Agriculture Reference
In-Depth Information
Soils have been shown to both produce and consume CH
4
(Topp and Pattey 1997;
Le Mer and Roger 2001). It is well known that forest soils are the most active sink
for CH
4
, followed by grass lands and cultivated soils, and that the CH
4
uptake poten-
tial of many upland soils is reduced by cultivation and application of ammonium-N
fertilizer (e.g., Topp and Pattey 1997; Le Mer and Roger 2001; Dutaur and Verchot
2007). These results suggest that synthetic N fertilizers used in conventional crop-
ping systems may increase CH
4
emissions in sub-Saharan Africa. By contrast, the
intercropping system uses less or no synthetic N fertilizer and may have the potential
to mitigate CH
4
emissions. Overall, GHG emissions from the agroforestry systems
either reduce benefits gained from enhanced soil C sequestration or add new benefits
from reduced N
2
O and CH
4
emissions. However, there is little data on GHG emis-
sions from agroforestry systems in eastern and southern Africa.
11.3.3.3 Suggested Future Studies
GHG emission in agroforestry has not been well understood, although it is recog-
nized that agroforestry can be a source of GHG emissions or mitigate GHG emis-
sions. First, studies quantifying the source and the mitigation capacity of GHG in
various agroforestry systems in eastern and southern Africa are urgently needed.
Especially, careful comparison of GHG emissions in agroforestry with monocrop-
ping will provide a better understanding of the contribution of agroforestry to miti-
gating GHG emissions. It is worth noting that field measurements of GHG emissions
in eastern and southern Africa should accurately observe peak GHG emissions fol-
lowing rewetting of dry soils (e.g., start/onset of the rainy season), since several
reports indicate that peak GHG emissions occur following soil rewetting in the areas
(e.g., Dick et al. 2006; Makumba et al. 2007), and these peak emissions may sig-
nificantly affect annual GHG budgets as has been shown in other areas (e.g., Lee et
al. 2004; Goldberg et al. 2010; Kim et al. 2012). These peaks could be measured by
using an automated measurement system (e.g., Wolf et al. 2010; Kim et al. 2010a) or
by increasing the frequency of manual chamber measurements during these periods
(e.g., Beare et al. 2009; Kim et al. 2010b). An area of significant promise involves
combining microbial community analyses and/or stable isotope techniques with flux
measurements. Models are promising tools for evaluating the importance of GHG
emissions in agroforestry systems. Initially, simple linear regressions and empirical
models can be developed on the basis of the relationships between environmental
factors, including soil moisture and/or soil temperature and soil GHG fluxes. With
improved understanding of C and N biogeochemistry and hydrological dynamics in
agroforestry systems, process-based models can be developed to more accurately
simulate GHG flux. It is critical to enhance the communication between field scien-
tists and the modeling community, as models can be used to generate hypotheses to
be tested in the field and laboratory (Kim et al. 2012).
11.4 CONCLUSIONS, CHALLENGES, AND FUTURE NEEDS
Including agroforestry species in smallholder cropping systems has well-documented
benefits in reduced land degradation and increased food production. Agroforestry also
has the potential to increase carbon storage in soils and aboveground wood biomass.
