Environmental Engineering Reference
In-Depth Information
and to give supplies of Ca
2
+
and Mg
2
+
for plant uptake (Robertson and Grace
2004
). CaCO
3
can react with soil Al
3
+
to form carbonic acid, raising the soil pH
by the following reaction (Robertson and Grace
2004
):
2Al
3
+
+
3CaCO
3
+
6H
2
O
→
3Ca
2
+
+
2Al
(
OH
)
3
+
3H
2
CO
3
(3.1)
Similarly, carbonic acid formed in the presence of CO
2
from root and microbial
respiration reacts with solid carbonates [ca. CaMg(CO
3
)
2
] to produce bicarbonate
by the following reaction (Eq.
3.2
) (Robertson and Grace
2004
):
CaMg
(
CO
3
)
2
+
2H
2
CO
3
→
Ca
2
+
+
Mg
2
+
+
4 HCO
3
−
(3.2)
A strong mineral acid such as nitric acid (HNO
3
) can react with carbonates
[e.g. CaMg(CO
3
)
2
] to produce CO
2
by the following reaction (Robertson and
Grace
2004
):
CaMg
(
CO
3
)
2
+
4 HNO
3
→
Ca
2
+
+
Mg
2
+
+
4 NO
3
−
+
2 CO
2
+
2H
2
O
(3.3)
Nitric acid is formed by nitrifying bacteria in most soils, including acid tropical
soils (Robertson
1982
; Sollins et al.
1988
).
(iii) Calcium-saturated groundwater can react with soil HCO
3
−
to produce CO
2
by the following reaction (Robertson and Grace
2004
; Schlesinger
1999
):
Ca
2
+
+
2 HCO
3
−
→
CaCO
3
+
H
2
O
+
CO
2
(3.4)
Carbonate reactions also occur when calcium-saturated groundwater is sprayed
on calcareous surface soils (Schlesinger
1999
). In arid regions groundwater often
contains as much as 1 % Ca and CO
2
(Robertson and Grace
2004
).
CH
4
has a microbial origin from natural (e.g. wetlands) and human-influencewd
sources, such as agricultural activities (rice and crops cultivation), enteric fermen-
tation, animal wastes and landfills (Mosier et al.
1991
,
1998
,
2004
; Robertson and
Grace
2004
; Smith et al.
2008
; IPCC
2001
; Watson et al.
1992
; Subak et al.
1993
;
Zuidema et al.
1994
; Crutzen et al.
1986
; Bingemer and Crutzen
1987
; Cicerone
and Oremiand
1988
; Robertson et al.
2000
). Methane is produced when organic
materials are decomposed in oxygen-deprived conditions, including fermentative
digestion by ruminant livestock, stored manures and rice grown under flooding
(Mosier et al.
1998
). A recent study estimates that agriculture accounts for 52 % of
the global anthropogenic CH
4
emissions (Smith et al.
2008
).
N
2
O emission by agricultural activities in soil is accounted for by microbial
nitrification, denitrification and chemo-denitrification, especially under wet condi-
tions. N
2
O is also produced by the microbial transformation of nitrogen in soil and
manure (IPCC
2007a
; Robertson and Grace
2004
; Smith et al.
2008
; Kreileman
and Bouwman
1994
; Mosier et al.
1989
,
1991
; Freney
1997
; Tsuruta et al.
1997
;
Stevens and Laughlin
1998
; Robertson et al.
2000
; Cavigelli and Robertson
2000
;
Xing et al.
2002
; Mahimairaja et al.
1994
; Smith and Conen
2004
; Oenema et al.
2005
). Natural sources of N
2
O have been estimated to be approximately 10 TgN/
yr in 1990. Soils account for about 65 % of the sources, oceans for about 30 %
(IPCC
2001
). It is estimated that agriculture accounts for 84 % of the global
anthropogenic N
2
O emissions (Smith et al.
2008
).
