Pedobiochemical indicators for eutrophication and the development of ”︁black spots” in tidal flat soils on the North Sea coast

The increasing eutrophication of tidal flat soils on the North Sea coast and the appearance of ”?black spots” gave rise to this study. The aim was to find pedobiochemical indicators for the development of ”?black spots”. Artificially eutrophicated soils were compared with untreated soils in the field and laboratory. The pH values of the artificially eutrophicated and natural soils often differed by nearly one unit. The treated soils mostly showed lower redox potentials (? —300 mV) than the untreated samples (? —250 mV). The mean sulfate concentrations were 2.2 mM in the eutrophicated laboratory soils and 13.1 mM in the eutrophicated field soils, compared with 12.5 mM and 20.2 mM in the untreated ones. Consequently, the SO₄^2—:Cl^— ratios and SO₄^2— differences were lower in the treated soils. Non‐eutrophicated soils showed methane concentrations of < 5 nmol cm^—3, whereas the eutrophicated soils showed up to 217.0 nmol cm^—3 in the field and 479.1 nmol cm^—3 in the laboratory. Differences between field and laboratory data were mainly due to a continuous sulfate supply and reoxidation process only possible in the field. Although all parameters showed differences between the eutrophicated and untreated soils, only the methane data did not overlap with their interquartile ranges. Those of the untreated soils were 2—7 nmol cm^—3 and those of the treated were 12—360 nmol cm^—3. Thus, threshold values can be defined. Methane concentrations of approximately > 10 nmol cm^—3 refer to the beginning eutrophication process and those in the range of > 100 nmol cm^—3 to advanced processes, phenologically forming ”?black spots”.