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The preferences of some woodland understorey species for ammonium and nitrate were investigated by measuring the potential nitrification (conversion of ammonium to nitrate) in the rhizosphere compared with the bulk soil. Less acid-tolerant species, which usually prefer nitrate or a mixture of ammonium and nitrate in hydroponic culture, should have a higher potential nitrification in the rhizosphere compared to the bulk soil due to a low uptake of ammonium (since ammonium is relatively immobile). Acid-tolerant species should have a high uptake of ammonium and thereby lower or equal potential nitrification in the rhizosphere compared to the bulk soil. The hypothesis was tested in a field investigation of five understorey herb species, Deschampsia flexuosa, Convallaria majalis, Poa nemoralis, Geum urbanum and Aegopodium podagraria performed in oak forests in southern Sweden. Overall, the two less acid-tolerant species, Geum urbanum and Aegopodium podagraria, had high potential nitrification in the rhizosphere compared to the bulk soil (indicating a relatively low uptake of ammonium), whilst the acid tolerant species, Deschampsia flexuosa and Convallaria majalis, had approximately equal potential nitrification in the rhizosphere compared to the bulk soil (indicating a relatively high uptake of ammonium). In the case of Poa nemoralis, a species which grows in both acid and less acid soils, we found the potential nitrification in the rhizosphere and in the bulk soil to be similar at low inorganic nitrogen concentrations, but the difference (rhizosphere > bulk) increased when nitrification in the bulk soil was enhanced (i.e. when the nitrogen availability increased). The potential nitrification in the bulk soil varied between 0 and 16 nmol g(-1) h(-1) and was positively correlated with pH. When species occurred at the same site, the potential nitrification in the bulk soil tended to be lower for the acid tolerant species. Despite a large variation in potential nitrification, the method offers a possibility of measuring the preference of plants for ammonium/nitrate in a soil system, under natural conditions. (C) 2000 Annals of Botany Company

Potential Nitrification as an Indicator of Preferential Uptake of Ammonium or Nitrate by Plants in an Oak Woodland Understorey