Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA.
Proc Natl Acad Sci U S A. 2011 Jan 4;108(1):214-9. doi: 10.1073/pnas.1011464108. Epub 2010 Dec 20.
Nitrous oxide (N(2)O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N(2)O via microbial denitrification that converts N to N(2)O and dinitrogen (N(2)). The fraction of denitrified N that escapes as N(2)O rather than N(2) (i.e., the N(2)O yield) is an important determinant of how much N(2)O is produced by river networks, but little is known about the N(2)O yield in flowing waters. Here, we present the results of whole-stream (15)N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N(2)O at rates that increase with stream water nitrate (NO(3)(-)) concentrations, but that <1% of denitrified N is converted to N(2)O. Unlike some previous studies, we found no relationship between the N(2)O yield and stream water NO(3)(-). We suggest that increased stream NO(3)(-) loading stimulates denitrification and concomitant N(2)O production, but does not increase the N(2)O yield. In our study, most streams were sources of N(2)O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y(-1) of anthropogenic N inputs to N(2)O in river networks, equivalent to 10% of the global anthropogenic N(2)O emission rate. This estimate of stream and river N(2)O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.
一氧化二氮（N2O）是一种强效温室气体，它导致气候变化和平流层臭氧破坏。人为向河流网络输入的氮（N）是通过微生物反硝化作用将 N 转化为 N2O 和氮气（N2）从而产生 N2O 的一个潜在重要来源。反硝化作用中未转化为 N2 而逸出的 N （即 N2O 得率）是决定河流网络产生多少 N2O 的一个重要因素，但关于流动水中的 N2O 得率知之甚少。在这里，我们展示了在美国多个土地利用类型的多个源头溪流中进行的全流（15）N 示踪剂添加的结果。我们发现，溪流反硝化作用以随溪流水中硝酸盐（NO3-）浓度增加而增加的速率产生 N2O，但只有 <1%的反硝化 N 转化为 N2O。与一些先前的研究不同，我们发现 N2O 得率与溪流水中的 NO3-之间没有关系。我们认为，增加的溪流 NO3-负荷刺激了反硝化作用和随之产生的 N2O 生成，但不会增加 N2O 得率。在我们的研究中，大多数溪流是大气中 N2O 的来源，在排水城市流域的溪流中观察到最高的排放速率。使用全球河流网络模型，我们估计微生物 N 转化（例如反硝化和硝化）将至少 0.68 Tg·y-1 的人为输入 N 转化为河流网络中的 N2O，相当于全球人为 N2O 排放速率的 10%。与政府间气候变化专门委员会的估计相比，该河流和溪流 N2O 排放的估计值高出三倍。
