Effect of plastic-film mulching on leaching of nitrate nitrogen in an upland field converted from paddy

A lysimeter experiment was conducted to examine the effects of plastic film mulching on the leaching rate of nitrate nitrogen (NO3-N) from chemical fertilizer that was applied to an upland field that had been converted from paddy rice production. Leaching was monitored in two lysimeters filled with sandy loam soil, which contained low soil organic matter content, under different surface mulch conditions. One was mulched only on the ridge (ridge-mulch treatment) and another one was mulched fully, including the furrow, with black plastic film (full-mulch treatment). Chemical fertilizer was mixed into the top 0.2 m of soil in the two lysimeters before installing the mulch. After transplanting broccoli, the amount of subsurface discharge water and the NO3-N concentrations in the discharge water were measured every day. Larger NO3-N discharges occurred in the ridge-mulch treatment for three days after heavy rainfalls in which cumulative precipitation exceeded 10 mm, and the daily NO3-N load was twice as large as the full-mulch treatment. The differences in the amount of subsurface discharge water and NO3-N discharged between treatments were not significant when there was no rainfall. Cumulative NO3-N loads for the ridge- and full-mulch treatment during the last month of the experimental period were 0.246 and 0.195 g m−2, respectively. The effect of mulching on the reduction of NO3-N discharge rate was higher for the full-mulch treatment. This result showed that a plastic-film mulching system would be effective as an appropriate fertilizer management to reduce nitrate-leaching losses.