Leaching of nutrient cations from the forest floor: Effects of nitrogen saturation in two long-term manipulations

Nitrogen saturation results in greater mobility of nitrate, which in turn is often correlated with concentrations of nutrient cations in soil solution and streamwater. At the Harvard Forest, U.S.A., under long-term NH₄NO₃ inputs, a Pinus resinosa Ait, forest has exhibited signs of N saturation more rapidly than a mixed-Quercus forest. We test the hypothesis that increased nitrate leaching causes increased concentrations of nutrient cations in soil solution. Over 2 years (years 6 and 7 of treatment) we measured SO₄^2-, NO₃^-, Cl^-, Ca²⁺, K⁺, Mg²⁺, Na⁺, H⁺, and NH₄⁺ in throughfall solution and in forest-floor (Oa) leachate. Concentrations of NO₃^- in forest-floor leachate increased with rates of N amendment and correlated positively with cation concentrations, with stronger overall correlations in the pine forest: r² values were 0.51 (pine forest) and 0.39 (oak forest) for Ca²⁺, 0.45 (pine) and 0.16 (oak) for K⁺, and 0.62 (pine) and 0.50 (oak) for Mg₂₊. In summer and fall, the oak forest showed some negative relationships between nutrient cation leaching and rate of N amendment. These contrasts showed retention of cations and N to occur together in an N-limited system, whereas increased nitrate mobility occurred with increased cation losses in an N-saturated system.