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 (N cr ), which is the minimum N concentration in shoot biomass (SB) required for maximizing growth. A reference curve of N cr decrease (Ref‐N cr ) with SB increase proposed for C 3 species (Ref‐N cr =48 SB ‐0.32 ) was validated for several crops growing without water deficiency in different sites and seasons; however, the validity of Ref‐N cr is uncertain when water is limiting. The objective was to assess whether water stress affects N cr . 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, N cr , available soil water, reference evapotranspiration (ET 0 ), and real evapotranspiration (RET) were estimated. N cr agreed well with Ref‐N cr when soil water was nonlimiting, but it was consistently lower than Ref‐N cr whenever crop RET was reduced (RET/ET 0 <1). Indeed, crop average N cr during an entire regrowth scaled linearly with the average level of water stress in the period: (N cr /Ref‐N cr ) avg = 0.83 (RET/ET 0 ) avg + 0.22 ( R 2 = 0.90, p < 0.0001). Hence, while Ref‐N cr 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 actualN cr .