Appendix: Nitrous oxide emissions from soils: How well do we understand the process and their controls?
Klaus Butterbach-Bahl1,2, Elizabeth M. Baggs3, Michael Dannenmann1,4, Ralf Kie1, Sophie Zechmeister-Boltenstern5
1 Karlsruhe Institute of Technology, Institute for Meteorology and Climate Rearch, Atmospheric Environmental Rearch (IMK-IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany
2 International Livestock Rearch Institute, Old Naivasha Road, Nairobi 00100, Kenya
3 Institute of Biological and Environmental Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB2
4 2TZ, UK
4 Institute of Forest Botany and Tree Physiology, Chair of Tree Physiology, University of Freiburg; Georges-Koehler-Allee 53/54, 79110 Freiburg, Germany
5 University of Natural Resources and Life Sciences Vienna, Department of Forest and Soil Sciences, Institute of Soil Rearch, Peter Jordan Str. 82, A-1190 Vienna, Austria
2. PRODUCTION AND CONSUMPTION PROCESSES OF N2O IN SOILS
(a) N2O formation by chemodenitrification
licentiousThe term “chemodenitrification” is summarizing abiotic pathways of the formation of NO, N2O and N2 in soil. Hence, nsu strictu it would also include the chemical decomposition of hydroxylamine and nitrite during nitrification, which was here classified under N2O formation by nitrification. However, usually chemodenitrification is equalled with nitrite reduction by organic or inorganic molecules to form NO, N2O or eventually N21,2,3 (Figure 2 main text). It should be noted that precursors for chemo-denitrification are predominantly formed by microbial process as shown in Figure 2 (main text).
Chemodenitrification is predominantly occurring under conditions of high nitrite concentrations in acidic soils (pH<5)4. Thus, its importance in natural environments may be restricted for example to acidic forest soils5. However, chemodenitrification has rarely been considered in N2O source partitioning experiments, and therefore needs to be further investigated1.
Another mechanism for abiotic N2O formation is the decomposition of ammonium nitrate in the prence of light, relative humidity and a surface such as soil or concrete6. The importance of this N
2O production process has hardly been assd yet, though first estimates for the US show that even at a regional scale this might be a significant source process for atmospheric N2O.
(b) N2O production via nitrification and nitrifier-denitrification
Autotrophic and heterotrophic nitrification are oxidative process involved in the formation of N2O. Both process u reduced N-forms (ammonia or organic N), and oxidize it to nitrite, thereby obligatorily requiring molecular O2. Hence, the process occur under aerobic
conditions. Both autotrophic and heterotrophic ammonia oxidizers can be found in various groups of bacteria and archaea7.
Chemo-litho-autotrophic ammonia and nitrite oxidizers perform the two-step-oxidation of ammonia (NH3) to nitrite (NO2-) and nitrate (NO3-), i.e. autotrophic nitrification5. Heterotrophic nitrification can be defined as the oxidation of organic compounds to ammonium and further to nitrite and nitrate (Figure 2 main text), although an inorganic N pathway similar to that of autotrophic nitrification may also exist1. Heterotrophic nitrification is carried out by bacteria and fungi that u organic compounds as both C and energy source. The organisms oxidize ammonia or the reduced nitrogen in organic compounds to hydroxylamine, nitrite and nitrate, using organic compounds for th
eir heterotrophic metabolism. Heterotrophic nitrification has been reported to dominate over autotrophic nitrification in veral soils ranging from various forest ecosystems8,9,10,11 to grassland12,13,14,15 and arable soils16.
There are two direct pathways of N2O formation by nitrifiers (Figure 2 main text). First, N2O can originate from the chemical decomposition of hydroxylamine, with NO as a precursor17,18 (Figure 2 main text). Second, N2O can be produced during nitrifier denitrification. Nitrifier denitrification is a different process from coupled nitrification-denitrification. While the latter requires the formation of nitrate, and subquent nitrate reduction by different microorganisms, the term “nitrifier denitrification” describes the oxidation of ammonia to nitrite and its subquent reduction to form NO and N2O by the same autotrophic ammonia-oxidizing organism under conditions of high N availability but low organic C and oxygen availability5,19,20. N2O production via this nitrite pathway is the dominating source of N2O from ammonia oxidizers, i.e. dominating over N2O production from the chemical decomposition of hydroxylamine5,21.
