Mechanisms of aluminum release in Chinese acid forest soils

Summary

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 aluminum release for a common Chinese forest soil type. Data from field investigations, laboratory experiments and soil extractions are compared and used to test several equilibrium models. The commonly used equilibrium with gibbsite (Al(OH)3) and an ion exchange model did not satisfactorily describe the observed data, neither did equilibrium reactions with other minerals. Better fit was obtained assuming equilibrium between dissolved Al3+ and aluminum extracted by pyrophosphate or copper chloride, often assumed to represent organically bound aluminum pools in the soil. The sizes of easily extractable aluminum pools were similar to values observed in Europe and North America. The smallest pools correspond to a few decades of the aluminum flux in soil water. In the laboratory experiments aluminum is re-supplied to easily extractable pools from more strongly bound phases, but it is not clear to what extent this occurs in the field. At a given pH, the aluminum concentrations were higher in samples collected in the field compared to the laboratory experiments, showing the importance of soil structure and water flow paths in the field. Our results indicate that it is possible to model aluminum chemistry in soil water from Chinese subtropical forest soils better than using gibbsite equilibrium. However, we are far from having a reliable, mechanistic model. This reduces the confidence in long-term model results.