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

Abstract

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.

Improved estimation of nitrogen uptake in grasslands using the nitrogen dilution curve

“…Since most observed data have had fertilizer treatment applied (i.e., reducing or removing N limitation), it is likely that the Lemaire-cutting method overestimates critical N uptake. Agnusdei et al (2010) also found the Lemaire reference curve to provide much higher N uptake values than critical N uptake derived from experimental data. By incorporating calibrated N dilution curves along with biomass yield and N concentration information at each cut, these methods move toward more realistic critical N uptake values.…”

Producción de forraje de agropiro alargado en función de la disponibilidad de nitrógeno y la posición topográfica

“…El uso de este insumo, ha demostrado ser una herramienta tecnológica de alto impacto para la producción de forraje en ambientes ganaderos de la Pampa Deprimida (Agnusdei, Assuero, Lattanzi y Marino, 2010), mejorando la eficiencia de uso de otros recursos, como por ejemplo el agua (Cooper, Gregory, Tully y Harris, 1987).…”

“…En este experimento, realizado en primavera avanzada, se registró efecto del agregado de N sobre la BA de agropiro alargado coincidiendo con lo reportado para esta especie y otras que habitualmente se utilizan en la región (Marino y Agnusdei, 2007;Agnusdei et al, 2010;Errecart, Agnusdei, Latanzi y Marino, 2012). Por otra parte, se registró efecto significativo del sitio, coincidiendo con lo observado por Vázquez, Costa, Monterubbianesi y Godz (2001).…”

Water Use Efficiency of Pastures in a Mollisol of the Semiarid Pampa

“…Sin embargo, en la práctica, a especies tan bien conocidas como agropiro o festuca (entre otras) se les asigna escasa tecnología y difícilmente llegan a expresar su capacidad productiva (Agnusdei et al, 2010;Parsons et al, 2010). El alto potencial de estas especies estaría relacionado con su plasticidad en cuanto a las condiciones de suelo y clima, lo cual asegura una alta persistencia en comparación con otras especies .…”