By Armwell Shumba, Regis Chikowo, Christian Thierfelder, Marc Corbeels, Johan Six, and Rémi Cardinael
The slow increase in grain production in sub-Saharan Africa (SSA) is largely the result of cropland expansion rather than an increase in crop yields, which have been stagnantly low (< 1.5 t ha-1). Sustainable intensification of crop production is therefore needed to feed a growing population whilst minimizing negative impacts on the environment, biodiversity, and climate. Full accounting of the net global warming potential (GWP) of management practices can provide a holistic approach for identifying cropping systems that promote sustainable agriculture intensification to ensure food security whilst mitigating climate change.
SSA is lying in the nexus of rapid population growth (UN, 2022), stagnantly low agricultural productivity and high vulnerability to climate change. Cropland intensification rather than expansion is needed to sustainably increase crop yields per unit area (Aramburu-Merlos et al., 2024; Falconnier et al., 2023) with a minimal negative environmental footprint (Zheng et al., 2023). Conservation agriculture (CA) has been widely proposed as one of the promising approaches to sustainable intensification of food production where it has been shown that CA can improve maize yields by 8.4% in SSA (Corbeels et al., 2020). However, the climate change mitigation potential of CA in terms of greenhouse gas (GHG) emissions and additional soil organic carbon (SOC) storage has not been fully explored in SSA (Corbeels et al., 2019; Li et al., 2023). Additionally, the respective contribution of each CA principle or their different combinations to GHG emissions and SOC storage remains understudied. Therefore, the aim of this research was to determine the net GWP of each of the respective CA principles and/or their different combinations. To evaluate the negative or positive climate impact of a cropping system, it is essential to quantify its net global warming potential (Six et al., 2004; Zheng et al., 2023) given as CO2 equivalents (CO2-eq). The net GWP provides an integrated index that allows for comparisons between different cropping systems and is done through accounting of GHG sources and sinks of a cropping system.
In addition, a measure of the intensity of GHG emissions associated with a cropping system is also critical (Zheng et al., 2023). This measure, the greenhouse gas intensity (GHGI), expresses GHG emissions per unit of output (emissions per unit of crop yield). Therefore, the objective of this paper is to share results on the potential climate change mitigation benefits from each CA principle or their combinations under low nitrogen (N) input in subhumid Zimbabwe.
Study description and methodology
This study was conducted at two long-term experimental sites established by CIMMYT in 2013 in Zimbabwe, on an Abruptic Lixisol at Domboshava Training Centre and a Xanthic Ferralsol at the University of Zimbabwe Farm (Fig. 1).
Six treatments replicated four times were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), no-tillage (NT), no-tillage with mulch (NTM), no-tillage with rotation (NTR), no tillage with mulch and rotation (NTMR). The main crop was maize (Zea mays L.) and treatments with rotation included cowpea (Vigna unguiculate L. Walp.). Nitrogen was split applied (basal and two top-dressings) at 58 kg N ha-1 yr-1 on maize rows. GHG samples were regularly collected using the static chamber method in the maize row and inter-row spaces (Fig. 2a) during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season (Shumba et al., 2023). SOC and bulk density were determined for samples taken from depths of 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75, and 75–100 cm from all treatments (Shumba et al., 2024). SOC stocks were calculated using the equivalent soil mass (ESM) approach to account for possible changes i soil bulk density between treatments (Ellert and Bettany, 1995).
Net GWP100 (CO2-equivalent; t CO2-eq ha−1 yr−1) at a 100-year time horizon was calculated for each treatment using Equation 1. The CT treatment was used as the reference treatment.
Equation 1: net GWP100 = ΔN2OGWP100 + ΔCH4GWP100–ΔSOCGWP100
The GWP100 for nitrous oxide (N2O), methane (CH4) and SOC were determined using Equations 2, 3 and 4. The net GWP100 for this study was calculated at plot scale hence the CO2-equivalent emissions associated with the manufacture and transportation of fertilizer was not included.
