Interactive effects of low pH and high ammonium levels responsible for the decline of Cirsium dissectum (L.) Hill

The decline of Cirsium dissectum in dessicatingwetlands is attributed to acidification and eutrophication. Experimentalevidence was obtained for the first time on ammonium toxicity under low pH. Inahydroculture experiment, interactive effects of nitrogen forms (250 micromol NH4 + or 250 micromolNO3 -) and pH (4, 5 or 6) were studied with regardtothe vitality of C. dissectum seedlings. The results showthat 250 micromol l-1 ammonium as sole nitrogensource only had negative effects on C. dissectum incombination with a low pH. Ammonium uptake at a rhizosphere pH of 4, resultedinlower nitrogen contents of both roots and shoots, lower internal pH of rootsandshoots and increased contents of basic amino-acids, resulting indecreased survival rate and biomass development. At higher pH, or when nitratewas the nitrogen source, these processes do not take place. This phenomenonstresses the importance of periodic influence of base rich groundwater duringthe winter in wet species-rich heathlands and grasslands, necessary torestore the acid neutralising capacity of the soil. Anthropogenic lowering ofthe groundwater table will lead to acidification enabling ammonium to becometoxic to herbaceous plant species such as C. dissectum.