土壤含水量对硝化和反硝化过程N_2O排放及同位素特征值的影响

【目的】通过室内培养试验,研究不同含水量对北京顺义潮褐土N_2O排放及同位素特征值(δ15Nbulk,δ18O和nitrogen isotopomer site preference of N_2O,简称SP)的影响,以期获得不同水分条件下土壤N_2O产生途径及变化规律,为农田土壤N_2O减排提供理论依据。【方法】结合稳定同位素技术与乙炔抑制法,以北京顺义潮褐土为试材,设置3个含水量梯度:67%、80%和95%WFPS(土壤体积含水量与总孔隙度的百分比或实际重量含水量与饱和含水量的百分比,简称WFPS),在此基础上设置无C2H2,0.1%(V/V)C2H2和10%(V/V)C2H2处理。将土壤装入培养瓶中培养2 h,之后收集培养瓶中的气体测定N_2O浓度及同位素特征值,并采集土样测定其NH+4-N和NO-3-N的含量。利用同位素二源混合模型计算硝化和反硝化作用对土壤N_2O排放的贡献率,对N_2O产生途径进行量化分析。【结果】根据室内土壤培养测定结果,高(95%WFPS)、中(80%WFPS)和低(67%WFPS)含水量土壤N_2O加权平均排放通量分别为1.17、0.27和0.08 mg N·kg-1·d-1,高含水量土壤N_2O排放量均显著高于中、低含水量处理,中含水量处理显著高于低含水量;整个培养周期,高、中和低含水量土壤N_2O+N_2累积排放量分别为培养初期总的无机氮含量的18.05%、5.27%和1.24%(N_2O+N_2累积排放量分别为19.61、5.72和1.35 mg N·kg-1;各处理NH+4-N+NO-3-N初始含量均为108.62 mg N·kg-1);与低含水量处理相比,高、中含水量土壤的N_2O+N_2累积排放量分别增加了13.53倍和3.24倍,高含水量土壤N_2O+N_2累积排放量比中含水量高2.43倍,表现为随着含水量的增加,土壤无机氮(NH+4-N+NO-3-N)以气态氮(N_2O+N_2)形式的损失量逐渐增加。3个含水量处理N_2O的δ15Nbulk加权平均值变化范围为-42.93‰—-4.07‰,且较高含水量处理显著低于较低含水量处理;10%(V/V)C2H2抑制土壤中N_2O还原成N_2的过程,各含水量土壤中,10%(V/V)C2H2处理组其N_2O的δ18O值显著低于0.1%(V/V)C2H2处理组,且N_2O/(N_2O+N_2)比率随土壤含水量增加而降低;各处理土壤中同时存在多个N_2O产生过程,对于培养第一周,土壤产生的N_2O的SP值于培养前4 d呈逐渐增加的趋势,之后又逐渐降低,低含水量土壤在第1—2天产生的N_2O的SP值为6.74‰—12.04‰,反硝化作用对土壤N_2O排放的贡献率为56.36%—66.15%,此培养阶段表现为土壤主要通过反硝化作用产生N_2O,之后,硝化作用贡献率(55.78%—100%)增强;中含水量土壤N_2O的SP加权平均值为10.26‰,该土壤中反硝化作用(40.90%—74.04%)占据主导地位;加10%(V/V)C2H2的高含水量处理,在整个培养第一周均具有较高的SP值,变化范围为7.61‰—21.11‰;与0.1%(V/V)C2H2处理组相比,10%(V/V)C2H2处理的高、中和低含水量土壤排放N_2O的SP加权平均值分别降低了0.10倍、0.33倍和0.06倍。【结论】土壤含水量增加促进N_2O排放,高含水量处理中N_2O排放量最高。67%WFPS处理中,N_2O排放前期以反硝化作用为主,后期以硝化作用为主;80%WFPS处理中,N_2O主要由反硝化过程产生;95%WFPS处理中,N_2O排放以硝化作用为主。

The Effects of Soil Water Content on N2O Emissions and Isotopic Signature of Nitrification and Denitrification