Equation 2: ΔN2OGWP100 = Δ t N2O-N ha-1 yr-1 x 44/28 x 265
Equation 3: ΔCH4GWP100 = Δ t CH4‑C ha-1 yr-1 x 44/12 x 28
Equation 4: ΔSOCGWP = Δ t SOC ha-1 yr-1 x 44/12
where 265 and 28 are GWP100 constants used to give the integrated radiative forcing of N2O and CH4, respectively, in terms of their CO2-equivalence at a 100-year time horizon (Myhre et al., 2013). ΔN2O, ΔCH4 and ΔSOC refers to the observed change of cumulative N2O and CH4 and SOC stocks compared to cumulative N2O and CH4 and SOC stocks from the reference treatment, CT. For CT, land was tilled using hand hoes, and fertilizer application, weeding and harvesting were all done manually as is typical practice on smallholder farms in Zimbabwe. Thus, the farming system represent very low indirect GHG emissions, hence they were not accounted, compared to mechanized and irrigated high-yielding maize systems in the developed world (e.g., Adviento-Borbe et al., 2007; Huang et al., 2013).
Lastly, the maize grain yield-GHGI (t CO2-eq t−1 yr−1) was calculated to determine the climatic impact of the maize systems by dividing the net GWP by the average maize grain yield (t maize grain ha−1 yr−1) for the 2019/20 and 2020/21 cropping seasons.
Cumulative soil N2O and CH4 emissions
There were no significant differences in cumulative N2O emissions between treatments at DTC (average of 2 years), cumulative N2O emissions ranged from 189-349 g N2O-N ha−1 yr−1 (Fig. 3). At UZF, NT in combination with rotation had significantly higher cumulative N2O emissions. Cumulative CH4 emissions were not different between treatments at both sites; however, DTC was a net emitter and UZF was a net sink of CH4 (Fig. 3).
SOC stocks
Results show that maize stover mulching under NT is the overarching factor explaining the increase of SOC in the topsoil (Fig. 4). The findings also highlight the importance of considering the entire soil profile where over 50% of the total SOC stocks are stored in the subsoil (30-100 cm).
Net Global Warming Potential (GWP) and Greenhouse Gas Intensity (GHGI)
No tillage alone had a positive GWP at DTC, suggesting a negative impact on climate change mitigation (Table 1). However, the combination of NT with at least one other CA component (mulching or rotation or both) where the net GWP is largely negative showed a positive climate benefit except for NTR at UZF. The greenhouse gas intensity (GHGI) followed a similar pattern as the GWP (Table 1). At DTC, NT emitted about 0.22 t CO2-eq t−1 of maize grain produced. In contrast, NT combined with rotation (NTR) showed the potential to capture between 0.57 to 0.65 t CO2-eq t−1 grain. At UZF, NTM had the highest global warming mitigation potential of -0.56 t CO2-eq t−1 grain yr−1 whilst NTR had a negative impact on global warming with a potential of releasing 0.07 t CO2-eq for every tonne of maize grain produced every year (Table 1).
Summary
Despite the high radiative forcing of N2O and CH4, their contributions to GWP were negligible compared to the benefits from additional SOC storage, resulting in an overall negative net GWP under NT in combination with at least one CA principle, especially with mulch addition. Moreover, the negative net GWP and GHGI of CA-based cropping systems (except NTR at UZF) were obtained despite an application of 58 kg N ha−1 yr−1. This fertilizer rate is more than three times the average rate used in SSA (Falconnier et al., 2023), and it suggests intensification of cropping systems in SSA to levels agreed at the recent Africa Fertilizer and Soil Health Summit in Nairobi (target of 50 kg N ha−1 yr−1) could be achieved with a limited impact on climate and with massive benefits on crop production. Provided some safeguards are put in place, this could also decrease pressure on natural ecosystems and reduce emissions related to land use change, an additional benefit on climate change mitigation not accounted for in this study. Our study clearly shows that field-based GHGs emissions were offset by additional SOC storage. Therefore, the climate benefit of CA in SSA is mainly driven by an increase in SOC stocks, and the trade-off with non-CO2 emissions is limited in this context of low N input systems. These findings obtained on two widespread soil types are also characteristic of smallholder farming systems in SSA, where CA practices with low fertilizer application are being promoted, such as the Pfumvudza/Intwasa program in Zimbabwe.
In brief, the study suggests that no-tillage systems especially with mulching, can bring some additional benefits in terms of climate change mitigation.
