Nitrate leaching loss under annual and perennial pastures with and without lime on a duplex (texture contrast) soil in humid southeastern Australia

Mineral N accumulates in autumn under pastures in southeastern Australia and is at risk of leaching as nitrate during winter. Nitrate leaching loss and soil mineral N concentrations were measured under pastures grazed by sheep on a duplex (texture contrast) soil in southern New South Wales from 1994 to 1996. Legume (Trifolium subterraneum)-based pastures contained either annual grass (Lolium rigidum) or perennial grasses (Phalaris aquatica and Dactylis glomerata), and had a control (soil pH 4.1 in 0.01 M CaCl 2 ) or lime treatment (pH5.5). One of the four replicates was monitored for surface runoff and subsurface flow (the top of the B horizon), and solution NO 3 - concentrations. The soil contained more mineral N in autumn (64-133 kg N ha -1 to 120cm) than in spring (51-96 kg N ha -1 ), with NO 3 - comprising 70-77%. No NO 3 - leached in 1994 (475 mm rainfall). In 1995 (697 mm rainfall) and 1996 (666 mm rainfall), the solution at 20 cm depth and subsurface flow contained 20-50 mg N l -1 as NO 3 - initially but < 1 mg N l -1 by spring. Nitrate-N concentrations at 120 cm ranged between 2 and 22 mg N l -1 during winter. Losses of NO 3 - were small in surface runoff (0-2 kg N ha -1 year -1 ). In 1995, 9-19 kg N ha -1 was lost in subsurface flow. Deep drainage losses were 3-12 kg N ha -1 in 1995 and 4-10 kg N ha -1 in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg N ha -1 year -1 under control and limed annual pastures, respectively, and 6 and 8 kg N ha -1 year -1 under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg N ha -1 under the four respective pastures. The increased loss of N caused by liming was of a similar amount to the decreased N loss by maintaining perennial pasture as distinct from an annual pasture.