Some effects of nitrate, ammonium and mycorrhizal fungi on the growth of Douglas fir and Sitka spruce
Investigations have been carried out into the effects of nitrate and ammonium on: the growth dynamics of mycor-rhizal fungi, the relative infection success of these fungi when associated with Douglas-fir and Sitka spruce seedlings, and the growth and nitrogen uptake by mycorrhizal and non-mycorrhizal seedlings. Fungal isolates were obtained from sporocarps collected in Douglas-fir or Sitka spruce stands. On the basis of growth rate and nitrogen utilisation patterns, Paxillus involutus and Lactarius rufus were chosen for further study. Both fungi were capable of rapid growth; but while P. involutus grew equally well when either ammonium or nitrate was supplied, L. rufus could only utilise ammonium. In aseptic culture with seedlings of Douglas-fir or Sitka spruce, both fungi readily formed ectotrophic mycorrhizas. These experiments were carried out in Erlenmeyer flasks. Although pure cultures of P. involutus grew well when either nitrate or ammonium was the nitrogen source, those supplied ammonium were always the heaviest. However, when the calculated energy cost of nitrate reduction was deducted from the ammonium cultures, the weights of nitrate and ammonium cultures became nearly identical. Furthermore, nitrogen form had no effect on the growth rate of P. involutus cultures. When the quantity of glucose supplied was increased from 0.5 g/1-1 to 2.0 g/1-1 , the culture weight increased proportionally, but increasing the glucose to 4.0 g/1-1 brought about a less than proportional weight gain. Regardless of original nitrogen form, or amount of glucose supplied, whenever P. involutus cultures entered the senescent phase of growth ammonium was released into the nutrient solution. In pure culture L. rufus grew well when ammonium was supplied, but was completely unable to utilise nitrate. Increasing the amount of glucose supplied brought about less than proportional increases in the weight of the colony, with the least glucose producing the greatest relative weight. Lactarius rufus never released nitrogen during the senescent phase of growth. The three previously mentioned mycorrhizal conditions (inoculated with P. involutus or L. rufus, or uninoculated) were factorially combined with nitrate and ammonium in a series of experiments that used Douglas-fir and Sitka spruce seedlings. Unlike the initial synthesis testing, these experiments used an improved culture unit that allowed the frequent replenishment of water and nutrient supplies. Sitka spruce was used in the first experiment. As expected the P. involutus/ammonium and the L. rufus/ammonium treatments formed abundant mycorrhizas (72.3% and 70.5% respectively). In contrast when nitrate was the nitrogen source, L. rufus formed no mycorrhizas and P. involutus infected only 6.4% of the short roots. Plants supplied ammonium produced more dry matter than those given nitrate, with the uninoculated plants being larger than either of the mycor-rhizal treatments. In the second experiment, the amount of nutrient was reduced by 80% and Douglas-fir seedlings were used. The same general patterns of growth and mycor-rhiza infection seen in the first experiment were repeated. Leaks in the culture units used in the first two experiments made the nitrogen uptake data unusable. In the third experiment, Sitka spruce seedlings were grown with the same reduced amount of nutrient as the plants in the second experiment. The same pattern of mycorrhizal infection was repeated, but unlike the other experiments the various treatments within the ammonium treatment were not statistically different. Although nitrogen uptake was similar among the different treatments within the nitrate group, mycorrhizal infection increased ammonium uptake. This increase resulted in a higher nitrogen content in the roots. The exact role of nitrate in the disruption of mycorrhizas was examined by alternating from ammonium to nitrate (and vice versa) and by adding root exudates and macerates to pure cultures of P. involutus and L. rufus. When the nitrogen supply was changed from nitrate to ammonium, the rate of formation of P. involutus mycorrhizas was apparently increased. However, when the nitrogen supply was changed from ammonium to nitrate the mycorrhizas already formed became unstable. In the case of the L. rufus and ammonium then nitrate treatment, the metabolism of the entire plant was apparently effected by the nitrogen form change. In contrast to work by others, the exudates and macerates did not stimulate or retard the growth of L. rufus or P. involutus.