Tải bản đầy đủ - 0 (trang)
1 Biological Nitrification and Denitrification—Physical Factors and Biological Processes

1 Biological Nitrification and Denitrification—Physical Factors and Biological Processes

Tải bản đầy đủ - 0trang

Nitrogenous Gas Emissions from Soils and Greenhouse Gas Effects



45



a minor role. The initial step of nitrification, ammonia oxidation, is performed by both bacteria (AOB) and archaea (AOA). Differences in the specific conditions under which each of these two groups predominates, as well

as differences in the catalyzing enzymes involved in the reaction, are thought

to differ, affecting the amounts of N2O produced during the conversion of

NH3 to NOÀ

3 . Nitrifiers are physiologically diverse leading to a range of soil

environmental conditions under which they can function and produce N2O

(Yao et al., 2013; Prosser and Nicol, 2012; Taylor et al., 2012).

In denitrification, NOÀ

3 is reduced in steps to N2 and, in the process, NO

and N2O can be formed as intermediates and released to the atmosphere.

The stepwise reactions are carried out by denitrifiers, predominantly heterotrophic microorganisms that are facultative anaerobes (Figure 3).

Capacity to perform this process is spread broadly across many taxa of

bacteria with as many as 50% of cultured phyla-containing organisms

capable of performing at least some steps of denitrification pathway (Prosser,

2007). As facultative anaerobes they cope with suboxic or anoxic conditions

by using NOÀ

3 in place of O2 as an electron acceptor. The sequence of

reactions in denitrification proceeds via the formation of NO, but NO is

usually not emitted from wet, anaerobic soils (Kool et al., 2010). The ratio

of NO-N/N2O-N decreases sharply with increasing water-filled pore space,

apparently because NO is more rapidly consumed than N2O by denitrifying

microorganisms (Firestone and Davidson, 1989).

Although produced by both nitrifiers and denitrifiers, N2O is also produced in the process of nitrifier-denitrification, in which NHÀ

3 is oxidized

to NO2 which in turn is reduced to N2O and N2 (Figure 4). This process

is carried out by one group of microorganisms, autotrophic NH3-oxidizing

bacteria, under decreasing O2 concentration and can be an important source

of N2O (Wrage et al., 2001). In contrast, ammonia-oxidizing archaea are not



Figure 3 Stepwise reactions involved in denitrification; showing enzymes involved in

each step.



46



Ed Gregorich et al.



Figure 4 Transformations of mineral N in soil that lead to emission of N2O and N2 via

different pathways. Adapted from Wrage et al. (2001).



believed to perform nitrifier-dentirification (Steiglmeier et al., 2014), limiting

their overall contribution to N2O production. Nitrifier-denitrification thus

differs from coupled nitrification-denitrification, in which various groups of

coexisting microorganisms transform NHÀ

3 ultimately to N2 (Wrage et al.,

2001). In some soils (e.g., nutrient-poor, sandy soil) nitrifier-denitrification

was a major contributor (up to 25e50%) to N2O emissions when soil water

content conditions are less than optimal for heterotrophic denitrification

(Kool et al., 2011; Venterea, 2007).

The O2 concentration in the soil matrix varies to the extent that there are

adjacent pockets of aerobic and anaerobic conditions. The close proximity

of these pockets can result in a coupling of nitrification and denitrification

processes, i.e., the production of N2O from nitrification occurring in aerobic

zones and N2O produced by denitrification occurring in the anaerobic

zones or by depletion of O2 by oxidation of NHỵ

4 followed by the anaerobic



reduction of NO3 . But the N2O may not be detected at the soil surface,

because N2O produced in either of these zones may diffuse to a welloxygenated zone where it can be reduced further to N2. The opposite

may also occur: the N2O produced within a saturated anoxic aggregate

can be quickly reduced further to N2. Thus the production and emission

of N gases derived from the processes like nitrification, denitrification,

and nitrifier-denitrification, depend heavily on the structure and water content of the soil.

