Reduced soil contributes to the anomalous occupation of dwarf communities in N‐richer habitats in a cool‐temperate mire

Spatial patterns of the oxidation–reduction potential (Eh) of soil, water level and chemistry were examined in the fen area of the Tohfutsu mire, northern Japan, where horse pasturage has been carried out throughout the growing season every year. The fen vegetation consisted mosaically of communities mono‐ or codominated by Eleocharis kamtschatica, Iris setosa and Carex lyngbyei . The highest concentration of total dissolved nitrogen of soil water was observed in the dwarf community dominated by E. kamtschatica with smaller biomass, whereas the nutrient level was lower in the community monodominated by C. lyngbyei with larger biomass. Principal component analysis denoted well that Eh, N, K, and pH contributed to the differentiation of these communities significantly. During the growing season that was investigated, the highest nitrogen concentration was observed in midsummer, when plants are expected to absorb nutrients intensely, indicating that the supply of dissolved nitrogen far exceeded the nutrient uptake of plants. Multiple habitats with reduced soil were formed by submergence and by an excess of nitrogen due to pasturage, and the soil reduction indirectly inhibited the incursion of species with large biomass at such sites. It was demonstrated that soil Eh contributed to the anomalous pattern of vegetation where the supply of available nitrogen exceeds the uptake by plants.