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

The preferences of some woodland understorey species for ammoniumand nitrate were investigated by measuring the potential nitrification(conversion of ammonium to nitrate) in the rhizosphere comparedwith the bulk soil. Less acid-tolerant species, which usuallyprefer nitrate or a mixture of ammonium and nitrate in hydroponicculture, should have a higher potential nitrification in therhizosphere compared to the bulk soil due to a low uptake ofammonium (since ammonium is relatively immobile). Acid-tolerantspecies should have a high uptake of ammonium and thereby loweror equal potential nitrification in the rhizosphere comparedto the bulk soil. The hypothesis was tested in a field investigationof five understorey herb species, Deschampsia flexuosa, Convallariamajalis, Poa nemoralis, Geum urbanum andAegopodium podagrariaperformed in oak forests in southern Sweden. Overall, the twoless acid-tolerant species, Geum urbanum and Aegopodium podagraria,had high potential nitrification in the rhizosphere comparedto the bulk soil (indicating a relatively low uptake of ammonium),whilst the acid tolerant species, Deschampsia flexuosa andConvallariamajalis , had approximately equal potential nitrification inthe rhizosphere compared to the bulk soil (indicating a relativelyhigh uptake of ammonium). In the case of Poa nemoralis, a specieswhich grows in both acid and less acid soils, we found the potentialnitrification in the rhizosphere and in the bulk soil to besimilar at low inorganic nitrogen concentrations, but the difference(rhizosphere > bulk) increased when nitrification in thebulk soil was enhanced (i.e. when the nitrogen availabilityincreased). The potential nitrification in the bulk soil variedbetween 0 and 16 nmol g^-1h^-1and was positively correlated withpH. When species occurred at the same site, the potential nitrificationin the bulk soil tended to be lower for the acid tolerant species.Despite a large variation in potential nitrification, the methodoffers a possibility of measuring the preference of plants forammonium/nitrate in a soil system, under natural conditions.Copyright 2000 Annals of Botany Company Ammonium uptake, nitrate uptake, nitrogen preference, potential nitrification, rhizosphere, Deschampsia flexuosa, Convallaria majalis, Poa nemoralis, Geum urbanum, Aegopodium podagraria