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Title: Oligotrophic peat as a nitrogen source for different tree species
Author: Morgan, J. L.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1990
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Nitrogen availability is limiting to the growth of certain tree species planted on oligotrophic peats in Britain and Ireland. Sitka spruce (Picea sitchensis (Bong.) Carr) requires repeated applications with fertilizer-N for the establishment of plantations on these sites. When spruce is planted in mixed stands with the pines or larches it can be grown successfully without fertilizer-N and with associated improvements in N status. Mixed stands accumulate greater biomass and nitrogen capitals than pure spruce plantations. Research indicates that greater N uptake in mixtures is derived from enhanced mineralization of the native peat organic matter. This thesis investigates whether certain tree species can access mineral-N from an oligotrophic peat substrate more readily than others and whether these trees have any influence on N release rates from peat. A large glasshouse experiment was planted with Sitka spruce, lodgepole pine (Pinus contorta Dougl.), Japanese larch (Larix leptolepis (Sieb and Zuc.)) and birch (Betula spp.). Trees were grown for two seasons in plots of Sphagnum peat, pH 3.1, fertilized with P, K at field rates. Treatments included plots with fertilizer-N and spruce in mixtures with the other species. There were no differences between the total N uptake for individual species or combinations of species in mixed plots (-N). In + N treatments the rate of N uptake by birch and larch was greater than for spruce and pine. In mixed plots birch was a more effective competitor for available N than spruce, larch was more effective in + N plots but not -N and pine was at a competitive disadvantage to spruce in all treatments. N use efficiency of biomass was similar for all species within the same N treatment and increased as N became limiting. Trees assimilated a mean of 220 μg N g^-1d.w. peat over two growing seasons. A 16 hr extraction with 2% acetic acid recovered less mineral-N than taken up by trees (130 μg N g-1d.w.). Approximately 40 percent of N uptake was derived from quantities of N which could only be extracted from peat after incubation for 6 weeks. Substrate N release rates during laboratory incubations did not show species-related differences. Planted peat N release rates were lower than for unplanted peats (40 vs 90 μgN g^-1 month^-1) but N release rates were considered to be a product of sample perturbation rather than an index of real N mineralization rates in-situ. Standing crops of fine roots were assessed in glasshouse plots (-N) during the second season. Root dry weight intensity reached a maximum of 1.3 mg cm^-3, equivalent to standing crops beneath forest stands. Birch had non-mycorrhizal roots, unlike other species, which rapidly exploited the whole plot volume. Root form for other species is described, with no change in morphology detected as a result of admixture. A separate glasshouse experiment confirmed that lodgepole pine roots were more tolerant of anerobic conditions in peat compared to Sitka spruce. However, the ability of pine roots to grow into an anaerobic peat substrate did not confer a significant advantage for N uptake. An experiment in 15 year old stands of lodgepole pine and Sitka spruce planted on a raised bog, investigated the response of fine roots to inorganic-N amendments to root ingrowth cores. Root production was not influenced by added N but production rates were different between stands. Trees fertilized with rock phosphate produced more root (782 kgha^-1yr^-1:pine, 913 kgha^-1yr^-1: spruce) than stands fertilized with superphosphate (524 kgha^-1yr^-1: pine, 374 kgha^-1yr^-1:spruce). Soil physical and chemical properties are described for each stand.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available