Agriculture Reference
In-Depth Information
water pH, soil and air temperature, cation-exchange capacity, H
+
-buffering capacity, N source, wind
speed, humidity, and NH
3
concentrations all affect the rate of NH
3
volatilization (Harper et all
1983; Boswell et al., 1985; Bouwmeester et al., 1985).
2.4 LOSSES THROUGH DENITRIFICATION
Denitrification is a microbial process of converting
NO
3
−
into gases N
2
O or N
2
. This process occurs
when soil is water saturated and microrganisms no longer have ready access to O
2
. Most microor-
ganisms depend on O
2
for energy conversion by utilizing O
2
as the last electron acceptor, thereby
converting O
2
into CO
2
. However, certain microorganisms can also utilize
NO
3
−
in the same way as
O
2
in anaerobic conditions (due to water saturation), thereby utilizing O
2
as the last electron acceptor
when converting
NO
3
−
into N
2
O or N
2
(Gerik et al., 1998).
Denitrification is a major loss of N from soil-plant system and it is mainly an anaerobic bacterial
respiration. Losses from irrigated soils in California ranged from 95 to 233 kg N ha
−1
year
−1
(Ryden
and Lund, 1980). The denitrification occurs in the following reductive ways:
NO nitrate
−
(
)
⇒
O nitrite
−
(
)
⇒
NO nitric oxide
(
)
⇒
NO nitrou
2
(
s oxide
)
⇒
N dinitrogen
2
(
)
3
2
N
2
O is produced from microbial transformation of N in soils and manures and is often enhanced
where the available N exceeds plant requirements, especially under wet conditions (Mosier et al.,
1991, 2006; Phillips, 2007; Buchkina et al., 2010; Collins et al., 2011). Manure applications may
cause relatively high N
2
O emissions when the soil contains NO
3
and the decomposition of organic
C in manure enhances denitrification or nitrification (Moller and Stinner, 2009; Sistani et al., 2010;
Collins et al., 2011).
Most denitrifying bacteria exist in the topsoil (0-30 cm) with the number exponentially decreas-
ing down to 120-150 cm (Parkin and Meisinger, 1989). Denitrification is influenced by several
factors such as soil pH, temperature, organic C supply, nitrate concentration, aeration, and water
status (Aulakh et al., 1992). Owing to the influence of several physical and chemical factors, the
exact quantity of N loss due to denitrification is difficult. However, Aulakh et al. (1992) reported
that, overall, N losses due to denitrification might be about 30% in an agroecosystem. In addition,
the estimation of global denitrification losses ranges from 83 Tg year
−1
(Stevenson, 1982) to 390
Tg yea r
−1
(Hauck and Tanji, 1982; Aulakh et al., 1992). A large part of N is ultimately returned to
the atmosphere through biological denitrification, thereby completing the cycle (Stevenson, 1982).
Denitrification usually occurs in soil high in organic matter, under extended periods of water-
logged conditions and as temperature rise. While N
2
is an inert gas that poses no known environ-
mental risk, N
2
O is one of the GHGs that contribute to the destruction of the Earth's protective ozone
layer (Fageria and Gheyi, 1999). Measured N
2
O emissions from fertilized cropland fall within a
wide range: 0.7-51.8 mg N
2
O-N m
−2
d
−1
(Sehy et al., 2003; Rochette et al., 2004; Venterea et al.,
2005; Drury et al., 2006; Molodovskaya et al., 2012). Among different vegetation covers, denitrifi-
cation was significantly higher in grasslands than in either hardwood or pine forest areas (Lowrance
et al., 1995).
About 6% of the total GHG emissions in the United States is contributed by agricultural activi-
ties (Greenhouse Gas Working Group, 2010; USEPA, 2011). The amount of CO
2
and N
2
O emissions
contributed by agriculture account for about 25% and 70%, respectively, of the total anthropogenic
emissions (Cole et al., 1997). Fossil fuel consumption, land conversion into cropland, lime applica-
tion, and N fertilization are major sources of agriculture CO
2
emissions while soil management
practices contribute about 92% of the total N
2
O emissions (USEPA, 2011).
A major part of the denitrification occurs in the surface soil. However, a small part also occurs
in the subsoil. The predominant N
2
O production process in the subsoil is the denitrification of
NO
3
−
and it has been suggested that, in some cases, rates up to 60-70 kg N ha
−1
year
−1
may be possible
