Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia) were grown for 71 d in flowing nutrient solutions containing N as 10 mmol m−3 NH4NO3, under artificial illumination, with shoots at 20/15°C day/night temperatures and root temperature reduced decrementally from 20 to 5°C. Root temperatures were then changed to 3, 7, 9, 11, 13, 17 or 25°C, and the acquisition of N by N2 fixation, NH4+ and NO3− uptake was measured over 14 d. Shoot specific growth rates (d. wt) doubled with increasing temperature between 7 and 17°C, whilst root specific growth rates showed little response; shoot: root ratios increased with root temperature, and over time at 11°C. Net uptake of total N per plant (N2 fixation + NH4+ + NO3−) over 14 d increased three-fold between 3 and 17°C. The proportion contributed by N2 fixation decreased with increasing temperature from 51% at 5°C to 18% at 25°C. Uptake of NH4+ as a proportion of NH4+ + NO3− uptake over 14 d varied little (55–62%) with root temperature between 3 and 25°C, although it increased with time at most temperatures. Mean rates of total N uptake per unit shoot f. wt over 14 d changed little between 9 and 25°C, but decreased progressively with temperature below 9°C, due to the decline in the rates of NH4+ and NO3− uptake, even though N2 fixation increased. The results suggest that N2 fixation in the presence of sustained low concentrations of NH4+ and NO4− is less sensitive to low root temperature than are either NH4+ or NO3− uptake systems.