Statistical analyses of data about forest sites in the soil monitoring network of Canton Zurich

In large parts of the Swiss Plateau, atmospheric deposition of nitrogen (N) compounds exceeds the critical loads. Excessive N-input into forests is believed to accelerate soil acidification. Hence, the Soil Protection Agency of Canton Zurich monitors soil acidity at some 180 forest sites in the canton. Soil samples were collected for a first time between 1995-1999. The sites were re-sampled 5 years later. Accredited analytical laboratories measured the pH, exchangeable cations and soluble heavy metals. A preliminary analysis of the data showed that pH decreased on sites with strongly acid soils but remained unchanged when pH above 4.

For quality control, each batch of samples sent to a lab contained specimens of up to 3 standard soils and some randomly chosen customary soil samples that had been analysed before. We modelled the fluctuations of these quality control measurements by robust analysis of variance (median polish). The estimated batch effects did not change smoothly in time, but fluctuated quite strongly

This suggests that there was no genuine change of the samples, as one would then expect more gradual variation. We corrected the data by the estimated batch effects, and this affected the estimated change: now we found that the pH increased on average from the first to the second sampling. Similar results were obtained for the other variables. Based on our findings we proposed to optimize the design of the quality control measurements. We further studied whether soluble heavy metals and exchangeable cations may be predicted by pedotransfer functions in the future. We compared robustly fitted linear regression and additive models (GAM) for the purpose. Validation with independent data showed no clear advantage of the GAMs

and therefore we recommend using the linear models to predict the variables of interest from site data and general soil attributes (pH, organic matter content, soil texture).

Team: Andreas Papritz
Funding: Soil Protection Agency of Canton Zurich
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