The recently developed nitrification inhibitor DMPP provides an opportunity to reduce environmental pollution and increase yields. The effect of adding DMPP to pig slurry and mineral fertilizer had already been significant in reducing N leaching in a previous pot experiment with ryegrass. Field experiments were conducted from 2005 to 2007 at Torreneral (Lleida, 41º44'N, Typic Calcixerept and irrigated) and Mas Badia (Girona, 42º3'N, Typic Xerofluvent and rainfed), to determine the effects of DMPP added to PS and ammonium sulfate (ANS). The soil was very deep at both sites. A second objective of this research was to determine the suitability of using a passive capillary lysimeter to estimate how much drainage water moved through the instruments and how much N was lost through leaching. The experimental design was a randomized complete block with three replications. Westerwold ryegrass (Lolium multiflorum Lam., CV. Trinova) was sown each year, in September. The five treatments were: control, ANS, Entec, Pig slurry, and PS with DMPP. On each plot, deep drainage was measured using Gee passive capillary lysimeters. No significant differences (p<0.05) in biomass, N uptake, nitrate plant content, CP, K uptake, micronutrient uptake and N volatilization due to DMPP were found at either of the sites. At the irrigated site, the total amount of P removed in the PS plus DMPP treatment was significantly higher than in the other treatments. The nitrogen efficiency indexes were also higher at Torreneral with DMPP. During the second year, the Gee passive capillary lysimeter was adapted to the field conditions (shallow soils and saline water table) at the Torreneral site. At Mas Badia, no water drainage was measured. Total N uptake ranged from 175 to 310 kgN/ha/yr at Mas Badia and from 160 to 415 kgN/ha/yr at Torreneral. (Research funded by the Spanish Ministry of Science and Education, PTR1995-0867-OP-02)