Consistent patterns of foliar N concentration and δ<sup>15</sup>N were found in age sequences of foliage. N concentration decreased with age and δ<sup>15</sup>N increased with age. Patterns of δ<sup>15</sup>N may be useful in studying nutrient retranslocation within the canopy because, as far as it is known, they are unaffected by contemporary changes in other leaf components. No evidence for increased efficiency of N recycling in spruce from mixed stands as compared to spruce from pure stands was found. The δ<sup>15</sup>N data appear to confirm that the amount of N retranslocated depends on the N concentration of the foliage. There was evidence that in a young site the δ<sup>15</sup>N value of externally available N in the mixed stands became increasingly enriched over time, while the δ<sup>15</sup>N value of externally available N in the mixed stands at a mature site remained constant. Foliar N concentrations and δ<sup>15</sup>N of spruce in mixtures with different nurse species and in different ratios were measured. The δ<sup>15</sup>N values of nurse foliage differed significantly between species, but no significant effects of nurse species on δ<sup>15</sup>N value of the spruce were found. Increasing the proportion of the nurse in a stand increased the foliar N concentration of spruce and caused an enrichment in the <sup>15</sup>N abundance. Soils from one site were subjected to a series of extractions to extract N pools of different liability. N concentrations of 4 pools were measured and δ<sup>15</sup>Nvalues calculated. Total soil N in mixed stands was enriched in δ<sup>15</sup>N relative to pure stands. It was not possible to identify the pools available to spruce in pure and mixed stands because of large errors in δ<sup>15</sup>N values. Organic N pools were consistently enriched relative to largely inorganic N pools. The foliar δ<sup>15</sup>N values of spruce in pure stands were close to those of the hot K<sub>2</sub>SO<sub>4</sub> extractable pool containing microbial N and amino acids, whereas in the mixed stands, the foliar δ<sup>15</sup>N values of both spruce and pine were close to those of the hot HCI extractable pool containing less labile organic N. The results are discussed in relation to the mixtures effect. It is concluded that patterns of <sup>15</sup>N natural abundance in pure and mixed stands of spruce support the hypothesis that in mixtures, spruce has access to a source of organic N not normally available to it in pure stands.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:339854 |
| Date | January 1997 |
| Creators | Horsburgh, Alison |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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