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.

• A multi-year monitoring of NO3–N and soil water content were measured under maize cropping at 6 sites.
• High soil nitrate concentrations were scored after fertilizer supply and ranged from 20 to 110 mg L−1.
• Mean annual leaching varied from 14 to 321 kg ha−1 year−1.
• High nitrate loss was primarily caused by N supply which largely exceeded crop demand.
• The double cropping system of silage maize and a catch crop reduced up to 90% the N leaching.