Nitrifier denitrification is contributing considerably to the total N2O emission from soils22,23,24,25,26, in particular when soil moisture and oxygen availability are limiting classical denitrification27, and in respon to increasing NO2- concentrations. Thus it is possible that the activ
ity of nitrite oxidizers is the key that regulates denitrification by ammonia oxidizing bacteria. Both autotrophic and heterotrophic nitrification may produce N2O by nitrification-denitrification20,25. Whether N2 can also be formed by nitrifier-denitrification is still uncertain5.
N2O production by denitrification
(c) Net
Denitrification is the stepwi reduction of nitrate to nitrite, NO, N2O and N2. Denitrification is part of the bioenergetic apparatus of the bacterial or fungal cell, where the N oxyanions nitrate and nitrite and the gaous N oxides NO and N2O rve in lieu of dioxygen (O2) as terminal acceptors for electron transport phosphorylation28. Denitrification as a heterotrophic process is depending on the availability of labile C substrates. However, the actual C regulation of denitrification, and in particular the significance of plant-derived root exudates as drivers of rhizosphere denitrification, is currently not yet understood1. On the one hand, denitrification is a major source of N2O, as it is an obligatory intermediate during the reduction process or even the end product under certain environmental conditions, or due to the lack of the gene encoding the N2O reducta in some microorganisms29. On the other hand, the last step of denitrification, catalyzed by the enzyme nitrous oxide reducta, i
2012专四真题s reducing N2O to inert N2. If gross N2O consumption is exceeding gross N2O production, denitrification can also act as a net consumption process for soil N2O30,31. The role of N2O reduction in controlling net soil surface emission or uptake of N2O may be esntial32, but–
chris evertdue to the enormous methodological difficulties in the quantification of N2 production - has not yet been sufficiently investigated33.
The ability to denitrify is widespread among bacteria, fungi (including ectomycorrhiza)34 and other eukaryotes1,7 and it is believed for soil archaea. In contrast to bacterial denitrification, the major end product of fungal denitrification may be N2O, since the fungal denitrification chain is often truncated ending with the formation of N2O29. The contribution of archaea to N2O production in soils remains to be directly proven.
Denitrification is generally thought to be an anaerobic process, while the N2O reducta may be most nsitive to oxygen1. However, after re-exposure to oxygen following anaerobic conditions all denitrification enzymes except the N2O reducta may remain active35,36. Furthermore, experiments with pure cultures revealed the occurrence of aerobic denitrification involving production of both N2O and N21,37,38. Currently, the actual relevance of aerobic denitrification in soils remains
mango是什么意思completely unclear1,37, although the occurrence of persistent in situ net N2O consumption under low soil moisture conditions may be explained by N2O reduction during aerobic denitrification39,40.
(d) Coupled nitrification-denitrification and fertilizer denitrification
In contrast to nitrifier denitrification, the term “coupled nitrification-denitrification” describes two distinct process, i.e. nitrification and denitrification, carried out by two distinct groups of microorganisms, i.e. nitrifiers and denitrifiers, in a concerted action2. This means that nitrite or nitrate produced by nitrifiers in aerobic microhabitats is rapidly denitrified in clo-by coexisting anaerobic, or rapidly fluctuating, microhabitats, while the N2O formation pathways correspond to the ones described for nitrification and denitrification above. Conquently, coupled nitrification-denitrification and associated N2O production is of high significance in environments where anaerobic and aerobic conditions co-exist at small scales, e.g. in hydromorphic soils or on the edge of cracks in the soil2.
stephenhawkingIn contrast to coupled nitrification-denitrification, the denitrification of soil nitrate, which was added externally to soil or significantly dislocated along hydrological pathways utilizes a N substrate not of immediate microbial origin. For this mechanism the term “fertilizer denitrification” was introduced26 and fertilizer denitrification may predominate as a N2O source over nitrifier denitrification when soil
moisture is at high levels26. Distinguishing fertilizer denitrification from coupled nitrification-denitrification and from nitrifier-denitrification has implications for the methodologies to be chon to investigate denitrification. The 15N gas flux method to determine denitrification gas products18,26,27 is usually bad on the application and tracing of 15N-enriched nitrate, and thus is an approach to directly measure fertilizer denitrification only. In contrast, the direct measurement of N2 and N2O in an (almost) N2-free helium/oxygen environment provides data on N gas formation from all contributing process39.
