Mitigation of climate change through soil organic carbon sequestration in smallholder farming systems of Zimbabwe

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<ul><li><p> Mitigation of climate change through soil organic carbon sequestration in smallholder </p><p>farming systems of Zimbabwe Mujuru La, Mureva Aa, Velthorst Eb. and Hoosbeek Mb </p><p>a Bindura University of Science Education, Department of Environmental Science, Private Bag 1020, Bindura, Zimbabwe; bDepartment of Soil Quality, Wageningen University, P. O Box 47, 6700AA, Wageningen, The Netherlands. </p><p> </p><p>ACKNOWLEDGMENTS We thank the Netherlands Fellowship Programme and the Climate Food and </p><p>Farming Network (CLIFF). We are also grateful to CIMMYT Zimbabwe and farmers in Shamva and Bindura. </p><p>INTRODUCTION Soil organic matter (SOM) represents a large, dynamic and complex </p><p>terrestrial reservoir of carbon (C). Soil management strategies </p><p>therefore become an important C mitigation approach through </p><p>mitigation measures involving both CO2 emissions reduction and </p><p>increasing C sinks (Food and Agriculture Organisation (FAO), 2010). </p><p>Land use practices in agro ecosystems affect the storage of organic </p><p>carbon in soils especially in sub- Saharan Africa, where crop farming is </p><p>characterised by mono cropping, frequent soil tillage and removal of </p><p>crop residues from fields through livestock grazing or burning </p><p>(Chigonda 2008). Conservation farming practices such as minimum or </p><p>no tillage minimise soil disturbance and utilises crop residues to retain </p><p>moisture and enrich the soil among the smallholder communal farming </p><p>systems. Addition of manure and other organic fertilisers improves </p><p>nutrient efficiency and enhances biomass yields (Nyamangara et al., </p><p>2003). Increasing biomass can improve soil organic carbon (SOC) </p><p>storage therefore becomes a major focal point for climate change </p><p>mitigation through accumulation of significant quantities of organic C. </p><p>This study evaluated the effects of tillage practices and fertility </p><p>amendments on SOC storage in sandy and clayey soils of Zimbabwe. </p><p>RESEARCH SITE AND METHODOLOGY Research was carried out in farmers fields in Bindura, Shamva and Murewa districts of Zimbabwe. Altitudinal ranges from 1000 to 1800 </p><p>m.a.s.l. with annual unimodal rainfall of 750-1000 mm. Soil samples </p><p>were collected at 0-10 and 10-30 cm depths in three tillage systems; </p><p>(conventional tillage (CT), Minimum tillage (MT) with a ripper, No </p><p>tillage (NT) using a direct seeder in Haplic Arenosols (sandy) in </p><p>Shamva and Rhodic Ferralsols (clayey) in Bindura. Minimum and no </p><p>tillage treatments received 2.5-3.0 Mg ha-1 organic inputs and the </p><p>three treatments received equal amounts of inorganic fertiliser. To </p><p>assess effects of agricultural land use on SOC, irrespective of </p><p>treatment, soil samples were also collected from adjacent natural </p><p>forests. In another experiment cattle manure and nitrogen fertiliser </p><p>were added to conventionally tilled soils and SOC was assessed. </p><p> RESULTS &amp; DISCUSSION: </p><p>CONCLUSIONS </p><p>When conventional tillage is the only available option, application of nitrogen fertiliser can be more beneficial for increasing C stocks in sandy soils </p><p>whereas application of organic fertiliser (cattle manure) has greater C benefits in clayey soils. Increased SOC improves crop production thus, </p><p>ensuring climate change mitigation and food security. Residue retention strategies need to be developed to improve environmental and productive </p><p>capacity of cropping systems in smallholder farming systems in arid and semi-arid areas where communal grazing rights are common. </p><p>Carbon storage under fertility treatments in </p><p>conventionally tilled soils Application of N fertiliser plus cattle manure significantly increased </p><p>the SOC stocks in soil compared to application of N Fertiliser alone at </p><p>all depths on clayey soils. On sandy soil, application of N Fertiliser </p><p>resulted in greater SOC than N Fertiliser + manure and control at all </p><p>depths except the 10-20 cm depth </p><p>Figure 3: Carbon storage in three density fractions (a) free light </p><p>fraction (fLF) (b) Occluded light fraction (oLF) (c) Mineral associated </p><p>heavy fraction (MaHF). </p><p>Figure 1: Carbon storage in bulk soils in three tillage systems and natural </p><p>forests on two contrasting soil types </p><p>Caborn storage in tillage systems On clayey soils, C storage was higher in minimum tillage (32 Mg ha-1) </p><p>than no tillage and conventional tillage which had similar C stocks (31 </p><p>Mg ha-1) at 0-30 cm. There were no significant difference in SOC stocks </p><p>among tillage systems in clayey soils. Sandy soils however, showed </p><p>more C under no tillage (11 Mg ha-1) than minimum tillage (10 Mg ha-1) </p><p>and conventional tillage (8 Mg ha-1). Lack of significantly different C </p><p>gains under conservation tillage practices (MT and NT) could be </p><p>attributed to limited residue cover which makes soils more vulnerable to </p><p>agents such as wind erosion compared to conventionally ploughed soils, </p><p>where the roughness created by tillage can reduce wind and water </p><p>erosion. </p><p>Figure 2 </p><p>CT NT MT </p><p>Depth distribution of soil organic carbon in tillage treatments Depth distribution showed significantly higher (F= 22.98; p</p></li></ul>