Agriculture Reference
In-Depth Information
NH
3
to nitrite (NO
−
) and then to nitrate (NO
−
) (Kowalchuk and Stephen 2001), and
denitrification, the stepwise reduction of NO
−
to NO
−
, NO, N
2
O, and ultimately N
2
.
In denitrification, facultative anaerobic bacteria use NO
−
as an electron acceptor in
the respiration of organic material under low oxygen (O
2
) conditions (Knowles 1982).
In nitrifier denitrification, which is carried out by autotrophic NH
3
-oxidizing bacte-
ria, NH
3
is oxidized to nitrite NO
−
, followed by the reduction of NO
−
to nitric oxide
NO, N
2
O, and molecular nitrogen (N
2
) (Wrage et al. 2001).
11.3.3.2 Emissions of GHGs in Agroforestry in Eastern and Southern Africa
Nitrogen-fixing tree and crop intercropping systems can be a sustainable agrofor-
estry practice in eastern and southern Africa (Makumba et al. 2006; Akinnifesi et al.
2010), and they can also contribute to mitigation of climate change through enhanced
soil C sequestration. Makumba et al. (2007) reported soil C and soil CO
2
emissions
in a 7-year-old
Gliricidia
and maize intercropping system, and a sole maize cropping
site in southern Malawi. They estimated that while soil C in the intercropping system
was about double that in the sole maize cropping site, soil CO
2
emissions from the
intercropping system were up to three times higher. The increased soil CO
2
emis-
sions in the intercropping system could be due to increased SOM and enhanced root
respiration in extended root systems from the intercropping system (Makumba et al.
2007). Using the data provided in Makumba et al. (2007), a C loss as soil CO
2
emis-
sions (51.2 ± 0.4 Mg C ha
−1
) was estimated, amounting to 67.4% of the sequestered
soil C (76 ± 8.6 Mg C ha
−1
in 0-2 m soil depth) for the first 7 years in the intercrop-
ping system (Kim 2012). These results suggest the need to account for the C loss as
soil CO
2
emissions in assessing the overall impact of the agroforestry system on soil
C dynamics.
Maize yields in the
Gliricidia
and maize intercropping systems without addi-
tional synthetic N fertilizer input were similar to the yields in the sole maize crop-
ping with 48 kg N ha
−1
year
−1
fertilizer applied in southern Malawi (Makumba et
al. 2006). These results support the premise that additional N is provided to the
crop through N fixation by
Gliricidia
(e.g., Makumba et al. 2006; Akinnifesi et al.
2010). These results also suggest that up to 48 kg N ha
−1
year
−1
of fertilizer could be
reduced in the intercropping system. Globally, 1% of applied N fertilizer converts to
N
2
O emission (IPCC 2006), and it was observed that 0.25%-4.1% of applied N fer-
tilizer converts to N
2
O emission in sub-Saharan Africa (Kim et al. 2012). Therefore,
the reduced N fertilizer use through the intercropping system may result in reduced
N
2
O emissions. In contrast, N
2
O emissions in the intercropping system may not be
lower than in the conventional cropping system where N fertilizer is applied. Soil
collected under N-fixing tree species produced significantly more N
2
O than soil
collected under non-N-fixing trees and N-fixing crop species in Senegal (Dick et
al. 2006). Nitrous oxide emissions from a maize field that previously had a 2-year
fallow of
A. angustissima
and
Sesbania
were significantly higher than those from
an unfertilized maize field in Zimbabwe (Chikowo 2004). Increased soil organic C
and N in the intercropping system can enhance the denitrification process, one of
the major processes that produce N
2
O gas in soil (e.g., Knowles 1982). It is there-
fore important to consider N
2
O emissions to better understand the contributions of
agroforestry to N
2
O dynamics.