【Objective】 The objective of this paper is to understand the contribution of microbial processes to N₂O production and its changing rules under different water contents to provide a theoretical basis for reducing agricultural N₂O emissions. 【Method】A microcosm experiment was performed to investigate the effects of different water-filled pore space on N₂O emissions and isotopic signatures (δ¹⁵N^bulk, δ¹⁸O and nitrogen isotopomer site preference of N₂O) of soil at Shunyi district, Beijing. The study combined stable isotope technique and gas inhibitor method to analyze N₂O flux and its isotope signatures that emitted from soil. The experiment was set up three different water content levels, including 67%, 80% and 95% WFPS, and with three inhibitor levels, (without C₂H₂, with 0.1% (V/V) C₂H₂ and with 10% (V/V) C₂H₂). After two hours incubation, the soil samples were collected to determine the concentrations of NH+ 4-N and NO- 3-N. The gas samples were collected to determine the isotope signatures, and the two end-members mixing model was applied to quantify the respective contributions of microbial processes to N₂O production. 【Result】According to the incubation of the soil, the weighted average N₂O flux of 95%, 80% and 67% WFPS were 1.17, 0.27 and 0.08 mgN·kg^-1·d^-1, respectively, and the N₂O emissions of 95% WFPS were significantly higher than that of both 80% and 67% WFPS, as well as the N₂O emissions of 80% WFPS were significantly higher than that of 67% WFPS. The cumulative emissions of (N₂O+N₂) in 95%, 80% and 67% WFPS were 18.05%, 5.27%, and 1.24% of initial mineral nitrogen, respectively, over the entire incubation period. The cumulative emissions of (N₂O+N₂) were 19.61, 5.72 and 1.35 mgN·kg^-1, respectively; the initial content of NH+ 4-N+NO- 3-N was 108.62 mgN·kg^-1. Compared with 67% WFPS, the cumulative (N₂O+N₂) emissions of 95% and 80% WFPS increased 13.53 and 3.24 times, respectively. The cumulative emissions of (N₂O+N₂) in 95% WFPS was 2.43 times greater than that of 80% WFPS. The values of reduced NH+ 4-N+NO- 3-N as gaseous nitrogen increased with the increase of the water content. The weighted average δ¹⁵N^bulk values varied from -42.93‰ to -4.07‰, and the higher level of soil water content showed significantly higher N₂O emissions. 10% (V/V) C₂H₂ would inhibit the reduction of N₂O to N₂. The δ¹⁸O values with 10% (V/V) C₂H₂ were significantly smaller than that of with 0.1% (V/V) C₂H₂ in three water content levels. And the ratio of N₂O/(N₂O+N₂) reduced with the increase of soil moisture. Multiple N₂O processes occurred simultaneously in all treatments. The values of SP increased during the initial four days and then decreased gradually with incubation time. The SP values of 67%WFPS treatment at the first two days ranged from 6.74‰ to 12.04‰, and the contribution of denitrification to N₂O production was from 56.36% to 66.15%, suggesting that denitrification was the dominant microbial process, then the contribution of nitrification (55.78%-100%) to N₂O production became greater. The weighted average SP value was 10.26‰ in 80% WFPS treatment, indicating denitrification (40.90%-74.04%) was the major N₂O production process. There were larger SP values in 95% WFPS treatment with 10% (V/V) C₂H₂ in the first seven incubation days, ranged from 7.61‰ to 21.11‰. Compared with 0.1% (V/V) C₂H₂, the weighted average SP values of N₂O under 95%, 80% and 67% WFPS treatments with 10% (V/V) C₂H₂ produced from soil reduced by 0.10, 0.33 and 0.06 times respectively.【Conclusion】 The increase of soil water content promotes N₂O emission, and the 95% WFPS treatment showed the highest N₂O emissions. In the 67% WFPS treatment, the initial stage of N₂O emission was dominated by denitrification, followed by nitrification. Denitrification was the dominate process in 80% WFPS treatment and nitrification was the dominate process in 95% WFPS treatment.