Seasonal variations of nitrous oxide emission in relation to nitrogen fertilization and energy crop types in sandy soil

Nitrous oxide (N₂O) is a greenhouse gas and agricultural soils are major sources of atmospheric N₂O. Its emissions from soils make up the largest part in the global N₂O budget. Research was carried out at the experimental fields of the Leibniz-Institute of Agricultural Engineering Potsdam-Bornim (ATB). Different types (mineral and wood ash) and levels (0, 75 and 150 kg N ha⁻¹) of fertilization were applied to annual (rape, rye, triticale and hemp) and perennial (poplar and willow) plants every year. N₂O flux measurements were performed 4 times a week by means of gas flux chambers and an automated gas chromatograph between 2003 and 2005. Soil samples were also taken close to the corresponding measuring rings. Soil nitrate and ammonium were measured in soil extracts.N₂O emissions had a peak after N fertilization in spring, after plant harvest in summer and during the freezing–thawing periods in winter. Both fertilization and plant types significantly altered N₂O emission. The maximum N₂O emission rate detected was 1081 μg N₂O m⁻² h⁻¹ in 2004. The mean annual N₂O emissions from the annual plants were more than twofold greater than those of perennial plants (4.3 kg ha⁻¹ vs. 1.9 kg ha⁻¹). During January, N₂O fluxes considerably increased in all treatments due to freezing–thawing cycles. Fertilization together with annual cropping doubled the N₂O emissions compared to perennial crops indicating that N use efficiency was greater for perennial plants. Fertilizer-derived N₂O fluxes constituted about 32% (willow) to 67% (rape/rye) of total soil N₂O flux. Concurrent measurements of soil water content, NO₃ and NH₄ support the conclusion that nitrification is main source of N₂O loss from the study soils. The mean soil NO₃-N values of soils during the study for fertilized soils were 1.6 and 0.9 mg NO₃-N kg⁻¹ for 150 and 75 kg N ha⁻¹ fertilization, respectively. This value reduced to 0.5 mg NO₃-N kg⁻¹ for non-fertilized soils.