Patterns of N concentration and '1'5N natural abundance in pure and mixed stands of Sitka spruce
Consistent patterns of foliar N concentration and δ15N were found in age sequences of foliage. N concentration decreased with age and δ15N increased with age. Patterns of δ15N 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 δ15N 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 δ15N value of externally available N in the mixed stands became increasingly enriched over time, while the δ15N value of externally available N in the mixed stands at a mature site remained constant. Foliar N concentrations and δ15N of spruce in mixtures with different nurse species and in different ratios were measured. The δ15N values of nurse foliage differed significantly between species, but no significant effects of nurse species on δ15N 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 15N 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 δ15Nvalues calculated. Total soil N in mixed stands was enriched in δ15N 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 δ15N values. Organic N pools were consistently enriched relative to largely inorganic N pools. The foliar δ15N values of spruce in pure stands were close to those of the hot K2SO4 extractable pool containing microbial N and amino acids, whereas in the mixed stands, the foliar δ15N 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 15N 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.