, Mg and K in organic and mineral soil compared to the other species; beech and hemlock tended to have the lowest concentrations of all nutrient cations in foliage.  Conversely, sugar maple had the highest concentration of Ca in organic soil samples and foliage.  For both sugar maple and oak, but not for other species, foliage was significantly (R² = 0.45 to 0.77; p=0.03 to 0.001) correlated with both organic and mineral soil Ca.  Loss of cations in soil solution was not related to the concentration of those cations in organic or mineral soil horizons.  Instead, there appear to be species-specific controls on processing of added N that influence losses of cations in soil solution.