Pelster, D.E., Rufino, M.C., Rosenstock, T., Mango, J., Saiz, G., Diaz-Pines, E., Baldi, G., Butterbach-Bahl, K., 2015. Smallholder African farms in western Kenya have limited greenhouse gas fluxes. Biogeosciences Discuss. 12, 15301-15336.
Abstract
Few field studies examine greenhouse gas (GHG) emissions from African
agricultural systems resulting in high uncertainty for national
inventories. We provide here the most comprehensive study in Africa to
date, examining annual CO2, CH4 and N2O
emissions from 59 plots, across different vegetation types, field
types and land classes in western Kenya. The study area consists of
a lowland area (approximately 1200 m a.s.l.) rising
approximately 600 m to a highland plateau. Cumulative annual
fluxes ranged from 2.8 to 15.0 Mg CO2-C ha−1,
−6.0 to 2.4 kg CH4-C ha−1 and −0.1 to
1.8 kg N2O-N ha−1. Management intensity of the
plots did not result in differences in annual fluxes for the GHGs
measured (P = 0.46, 0.67 and 0.14 for CO2, N2O and
CH4 respectively). The similar emissions were likely related
to low fertilizer input rates (≤ 20 kg ha−1). Grazing
plots had the highest CO2 fluxes (P = 0.005); treed plots
were a larger CH4 sink than grazing plots (P = 0.05); while
N2O emissions were similar across vegetation types (P =
0.59). This case study is likely representative for low fertilizer
input, smallholder systems across sub-Saharan Africa, providing
critical data for estimating regional or continental GHG
inventories. Low crop yields, likely due to low inputs, resulted in
high (up to 67 g N2O-N kg−1 aboveground N uptake)
yield-scaled emissions. Improving crop production through
intensification of agricultural production (i.e. water and nutrient
management) may be an important tool to mitigate the impact of African
agriculture on climate change.
Borges et al. 2015. Globally significant greenhouse-gas emissions from African inland waters
Borges A V, Darchambeau F, Teodoru C R, Marwick T R, Tamooh F, Geeraert N, Omengo F O, Guerin F, Lambert T, Morana C, Okuku E and Bouillon S 2015 Globally significant greenhouse-gas emissions from African inland waters. Nature Geosci doi:10.1038/ngeo2486.
Carbon dioxide emissions to the atmosphere from inland waters—streams, rivers, lakes and reservoirs—are nearly equivalent to ocean and land sinks globally. Inland waters can be an important source of methane and nitrous oxide emissions as well, but emissions are poorly quantified, especially in Africa. Here we report dissolved carbon dioxide, methane and nitrous oxide concentrations from 12 rivers in sub-Saharan Africa, including seasonally resolved sampling at 39 sites, acquired between 2006 and 2014. Fluxes were calculated from published gas transfer velocities, and upscaled to the area of all sub-Saharan African rivers using available spatial data sets. Carbon dioxide-equivalent emissions from river channels alone were about 0.4 Pg carbon per year, equivalent to two-thirds of the overall net carbon land sink previously reported for Africa. Including emissions from wetlands of the Congo river increases the total carbon dioxide-equivalent greenhouse-gas emissions to about 0.9 Pg carbon per year, equivalent to about one quarter of the global ocean and terrestrial combined carbon sink. Riverine carbon dioxide and methane emissions increase with wetland extent and upland biomass. We therefore suggest that future changes in wetland and upland cover could strongly affect greenhouse-gas emissions from African inland waters.
Abstract
Carbon dioxide emissions to the atmosphere from inland waters—streams, rivers, lakes and reservoirs—are nearly equivalent to ocean and land sinks globally. Inland waters can be an important source of methane and nitrous oxide emissions as well, but emissions are poorly quantified, especially in Africa. Here we report dissolved carbon dioxide, methane and nitrous oxide concentrations from 12 rivers in sub-Saharan Africa, including seasonally resolved sampling at 39 sites, acquired between 2006 and 2014. Fluxes were calculated from published gas transfer velocities, and upscaled to the area of all sub-Saharan African rivers using available spatial data sets. Carbon dioxide-equivalent emissions from river channels alone were about 0.4 Pg carbon per year, equivalent to two-thirds of the overall net carbon land sink previously reported for Africa. Including emissions from wetlands of the Congo river increases the total carbon dioxide-equivalent greenhouse-gas emissions to about 0.9 Pg carbon per year, equivalent to about one quarter of the global ocean and terrestrial combined carbon sink. Riverine carbon dioxide and methane emissions increase with wetland extent and upland biomass. We therefore suggest that future changes in wetland and upland cover could strongly affect greenhouse-gas emissions from African inland waters.
Kimaro et al. 2015. Is conservation agriculture ‘climate-smart’for maize farmers in the highlands of Tanzania?
Kimaro, A. A., Mpanda,
M., Rioux, J., Aynekulu, E., Shaba, S., Thiong’o, M., ... &
Rosenstock, T. S. (2015). Is conservation agriculture ‘climate-smart’for
maize farmers in the highlands of Tanzania?. Nutrient Cycling in Agroecosystems, 1-12.
