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.