Nitrous oxide emissions from cropping systems producing biomass for future bio-refineries

A growing bioeconomy demands high, but sustainable production of biomass. In contrast to annual crops, perennial crops may combine high biomass yields with low nitrogen (N) leaching losses despite intensive management. However, there is an important trade-off between N fertilization to increase biomass production and the associated nitrous oxide (N2O) emissions when estimated with the IPCC default emission factor (EF) of 1% of applied N. Actual N2O emissions may vary with crop species and site conditions, and a field experiment was therefore conducted to determine N2O emissions and biomass yields from selected perennial crops. Three perennial crops were established five years prior to this study, including festulolium (x Festulolium braunii) and tall fescue (Festuca arundinacea), both receiving 425 kg N ha(-1) in NPK fertilizer with 60% ammonium-N and 40% nitrate-N, and grass-clover (Lolium perenne-Trifolium pretense) receiving PK fertilizer only. Maize (Zea mays) receiving 140 kg N ha(-1) in NPK was included as a reference annual crop; all treatments had unfertilized subplots. The total biomass yields of fertilized maize (14.3 Mg DM ha(-1)), festulolium (15.3 Mg DM ha(-1)) and tall fescue (16.2 Mg DM ha(-1)) were similar, and higher than grass-clover yield (8.4 Mg DM ha(-1)). There were three cuts in perennial crops, and peak emissions of N2O occurred after cutting and fertilization. Unexpectedly, PK fertilization increased N2O emissions by 89% compared to unfertilized grass-clover. A mixed-effect model examining drivers of N2O emissions after each fertilization indicated that temperature and nitrate were more important for N2O emissions than soil wetness, whether this was expressed as water-filled pore space or relative gas diffusivity, D-p/D-0. The overall highest emissions of N2O, which occurred in festulolium and tall fescue after the 1st cut and 2nd fertilization, coincided with rainfall after a dry period. Measurements of soil respiration indicated that these high N2O emissions were triggered by a release of labile carbon after the rapid wetting, and hence that denitrification of nitrate in the fertilizer applied was a significant source. Fertilization increased N2O emissions two- to three-fold compared to treatments without fertilization, but the annual emission factors for fertilizer N were consistently below the default IPCC emission factor of 1% with 0.23 +/- 0.04, 0.32 +/- 0.03 and 0.54 +/- 0.13% for, respectively, festulolium, tall fescue and maize. Hence, this study suggests that intensively managed perennial crops can be used for biomass production on sandy loam soil without excessive N2O emissions compared to annual crops.