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 (ET0), 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/ET0 < 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/ET0)(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 actual N-cr.