Mineral nitrogen fluctuations in soils under improved pasture in Southern New South Wales

Abstract

Soils under improved pasture on the Southern Tableland of New South Wales accumulated nitrate in substantial quantities during the summer and autumn. In this respect they behaved quite unlike the pasture soils which have been studied in most of the earlier literature, and they resembled cultivated fallow soils, which usually accumulate nitrate at the same time of the year. The nitrate was produced mainly in the top inch of soil; ammonium also accumulated under certain conditions. The precise sequence of climatic events, particularly the period of drying between consecutive wettings, was of primary importance in the nitrate accumulation. The nitrate produced during summer and early autumn disappeared from the topsoil after heavy rain in autumn and winter. The seasons could therefore be distinguished as a nitrogen-rich summer and autumn, a nitrogen-depleting winter, and a nitrogen-poor early spring, with an increasing supply of mineral nitrogen during late spring. No appreciable fluctuations in mineral nitrogen were found in soils resown directly from native pasture with less than 0.10% total nitrogen at 0-2 in.