Abstract
Conservation
agriculture (CA) is promoted extensively to increase the productivity
and environmental sustainability of maize production systems across
sub-Saharan Africa and is often listed as a climate-smart agriculture
(CSA) practice.
However, the impacts of CA on food security,
resilience/adaptive capacity and climate change mitigation are
location-dependent and it is unknown whether CA can simultaneously
address CSA’s multiple objectives. Here we evaluate four variations of
CA: reduced tillage plus mulch (mulch), reduced tillage plus mulch and
leguminous cover crop (Lablab), reduced tillage plus mulch and
leguminous trees (CAWT), and reduced tillage plus mulch and nitrogen
fertilizer (CA + F)—for their effect on CSA-relevant outcomes in
highland Tanzania maize production. By comparison to conventional
practice in the region, intensification of maize production by Lablab,
CAWT, and CA + F significantly increases yields by 40, 89 and 77 %,
respectively. Likewise, rainfall use efficiency was highest in these
three treatments and significantly greater than conventional practices
in 7 of 12 comparisons. Seasonal and annual greenhouse gas fluxes were
similar across all treatments; however, yield-scaled global warming
potential (Mg CO2 eq Mg grain−1) was lower in CAWT
(2.1–3.1) and CA + F (1.9–2.3) than conventional practice (1.9–8.3),
averaging 62 and 68 % of the emission intensity of conventional
practice, respectively. The findings demonstrate that CA can deliver
benefits consistent with the objectives of CSA for farmers in this
region, particularly when soil nitrogen limitation is alleviated,
providing other constraints to adoption are removed.
http://link.springer.com/article/10.1007/s10705-015-9711-8
Goenster et al. 2015. Gaseous emissions and soil fertility of homegardens in the Nuba Mountains, Sudan
Goenster, S., Wiehle, M., Predotova, M., Gebauer, J., Ali, A. M., and Buerkert, A.: Gaseous emissions and soil fertility of homegardens in the nuba mountains, sudan, J. Plant Nutr. Soil Sci., 178, 413-424, 10.1002/jpln.201400292, 2015.
Abstract
Intensification
of homegardens in the Nuba Mountains may lead to increases in C and
nutrient losses from these small-scale land-use systems and potentially
threaten their sustainability. This study, therefore, aimed at
determining gaseous C and N fluxes from homegarden soils of different
soil moisture, temperature, and C and N status. Emissions of CO2, NH3, and N2O
from soils of two traditional and two intensified homegardens and an
uncultivated control were recorded bi-weekly during the rainy season in
2010. Flux rates were determined with a portable dynamic closed chamber
system consisting of a photo-acoustic multi-gas field monitor connected
to a PTFE coated chamber. Topsoil moisture and temperature were recorded
simultaneously to the gas measurements. Across all homegardens
emissions averaged 4,527 kg CO2-C ha−1, 22 kg NH3-N ha−1, and 11 kg N2O-N ha−1
for the observation period from June to December. Flux rates were
largely positively correlated with soil moisture and predominantly
negatively with soil temperature. Significant positive, but weak (rs < 0.34) correlations between increasing management intensity and emissions were noted for CO2-C. Similarly, morning emissions of NH3 and increasing management intensity were weakly correlated (rs
= 0.17). The relatively high gaseous C and N losses in the studied
homegardens call for effective management practices to secure the soil
organic C status of these traditional land-use systems.
Hickman et al. 2015. A potential tipping point in tropical agriculture: Avoiding rapid increases in nitrous oxide fluxes from agricultural intensification in kenya
Hickman, J.
E., Tully, K. L., Groffman, P. M., Diru, W., and Palm, C. A. C. J. G.: A
potential tipping point in tropical agriculture: Avoiding rapid increases in
nitrous oxide fluxes from agricultural intensification in Kenya, Journal of
Geophysical Research: Biogeosciences, doi: 10.1002/2015jg002913, 10.1002/2015jg002913,
2015.
