Some effects of combined nitrogen on the nodule symbioses of Casuarina and Ceanothus

Abstract

1. The effect of ammonium-nitrogen on the further growth and activity in fixation of nodules already present at the commencement has been studied inCasuarina cunninghamiana growing in water culture and inCeanothus velutinus var.laevigatus in Peralite culture. 2. In Casuarina, at a low level of ammonium-nitrogen nodule growth remained similar to that in plants in nitrogen-free solution, but was stimulated in Ceanothus. In both genera nodule growth was strongly retarded at higher levels. 3. Fixation of nitrogen fell continuously in Casuarina as the level of ammonium-nitrogen was increased, while that in Ceanothus was unaffected at a low level but markedly decreased at a higher one. These effects were compounded from the changes noted in 2 and a tendency for the efficiency of nodule tissues in fixation to fall in the presence of ammonium-nitrogen, though this was not always shown. 4. In both genera but especially in Casuarina the growth of plants entirely dependent on nodule nitrogen was inferior to that of plants additionally supplied with ammonium-nitrogen. The reasons are discussed. 5. The results indicate the effects likely to be produced in the field as the soil nitrogen level rises through the action of the nodules of these species.