Increasing N uptake in elevated [CO₂] conditions through manipulation of NO₃:NH₄ ratio in fertigated tomatoes

Elevated [CO₂] (e[CO₂]) causes an increase in biomass but a decrease in plant N concentration, leading to increased N deficiency. While increasing N fertilization may ameliorate e[CO₂] induced N deficiency (eCIND), it is not necessarily environmentally feasible. In this study, we examine whether manipulation of the NO₃:NH₄ ratio can ameliorate eCIND. Tomato plants were grown in greenhouse chambers with 2 levels of [CO₂] (400, 850 ppm) and 4 levels of NO₃:NH₄ in the irrigation water (100:0, 67:33, 33:67, 0:100). Biomass and leaf N concentration were measured. Total leaf N uptake and nitrogen use efficiency (NUE) were calculated. We found that e[CO₂] caused a decline in N concentration. In the low N concentrations used in this experiment, manipulating the NO₃:NH₄ ratio did not change the N concentration in the leaves nor did it ameliorate the negative effect of e[CO₂] on leaf N concentration. However, treatments that included a mix of NO₃ and NH₄ lead to greater N uptake in both ambient [CO₂] and in elevated [CO₂], leaving open the possibility of using NO₃:NH₄ manipulation as a tool for ameliorating eCIND in future e[CO₂] conditions, perhaps in higher N conditions.