Abstract
There are national and regional efforts aimed at increasing fertilizer use in sub-Saharan Africa, where nitrogen (N) inputs must be increased by an order of magnitude or more to reach recommended rates. Fertilizer inputs increase N availability and cycling rates and subsequently emissions of nitrous oxide (N2O), a powerful greenhouse gas and the primary catalyst of stratospheric ozone depletion. We established experimental maize (Zea mays L.) plots in western Kenya to quantify the relationship between N inputs and N2O emissions. Mean N2O emissions were marginally, but not significantly, better described by an exponential model relating emissions to N input rate in 2011; in 2012, an exponential relationship provided the best fit compared to linear and other nonlinear models. Most N2O fluxes occurred during the 30 days following the second fertilizer application. Estimates of fertilizer N lost as N2O annually were well below the 1% Intergovernmental Panel on Climate Change default emission factor, ranging from 0.07% to 0.11% in 2011 and from 0.01% to 0.09% in 2012. In both years, the largest impact on annual N2O emissions occurred when inputs increased from 100 to 150 kg N ha−1: fluxes increased from 203 to 294 g N2O-N ha−1 yr−1 in 2011 and from 168 to 254 kg N ha−1 in 2012. Our results suggest that exponential emission responses are present in tropical systems and that agricultural intensification in western Kenya may be managed for increasing crop yields without immediate large increases in N2O emissions if application rates remain at or below 100 kg N ha−1.Sommer et al. 2015. Nitrogen dynamics and nitrous oxide emissions in a long-term trial on integrated soil fertility management in western Kenya
Sommer, R., Mukalama, J., Kihara, J., Koala, S., Winowiecki, L., and Bossio, D.: Nitrogen dynamics and nitrous oxide emissions in a long-term trial on integrated soil fertility management in western kenya, Nutrient Cycling in Agroecosystems, 10.1007/s10705-015-9693-6, 2015.
Abstract
Integrated
soil fertility management (ISFM) is a concept that includes the
management of organic matter in smallholder farming systems for
sustainable intensification. To determine whether ISFM is also
eco-efficient, we measured and simulated nitrogen (N)-dynamics and
nitrous oxide (N2O) emissions in an ISFM long-term maize
trial in Western Kenya. The total annual N-balance averaged over
10.5 years was negative for all continuous maize treatments that
received only inorganic N-fertilizer. The N-balance was zero or positive
when maize was grown in rotation with the green manure cover crop, Tephrosia candid, and/or to which 4 Mg ha−1 season−1
farm yard manure (FYM) added. These results thus substantiate the
importance of organic matter management in tropical ecosystems. They
also underpin that mineral N-fertilizer application alone does not
guarantee agro-ecosystem sustainability, which should be considered in
fertilizer (subsidy) policies. Treatments that included Tephrosia and
FYM application emitted the largest amounts of N2O. Highest emissions (12.0 kg N2O–N ha−1) were simulated for the maize–Tephrosia rotation to which FYM and 30 kg ha−1 of mineral fertilizer N was added and 2 Mg ha−1
maize stovers retained. Such treatments had the highest N-emission
intensity. The slope of the linear regression equation describing the N2O
emission–N-input relationship of all considered treatments (0.023) was
twice as high as the IPCC-Tier-1 emission factor. Maize–Tephrosia
treatments had the highest seasonal maize yields. These were, however,
not high enough to compensate for the inclusion of Tephrosia into the
system as compared to growing maize continuously, compromising adoption
by smallholder farmers.
Valentini et al., 2014. A full greenhouse gases budget of Africa: synthesis, uncertainties, and vulnerabilities
Valentini, R., Arneth, A., Bombelli, A., Castaldi, S., Cazzolla Gatti, R., Chevallier, F., Ciais, P., Grieco, E., Hartmann, J., Henry, M., 2014. A full greenhouse gases budget of Africa: synthesis, uncertainties, and vulnerabilities. Biogeosciences 11, 381-407.
Abstract
This paper, developed under the framework of the
RECCAP initiative, aims at providing improved estimates of
the carbon and GHG (CO2, CH4 and N2O) balance of continental
Africa. The various components and processes of
the African carbon and GHG budget are considered, existing
data reviewed, and new data from different methodologies
(inventories, ecosystem flux measurements, models, and
atmospheric inversions) presented. Uncertainties are quantified
and current gaps and weaknesses in knowledge and monitoring
systems described in order to guide future requirements.
The majority of results agree that Africa is a small sink of carbon on
an annual scale, with an average value of −0.61±0.58 PgC yr−1.
Nevertheless, the emissions of CH4
and N2O may turn Africa into a net source of radiative forcing
in CO2 equivalent terms. At sub-regional level, there is
significant spatial variability in both sources and sinks, due
to the diversity of biomes represented and differences in the
degree of anthropic impacts. Southern Africa is the main
source region; while central Africa, with its evergreen tropical
forests, is the main sink. Emissions from land-use change
in Africa are significant (around 0.32±0.05 PgC yr−1), even
higher than the fossil fuel emissions: this is a unique feature
among all the continents. There could be significant carbon
losses from forest land even without deforestation, resulting
from the impact of selective logging. Fires play a significant
role in the African carbon cycle, with 1.03±0.22 PgC yr−1
of carbon emissions, and 90% originating in savannas and
dry woodlands. A large portion of the wild fire emissions are
compensated by CO2 uptake during the growing season, but
an uncertain fraction of the emission from wood harvested
for domestic use is not. Most of these fluxes have large interannual
variability, on the order of ±0.5 PgC yr−1 in standard
deviation, accounting for around 25% of the year-toyear
variation in the global carbon budget.
Despite the high uncertainty, the estimates provided in this
paper show the important role that Africa plays in the global carbon
cycle, both in terms of absolute contribution, and as a key source of
interannual variability.
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