Measuring and modelling nitrate fluxes in a mature commercial apple orchard

Leaching of nitrogen from intensive agriculture to groundwater and waterways is a source of environmental harm. Nitrate leaching beneath a commercial 10-year-old ‘Galaxy’ apple orchard was investigated using a combined measurement and modelling approach. Measurement of sapflow, climate, soil moisture, deep drainage and nitrate leaching beneath the root zone were monitored for 30 months in order to parameterise and calibrate the Soil Plant Atmosphere System Model (SPASMO). Sap flow peaked at around 15 L tree−1 day−1 in late January with average sap flow for the 2013/14 season of 8.58, 6.39 and 6.47 L tree−1 day−1 for the high (4.0 L ha−1), medium (2.3 L ha−1) and low (1.6 L ha−1) irrigation treatments, respectively. Deep drainage and nitrate leaching were highly variable and did not appear to be influenced by irrigation rate. Over the 30-month monitoring period, drainage beneath the rootzone averaged 89 mm yr−1, whilst nitrate leaching averaged 33.2 kg-NO3 ha−1 yr−1, equivalent to 55% of the applied fertilizer. Average nitrate concentration in the leachate exceeded the World Health Organisation threshold for drinking water of 50 mg L−1 in 41 out of 279 samples. Long-term modelling indicated the average amount of drainage and nitrate leaching were 136 mm yr−1 and 11.4 kg-NO3 ha−1 yr−1, respectively. Scenario analysis indicated that few options existed to reduce nitrate leaching at the site through irrigation and fertigation management.