Dissimilatory reduction of nitrate to ammonia (DRNA) occurs through

À

intermediary NOÀ

2 . The reduction of NO2 may produce N2O, but DRNA

is often overlooked in process-scale prediction of N2O emissions (Baggs,



Nitrogenous Gas Emissions from Soils and Greenhouse Gas Effects



47



2010). Although DRNA is usually deemed an important process in tropical

systems (Rϋtting et al., 2008), where C is abundant compared to N and conditions are highly reducing, recent information suggests that it may occur

also in temperate arable soils (Schmidt et al., 2011). Both DRNA and denitrification are promoted by anaerobic conditions and C availability, but

DRNA conserves N in the system by producing NHỵ

4 rather than N2 which

is lost to the atmosphere. Therefore DRNA is particularly important in

N-limited systems.

Changes in microbial biomass can affect N2O emissions by increasing

the abundance of N2O producers and consumers (i.e., the potential for

N2O emission) and conversely by increasing the capacity of organisms

that do not produce N2O to immobilize substrates for nitrification and

denitrification (C and N). A change in community composition can also

affect N2O emissions because individuals within these functional groups

can differ physiologically and thereby affect the amount of N2O produced

during transformations (Webster et al., 2005; Prosser and Nicol, 2012). In

the long-term Broadbalk Wheat Experiment, Clark et al. (2012) observed

that the relationship between denitrification fluxes and denitrifier

abundance or community composition (nirK, nirS, nosZ) was influenced

by management through links with soil C and N. Thus, the interaction

of management, soil type, and environment has complex effects on the

dynamics of N2O-producing organisms over wide-ranging scales of time

and space that ultimately lead to unpredictable differences in N2O

production.



3.2 Conceptual Models: How Factors, Processes, and Levels

Regulate N Gas Emission from Soil Hole-In-Pipe Model

The hole-in-the-pipe model (Firestone and Davidson, 1989) uses an analogy of a leaky pipe in which NO and N2O are emitted (Figure 5). The

rate of flow of N through the pipes is analogous to the rates of nitrification

and denitrification. Several factors regulate the flow rate of N through the

pipes: the amount of mineral soil N, soil temperature, and soil water content. The NO and N2O gases leak out of holes in the pipes; the size of which

is determined primarily by the water content of the soil. Other factors also

control the proportions of NO and N2O produced; these include mineral N

concentrations, pH, and available C. Another set of factors controls the consumption of NO and N2O within the soil pore space; these include diffusion

and mass flow, which in turn are controlled by soil structure, air-filled pore

space, and temperature.



48



Ed Gregorich et al.



Figure 5 Diagram of hole-in-the-pipe conceptual model. Adapted from Davidson

(2000).



3.2.1 N Gas Emission as Function of Soil Pore Space Properties

The production and emission of N gases from soil are primarily the result of

biological activity, but that N-gas generating activity can be regulated to a

large extent by soil structural properties (Ball, 2013). The production and

emission of N2O from soil are regulated by three main factors: substrate

availability, aeration status, and temperature (Smith, 1980). These three

factors interact and are affected by the timing of weather events and soil

physical conditions.

Linn and Doran (1984) proposed a simple model, which illustrates the

extent to which water content, expressed as water-filled pore space, controls

the production of N2O and CO2. In their model, the critical point at which

dominant aerobic/anaerobic processes begin to exert control on the production of these gases is at 60% water-filled pore space, which approximates

field capacity for most soils with a loam texture. Field capacity is defined

as the soil water content after excess water has been drained from the soil;

at this water content the soil macropores have drained and are air-filled

but the micropores are still water-filled. This breakpoint of 60% water-filled

pore space is assumed to be the point of transition at which oxidative and

reductive processes are active.

This model is useful for a general understanding of the effects of soil

structure, as characterized by water-filled pore space, on N gas emissions



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

1 Biological Nitrification and Denitrification—Physical Factors and Biological Processes

Tải bản đầy đủ ngay(0 tr)

×