Post-harvest N₂O and CO₂ emissions related to plant residue incorporation of oilseed rape and barley straw depend on soil NO₃^- content

The sustainable production of bioenergy from crops like oilseed rape, barley, and maize presents a significant option to mitigate climate change by reducing fossil CO₂ emissions. Greenhouse gas emissions (specifically N₂O) during the energy crop production need to be quantified precisely for reliable life cycle analysis of bioenergy cropping systems. Energy crops (specifically oilseed rape) have a very high N demand, which results in a higher N-fertilizer application and thus higher risk of N₂ O emissions not only during the vegetation period but also after crop harvest due to i) incorporation of N rich plant residue to soil and/or ii) residual N. An incubation experiment was conducted under conditions favoring denitrification (80% water-holding capacity), to study the drivers of N₂O emissions specifically during the post-harvest period. Here we compared two different plant residues varying in C/N ratio (oilseed rape (RST) and barley straw (BST)) with or without N supply and measured CO₂, and N₂O emissions. Stable isotope labeling (¹⁵N) was used to quantify soil- and residue-born N₂O. Incorporation of both plant residues alone induced significant increases in CO₂ emissions compared to control soil without straw addition (p < .05). However, the increase in CO₂ emissions was less pronounced when straw was incorporated in conjunction with mineral N. There was a clear increase in cumulative N₂O emissions (p < .05) when soil amended with BST or RST (6- and 9-fold) was compared to control, however, the increase of cumulative N₂O emissions was drastic when mineral N was added (15- and 23-fold). No significant differences in N₂O emission were observed when comparing residue types (p > .05). Stable isotope labeling of barley straw clearly showed that the share of residue-born N₂O was very low (1.35 or 0.4%) in the overall N₂O fluxes in BST and BST+N. The present study suggests that N fertilization in autumn should be avoided to minimize N₂O fluxes regardless of type of straw.