Ion dynamics in acid forest soils as affected by addition of Ca fertilizers

Abstract

Ca fertilizers may be used as tools to improve the chemical status of acidic, base poor forest soils. The downward movement of ions after addition of CaCO 3, CaCO 3 + MgO, and CaS0 4 · 2H 2O was studied by monitoring the leachate composition from three types of columns — A1/B, mull/A1/B and moder/A1/B — reconstructed from an acid brown forest soil profile. The fertilizers were surface-applied at rates of 0, 0.56, 2.80 and 5.60 t ha −1 equivalent CaO, and the treated columns were leached with local rainfall in an open-air nursery during a 20-month period. For all column types and rates of application, the leaching of Ca increased in the order: control < CaCO 3 + MgO < CaCO 3 < CaSO 4 · 2H 2O. Retention of solubilized Ca from gypsum was mainly achieved by simple exchange with resident cations, with no major pH change relative to the control. The elution of Ca and desorbed cations with predominantly SO 4 2− anions resulted in an important increase in total cationic mobilization compared to the control. Due to kinetic restraints and strong buffering by the soil, the downward movement of Ca (from CaCO 3) and Mg (from MgO) ions was greatly limited. The mobility of Mg ions from CaCO 3 + MgO was however much greater than that of Ca, despite considerably larger inputs of the latter. In the leachates from the columns without humus, total Al concentrations decreased exponentially with increase in pH. For the other column types the reduced Al leaching, when observed, was not associated to any major pH increase. Gypsum application depressed nitrification but increased DOC leaching relative to the control. Liming generally had no or little influence on either NO 3 − or DOC leaching, but increased SO 4 2− mobilization in some cases. Total cation leaching from the non gypsum-treated columns was chiefly affected by NO 3 − dynamics. Varying the rate of fertilizer application did not modify the mechanisms involved in the downward movement of the various fertilizers, but could influence the intensity of the processes as well as the rate of soil chemical evolution.