Mechanisms for aluminum release in Chinese acid forest soils

With acid rain as an increasing environmental problem in China there is an urgent need for evaluation and improvement of soil acidification models. We have evaluated the applicability of a range of simple models describing al uminum release for a common Chinese forest soil type. Data from field inves tigations, laboratory experiments and soil extractions are compared and use d to test several equilibrium models. The commonly used equilibrium with gi bbsite (Al(OH)(3)) and an ion exchange model did not satisfactorily describ e the observed data, neither did equilibrium reactions with other minerals. Better fit was obtained assuming equilibrium between dissolved Al3+ and al uminum extracted by pyrophosphate or copper chloride, often assumed to repr esent organically bound aluminum pools in the soil. The sizes of easily ext ractable aluminum pools were similar to values observed in Europe and North America. The smallest pools correspond to a few decades of the aluminum fl ux in soil water. In the laboratory experiments aluminum is re-supplied to easily extractable pools from more strongly bound phases, but it is not cle ar to what extent this occurs in the field. At a given pH, the aluminum con centrations were higher in samples collected in the field compared to the l aboratory experiments, showing the importance of soil structure and water f low paths in the field. Our results indicate that it is possible to model a luminum chemistry in soil water from Chinese subtropical forest soils bette r than using gibbsite equilibrium. However, we are far from having a reliab le, mechanistic model. This reduces the confidence in long-term model resul ts. (C) 1999 Elsevier Science B.V. All rights reserved.