Limitations imposed by nitrogen deficiency on the productivity of subterranean clover-based annual pasture in southern Australia

Abstract

Potential productivity of pasture in the Mediterranean environment of southern Australia and limitations imposed on productivity by nitrogen deficiency were studied on grazed subterranean clover-based annual pasture at Kybybolite, S.A. Estimates of monthly growth, botanical composition and nitrogen uptake were made on the basis of growth in open and closed quadrats. Ammonium and nitrate nitrogen in the top 10cm of soil were measured frequently, and six estimates of the response of pasture to nitrogen were made during the growing season. Potential herbage productivity was assumed to be 5 % of photosynthetically active radiation. Grazed pasture yielded 12.6 t of dried herbage per hectare compared with a calculated potential of 28.7 t ha-1. It was calculated that nitrogen deficiency probably accounted for about 59 % of the shortfall. Maximum rates of growth in mid spring (October) reached 160 kg ha-1 day-1 in the presence of added nitrogen compared with a calculated potential of 180 kg ha-1 day-1, and measured growth of only 98 kg ha-1 day-1 in the absence of nitrogen. Pasture responded least to added nitrogen in autumn even though, in that season, grasses and herbs dominated subterranean clover. Low responsiveness was associated with high levels of mineral nitrogen in the soil before and for 2 weeks after the first autumn rains. Thereafter mineral nitrogen remained very low, and pasture increasingly responded to nitrogen until it matured in late spring. During this time the proportion of legume steadily increased until, by the end of spring, it reached 50 %. In all, it was calculated that soil released 184 kg nitrogen, and clover fixed 146 kg nitrogen, per hectare over the growing season. In order to reach its potential productivity the pasture needed a further 406 kg nitrogen per hectare. In the absence of fertilizer nitrogen, economically unacceptable on the annual pastures of southern Australia, subterranean clover will need lo fix this extra nitrogen if annual pastures are to reach their potential productivity.