The influence of N and P supply and genotype on N remobilization in containerized Pinus radiata plants
A large proportion of the nitrogen (N) used in the current-year growth of the widely grown plantation species Pinus radiata D. Don is N that was stored in plant tissues the previous year. However, the extent to which an imbalance between levels of phosphorus (P) and N may change the capacity of plants to remobilize N is unknown. In this study, the N remobilization responses of four P. radiata genotypes to a factorial combination of N and P additions were assessed in containerized plants in a two-year greenhouse experiment. N supply was enriched with 15N at 2.5%o(labeled N) during the first year. Plants were then transferred to clean sand and grown for another year with 15N at levels close to natural levels (0.3664899 atom percent 15N, δ15N 0.5115%o). Calculations of N storage and remobilization were based on the recovery of labeled N from new tissues during the second year of growth. Over the second year, N remobilization for the high-N high-P supply regime (953 mg N plant-1) was five-fold that of the N remobilization for the low-N low-P supply regime (199 mg N plant-1), with the unbalanced high-N low-P (422 mg N plant-1) and low-N high-P (228 mg N plant-1) treatments showing intermediate N remobilization. Sixty-five percent of the plant N content at the end of the first year of growth was remobilized during the second year in the high-N high-P supply treatment, compared to 42-48% for the other N and P addition treatments. The ratio of N uptake to N remobilization was greater for the high-N supply regimes compared to the low-N supply regimes, suggesting that trees may rely progressively more on remobilization than uptake as N fertility declines. Most N remobilization (77%) occurred during the spring-summer periods and coincided with the largest proportion of needle development (80%), suggesting that N remobilization was driven by sink-strength. Old foliage was the main source for internal cycling, while roots were the main sink. Faster-growing genotypes did not exhibit an enhanced capacity for N remobilization, suggesting that genetically induced growth performance is not explained by N internal cycling. In conclusion, trees growing with an abundant and balanced nutrient supply exhibited greater capacity for nitrogen remobilization than trees with N and/or P deficiencies however, faster-growing clones did not exhibit an enhanced capacity for N remobilization.