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

Abstract

Nitrous oxide (N₂O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N₂O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N₂O include the application of biochar, but efficacy and mechanisms of N₂O suppression are elusive. We measured N₂O 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^-1) 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 N₂O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N₂O emissions (1.71-2.78 kg N ha^-1 yr^-1). Likewise, no significant effects were found on crop yield, yield-scaled N₂O 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 N₂O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N₂O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha^-1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N₂O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N₂O 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 N₂O emissions.

Keywords: Biodiesel; Denitrification; Emission factor; Manure fibre; N(2)O; Wheat straw.