Ion leaching from a sugar maple forest in response to acidic deposition and nitrification

Abstract

Year-to-year variation in acidic deposition within a mature sugar maple-dominated forest and in leaching of ions from the associated podzolic soil were examined at the Turkey Lakes Watershed between 1981 and 1986. Below-canopy inputs to the soil of SO₄ ²⁻ and NO₃ ⁻ in throughfall averaged 640 and 295 eq. ha⁻¹ yr⁻¹; the corresponding ranges were 493–917 and 261–443 eq. ha⁻¹ yr⁻¹. The contribution of atmospheric deposition to SO₄ ²⁻ NO₃ ⁻ and Ca²⁺ leaching decreased over the six years. During the study period, the mean annual volume-weighted NO₃ ⁻ concentration decreased in throughfall and forest-floor percolate and increased in the mineral-soil solution collected below the effective rooting zone. A substantial shift in the balance between SO₄ ²⁻ and NO₃ ⁻leaching from the mineral soil was observed; leaching of SO₄ ²⁻decreased and NO₃ ⁻ leaching increased with time. Leaching of Ca²⁺ and Mg²⁺ from the soil was increased as a result of excess NO₃ ⁻ production in the soil. The calculated output of NO₃ ⁻ from the soil, which averaged 1505 eq. ha⁻¹ yr⁻¹, considerably exceeded the atmospheric deposition of NO₃ ⁻, whereas SO₄ ²⁻ outputs were only moderately greater than inputs.