Quantitative estimates of uptake and internal cycling of 14N-labeled fertilizer in mature walnut trees

Uptake and internal cycling of nitrogen (N) in mature walnut trees was studied over a period of 6 years using 15N-depleted fertilizer and full-canopied walnut (Juglans regia L. cv Hartley) trees. The magnitude of internal N cycling, i.e., the availability of N for new growth from internal N pools, was quantified using both the percent annual depletion (PAD) and the N balance budget approaches. There was good agreement between the two measures, and about 60% of annual N demand was derived from N redistribution from internal pools. The remaining 40% of annual 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 determine total tree N content and labeled N recovery. Trees recovered 29.4% of the labeled N applied and, based on previous evidence, we assumed 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 support development of annual organs in subsequent years. In the first year, the early maturing catkins did not accumulate labeled N, indicating their exclusive reliance on internal N. Using the atom% 14N excess values of catkins and an exponential decline equation to determine turnover 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 in the tree was estimated to be about 50% of the total N content of perennial tree parts. Our data support the hypotheses that: (1) in any given year, mature walnut trees store the majority of soil and fertilizer 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.