NITRATE LEACHING FROM SOILS UNDER A MAIZE-WHEAT-MAIZE SEQUENCE, 2 IRRIGATION SCHEDULES AND 3 TYPES OF FERTILIZERS

To implement measures to tackle the nitrate leaching problem it is nec essary to assess the effect of alternative agricultural practices in v ulnerable zones. A 3-year experiment was conducted at La Poveda Field Station (30 km southeast of Madrid, Spain) to assess the effects of ir rigation schedules (conventional vs. efficient)]and fertiliser types ( unfertilised control, urea, Floranid32 (a slow-release N fertiliser), and municipal organic waste (MOW) compost) on nitrate leaching, grain yield and N uptake by plants. Treatments were combined in a factorial model and distributed to 24 plots. Chemical fertilisers were applied a t the rate of 150 kg N ha(-1) to maize (1993 and 1995 growing seasons) and 100 kg N ha(-1) to wheat (1994 growing seasons). The MOW compost was applied at a single dose before 1993 maize planting at the rate of 27.5 Mg ha(-1) Conventional irrigated plots were overwatered and the water input in efficient irrigated plots was dosed according to plant consumption. Nitrate discharge was calculated by multiplying the seaso nal drainage volume by the concentration of nitrates in soil water sol ution, both at 1.4 m soil depth. Seasonal drainage was calculated by u sing the water balance equation after measuring water movement (tensio meters) and water storage throughout the soil profile (neutron probe), Nitrate concentration was measured in samples extracted with ceramic cups. Drainage losses throughout the experiment were 184 and 38 mm in plots irrigated with conventional and efficient schedules, respectivel y. Corresponding nitrate-nitrogen discharge in both sets of plots were 128 and 25 kg N ha(-1) across fertiliser types. The nitrate discharge in unfertilised plots from the soil N reserve was on average half tha n that in fertilised ones. The MOW compost treatment combined with eff icient irrigation showed promise in controlling nitrate discharge, Mai ze yields were unaffected by fertiliser type, but wheat yields decreas ed by 33% in unfertilised plots. It is concluded that, at a given rate of chemical fertiliser, nitrate loading to the aquifer can be traced to drainage discharge and that safe loading of MOW compost to soils re quires control of water inputs. (C) 1997 Elsevier Science B.V.