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

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Abstract
Summary: 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 aquaticaandDactylis glomerata), and had a control (soil pH 4.1 in 0.01 mCaCl2) or lime treatment (pH 5.5). 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 (64–133 kg N ha−1to 120 cm) than in spring (51–96 kg N ha−1), with NO3–comprising 70–77%. No NO3–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−1as NO3–initially but < 1 mg N l−1by spring. Nitrate‐N concentrations at 120 cm ranged between 2 and 22 mg N l−1during winter. Losses of NO3–were small in surface runoff (0–2 kg N ha−1year−1). In 1995, 9–19 kg N ha−1was lost in subsurface flow. Deep drainage losses were 3–12 kg N ha−1in 1995 and 4–10 kg N ha−1in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg N ha−1year−1under control and limed annual pastures, respectively, and 6 and 8 kg N ha−1year−1under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg N ha−1under 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.