Nitrogen uptake, fertilizer use efficiency, and physiological responses of Coix lacryma-jobi L. under different N application rates: a ¹⁵N tracer study

Abstract

Coix lacryma-jobi L. (commonly known as Job’s tears or adlay) is getting more attention as an emerging functional food crop, but species-specific nitrogen (N) management information remains limited. Most growers currently extrapolate recommendations from other cereals. We hypothesized that increasing N supply would enhance biomass and N accumulation but reduce nitrogen use efficiency (NUE) beyond an optimal rate, and that physiological parameters including net photosynthetic rate (Pn) and SPAD value would show saturation at higher N levels. To test this, we grew C. lacryma-jobi in pots with four N rates (0, 0.5, 1.0, 2.0 g N plant− 1) under greenhouse conditions and used 15N-labeled KNO₃ to track fertilizer N fate. Total biomass increased with N supply, reaching 19.74 g plant− 1 at the highest rate—35% above the unfertilized control. Most of this extra mass accumulated in stems rather than leaves or roots. Leaf and root N contents rose with N supply, while stem N concentration remained stable. Only 4.8–15.1% of plant N derived from fertilizer, indicating that soil supplied 85–95% of total plant N even at the highest application rate. Nitrogen use efficiency (NUE) peaked at 13.0% at 1 g N and declined to 10.1% at 2 g N, confirming our hypothesis of diminishing returns at higher N inputs. Leaf photosynthetic rate (Pn) and SPAD values increased with N supply up to 1 g N, with only modest further gains at 2 g N. Leaf nitrate concentration at 2 g N reached 1685.7 mg kg−¹ fresh weight. We conclude that under the conditions of this study, N application rates above approximately 1 g plant−¹ provide limited additional physiological or N uptake benefit and reduce fertilizer efficiency. These findings provide a scientific basis for optimizing N fertilization in this emerging functional crop.