Soil nitrogen changes under continuously grazed legume-grass pastures in subtropical coastal Queensland

Abstract

Net changes in total soil nitrogen were measured in 1.2 to 2.4 hectare paddocks of legume-grass pastures at three sites. The sites and pastures were (a) Rodd's Bay-a pasture of Stylosanthes humilis and Heteropogon contortus, (b) Samford-a pasture of Phaseolus atropurpureus cv. Siratro, Digitaria didactyla, and Axonopus affinis and (c) Beerwah-a pasture of Desmodiam intorum cv. Greenleaf, D. uncinatum cv. Silverleaf, Lotononis bainesii, Trifolium repens, Digitaria decumbens, Chloris gayana, Paspalum dilatatum, and P. commersonii. At Rodd's Bay, the changes under pasture containing Stylosanthes humilis varied greatly according to seasonal conditions. During a period of 29 months that covered three successive growing seasons with below-average rainfall, short-term fluctuations in total soil nitrogen gave a net decrease of 70 kg ha-1 (0.95 confidence limits ¦30), an average of 29 (¦ 12) kg ha-1 year-1. In the following three-year period, which included two very wet growing seasons and one very dry one, soil nitrogen increased by an average of 30 ¦ 10 kg ha-1 year-1. Over a period of four years, soil nitrogen increased by an average of 44 ¦ 9 kg ha-1 year-1 under Phaseolus atropurpureus at Samford, and 81 ¦ 10 kg ha-1 year -l under the mixture of legumes at Beerwah.