The effect of pasture composition on the soil water dynamics beneath native pastures in the high rainfall zone of south-eastern Australia

Transitions in native pasture botanical composition due to an intensification of management (particularly grazing), is a common occurrence in Australian grasslands, as in other parts of the world. In the high rainfall zone of temperate south-eastern Australia, the loss of perennial species has caused increased recharge and dryland salinity. Farmers are now encouraged to retain and restore perennial vegetation into grazing systems. In the less arable areas, topographic, edaphic and economic constraints prevent the sowing of introduced perennial pasture species, and current perennial options are limited to the encouragement of native grassland vegetation to fulfil the role of reducing recharge. Little however is known about the hydrologic impact of varying pasture compositions in this region. A field experiment was conducted to examine the seasonal soil water use patterns of three native perennial pastures varying in composition of native grass species (C4 only, C3/C4 mixed and C3 only) combined with annual ryegrass. Morphological attributes of the pastures were examined to help explain what made these patterns occur. All perennial pastures used considerably more soil water than annual ryegrass plots despite soil limitations preventing any rooting advantage to the perennial growth habit. The C4 pasture dominated by Themeda triandra created the largest soil water deficit using on average 82 mm more water than annual ryegrass. The C3/C4 pasture dominated by Microlaena stipoides, Austrodanthonia spp. and Bothriochloa macra showed soil water deficits comparative to the C3 pasture dominated by Microlaena stipoides and Austrodanthonia spp. only (approximately 54 mm greater than annual ryegrass). The prolonged maintenance of green leaf and increased summer activity were recognised as key factors influencing soil water use. Native pastures should therefore be assessed for both abundance of perennial species and their level summer activity when estimating potentialuse.