Effects of enhanced N deposition as (NH4)2SO4 and HNO3 on base cation leaching from podzol microcosms

An upland moorland podzol profile was reconstructed in 100 cores. Cores were subjected, over 20 weeks, to simulated ambient rainfall, and rainfall containing enhanced N (2- and 6-fold increases) either as (HNO3) or as (NH4)2SO4. Selected cores were destructively analysed every two weeks, to quantify the dynamics of soil change in each horizon. The drainage water was analysed on a biweekly basis. Enhanced NH4+ deposition caused substantial mobilization of base cations from upper horizons with subsequent accumulation in lower horizons. Base cation concentrations in drainage water increased generally for both ammonium and nitrate enhanced N treatments. The results suggest that deposition of (NH4)2SO4 can be just as damaging as deposition of HNO3, and that competition between NH4+ and base cations for cation exchange sites is an additional damaging aspect of N deposition which is currently being largely ignored in the quantification of risk of N damage to soils.