Towards a complete nitrogen budget from subtropical dairy farms: three years of pasture nitrogen losses in surface runoff

Dairy represents one of the most intensive and nitrogen (N) loaded production systems in the high rainfall regions of Queensland, Australia. Fertiliser application rates during the winter rye grass season (April-October) frequently surpass 300 kg N ha-1 year-1 yet the fate of much of the applied N is uncertain. The high (>1200 mm year-1) and intensive rainfall and the proximity to environmentally sensitive areas such as the Great Barrier Reef make losses in surface water run-off of particular interest to the industry. Two run-off plots (416 m2) were installed on an intensively irrigated and fertilised rye-grass/kikuyu pasture rotation near Gympie, Queensland and monitored over three (1 June to 31 May) measurement years. Runoff was measured using a tipping bucket and nutrients collected via an automated sampler. Runoff and losses were largest during the 2012-13 season when five of the nine runoff events over the measurement period occurred and total runoff exceeded 480 mm, corresponding to 37% of the annual rainfall. Total N load was dominated by NO3–, with largest losses during a four day, 448 mm rain event in January 2013 following an extended dry period resulting in 280 mm of runoff and 16.5 kg ha-1 of N losses. Total N losses over the remaining periods were typically negligible (-1­ event-1), with annual losses of 5.0 and 0.7 kg N ha-1 for 2013-14 and 2014-15 respectively. These results indicate that under current management systems intensive pastures contribute only minor nutrient loads, though losses can be high following extended dry periods