Nitrogen and phosphorus interactions at a 21 nitrogen:1 phosphorus Redfield-like ratio impact growth and seed yield in wheat (Triticum aestivum L.)

Efficient use of nitrogen (N) and phosphorus (P) is essential to reduce fertilizer costs and nutrient pollution and to lower the carbon footprint of agriculture. This requires a better understanding of N and P limitations on photosynthesis and biomass generation in one of the world's most important crops, wheat (Triticum aestivum). In a fully factorial nutrient experiment, we find that the limiting nutrient, N or P, at a Redfield-like N:P ratio (21N:1P molar) produces the ‘effective dose of fertilizer’ for the generation of above-ground biomass (seed or vegetative). Best-fit structural equation models showed that neither N nor P applied in excess of 21N:1P led to any increase in biomass. The light (qP, NPQ) and dark (A_max, V_cmax, J_max) reactions of photosynthesis are also constrained by the limiting nutrient at the 21N:1P Redfield-like ratio. Beyond this ratio, ‘excess N’ or ‘excess P’ had no effects. The direct effects of the ‘effective dose of fertilizer’ on the accumulation of biomass were stronger than its indirect effects via photosynthesis, likely driven by the N and P costs in building nucleic acids needed for cell division, cell maintenance and RNA transcription. Modern composite fertilizers are typically provided at a much higher P content than a 21N:1P ratio, potentially resulting in a huge global wastage of P, a finite resource, with all the concomitant costs to the farmer, consumer and the environment.