The long-term impact of two pasture production systems on soil acidification in southern Queensland

Abstract

Summary. Soil samples to a depth of 90 cm were collected in 1996 from adjacent 1 ha paddocks which had either been established to a leucaena-based or nitrogen-fertilised and irrigated pasture system in 1960 at Samford, south-eastern Queensland, to assess the impact of these production systems on soil chemical properties. The chemical properties were compared with those of soil samples collected from the site in 1960. Soil pH under both the nitrogen-fertilised and leucaena-based systems have declined significantly to a depth of 40 cm against that of the soil samples collected in 1960. However, the extent of acidification was higher under the more intensively managed nitrogen-fertilised system compared with the leucaena-based system. The acidification rates for the nitrogen-fertilised and leucaena-based pasture systems over the 36-year period are 5.1 and 1.0 kmol H⁺/ha.year, respectively, and are similar to values observed under annual pasture systems of temperate southern Australia. Associated with the decline in pH there was a decline in exchangeable Ca²⁺ in the nitrogen-fertilised system and a corresponding increase in exchangeable acidity.

The contribution of the nitrogen cycle to net acidification for the leucaena-based pasture system was estimated to account for 20% of the total net acid addition. This result is discussed in the light of a similar study undertaken in the semi-arid tropics where the contribution of the nitrogen cycle by a similar leucaena-based system was estimated to be 78% of the net acidification.