Agriculture Reference
In-Depth Information
11.3.3 ghg e
miSSionS
Decomposition of SOM in cropping systems releases nutrients for crop production,
but also returns carbon dioxide (CO
2
) to the atmosphere. At the same time, some
of the C from decomposition of organic matter is retained in soil aggregates and
adsorbed to soil colloids, some of which will later be eroded (Lal 2004). Addition of
agroforestry species has the potential to either enhance or reduce soil C storage (Kim
2012) and GHG emissions. Thus, the study of GHG emissions is critical to describ-
ing the trade-offs between smallholder and ecosystem benefits from agroforestry.
11.3.3.1 Soil Carbon Dioxide, Methane, and Nitrous Oxide Emissions
Carbon dioxide (CO
2
) is the dominant pathway of C loss in most terrestrial ecosys-
tems, as well as the most important GHG in the atmosphere (Forster et al. 2007). Soil
CO
2
is produced primarily by both heterotrophic (i.e., decomposer organisms) and
autotrophic activity (i.e., living roots and mycorrhizae) (Raich and Schlesinger 1992;
Schlesinger and Andrews 2000). Soil CO
2
efflux amounts to 75-80 Pg of CO
2
per
year globally (Raich and Potter 1995; Raich et al. 2002) and made up 20%-40% of the
total annual input of CO
2
into the atmosphere in the 1990s (Raich and Schlesinger
1992; Schimel 1995). Soil temperature, soil moisture, soil C content, litter quality,
root dynamics, and plant photosynthesis or growth are known control factors for soil
CO
2
flux (e.g., Raich and Schlesinger 1992; Rustad and Fernandez 1998; Davidson et
al. 2000; Vargas and Allen 2008).
Methane (CH
4
) has the second-largest radiative forcing of the long-lived GHGs
after CO
2
(Forster et al. 2007). The net CH
4
flux is the result of the balance between
the two offsetting processes of methanogenesis (microbial production under anaero-
bic conditions) and methanotrophy (microbial consumption) (Dutaur and Verchot
2007). Methanogenesis occurs via the anaerobic degradation of organic matter by
methanogenic archaea within the archaeal phylum Euryarchaeota
(Thauer 1998).
Methanotrophy occurs through methanotrophs metabolizing CH
4
as their source of
C and energy (Hanson and Hanson 1996). In anoxic soils, emergent vegetation also
influences CH
4
flux to the atmosphere, as plants enable oxygen transport to the rhi-
zosphere, through aerenchymateous tissue, and through the production of labile sub-
strates via root exudation (Joabsson et al. 1999). In general, CH
4
production rates are
controlled by the availability of suitable substrates, alternative electron acceptors for
competing redox reactions (i.e., sulfate reduction), the nutritional status of the eco-
system (i.e., bog vs. fen), water table position or soil moisture content, temperature,
and soil salinity (Hanson and Hanson 1996; Dutaur and Verchot 2007).
Atmospheric nitrous oxide (N
2
O) contributes to both the greenhouse effect (Wang
et al. 1976) and ozone layer depletion (Crutzen 1970). Nitrous oxide has a relatively
high global warming potential (i.e., 298 times greater than CO
2
in a 100-year time
horizon; Intergovernmental Panel on Climate Change [IPCC] 2006; Forster et al.
2007) and agricultural soils provide 3.5 Tg N
2
O-N year
−1
of total anthropogenic
N
2
O emissions (5.7 Tg N
2
O-N year
−1
) (IPCC 2006). Use of N fertilizers and ani-
mal manure are the main anthropogenic N
2
O sources, which together account for
roughly 24% of total annual emissions (Bouwman 1996; Forster et al. 2007). The
main processes that produce N
2
O in soils are nitrification, the stepwise oxidation of
