Nitrogen nutrition of mycorrhizal pines : A thesis presented for the degree of Doctor of Philosophy in the University of Canterbury [Lincoln College]

Nitrogen has been central to many of these speculations regarding the function of mycorrhizas in tree nutrition and growth. Historically it was felt that mycorrhizas were more efficient in absorbing nitrogen from humus than were non-mycorrhizal roots because the fungal components could use the complex organic nitrogen in humus (Frank, 1885; Von Tubeuf, 1903; Falck, 1923; Francke, 1934; all cited in Hatch, 1937). Hatch (1937) stated that "Nearly all work on the physiology of tree mycorrhizae, recorded in the literature, has been carried out with the object of elucidating the organic nitrogen theory." Such experiments (Melin, 1927; McArdle, 1932; Rayner, 1934) were either inconclusive or failed to support such a theory. Subsequent to Hatch's review of the early literature there have been a number of studies of mycorrhizal and non-mycorrhizal tree seedlings for which tissue nitrogen analyses have been reported (Hatch, 1937; Mitchell, Finn and Rosendahl, 1937; McComb, 1938; Finn, 1942; Routien and Dawson, 1943). These all demonstrate that mycorrhizal plants contain more nitrogen than uninfected plants. Almost the only information we have however about the mechanism whereby such increases in nitrogen may be occurring is contained in two papers by Carrodus (1966, 1967) in which beech mycorrhizas are shown to absorb ammonium rapidly and incorporate it into the amide glutamine, while nitrate is little absorbed if at all. That the importance of mycorrhizas to the nitrogen nutrition of forest trees remains considerable yet the mechanism of stimulated absorption remains largely unresolved is not entirely paradoxical. Experimental procedures for following nitrogen in its various forms from soil or solutions into and through plants are complex and only relatively recently developed. No suitable radioisotopes have been available for tracer experiments which left only the cumbersome and time consuming technique involving the mass isotope ¹⁵N. It was from this background of fact and speculation that the present investigations on the nitrogen nutrition of Pinus radiata D. Don developed. These investigations were given added point by the knowledge that New Zealand's fourth highest export earnings are based on Pinus radiata monocultures, the greatest area of which are found on the nitrogen deficient, rapidly weathering pumice soils of the central volcanic plateau in the North Island.