THE IMPORTANCE OF THE CARBON-CYCLE TO ACIDIFICATION OF A GRAZED ANNUAL PASTURE

Soil samples to a depth of 60 cm were collected from 3 fields of a 73-year-old experiment in north-eastern Victoria. One field (unfertilised) had never received fertiliser, whereas, the other 2 fields (fertilised) had received 4.5 t/ha superphosphate. One of the fertilised fields also had a lime application history (fertilised and limed). The fields were particularly useful for estimation of the amount of acid added by the carbon cycle as records of lamb, wool and hay removal over a long period were available. The soil pH of the fertilised field had declined relative to the unfertilised field to a depth of at least 30 cm. The field receiving fertiliser and lime had a similar pH profile to that of the unfertilised field. Pasture improvement resulted in much higher stocking rates and consequent product removal (hay, wool and meat) from the fertilised and limed fields compared with the unfertilised field (68, 72 and 12 kmol H+/ha, respectively). Carbon and nitrogen cycle acidification accounted for 65 and 35%, respectively, of the net acid addition on the fertilised field. The acidification rates for the fertilised, fertilised and limed, and unfertilised fields over the 73-year period were 1.42, 2.37 and 0.16 kmol H+/ha.year. These acid addition rates are likely to be underestimates because the laboratory pH buffering capacity method used did not account for slow buffering reactions in the field (Ridley et al. 1990).