EFFECT OF PERENNIAL PASTURE SPECIES ON SURFACE SOIL-MOISTURE AND EARLY GROWTH AND SURVIVAL OF SUBTERRANEAN CLOVER (TRIFOLIUM-SUBTERRANEUM L) SEEDLINGS

Subterranean clover seedling numbers and growth in swards containing 1 of 5 perennial pasture species [phalaris (Phalaris aquatica) cv. Siro lan, cocksfoot (Dactylis glomerata) cv. Currie, lucerne (Medicago sati va) cv. Aquarius, wallaby grass (Danthonia richardsonii) cv. Taranna, and lovegrass (Eragrostis curvula) cv. Consol] were compared with thos e in typical annual pastures and pure clover swards in the wheatbelt o f eastern Australia. Presence of a perennial species or the volunteer annual grass (Eragrostis cilianensis) increased the rate of drying of the soil surface (0-5 cm) after late February and May rain, compared w ith subterranean clover swards. Perennials differed in the rate they d ried the soil surface, with the more summer-active lucerne and consul lovegrass drying the profile more rapidly than phalaris. The amount of water in the surface 5 cm, 6 days after the rainfall event on 27-28 F ebruary, was strongly negatively correlated (r = -0.75, P < 0.01) with the amount of green perennial biomass, but not related to standing de ad material or surface residues. Where perennials were present, a smal ler proportion (2-4%) of the clover seed pool produced seedlings in re sponse to late summer rain, compared with pure clover swards (18%). A higher proportion of the seed pool produced seedlings (19-36%) followi ng rain in late autumn but there was no difference between species. Th e more summer-active perennials (cocksfoot, danthonia, and lucerne) ma rkedly depressed the survival of emerged clover seedlings following bo th germinations. Of the seedlings that emerged in early March, the pro portion remaining by 29 March was 57% in phalaris, 21% in lucerne, 13% in danthonia, and 1% in cocksfoot, compared with a 78% increase in se edlings in pure subterranean clover swards. By 15 May, all perennials had <2 clover seedlings/m(2) surviving, compared with 37 in the annual pasture and 964 plants/m(2) in pure subterranean clover. Following th e May germination, the highest proportion of emerged seedlings survivi ng until 29 May was in the phalaris swards (40%) and least in the cock sfoot and danthonia swards (2-4%). Presence of a perennial or annual g rass decreased (P < 0.05) relative water content of clover seedlings o n 15 March from 74% in pure clover swards, to 48% in annual pasture, 3 4% in phalaris, and 29% in lucerne swards. Clover seedlings growing in pure subterranean swards on 15 March (17 days after germinating rain) were 4 times larger than those in lucerne and twice as large as those in either phalaris or annual pasture. Seed size did not differ betwee n treatments, but available mineral soil nitrogen was significantly hi gher (P < 0.001) in pure subterranean clover sa ards (32 mu g N/g) com pared with perennials (3-13 mu g N/g). Strategies such as heavy grazin g in late summer to reduce green biomass of the perennials or sowing t he perennials at lower densities may reduce the adverse effects that p erennials have on subterranean clover seedlings in these drier environ ments.