(e) N2O formation by co-denitrification
Nitrous oxide can be formed via a microbially mediated N-nitrosation reaction by one N atom of NO originating from denitrification and one N atom from a co-substrate, e.g. ammonia, hydroxylamine or a range of monomeric organic N compounds (Figure 2 main text)37,40. Since this process simultaneously occurs with denitrification, and is at least partially conducted by
similar or the same microorganisms40, it was termed “co-denitrification”. The potential N2O formation of co-denitrification (2 mol N2O per 2 mol nitrate) exceeds that of conventional denitrification (1 mol N2O per 2 mol nitrate) by a factor of two. Similar to denitrification, the ability to
co-denitrify occurs in bacteria and fungi40, but so far it has not been shown for archaea. There is evidence, that co-denitrification is significant not only in pure cultures, but also in soils42,43. Despite the theory that co-denitrification is a common process in terrestrial environments40, the actual relevance of this process as a source within the global N2O budget remains uncertain37.
(f) N2O production by dissimilatory nitrate reduction to ammonium (DNRA) or强调的意思
nitrate ammonification
establishedDNRA is the dissimilatory reduction of nitrate to nitrite and ammonium. Thus, DNRA is a pathway of internal N cycling in ecosystems similar to microbial immobilization and remineralization, leading to nitrogen retention and nutrient conrvation44. DNRA is a strictly anaerobic process, conducted by facultative and obligate anaerobic microorganisms, which occur in a wide range of environments1. In contrast to earlier assumptions, that DNRA is mainly restricted to wetland ecosystems, recent evidence suggests that it can also be significanct in a range of upland soils15,44,45. The formation of nitrous oxide within DNRA is a biotic process and occurs due to reduction of nitrite to N2O by the bacteria conducting DNRA1,45. The quantity of N2O produced during DNRA varies with C-to-N ratio, and is not correlated with the production of ammonium46. The biochemical regulation of the relative
proudestamount of N2O evolution during DNRA is not yet understood1, and an approach has still to be developed for direct quantification of DNRA-N2O in situ.
(g) ANAMMOX and biological dinitrogen fixation
Bad on our current understanding the microbial process of anaerobic ammonia oxidation (ANAMMOX), i.e. the conversion of nitrite and ammonium directly into dinitrogen gas, and biological dinitrogen fixation are not involved in N2O formation.
(h) Denitrification and biological N2 fixation as N2O consumption process
masterThe most relevant process of N2O consumption is the reduction of N2O to N2 during denitrification47. This is occurring when genetic configuration or environmental conditions facilitate denitrifiers to express the full chain of enzymes including the final enzyme N2O reducta, catalyzing N2O reduction to N2. Denitrifiers may u exogenous N2O too. Such external N2O may originate from diffusion of N2O from sites of net N2O production in deeper soil layers to the soil surface32,40. There is experimental evidence that up to 2/3 of the produced N2O may be removed from the soil atmosphere by its reduction to N2 while diffusing from deeper soil layers to the topsoil48,49. Thus, N2O consumption may account for the largest part of N2O produced, with estima
翻跟斗亲亲tes ranging from 30-80%49,50,51. Hence, a better understanding of N2O reduction to N2 in soils is of outstanding importance for the quantification of both net N2O relea to the atmosphere and for an understanding of N2 production and emission from terrestrial systems. The N2O reduction to N2 currently remains the largest uncertainty of the N cycle at all scales48,52.
Since veral decades it is known that biological dinitrogen fixation (BNF) can u N2O as substrate53, thereby reducing N2O either directly or via N2 as intermediate to NH347. However, magnitudes of N2O consumption due to BNF are unclear and since BNF varies significantly between different ecosystems54 the same can be assumed for N2O consumption by BNF, though this assumption has not further been explored so far.
(i) N2O production associated with plant N metabolism
There is evidence that N2O production is correlated with plant leaf nitrate assimilation activity. Bad on experiments with stable isotopes Smart and Bloom55 showed in vitro production of N2O by both intact chloroplasts and nitrite reducta, but not by nitrate reducta. This indicated that N2O is produced by leaves during photoassimilation of NO2 in the chloroplast.
