Critical Nitrogen Concentration Declines with Soil Water Availability in Tall Fescue

The diagnosis of the N status of crops is based on the concept of critical N concentration (Ncr), which is the minimum N concentration in shoot biomass (SB) required for maximizing growth. A reference curve of Ncr decrease (Ref‐Ncr) with SB increase proposed for C3 species (Ref‐Ncr=48 SB‐0.32) was validated for several crops growing without water deficiency in different sites and seasons; however, the validity of Ref‐Ncr is uncertain when water is limiting. The objective was to assess whether water stress affects Ncr. Five regrowths of a temperate‐type tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] were followed during autumn, spring, and summer in Balcarce, Argentina. Several N rates were applied and SB accumulation and N concentration were measured in each of four to six sequential SB harvests performed at every regrowth. SB, Ncr, available soil water, reference evapotranspiration (ET0), and real evapotranspiration (RET) were estimated. Ncr agreed well with Ref‐Ncr when soil water was nonlimiting, but it was consistently lower than Ref‐Ncr whenever crop RET was reduced (RET/ET0<1). Indeed, crop average Ncr during an entire regrowth scaled linearly with the average level of water stress in the period: (Ncr/Ref‐Ncr)avg = 0.83 (RET/ET0)avg + 0.22 (R2 = 0.90, p < 0.0001). Hence, while Ref‐Ncr remains appropriate for assessing crop N status under adequate water availability conditions, the N nutrition management of water stressed crops should be guided by their actualNcr.