Feldspar weathering as the key to understanding soil acidification monitoring data; a study of acid sandy soils in the Netherlands

Monitoring activities pose special demands on the type of survey results needed. In the early 1990s a soil acidification monitoring methodology was adopted in the Netherlands that leaned heavily on methods developed in more fundamental research, most notably the use of proton budgets. Consequently, various controversies still not resolved in the scientific debate reflect on the current practice of soil acidity monitoring and complicate interpretation of the monitoring results. In a pilot study we address the most pressing issues: capacity versus intensity parameters, choice of monitoring objective, and natural variation in the compartment to be monitored. Focus is on the major source of buffering, the possible usefulness of the historic approach, and the regional patterns present in the sandy soils of the Netherlands. In a field campaign 92 locations in sandy regions all over the country were sampled at two depths. The solid phase, the displaced soil solution, and solid phase extractions with 0.01 M CaCl2 and 0.43 M HNO3, for the 184 samples were analyzed by a variety of methods. Aluminum release is the major source of buffering and is shown to contribute substantially to acid buffering already under natural conditions. The predominant Al bearing phases in Dutch sandy soils are feldspars and secondary Al minerals; feldspars are found to be the determinative phase in acid buffering. Application of the historic approach using the subsoil as a proxy for the initial composition of the topsoil proved feasible for this regional dataset. The average depletion of the ANC(s) of 230 mmolc kg¿1 in the topsoil matches well with estimates of the total proton load since the last ice age, with the anthropogenic contribution being between 20% and 50%. Fuzzy c-means cluster analyses of the solid phase and soil solution data show a distinct regionality that was also reflected in the parameters generally used to indicate the acidity status of soils, ¿ANC(s) and Al/BC ratios. A combined insight into both solid phase and soil solution, based on a comprehensive set of parameters, proves essential for interpreting soil acidity monitoring data.