Nitrate leaching under maize cropping systems in Po Valley (Italy)

Intensive crop production in Po Valley (Northern Italy) is associated to high risk of nitrate leaching. A multi-year monitoring of soil solution nitrogen was conducted at 6 sites under the ordinary farm management of maize crop (Zea mays L.) in order to assess NO3–N leaching. The amount of N fertilizer (organic + mineral) varied from 209 to 801 kg N ha−1 year−1. Maize biomass ranged from 15 to 32 t ha−1 and N removal from 150 to 400 kg ha−1. Soil water solution was sampled at five depths along the soil profile (from 0.3 to 1.5 m) at time intervals of 7–30 days using suction cups. Soil water content (SWC) was measured daily by TDR at the same depths of suction cups. Soil water NO3–N concentrations varied from 0 to 110 mg L−1, with the highest concentrations measured after fertilizer application. Once validated on measured SWC data, SWAP model was applied to estimate the drainage flux. Annual leaching was calculated by multiplying drainage flux by soil water NO3–N concentration. N Leaching ranged from 14 to 321 kg ha−1 year−1, according to fertilization, crop N removal, rainfall, irrigation management, and it was mainly affected by N surplus.