Nitrogen and phosphorus in senescent leaves of field-grown common bean cultivars and their contribution to crop nutrient budget

Leaf senescence and its concomitant remobilization of nutrients supplies part of seed demand, and leaf decay also contributes to nutrient cycling. However, a nutrient budget of common bean (Phaseolus vulgaris) crop that considers senescent leaves is lacking. The purpose of this study was to measure the amounts of N and P in senescent leaves of common bean cultivars and their contribution to the crop nutrient budget. Seven bean cultivars were grown in a field experiment, where shoot biomass was sampled at each week after flowering and senesced leaves were collected twice weekly. Grain yield and the actual and apparent harvest indices (respectively including or not senescent leaves) were measured at maturity. Leaf decay in bean crop was low until 3 weeks after flowering and increased until grain maturity. Senesced leaves collected throughout the growth cycle reached 96.4, 1.94 and 0.15 g m−2 of dry mass, N and P, respectively, which corresponded to 29%, 28% and 19% of the total biomass, N and P accumulated by the crop. Bean cultivars differed in the amounts of mass, N and P deposited by senesced leaves, for apparent harvest indices of biomass, N and P, but not for actual harvest indices. Of the amount present in the leaf canopy 2 weeks after flowering, 44% of N and 51% of P was further remobilized to seeds, which corresponded to 33% of N and 25% of P accumulated by grains at maturity. A subsequent decomposition study with litterbags provided half-lives of 80, 179 and 64 days, respectively, for senesced leaves, stems and podwalls produced after pod threshing, with similar patterns of N and P release. The results illustrate the remobilization of N and P from leaves to seeds during pod-filling and stress the relevance of senescent leaves in the nutrient budgets of legume crops.

► Amounts of N and P in senescent leaves of bean cultivars were measured in the field.
► Aboveground crop N and P budgets were estimated with experimental data.
► Senesced leaves corresponded to 28 and 19% of N and P accumulated by the crop.
► Leaves transported half of their N and P to seeds, or 33 and 25% of N and P in grains.
► Decomposition dynamics of crop residues including senescent leaves was measured.