QUANTITATIVE ESTIMATES OF UPTAKE AND INTERNAL CYCLING OF N-14-LABELEDFERTILIZER IN MATURE WALNUT TREES

Uptake and internal cycling of nitrogen (N) in mature walnut trees was studied over a period of 6 years using N-15-depleted fertilizer and f ull-canopied walnut (Juglans regia L. cv Hartley) trees. The magnitude of internal N cycling, i.e., the availability of N for new growth fro m internal N pools, was quantified using both the percent annual deple tion (PAD) and the N balance budget approaches. There was good agreeme nt between the two measures, and about 60% of annual N demand was deri ved from N redistribution from internal pools. The remaining 40% of an nual tree N demand was met by an influx of N from the soil/fertilizer pool. Trees were excavated, processed and analyzed after 6 years to de termine total tree N content and labeled N recovery. Trees recovered 2 9.4% of the labeled N applied and, based on previous evidence, we assu med that tree accumulation of labeled N occurred entirely in the First year. Labeled N in the fruits and leaves harvested in the first year represented 26% of the total labeled N accumulated, and the remaining 74% of the labeled N accumulated that year was stored and used to supp ort development of annual organs in subsequent years. In the first yea r, the early maturing catkins did not accumulate labeled N, indicating their exclusive reliance on internal N. Using the atom% N-14 excess v alues of catkins and an exponential decline equation to determine turn over rate, the Mean Residence Time (MRT) of storage N in the tree was estimated to be 2.0 years. The size of the cycling pool of storage N i n the tree was estimated to be about 50% of the total N content of per ennial tree parts. Our data support the hypotheses that: (1) in any gi ven year, mature walnut trees store the majority of soil and fertilize r N absorbed and within 2 years following uptake the N is remobilized and used for new growth, and (2) about half of the total N content of the perennial parts of mature walnut trees is present as nonstructural N and is available for recycling.