Mitigation of climate change through soil organic carbon sequestration in smallholder
farming systems of Zimbabwe Mujuru La, Mureva Aa, Velthorst Eb. and Hoosbeek Mb
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.
ACKNOWLEDGMENTS We thank the Netherlands Fellowship Programme and the Climate Food and
Farming Network (CLIFF). We are also grateful to CIMMYT Zimbabwe and farmers in Shamva and Bindura.
INTRODUCTION Soil organic matter (SOM) represents a large, dynamic and complex
terrestrial reservoir of carbon (C). Soil management strategies
therefore become an important C mitigation approach through
mitigation measures involving both CO2 emissions reduction and
increasing C sinks (Food and Agriculture Organisation (FAO), 2010).
Land use practices in agro ecosystems affect the storage of organic
carbon in soils especially in sub- Saharan Africa, where crop farming is
characterised by mono cropping, frequent soil tillage and removal of
crop residues from fields through livestock grazing or burning
(Chigonda 2008). Conservation farming practices such as minimum or
no tillage minimise soil disturbance and utilises crop residues to retain
moisture and enrich the soil among the smallholder communal farming
systems. