Greenhouse gas emissions from the energy crop oilseed rape (Brassica napus); the role of photosynthetically active radiation in diurnal N₂O flux variation

Abstract

Oilseed rape (OSR, Brassica napus L.) is an important feedstock for biodiesel; hence, carbon dioxide (CO2), methane (CH4) and particularly fertilizer‐derived nitrous oxide (N2O) emissions during cultivation must be quantified to assess putative greenhouse gas (GHG) savings, thus creating an urgent and increasing need for such data. Substrates of nitrification [ammonium (NH4)] and denitrification [nitrate (NO3)], the predominant N2O production pathways, were supplied separately and in combination to OSR in a UK field trial aiming to: (i) produce an accurate GHG budget of fertilizer application; (ii) characterize short‐ to medium‐term variation in GHG fluxes; (iii) establish the processes driving N2O emission. Three treatments were applied twice, 1 week apart: ammonium nitrate fertilizer (NH4NO3, 69 kg‐N ha−1) mimicking the farm management, ammonium chloride (NH4Cl, 34.4 kg‐N ha−1) and sodium nitrate (NaNO3, 34.6 kg‐N ha−1). We deployed SkyLine2D for the very first time, a novel automated chamber system to measure CO2, CH4 and N2O fluxes at unprecedented high temporal and spatial resolution from OSR. During 3 weeks following the fertilizer application, CH4 fluxes were negligible, but all treatments were a net sink for CO2 (ca. 100 g CO2 m−2). Cumulative N2O emissions (ca. 120 g CO2‐eq m−2) from NH4NO3 were significantly greater (P < 0.04) than from NaNO3 (ca. 80 g CO2‐eq m−2), but did not differ from NH4Cl (ca. 100 g CO2‐eq m−2) and reduced the carbon sink of photosynthesis so that OSR was a net GHG source in the fertilizer treatment. Diurnal variation in N2O emissions, peaking in the afternoon, was more strongly associated with photosynthetically active radiation (PAR) than temperature. This suggests that the supply of carbon (C) from photosynthate may have been the key driver of the observed diurnal pattern in N2O emission and thus should be considered in future process‐based models of GHG emissions.

N2O flux short-term response to temperature and topsoil disturbance in a fertilized crop: An eddy covariance campaign

“…Similar observations were made by Laville et al (2011) for a fertilized crop over a period of 9 days, while others found a correlation with deeper temperatures (Alves et al, 2012;Livesley et al, 2008). Keane et al (2017) observed a The daily pattern observed during the second and third peak episodes was less obvious (Table 4). On one hand, we observed larger night emissions than during the first peak episode, which could originate from a higher range of temperatures in May and June promoting nightly microbial activity.…”

Assessment of nitrous oxide emission factors for arable and grassland ecosystems

“…Peak N 2 O fluxes were generally observed at the beginning of April (Figure 2(a-d)). Keane et al (2018) 651 g N ha −1 d −1 in WOSR fertilized, with ammonium nitrate and ammonium only, respectively. They observed an increase in N 2 O fluxes associated with higher levels of photosynthetically active radiation.…”

Real‐time monitoring of greenhouse gas emissions with tall chambers reveals diurnal N2O variation and increased emissions of CO2and N2O fromMiscanthusfollowing compost addition

“…The peak in N 2 O emissions seen here was equivalent to ca. 2000 µg m −2 hr −1 , which is more than ten times the magnitude of fluxes reported from Miscanthus in a long-term study at this site (Drewer et al, 2012) and approximately half the maximum rate seen at this farm following the addition of 120 kg N/ha ammonium nitrate fertilizer to oilseed rape (OSR, Brassica napus L.; Keane et al, 2018). While fluxes of a similar magnitude have been reported from unfertilized Miscanthus elsewhere (Guzman, Ussiri, & Lal, 2017), N 2 O emissions from established Miscanthus plantations are widely thought to be much lower than those from conventional cropping .…”

“…Whilst diurnal patterns of N 2 O emission have been seen several times before (e.g. Alves et al, ; Ryden, Lund, & Focht, ; Shurpali et al, ; Yamulki et al, ), and were very recently observed in OSR at the same farm (Keane et al, ), the diurnal variation reported by these authors was defined by a peak in emissions during the day and lower emissions during the night. The sole exception was reported by Shurpali et al (), who saw higher emissions at night under N‐limiting conditions.…”

“…The sole exception was reported by Shurpali et al (2016), who saw higher emissions at night under N-limiting conditions. Of these studies, all but Keane et al (2018) attribute the pattern to soil temperature, and to our knowledge, there are no reports of a transition between N 2 O uptake during the day and production during the night.…”