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Values for the various fluxes of nitrogen through the tree–soil system were calculated using models derived from measurements of rates of input, accumulation, and transfer in plots of 11-m-tall Pinusnigra var. maritima (Ait.) Melv. that were showing growth responses to varying rates of nitrogen fertilizer. As the result of continuing immobilization in trees and humus, growth of the unfertilized trees was declining and the trees were having to sacrifice older tissues to mobilize sufficient nitrogen for new growth. In the fertilized trees, there was similar mobilization but of excess nitrogen stored during the period that fertilizers were applied. Models of the flux of nitrogen, phosphorus, calcium, and magnesium at optimum growth showed that tight cycling and low rates of immobilization, coupled with effective retention of relatively high atmospheric inputs, probably represents the adaptive mechanism that enables trees to thrive on soils low in potassium and magnesium. However, high rates of immobilization and low rates of atmospheric input relative to tree uptake mean that despite the maintenance of tight cycles, the trees continued to make significant demands on soil reserves of nitrogen, phosphorus, and calcium throughout their life cycle.

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