Unité Agropédoclimatique de la zone Caraïbe, INRA, Guadeloupe, France
Irrigated microplots of dense stands of Dichanthium aristatum in Guadeloupe were subject to 100%, 56% and 33% levels of incoming radiation, respectively. Two regrowth cycles were followed: the first with the equivalent of 200 kg/ha N and the second without applied N.
Reduced radiation depressed grass growth at both levels of N supply. However, under low N availability, the shaded stands compensated for the low irradiance levels by increasing the leaf area index to values equivalent to that of the stand under full sun. The nitrogen nutrition of stands, as expressed in the changing relationship of N uptake to biomass accumulation, was not significantly modified by shade, although the N concentration of leaves under N-limiting conditions was slightly increased by shade. The large increase in radiation use efficiency (RUE) calculated on herbage biomass for shaded stands could not be explained in terms of changed N nutrition and it is postulated that changes in the shoot:root ratio of shaded stands may result in an overestimation of RUE.
The calculated RUE of full sunlight stands confirms the potential productivity under high N fertilisation previously reported for this species and other C4 grasses.