Foliar retention of ¹⁵N-nitrate and ¹⁵N-ammonium by red maple (Acer rubrum) and white oak (Quercus alba) leaves from simulated rain

Studies of nitrogen cycling in forests indicate that trees assimilate atmospheric nitrate and ammonium and that differences between atmospheric deposition to the forest canopy and deposition measured in forest throughfall can be attributed to the removal of these ions from rain by tree leaves. Red maple and white oak leaves were exposed to artificial rain solutions (pH 4.1) containing ¹⁵N-labeled nitrate (3.5 μg N/ml) or ammonium (2.2 μg N/ml). At two time intervals after exposure (2 hr and 2 days) an exposed leaf and a control (non-exposed) leaf were removed from replicate seedlings. Based on results from ¹⁵N analysis, most of the nitrate applied to tree leaves was removed by washing with water; the mean per cent removal (± standard error, N = 4) was 87 ± 1 and 73 ± 4% of the ¹⁵NO-N Applied to red maple and white oak leaves, respectively. Relative retention of ¹⁵NH₄-N by the leaves was greater than that observed for ¹⁵NO₃-N. In red maple and white oak leaves, 58 ± 9 and 84 ± 7% (mean ± standard error, N = 4), respectively, of the applied ammonium was not removed by washing treatments. Our results show that the foliar uptake of ¹⁵NH₄⁺ from simulated rain by deciduous tree leaves is greater than that for ¹⁵NO₃^-. Greater retention of NH₄⁺ than NO₃^- ions by red maple and white oak leaves from simulated rainfall is consistent with field observations showing a preferential retention of ammonium from rainfall by forest canopies. As nitrogen chemistry and the relative importance of nitrogen compounds in the atmosphere change in response to proposed emission reductions (and possibly climate change), an improved understanding of the fate of airborne nitrogen compounds in forest biogeochemical cycles will be necessary.