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

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 15N 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.

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