Alternate partial root-zone N-fertigation increases water use efficiency and N uptake of barley at elevated CO2

Abstract Elevated atmospheric CO2 concentration (e[CO2]) increases water use efficiency (WUE) while reducing nitrogen (N) concentration of crops particularly under drought conditions; yet the combined effects of e[CO2] and different N-fertigation regimes on WUE and crop N nutrition remain largely elusive. In this experiment, the growth and physiological responses of two barley genotypes, wild type barley Steptoe (WT) and its correspondent ABA-deficient mutant barley Az34, to three N-fertigation regimes at ambient CO2 (a[CO2]) (400 ppm) and e[CO2] (800 ppm) were investigated. From tillering to grain filling stage, the plants were subjected to three N-fertigation regimes: 1) N-fertigation at full irrigation volume (FIN); 2) N-fertigation at reduced irrigation volume (DIN); 3) alternate N-fertigation at reduced irrigation volume (PRDN). Although e[CO2] had little effect on gs, Tr and plant water use of WT, especially under DIN and PRDN, it increased An, resulting in an increased WUE at stomatal, leaf and whole plant levels. For Az34, the positive effect of e[CO2] on WUE was attributed to both significantly enhanced An and lowered gs and Tr. For both genotypes, e[CO2] increased 100-grain weight and shoot dry biomass but didn’t affect grain yield and WUE for grain production (WUEg). PRDN increased grain yield, HI and WUEg of both genotypes regardless of [CO2], compared to FIN. DIN and PRDN increased N uptake of both genotypes at e[CO2] compared to FIN. Compared to a[CO2], e[CO2] increased 15N uptake and 15N recovery rate of both genotypes by enhancing plant biomass. In addition, both genotypes grown under DIN and PRDN allocated more N to the grain compared to the FIN plants. Collectively, N-fertigation at reduced irrigation volume promoted N allocation to the grain and increased WUE, particularly under e[CO2]. Such information is conductive for optimizing WUE and N nutrition of crops in a future water-limited and CO2-enriched environment.