N_2:Ar法直接测定淹水环境反硝化产物N_2的产生速率

N_2:Ar Technique for Direct Determination of Denitrification Rate of Aquatic Ecosystems Using Membrane Inlet Mass Spectrometry

反硝化是活性氮变为惰性氮返回大气的最主要过程,对于控制生态系统氮平衡至关重要,然而反硝化速率的测定,特别是淹水环境的反硝化速率测定一直是一个难题。为深入研究淹水环境反硝化过程及机理,建立了可以模拟原位淹水环境的培养装置,结合可以精确测定水体N2∶A(r物质的量比)的膜进样质谱仪(MIMS),实现了直接对原状沉积物反硝化速率的精确测定。实验结果表明,MIMS在长时间(10h)连续测定情况下仍能保持良好的信号稳定性,水温12℃和30℃标样的N2∶Ar变异系数(CV)分别为0.26%和0.08%。整个实验体系(培养装置结合MIMS)重复性较好,测得不同NO-3-N浓度处理的3个平行沉积物柱样反硝化速率的CV<9.05%;培养实验所取平行水样标准偏差(SD)<0.1μmol·L-1,远小于培养期内N2浓度的实际增加量(4.5μmol·L-1),说明该系统可以满足对淹水环境反硝化速率的测定要求。应用该方法得到的沉积物反硝化速率与NO-3-N浓度关系符合米氏方程(R2=0.9992,P<0.0001);该方法和NO-3-N消失法测定的结果具有显著的线性关系(R2=0.9998,P<0.0001)。表明通过该实验体系所建立的N2∶Ar法在今后深入开展水体氮循环研究中具有良好的应用前景。

High background concentrations of N2 and high spatio-temporal heterogeneity of sediments make it difficult to quantify denitrification in aquatic ecosystems. Traditional methods such as C2H2 inhibition, 15N tracer and mass balance measure denitrification in an indirect way, which is insufficiently accurate and time-consuming. The development of membrane inlet mass spectrometry（MIMS） makes it possible to measure N2∶Ar ratio precisely. In this study, we developed an undisturbed sediments incubation system combined with MIMS to measure net N2 fluxes directly and precisely. The incubation and sampling protocol were developed in this paper. Laboratory experiments showed that the instrument yielded high signal stability. The coefficient variations（CVs） of the measured N2∶Ar ratio of two standard solutions（with water temperature of 12 ℃ and 30 ℃） during 10 h continuous measurements were 0.26% and 0.08%. The net N2 fluxes of three replicate sediment cores also showed high reproducibility of the method（CV〈9.05%）. The standard deviation of N2 concentration in triplicate water samples collected from our undisturbed sediments incubation system is less than 0.1 μmol·L-1, which was far less than the increment of dissolved N2（μmol·L-1） during the incubation（4.5 μmol·L-1 within 9 h）. Denitrification rates obtained from this method showed a Michaelis-Menten relationship with NO-3-N concentrations（R2=0.999 2, P〈0.000 1）. In addition, there was a significant linear relationship between results of the N2∶Ar method and the NO-3-N disappearance method（R2=0.999 8, P〈0.000 1）. This study suggests good applicability of the N2∶Ar method combined with our undisturbed sediments incubation system in determining denitrification rate of wetlands.