Potential nitrification as an indicator of preferential uptake of ammoniumor nitrate by plants in an oak woodland understorey

The preferences of some woodland understorey species for ammonium and nitra te were investigated by measuring the potential nitrification (conversion o f ammonium to nitrate) in the rhizosphere compared with the bulk soil. Less acid-tolerant species, which usually prefer nitrate or a mixture of ammoni um and nitrate in hydroponic culture, should have a higher potential nitrif ication in the rhizosphere compared to the bulk soil due to a low uptake of ammonium (since ammonium is relatively immobile). Acid-tolerant species sh ould have a high uptake of ammonium and thereby lower or equal potential ni trification in the rhizosphere compared to the bulk soil. The hypothesis wa s tested in a field investigation of five understorey herb species, Descham psia flexuosa, Convallaria majalis, Poa nemoralis, Geum urbanum and Aegopod ium podagraria performed in oak forests in southern Sweden. Overall, the tw o less acid-tolerant species, Geum urbanum and Aegopodium podagraria, had h igh potential nitrification in the rhizosphere compared to the bulk soil (i ndicating a relatively low uptake of ammonium), whilst the acid tolerant sp ecies, Deschampsia flexuosa and Convallaria majalis, had approximately equa l potential nitrification in the rhizosphere compared to the bulk soil (ind icating 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 pote ntial nitrification in the rhizosphere and in the bulk soil to be similar a t 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 b ulk soil varied between 0 and 16 nmol g(-1) h(-1) and was positively correl ated with pH. When species occurred at the same site, the potential nitrifi cation in the bulk soil tended to be lower for the acid tolerant species. D espite a large variation in potential nitrification, the method offers a po ssibility of measuring the preference of plants for ammonium/nitrate in a s oil system, under natural conditions. (C) 2000 Annals of Botany Company.