Nitrous oxide emissions from oilseed rape cultivation were unaffected by flash pyrolysis biochar of different type, rate and field ageing.

Nitrous oxide (NO) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an NO emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of NO include the application of biochar, but efficacy and mechanisms of NO suppression are elusive. We measured NO emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017-2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on NO production and consumption by denitrification. Biochar treatments had no significant effects on cumulative NO emissions (1.71-2.78 kg N ha yr). Likewise, no significant effects were found on crop yield, yield-scaled NO emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed NO production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate NO emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant NO mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their NO mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased NO emissions.