Herbage production and the accumulation of soil nitrogen under irrigated pastures on the Riverine Plain

Abstract

Herbage yield, herbage nitrogen, and soil nitrogen accumulation were followed over a five-year period (1958-63) under irrigated annual and perennial pastures established initially with different clovergrass proportions. Species sown in the annual pasture treatments were subterranean clover (Trifolium subterraneum cv. Tallarook) and annual ryegrass (Lolium rigidum cv. Wimmera). Those used in the perennial mixtures were white clover (T. repens cv. Irrigation) and perennial ryegrass (L. perenne cv. Victorian). In the treatments sown to annual or perennial ryegrass only, nitrogenous fertilizer as urea was applied annually at four rates. Total soil nitrogen (mat + 0-91 cm of soil) after five years did not differ significantly for annual pasture and bare ground. In contrast, total soil nitrogen under all perennial pasture treatments, particularly those with a white clover component, was significantly greater than for bare soil (5365 cf. 4181 kg ha-1). Where white clover was sown, nitrogen accumulated at the rate of 258 kg ha-1 per annum compared with 101 kg ha-1 per annum under subterranean clover, the latter barely matching the accumulation rate under perennial ryegrass sown alone without applied nitrogen (105 kg N ha-1 p.a.). Nitrogen accumulation (soil plus mat) was related to both legume and non-legume nitrogen. Perennial pasture, particularly if white clover was present, generally outyielded annual pasture. The total yield of white clover over five years was 29,970 kg ha-1 compared with 11,614 kg ha-1 for subterranean clover. Annual and perennial ryegrasses showed similar yield responses to applied nitrogen. Irrespective of the rate of urea application, nitrogen recovery was low (21-23 per cent).