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 greases (Phalaris aquatica and Dactylis glomerata), and had a control (soil pH4.1 in 0.01M CaCl2) 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 NO3- concentrations. The soil contained more mineral N in autumn (64-133kgNha(-1) to 120cm) than in spring (51-96kgNha(-1)), with NO3- comprising 70-77%. No NO3- leached in 1994 (475mm rainfall). In 1995 (697 mm rainfall) and 1996 (666 mm rainfall), the solution at 20 cm depth and subsurface flow contained 20-50 mg Nl(-1) as NO3- initially but < 1 mgNl(-1) by spring. Nitrate-N concentrations at 120 cm ranged between 2 and 22 mgNl(-1) during winter. Losses of NO3- were small in surface runoff (0-2 kgNa(-1) year(-1)). In 1995, 9-19 kg N ha(-1) was lost in subsurface flow. Deep drainage losses were 3-12 kg Nha(-1) in 1995 and 4-10 kg Nha(-1) in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg Nha(-1) year(-1) under control and limed annual pastures, respectively, and 6 and 8 kgNha(-1) year(-1) under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg Nha(-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.