氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3~--N与还原态NH_4~+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm~(-2)a~(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4~+-N的积累效应显著高于NO_3~--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4~+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3~--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4~+和NO_3~-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。

The effects of types and doses of nitrogen addition on soil N2O flux in a cold-temperate coniferous forest, northern China

Increasing atmospheric nitrogen (N) deposition would change soil N availability, and thereby changes N₂O production and emission from N-limiting forest soils. However, it is still unclear about the contrasting effects of different N ion (i.e., oxidized NO₃^--N and reduced NH₄⁺-N) deposition on soil N₂O emission in boreal forests. In the present study, a cold-temperate coniferous forest in the Great Khingan region was selected for conducting the manipulative N addition experiment, including three forms of N fertilizers (NH₄Cl, KNO₃, and NH₄NO₃) and four rates of N addition (0, 10, 20, and 40 kg N hm^-2 a^-1). Soil-atmospheric N₂O exchanging fluxes were measured four times per month using static chamber-gas chromatography. Simultaneously, soil temperature, moisture, and inorganic N contents were measured to explore how the main factors affect soil N₂O emission. The results showed that the types and doses of N addition significantly changed soil inorganic N contents, and the accumulation of soil NH₄⁺-N was significantly higher than that of soil NO₃^--N due to N addition. N addition increased N₂O emission from the boreal forest soil, and the NH₄NO₃ addition treatments promoted high soil N₂O fluxes, with an increase of 442% to 667%, which was higher than the global average (134%). There were positive correlations between soil N₂O fluxes and soil temperature, and between soil N₂O fluxes and soil NH₄⁺-N contents in the litter layer. Soil N₂O fluxes were driven by soil temperature, followed by soil NH₄⁺-N contents in the litter layer. These findings suggest that atmospheric N deposition cannot lead to a significant soil NO₃^--N leaching and loss in boreal forests over the short term, but it can significantly promote soil N₂O emission. Furthermore, exogenous NH₄⁺ and NO₃^- inputs into boreal forests have synergic effects on soil N₂O emission, which should be distinguished in N cycle and balance in terrestrial ecosystems using ¹⁵N tracer methods in the future.