Nitrate leaching under phalaris, cocksfoot, and annual ryegrass pastures and implications for soil acidification

Nitrogen uptake and nitrate (NO3-) leaching below 1.1 m was estimated under phalaris, cocksfoot, and annual ryegrass pastures and under bare fallow in a 4-year field experiment under control and high N (500 kg N/ha) treatments in north-eastern Victoria (693 mm/year rainfall for the study period). The perennial grasses, particularly phalaris, took up more N in herbage than annual ryegrass. High concentrations of NO3-N were measured at 1 m depth below all treatments, suggesting that NO3- losses from pastures have potential to contaminate streams and/or groundwater. Perennial pastures were only able to reduce NO3- leaching compared with annuals in drier than average years. Values calculated for acid addition due to NO3- leaching resulted in a net annual input of approximately 1 kmol H+/ha.year under the phalaris pasture compared with 2 kmol H+/ha.year under annual ryegrass. Adding these figures to carbon cycle addition data of 1 kmol H+/ha.year (measured in a previous study) corresponds to a lime rate of 100 and 150 kg lime/ha.year being required to stop further acidification under these pasture types. A 1 unit pH decline to 30 cm depth was estimated to take 42 years under annual pasture or 67 years under perennial grasses. Whilst perennial pastures have a role in reducing soil acidification, lime application is the most important management option in balancing soil acidification caused by agriculture.