Net N2O and CH4 soil fluxes of annual and perennial bioenergy crops in two central German regions.

The area used for bioenergy feedstock production is increasing because substitution of fossil fuels by bioenergy is promoted as an option to reduce greenhouse gas (GHG) emissions. However, agriculture itself contributes to rising atmospheric nitrous oxide (N 2 O) and methane (CH 4 ) concentrations. In this study we tested whether the net exchanges of N 2 O and CH 4 between soil and atmosphere differ between annual fertilized and perennial unfertilized bioenergy crops. We measured N 2 O and CH 4 soil fluxes from poplar short rotation coppice (SRC), perennial grass-clover and annual bioenergy crops (silage maize, oilseed rape, winter wheat) in two central German regions for two years. In the second year after establishment, the N 2 O emissions were significantly lower in SRC (<0.1 kg N 2 O–N ha −1 yr −1 ) than grassland (0.8 kg N 2 O–N ha −1 yr −1 ) and the annual crop (winter wheat; 1.5 kg N 2 O–N ha −1 yr −1 ) at one regional site (Reiffenhausen). However, a different trend was observed in the first year when contents of mineral nitrogen were still higher in SRC due to former cropland use. At the other regional site (Gierstädt), N 2 O emissions were generally low (<0.5 kg N 2 O–N ha −1 yr −1 ) and no crop-type effects were detected. Net uptake of atmospheric CH 4 varied between 0.4 and 1.2 kg CH 4 –C ha −1 yr −1 with no consistent crop-type effect. The N 2 O emissions related to gross energy in the harvested biomass ranged from 0.07 to 6.22 kg CO 2 equ GJ −1 . In both regions, Gierstädt (low N 2 O emissions) and more distinct Reiffenhausen (medium N 2 O emissions), this energy yield-related N 2 O emission was the lowest for SRC. [ABSTRACT FROM AUTHOR]