NODULATION AND NITROGEN-FIXATION BY SUBTERRANEAN CLOVER IN ACID SOILSAS INFLUENCED BY LIME APPLICATION, TOXIC ALUMINUM, SOIL MINERAL N, AND COMPETITION FROM ANNUAL RYEGRASS

The effects of low soil pH and lime addition on nodulation and nitroge n fixation in subterranean clover (Trifolium subterraneum L.) were exa mined in field and glasshouse experiments in south-west Australia. Fie ld data from a number of broadacre pastures indicated that soil acidit y combined with high soil aluminium reduced clover dependence on nitro gen fixation (% Ndfa) and that addition of lime, generally, promoted n odulation and increased % Ndfa. In a detailed study at one site additi ons of lime increased soil pH, reduced extractable aluminium concentra tions below phytotoxic concentrations and increased plant nodulation. Glasshouse experiments also demonstrated positive effects of liming in reducing extractable aluminium and increasing total Nz fixation and % Ndfa, but beneficial effects on N-2 fixation were partly offset by co unteractive effects on symbiosis through increased nitrogen mineralisa tion at high lime application rates. Where companion ryegrass (Lolium rigidum Gaudin) was pot cultured with subterranean clover soil mineral N released following liming was absorbed preferentially by the grass, resulting in increases in % Ndfa of the associated clover. Copyright (C) 1996 Elsevier Science Ltd