Temporal patterns of symbiotic nitrogen fixation and soil mineral nitrogen uptake differ among faba bean, chickpea, field pea and lentil

While pulse crops are valued for their symbiotic nitrogen-fixing capacity, the dynamics of N fixation during crop development are not well quantified in cool-season pulses. Single-time measurements, typically at peak biomass, limit our understanding of how N source acquisition shifts with crop ontogeny. We aimed to quantify the temporal dynamics of N fixation, soil mineral N uptake and shoot dry matter accumulation in chickpea, faba bean, field pea and lentil grown under two seasons, with seasonal rainfall of 267 mm and 196 mm. We randomly sampled shoots weekly between late vegetative stage and physiological maturity. Samples were analysed for shoot N content and δ 15 N for both pulses and for a non-fixing reference crop (canola), and quantified N fixation using the 15 N natural abundance method. Sigmoid and logistic models characterised temporal growth patterns and allometric relationships between N source and shoot dry matter accumulation. Across species and seasons, peak N fixation occurred between 1000 and 1100 °C d after sowing—typically 100–200 °C d before flowering and 350–500 °C d before podding—but its synchrony with maximum growth rate varied among species. In faba bean, close temporal coupling between N fixation and growth was associated with the seasonal-dependent peak dry matter. Chickpea had pronounced asynchrony, with growth peaking 270–370 °C d after N fixation and a consistent reliance on soil N that accounted for 66–77 % of total N accumulation. In field pea, synchrony between N fixation and growth was stronger under drier conditions, while lentil maintained stable asynchrony across seasons. Allometric scaling between shoot dry matter and N sources were species-specific. Faba bean accumulated nitrogen from both N fixation and soil N proportionally with plant growth. Chickpea was more reliant on soil N relative to N fixation in meeting N requirements, while field pea and lentil had reduced N fixation with plant development. Interspecific variation in N fixation was associated with temporal synchronisation with growth rather than the absolute timing of N fixation. Consistent thermal timing of peak N fixation but species-specific synchrony with growth requires reconsidering species-specific: (i) sampling protocols for N fixation quantification, (ii) allometric models that account for distinct relationships between N source and growth, and (iii) N acquisition strategies with practical implications for nitrogen management. • We quantified temporal dynamics of N fixation, soil N uptake, and growth in pulses. • Across species, peak N fixation occurred at 350–500 ℃d before podding. • Faba bean N fixation synchronised with growth in favourable environment. • Chickpea relied on soil N (66–77 %) with asynchronous growth patterns. • Field pea improved synchrony under stress; lentil maintained asynchrony.