Soil pH-increase strongly mitigated NO emissions following ploughing of grass and clover swards in autumn: A winter field study.

Emissions from crop residues contribute largely to the total estimated NO emissions from agriculture. Since low soil pH increases NO production by impairing the last denitrification step, liming has been suggested as a mitigation strategy; however, it may also increase NO emissions by enhancing mineralization and nitrification. To gain field-based empirical knowledge, we measured NO fluxes with an autonomous field-flux robot in limed and control plots before and after autumn ploughing of 3-year-old grass, clover grass or red clover swards under different N fertilization regimes. Dolomite applied before establishing the swards raised soil pH from ~4.8 to ~5.8 in limed plots. Higher pH halved emissions from ploughed leys despite higher soil mineral N contents. It also reduced emissions before ploughing. We observed substantial NO fluxes after ploughing, with peaks during a relatively warm wet period after freezing and higher peaks during diurnal snow melt over frozen soil. Average NO fluxes were strongly positively correlated with high herbage yields in the preceding growing seasons rather than with the presence of clover. The yield-scaled average NO fluxes were strongest in low pH soils at all yield levels; this was a true effect of soil pH on NO, as herbage yields were not increased by liming. Here, yield-scaled flux was defined as the average NO flux after ploughing divided by the dry matter. Fluxes in red clover plots were similar to those in grass plots, despite the lower C/N ratio and higher total amount of N in clover residues. However, clover in mixtures with grass increased yields and NO emissions. This suggests that higher ley production enhanced microbial activity, including nitrifiers and denitrifiers, and that the pH effect on facilitating complete denitrification to N overrode any effect on mineralization and nitrification, thus resulting in NO mitigation.