Mr. Shumba (e-mail: armwellshumba123@gmail.com) is with the Department of Plant Production Sciences and Technologies, University of Zimbabwe, Harare, Zimbabwe; French Agricultural Research Centre for International Development (CIRAD), UPR AIDA, Harare, Zimbabwe; and Fertilizer, Farm Feeds and Remedies Institute, Department of Research and Specialist Services, Ministry of Lands, Agriculture, Fisheries, Water and Rural Development, Harare, Zimbabwe. Dr. Chikowo is Professor, Department of Plant Production Sciences and Technologies, University of Zimbabwe; International Maize and Wheat Improvement Center (CIMMYT), Harare, Zimbabwe. Dr. Thierfelder is with CIMMYT, Harare, Zimbabwe. Dr. Corbeels is with AIDA, Univ Montpellier, CIRAD, Montpellier, France; and International Institute of Tropical Agriculture (IITA), Nairobi, Kenya. Dr. Six is with the Department of Environmental Systems Science, ETH Zurich, Zürich, Switzerland. Dr. Cardinael is with the Department of Plant Production Sciences and Technologies, University of Zimbabwe, Harare, Zimbabwe; CIRAD, UPR AIDA, Harare, Zimbabwe; and AIDA, Univ Montpellier, CIRAD, Montpellier, France.
Cite this article
Shumba, A., Chikowo, R., Thierfelder, C., Corbeels, M., Six, J., Cardinael, R. 2024. Soil Organic Carbon Storage, Nitrous Oxide Emission and Net Climate Benefit of Conservation Agriculture: Insights from Two Long-Term Experiments in Subhumid Zimbabwe, Growing Africa 3(1), 21-26. https://doi.org/10.55693/ga31.IMVF9429
REFERENCES
Adviento-Borbe, M.A.A., et al. 2007. Soil greenhouse gas fluxes and global warming potential in four high-yielding maize systems. Glob. Chang. Biol. 13, 1972–1988.
Aramburu-Merlos, F., et al. 2024. Adopting yield-improving practices to meet maize demand in Sub-Saharan Africa without cropland expansion. Nat. Commun. 15.
Corbeels, M., et al. 2019. The 4 per 1000 goal and soil carbon storage under agroforestry and conservation agriculture systems in sub-Saharan Africa. Soil Tillage Res. 188, 16–26.
Corbeels, M., et al. 2020. Limits of conservation agriculture to overcome low crop yields in sub-Saharan Africa. Nat. Food 1, 447–454.
Ellert, B.H., Bettany, J.R. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75, 529–538.
Falconnier, G.N., et al. 2023. The input reduction principle of agroecology is wrong when it comes to mineral fertilizer use in sub-Saharan Africa. Outlook Agric. Submitted.
Huang, T., et al. 2013. Net global warming potential and greenhouse gas intensity in a double-cropping cereal rotation as affected by nitrogen and straw management. Biogeosciences 10, 7897–7911.
Li, Y., et al. 2023. The role of conservation agriculture practices in mitigating N2O emissions: A meta-analysis. Agron. Sustain. Dev. 43.
Myhre, G., et al. 2013. Anthropogenic and natural radiative forcing, In: Stocker, T.F., et al. (Eds.), Climate Change 2013: The Physical Science Basis. Contribution of Working Group to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambrige and New York, pp. 659–740.
Shumba, A., et al. 2023. Long-term tillage, residue management and crop rotation impacts on N2O and CH4 emissions on two contrasting soils in sub-humid Zimbabwe. Agric. Ecosyst. Environ. 341, 1–17.
Shumba, A., et al. 2024. Mulch application as the overarching factor explaining increase in soil organic carbon stocks under conservation agriculture in two 8-year-old experiments in Zimbabwe. Soil 10, 151–165
Six, J., et al. 2004. The potential to mitigate global warming with no-tillage management is only realized when practised in the long term. Glob. Chang. Biol. 10, 155–160.
UN Department of Economic and Social Affairs; Population Division, 2022. World Population Prospects 2022: Summary of Results, United Nations.
Zheng, J., et al. 2023. Cropland intensification mediates the radiative balance of greenhouse gas emissions and soil carbon sequestration in maize systems of sub-Saharan Africa. Glob. Chang. Biol. 29, 1514–1529.