Complex controls on nitrous oxide flux across a large-elevation gradient in the tropical Peruvian Andes
Complex controls on nitrous oxide flux across a large-elevation gradient in the tropical Peruvian Andes
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DOI：
10.5194/bg-14-5077-2017
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发表时间：
2017-11
期刊：
影响因子：
4.9
通讯作者：
T. Diem;N. Morley;Adan J. Q. Ccahuana;Lidia Priscila Hauraca Quispe;E. Baggs;P. Meir;M. Richards;Pete Smith;Y. Teh
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作者：
T. Diem;N. Morley;Adan J. Q. Ccahuana;Lidia Priscila Hauraca Quispe;E. Baggs;P. Meir;M. Richards;Pete Smith;Y. Teh
抽象。目前的自下而上的过程模型表明，山地热带生态系统的N2 O弱的大气来源，虽然最近的实证研究，从秘鲁南部安第斯山脉挑战了这一想法。在这里，我们报告的N2 O通量相结合的领域和实验室的实验，调查过程为基础的控制N2 O通量从山地生态系统跨越大海拔梯度（600-3700米a.s.l.）。在秘鲁南部的安第斯山脉。在2011年1月至2013年6月的30个月期间，对四个主要生境（山前森林、低山森林、高山森林和山地草原）的一氧化二氮通量和环境变量进行了每月一次的量化。通过15 N示踪实验研究了土壤含水量对N2 O通量的调节作用。通过野外凋落物处理实验和实验室15 N-NO3-添加研究，分别研究了基质可用性（不稳定有机质，NO3-）在调节N2 O通量中的作用。该地区的生态系统是N2 O的净大气源，未加权平均通量为0.27 ± 0.07 mg N-N2 O m−2 d−1。加权外推法考虑了栖息地之间陆地表面积的差异以及季节之间通量的变化，预测平均年通量为1.27 ± 0.33 kg N2 O-N ha-1 yr-1。一氧化二氮通量最大的是山前森林，未加权平均通量为0.75 ± 0.18 mg N-N2 O m−2 d−1，换算为加权年通量为0.66 ± 0.16 kg N2 O-N ha−1 yr−1。相比之下，N2 O通量显着较低的其他生境。山地下层森林、山地草地和山地上层森林的未加权平均通量分别为0.46 ± 0.24 mg N-N2 O m−2 d−1、0.07 ± 0.08 mg N-N2 O m−2 d−1和0.04 ± 0.07 mg N-N2 O m−2 d−1。这相当于加权年通量分别为0.52 ± 0.27 kg N2 O-N ha−1 yr−1、0.05 ± 0.06 kg N2 O-N ha−1 yr−1和0.04 ± 0.07 kg N2 O-N ha−1 yr−1。整个区域的氧化亚氮通量表现出较弱的季节性变化，只有较低的山地森林表现出显着较高的N2 O通量在旱季相比，雨季。在实验室中的土壤水分含量的操纵表明，N2 O通量显着影响的变化，充满水的孔隙空间（WFPS）。N2 O通量和WFPS之间的关系是复杂的和非线性的，偏离WFPS如何与N2 O通量的理论预测。硝化作用对N2 O通量的贡献可以忽略不计，与土壤含水量无关，表明硝酸盐还原是N2 O的主要来源。对汇总数据的分析表明，N2 O通量在90和50%WFPS时最大，在70和30%WFPS时最低。N2 O通量的这种趋势表明WFPS和硝酸盐还原过程（即反硝化，异化硝酸盐还原为铵）之间的复杂关系。在不稳定的有机质输入的变化，通过操纵叶凋落物，没有改变N2 O通量。对现场和实验室数据的综合分析表明，NO3−有效性的变化强烈地限制了N2 O通量。栖息地-在野外条件下NO3-可用性的代理-是N2 O通量的最佳预测因子，富氮栖息地（山前森林，低山地森林）显示出显着高于N-贫氮栖息地（上山地森林，山地草原）的N2 O通量。然而，N2 O通量对NO3−浓度的短期变化没有反应。
Abstract. Current bottom–up process models suggest that montane tropical ecosystems are weak atmospheric sources of N2O, although recent empirical studies from the southern Peruvian Andes have challenged this idea. Here we report N2O flux from combined field and laboratory experiments that investigated the process-based controls on N2O flux from montane ecosystems across a large-elevation gradient (600–3700 m a.s.l.) in the southern Peruvian Andes. Nitrous oxide flux and environmental variables were quantified in four major habitats (premontane forest, lower montane forest, upper montane forest and montane grassland) at monthly intervals over a 30-month period from January 2011 to June 2013. The role of soil moisture content in regulating N2O flux was investigated through a manipulative, laboratory-based 15N-tracer experiment. The role of substrate availability (labile organic matter, NO3−) in regulating N2O flux was examined through a field-based litter-fall manipulation experiment and a laboratory-based 15N–NO3− addition study, respectively. Ecosystems in this region were net atmospheric sources of N2O, with an unweighted mean flux of 0.27 ± 0.07 mg N–N2O m−2 d−1. Weighted extrapolations, which accounted for differences in land surface area among habitats and variations in flux between seasons, predicted a mean annual flux of 1.27 ± 0.33 kg N2O–N ha−1 yr−1. Nitrous oxide flux was greatest from premontane forest, with an unweighted mean flux of 0.75 ± 0.18 mg N–N2O m−2 d−1, translating to a weighted annual flux of 0.66 ± 0.16 kg N2O–N ha−1 yr−1. In contrast, N2O flux was significantly lower in other habitats. The unweighted mean fluxes for lower montane forest, montane grasslands, and upper montane forest were 0.46 ± 0.24 mg N–N2O m−2 d−1, 0.07 ± 0.08 mg N–N2O m−2 d−1, and 0.04 ± 0.07 mg N–N2O m−2 d−1, respectively. This corresponds to weighted annual fluxes of 0.52 ± 0.27 kg N2O–N ha−1 yr−1, 0.05 ± 0.06 kg N2O–N ha−1 yr−1, and 0.04 ± 0.07 kg N2O–N ha−1 yr−1, respectively. Nitrous oxide flux showed weak seasonal variation across the region; only lower montane forest showed significantly higher N2O flux during the dry season compared to wet season. Manipulation of soil moisture content in the laboratory indicated that N2O flux was significantly influenced by changes in water-filled pore space (WFPS). The relationship between N2O flux and WFPS was complex and non-linear, diverging from theoretical predictions of how WFPS relates to N2O flux. Nitrification made a negligible contribution to N2O flux, irrespective of soil moisture content, indicating that nitrate reduction was the dominant source of N2O. Analysis of the pooled data indicated that N2O flux was greatest at 90 and 50 % WFPS, and lowest at 70 and 30 % WFPS. This trend in N2O flux suggests a complex relationship between WFPS and nitrate-reducing processes (i.e. denitrification, dissimilatory nitrate reduction to ammonium). Changes in labile organic matter inputs, through the manipulation of leaf litter-fall, did not alter N2O flux. Comprehensive analysis of field and laboratory data demonstrated that variations in NO3− availability strongly constrained N2O flux. Habitat – a proxy for NO3− availability under field conditions – was the best predictor for N2O flux, with N-rich habitats (premontane forest, lower montane forest) showing significantly higher N2O flux than N-poor habitats (upper montane forest, montane grassland). Yet, N2O flux did not respond to short-term changes in NO3− concentration.