Seasonal differences in the soil water balance under perennial and annual pastures on an acid Sodosol in southeastern Australia

Replacement of native deep-rooted grasses by shallow-rooted ones has result ed in greater losses of water and nitrogen by drainage. To counter this eff ect we have tested the hypothesis that liming, and the conversion of annual grass pastures to perennial grass pastures, could improve the sustainabili ty of grazing systems in the high rainfall zone (> 600 mm per annum) in sou theastern Australia, through better use of water and nitrogen. A field expe riment consisting of sixteen 0.135ha (30m x 45m) grazed paddocks representi ng four pasture combinations (annual pasture (mainly Lolium rigidum) withou t lime (AP-); annual pasture with lime (AP+); perennial pasture (mainly Pha laris aquatica) without lime (PP-), and perennial pasture with lime (PP+)) was carried out from 1994 to 1997 on an acid Sodosol (Aquic Hapludalf) in s outhern New South Wales, Australia. Measurements were made of surface runof f, subsurface flow (on top of the B horizon) and soil water content. The re sults showed that perennial grass pastures, especially PP+, extracted appro ximately 40 mm more soil water each year than the annual grass pastures. As a result, surface runoff, subsurface flow and deep drainage were at least 40 mm less from the perennial pastures. These measurements were further sup ported by a simulation of soil water deficit and deep drainage for AP- and PP+ paddocks, using 10 years' past meteorological records. Overall, the res ults suggested that well-grown, phalaris-based pastures could reduce rechar ge to groundwater and make pastoral systems more sustainable in the high ra infall zone.