Critical N concentration can vary with growth conditions in forage grasses: implications for plant N status assessment and N deficiency diagnosis

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 biomass- was 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 (150-200A degrees 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.