Nitrogen uptake, partitioning and internal cycling inPicea sitchensis(Bong.) Carr. as influenced by nitrogen supply

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Abstract
SUMMARY: Four‐year‐old seedlings ofPicea sitchensis(Bong.) Carr. were grown in sand culture throughout 1989 and irrigated with nutrient solutions containing either 1.0 mol N m−3(Low N) or 6.0 mol N m−3(High N), to precondition their growth and capacity for N storage. During 1990 N enriched with15N was supplied, either from 15 March to 27 June, or 28 June to 20 November. Recovery of unlabelled N was used to determine the storage and immobilization of N for foliage growth, and the partitioning of labelled N taken up during the two periods was measured.Initial growth of trees in 1990 was unaffected by the current N supply and determined only by the N supplied the previous year. High N throughout increased the number of needles grown in 1990 compared to low N‐treated trees, but had little effect on the dry weight of individual needles. When preconditioned with High N, trees responded to Low N in 1990 by a reduction in needle dry weight, without altering the number of needles produced. Low N trees supplied with High N in 1990 responded by increasing both needle numbers and dry weight, compared with trees supplied with Low N throughout. The amount of unlabelled N remobilized to foliage growth in 1990 was unaffected by the current N supply but reflected the amount of N in store, as determined by the N supply the previous year. The majority of N was remobilized from the 1989 foliage and none from roots. Partitioning of labelled N taken up during 1990 altered during the year, with a greater proportion of N taken up after 28 June recovered in the roots in all treatments, due to root growth as opposed to allocation of N to storage during the autumn, since root N concentrations fell between 17 June and the final harvest on 20 November.