THE EFFECTS OF SOIL TEMPERATURE ON PLANT GROWTH, NODULATION AND NITROGEN FIXATION IN CASUARINA CUNNINGHAMIANA MIQ.

Summary The effects of soil temperatures between 15 and 30°C on plant growth, nodulation and nitrogen fixation in seedlings of Casuarina cunninghamiana Miq. inoculated with Frankia from two different sources were examined. The optimum soil temperature for the growth of plants dependent on symbiotic nitrogen fixation was 25°C. Decreasing the soil temperature below 25°C markedly decreased plant growth that was reliant on symbiotically fixed nitrogen; effects on the growth of plants supplied with mineral nitrogen were much smaller. At 15°C there was no response in plant growth to inoculation after 148 d, whereas plants supplied with nitrogenous fertilizer were 10 times the weight of uninoculated plants. Nodulation was delayed at 15 and 20°C with nodules formed at 15°C fixing no nitrogen in these studies. The production of fewer nodules at 20°C than at 25°C was partly compensated by the production of larger nodules. Nodule growth at 20 to 30°C was a prime determinant of nitrogen fixed, with the exception of one Frankia at 20°C. The amount of nitrogen‐fixed g−1 nodule was the same for the two Frankia sources at 25 and 30°C, differences in effectiveness being due to nodule development. However, differences in the effectiveness of the two Frankia sources at 20°C were related to differences both in nodule development and in nitrogen‐fixing ability. The absence of nitrogen fixation at 15°C would be expected to limit the natural distribution of Casuarina species reliant on symbiotically fixed nitrogen to areas where soil temperatures exceed 15°C for a major part of the potential growing season.