Abstract
Abstract. Intercropping with legumes is an important component of climate-smart agriculture (CSA) in sub-Saharan Africa, but little is known about its effect on soil greenhouse gas (GHG) exchange. A field experiment was established at Hawassa in the Ethiopian rift valley, comparing nitrous oxide (N2O) and methane (CH4) fluxes in minerally fertilized maize (64 kg N ha−1) with and without Crotalaria (C. juncea) or lablab (L. purpureus) as intercrops over two growing seasons. To study the effect of intercropping time, intercrops were sown either 3 or 6 weeks after maize. The legumes were harvested at flowering, and half of the aboveground biomass was mulched. In the first season, cumulative N2O emissions were largest in 3-week lablab, with all other treatments being equal to or lower than the fertilized maize mono-crop. After reducing mineral N input to intercropped systems by 50 % in the second season, N2O emissions were comparable with the fully fertilized control. Maize-yield-scaled N2O emissions in the first season increased linearly with aboveground legume N yield (p=0.01), but not in the second season when early rains resulted in less legume biomass because of shading by maize. Growing-season N2O-N emission factors varied from 0.02 % to 0.25 % in 2015 and 0.11 % to 0.20 % in 2016 of the estimated total N input. Growing-season CH4 uptake ranged from 1.0 to 1.5 kg CH4-C ha−1, with no significant differences between treatments or years but setting off the N2O-associated emissions by up to 69 %. Our results suggest that leguminous intercrops may increase N2O emissions when developing large biomass in dry years but, when mulched, can replace part of the fertilizer N in normal years, thus supporting CSA goals while intensifying crop production in the region.
Highlights
With a rapidly increasing population and declining agricultural land in sub-Saharan Africa (SSA), increasing productivity per area is the only viable alternative for producing sufficient food and feed (Hickman et al, 2014a)
Emission fluxes were generally larger for the 3-week intercropping treatments; the 3-week Crotalaria treatment emitted N2O at rates of 1.7–34.3 and the 3-week maize–lablab emitted 1.9–62.7 μg N m−2 h−1, whereas the 6-week maize
The generally low emission rates in the 6-week lablab intercropping systems corresponded to poor growth of lablab due to shading by the maize plants
Summary
With a rapidly increasing population and declining agricultural land in sub-Saharan Africa (SSA), increasing productivity per area (intensification) is the only viable alternative for producing sufficient food and feed (Hickman et al, 2014a). Climate-smart agriculture (CSA) is an approach to transform agricultural practices in a changing climate with the triple objective of increasing agricultural productivity, building climate resilience and reducing greenhouse gas (GHG) emissions (Neufeldt et al, 2013). In 2016, mulching of the 3-week legumes was followed by rainfall, increasing the WFPS to 50 % (Fig. 1h) but without resulting in elevated N2O emission rates (Fig. 1f, g). Together, this suggests that the direct effect of mulching on N2O emissions is highly dependent on soil moisture and the amount of mulch and cannot be generalized, contrary to our hypothesis that legume intercrops would invariably increase N2O emissions
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