processes (Touraine et al., 1994), a high N requirement during late winter and early spring has been observed For some species, mathematical models have been developed to for annual ryegrass (Salette et al., 1984). However, since describe tissue N dilution during crop growth and to estimate the the rate of soil N mineralization varies throughout the plant N status applying the N nutrition index (NNI), the ratio betweenyear, mainly in response to soil temperature and water the actual tissue N concentration and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (criti-

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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.

The N nutrition index (NNI, defined as the actual shoot N concentration relative to a reference critical value, N ref ) is a valuable tool to analyze the N nutrition status of crops. We present empirical evidence obtained for a range of forage grass species and environmental conditions with the aim to contribute to verify the NNI as a tool to diagnose N deficiency and to examine responses to N fertilization. Ten N fertilization experiments were conducted in the SE of the Pampa region (Argentina) and included (1) perennial (Festuca arundinacea Schreb. Thinoppyron ponticum (Podp.) Barkworth and Dewey) and annual (Lolium multiflorum Lam., Avena sativa L. Bromus catharticus H.B.K.) grasses, (2) autumn, winter and spring regrowths, and (3) good and poor quality soils. Experiments comprised four to six N fertilization rates, each including five to seven sequential harvest dates. Plots were replicated in three or four complete randomized blocks. All experiments were rainfed and adequately provided with phosphorus. The critical N concentration (N cr ) -that is the minimum plant N concentration required to reach maximal accumulated shoot biomasswas determined for each harvest date. Observed N cr were then compared to values predicted by the general reference curve (N ref ) proposed elsewhere. In six of the ten experiments measured N cr agreed with predicted N ref . Thus, the present study extends to marginal soils, to species adapted to such environment, and to winter growth conditions the empirical support for the use of N ref in the quantification of pasture N deficiency. But N cr was lower than N ref in four experiments in which growth conditions were not adequate for reaching potential growth rates. Therefore, it is suggested that when factors other than N are expected to limit pasture growth, an N cr lower than N ref should be considered. In all experiments, a direct relationship between accumulated shoot biomass and NNI was found, radiation use efficiency (RUE) being more negatively affected than photosinthetically active radiation interception (PARi) by N deficiency. Reductions in the latter were relevant only from moderate to severe N deficiency. Notably, the NNI achieved at initial stages of regrowth (150-200°C day after the fertilization date) was highly associated with the N nutritional status along the regrowth and, therefore, confirming NNI as a promising tool for diagnostic purposes.

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