Nitrogen uptake capacities of maize and sorghum crops in different nitrogen and water supply conditions

The dynamics of dry-matter accumulation and nitrogen uptake of sorghum and maize crops were compared over two successive years under dry and irrigated conditions. In non-limiting situations for growth, with irrigation and high nitrogen fertilization, maize growth was higher than that of sorghum because the leaf area index of maize developed earlier, leading to a larger quantity of intercepted radiation. The efficiency of transforming intercepted energy into aerial biomass varied little between the two species. In situations where nitrogen was limiting, the capacity of sorghum crops to take up nitrogen from the soil was always higher than that of maize. This feature was not linked to the greater planting density of the sorghum crop (50 cm between the rows) compared with the maize crop (75 cm), but seemed to be a characteristic of the species. A careful study of the evolution of the nitrogen nutrition level of the different crops throughout their growth showed that sorghum was capable of taking up much larger quantities of nitrogen from the soil than maize before its nitrogen nutrition became limiting. Drought reduced nitrogen availability in the soil for both crops. Maize was more sensitive to this restriction in nitrogen nutrition, and in addition to the direct effect of a water deficit on dry-matter accumulation in maize there was also this indirect effect of a nitrogen-nutrition deficit. All the results obtained in this series of experiments showed that the potential production of sorghum was lower than that of maize, but that its adaptation to limiting growth conditions, drought and limited nitrogen input should enable this species to grow in extensive cropping conditions. Moreover, the higher N uptake/water consumption of sorghum compared to maize indicates that the risk of nitrate leaching in the winter following a sorghum crop should be lower than after maize. nitrogen uptake capacity / nitrogen nutrition index / water-nitrogen interaction Rsum — Capacits de prlvement d'azote de cultures de mas et de sorgho dans diffrentes conditions d'alimentation hydrique. Durant 2 annes successives, les dynamiques de croissance en matire sche et de prlvement d'azote de cultures de sorgho et de mas ont t compares en conditions sches et irrigues. En situations non limitantes pour la croissance, irrigation et fertilisation leve en azote, la croissance du mas est suprieure celle du sorgho du fait d'une mise en place plus prcoce de l'indice foliaire de la culture conduisant une plus grande quantit de rayonnement intercept. Les efficiences de transformation de l'nergie intercepte en biomasse arienne restent peu diffrentes entre les deux espces. En situations d'apport limit en azote, les capacits de prlvement de l'azote du sol par les cultures de sorgho ont toujours t suprieures celles du mas. Cette proprit n'est pas lie la densit de plantation plus importante de la culture du sorgho (50 cm entre les rangs) par rapport celle du mas (75 cm), mais semble bien une caractristique de l'espce. La dtermination de l'volution du * Correspondence and reprints niveau de nutrition azote des diffrentes cultures au cours de leur croissance a permis de montrer que le sorgho tait capable d'extraire des quantits d'azote du sol bien suprieures celles du mas avant de voir sa nutrition azote devenir limitante. La scheresse diminue la disponibilit en azote du sol pour les deux cultures. Le mas est plus sensible cette restriction de nutrition azote, et, l'effet direct du dficit hydrique sur la croissance en matire sche de cette espce, s'ajoute un effet indirect de dficit de nutrition azote. L'ensemble des rsultats obtenus dans cette srie d'exprimentations montre que la production potentielle du sorgho est plus faible que celle du mas, mais que son adaptation des conditions limitantes de croissance, scheresse et apports limits d'azote doit permettre cette espce de se dvelopper dans des conditions de culture extensive. De plus, le rapport plus lev pour le sorgho que pour le mas entre le prlvement d'azote et la consommation d'eau indique que les risques de lessivage de nitrate doivent tre rduits aprs une culture de sorgho en comparaison avec le mas. capacit de prlvement d'azote / index de nutrition azote / interaction eau-azote