Long-term field observations showed that N2O fluxes observed shortly after N application were not significantly affected by elevated CO2 in the Giessen Free Air Carbon dioxide Enrichment (FACE) study. To further investigate this unexpected result a 15N tracer study was carried out under controlled conditions where in parallel treatments either the NH4+ pool (15NH4NO3) or the NO3 pool (NH415NO3) was enriched with 15N. Fluxes of CO2, CH4, and N2O as well as the 15N enrichment of the N2O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N2 + N2O) and N2-to-N2O ratios were quantified in separate experiments. Over the 57 day incubation, N2O fluxes averaged 0.090 ng N2O-N g?1 h?1 under ambient and 0.083 ng N2O-N g?1 h?1 under elevated CO2 (not significantly different). The N2O production processes were identified by a two-source model. Results showed that N2O must have also been produced by a third source – possibly related to organic N transformation – which was stimulated by elevated CO2. Soil CO2 fluxes were approximately 20 % higher under elevated CO2 than soil from ambient but the differences were not significant. CH4 oxidation rates were on average ?1.75 ng CH4-C g?1 h?1 in the elevated and ?1.17 ng CH4-C g?1 h?1 in the ambient indicating that elevated CO2 increased the CH4 oxidation by 49 % compared to ambient CO2 under controlled conditions. N fertilization increased CH4 oxidation by 3-fold in both CO2 treatments. CO2 did not have any significant effect on DEA while total denitrification and N2-to-N2O ratios increased by 36 and 33 %, respectively. The results indicate that shortly after N application elevated CO2 must have stimulated both the N2O production and reduction to N2 to explain the increased N2-to-N2O ratio and at the same time explain the non-responsiveness of the N2O emissions. Thus, the observed variation of the CO2 effect on N2O emissions throughout the year is possibly governed by the dynamics of the N2O reductase activity.
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机译：长期场景观察结果表明，在N施用后不久观察到N2O助熔剂在Giessen游离空气二氧化碳富集（Face）研究中没有显着影响。为了进一步研究这种意外结果，在受控条件下进行15N示踪研究，其中在并行处理中NH 4 +池（15nH 4 NO 3）或NO3池（NH415NO3）富含15N。测量CO 2，CH4和N2O的通量以及N2O的15N富集。在单独的实验中定量反硝化酶活性（DEA），总脱氮（N2 + N 2 O）和N2-〜N 2 O比。在57天孵育中，N 2 O助熔剂在环境温度下平均为0.090ng N 2 -N-N G 1 H 1 H 1 H 1 H 2。在升高的CO 2（不显着不同）下0.083ng N2O-N G 1 H 2 1。通过双源模型识别N2O生产过程。结果表明，N2O也必须由第三源产生 - 可能与有机N转化有关 - 由升高的CO 2刺激。在二氧化碳升高的二氧化碳下的土壤二氧化碳助熔剂比来自环境的土壤约为20％，但差异并不显着。 CH4氧化率平均为α1.75ngch4-cg≤1h≤1在升高中，α1.1.17ngch 4 -cg≤1h≤1，表明升高的二氧化碳增加了49％的CH4氧化增加了49％受控条件下的环境二氧化碳。氮肥在二氧化碳处理中增加3倍的CH 4氧化。 CO2对DEA没有任何显着影响，而总抵消和N 2-N2O比分别增加36％和33％。结果表明，在N施加升高的CO 2后不久，必须刺激N2O产生和降低到N2，以解释增加的N2-〜N2O比，同时解释N2O排放的不响应性。因此，通过N2O还原酶活性的动态，观察到对N2O排放的CO 2对N2O发射的影响。
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