Tillage, compaction and wetting effects on NO₃, N₂O and N₂ losses

Abstract

Denitrification is sensitive to changes in soil physical properties that affect solute transport, air content and gas diffusion. Using lysimeters, containing intact soil from intensively tilled (IT) and no-tilled (NT) soil used to grow forage crops, we examined how simulated animal treading at different moisture contents (above and below field capacity >FC and <FC respectively) affected losses of nitrous oxide (N₂O), dinitrogen (N₂) and nitrate (NO₃). We applied ¹⁵N-labelled NO₃ (250 kg N ha⁻¹) to the soil surface after treading (applied at 220 kPa to 40% of the soil surface), or to untrodden soil. Drainage occurred following weekly application of water over the experiment (two pore volumes over 84 days). Treading at >FC greatly increased denitrification, especially from IT soil and produced the greatest amount of N₂ (64 kg N ha⁻¹ ), N₂O (8.2 kg N ha⁻¹), as well as the lowest N₂O to N₂O + N₂ ratio (0.08) and NO₃ leaching (136 kg N ha⁻¹ below 30 cm). In both the uncompacted or compacted soils <FC, emissions of N₂O were greater (1.5-2.7% of N applied) and the N₂O to N₂O + N₂ ratios were closer to 0.2 compared to compaction at >FC. Treading at <FC had minimal or no effect on denitrification compared to untrodden soil. Fluxes of N₂ and N₂O were strongly influenced by the weekly irrigation-drainage cycle. The N₂ production and reduction in NO₃ leaching were best correlated with increases in microporosity and reduced saturated hydraulic conductivity following treading. Although recovery of 15 N was high (84.3%), the remainder of the balance was likely lost as either N₂ or, of greater concern, as N₂O. Practically, animal trampling on wet soils, especially when recently cultivated, should be avoided.