Nitrate leaching and nitrous oxide emissions from maize after grass-clover on a coarse sandy soil: Mitigation potentials of 3,4-dimethylpyrazole phosphate (DMPP).

Cropping of maize (Zea mays L.) on sandy soil in wet climates involves a significant risk for nitrogen (N) losses, since nitrate added in fertilizers or produced from residues and manure may be lost outside the period with active crop N uptake. This one-year lysimeter experiment investigated the potential of Vizura®, a formulation for liquid manure (slurry) with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP), to mitigate nitrous oxide (NO) emissions and nitrate (NO) leaching from a coarse sandy soil cropped with maize. Maize followed grass-clover (Lolium perenne L.-Trifolium pratense L.) with spring incorporation and was fertilised with cattle slurry. A total of 12 treatments in triplicate were included in a factorial experiment with 1 m large and 1.4 m deep lysimeters: 1) with or without spraying the above-ground biomass of grass-clover with DMPP before incorporation; 2) application of cattle manure with or without DMPP, or no fertilization; and 3) natural rainfall or extra rain events to represent wet spring conditions, which were simulated with an automated and programmable irrigation system. Around 20 kg N ha was returned to the soil in grass-clover above-ground biomass, and 145 kg N ha in cattle manure. Cumulative annual NO emissions ranged from 0.4 to 1.3 kg N ha, with between 49 and 86% of emissions occurring during spring. Manure application increased NO emissions, while extra rainfall had no effect. The mitigation of NO emissions by DMPP ranged from 46 to 67% under natural, and from 44 to 48% under high rainfall conditions. Total annual NO leaching ranged from 65 to 162 kg N ha. The extent of NO leaching to 1.4 m depth during spring was low, and instead most (72-83%) of total annual NO-N leaching was recorded during autumn before harvest. The extra rainfall during spring increased NO-N leaching in the pre-harvest period, but it is not clear to what extent this was associated with the N in grass-clover residues or manure applied in spring, or from N mineralisation below the root zone. Despite evidence for a reduction of NO leaching in three of four scenarios, overall this effect was not significant. No DMPP was detected in leachates. In conclusion, DMPP significantly reduced NO emissions from cattle manure on this sandy loam soil independent of rainfall, while there was no significant effect on NO leaching. The results indicate that NO emissions and NO-N leaching were partly derived from below-ground sources of N not affected by DMPP, which should be further investigated to better predict the mitigation potential of nitrification inhibitors.