Suppression of Litter Decomposition by Mycorrhizal Roots of Pinus Radiata

Two experiments, designed to give more information about the suppression of litter decomposition by radiata pine (Pinus radiata D. Don) roots and their associated microflora, were carried out. In the first, root activity was reduced by cutting the boundaries of small areas of the forest floor in an unthinned stand. Samples collected after 3, 6, 9, and 12 months showed that less litter accumulated in areas where root activity was reduced and that the development of mycelial mats in the litter was greatest where mycorrhizal roots were present. Soil moisture content was increased where treatment had involved soil disturbance, but no significant differences attributable to treatment were observed in net amounts of major nutrients or pH. In the second experiment, litter decomposition in the presence of mycorrhizal and non-mycorrhizal roots was studied in the laboratory. Decomposition was slower when mycorrhizal roots were present. It was concluded that the presence of external mycelia of mycorrhizal fungi suppressed litter decomposition. The mechanism of suppression is not clear but moisture and pH did not play an important part in it. Competition for nutrients may have been involved. It is suggested that suppression of decomposition by mycorrhizas may be a major factor in the formation of raw humus. INTRODUCTION Gadgil and Gadgil (1971) described an experiment in which the roots of radiata pine (Pinus radiata D. Don) were partially excluded from small areas of the forest floor. This showed that the presence of living roots of radiata pine and their associated biota suppressed litter decomposition in some way. In the area where the experiment was carried out, there was an intense development of mycorrhizal roots and of fungal mycelia which were shown to be associated with them. This is typical of raw litter of temperate coniferous forests (Melin, 1925; Romell, 1935; Harley, 1940; Meyer, 1964). The mechanism by which the living roots suppressed the activity of litter-decomposing organisms was not clear from the 1971 experiment. In a stand of this type where the upper soil layers are almost fully exploited, it can be assumed that the mycorrhizal root/fungus partnership is the dominant biotic influence in the soil of the main rooting zone. The reduced activity of the litter-decomposing organisms may therefore be the result of: (a) The mycorrhizal roots, their external mycelia, or both being more successful competitors for nutrients or moisture; (b) The exclusion of obligate saprophytes by direct physical or chemical antagonism by the mycorrhizal roots, their mycelia, or both. N.Z. JI For. Sci. 5 (1): 33-41 34 New Zealand Journal of Forestry Science Vol. 5 This paper describes two experiments designed to give more information about the suppression mechanism. The first was a field trial in which the relationships between litter accumulation and certain chemical, physical, and microbiological characteristics of soil and litter samples were studied. In the second experiment, litter decomposition in the presence of mycorrhizal and non-mycorrhizal roots was measured in the laboratory.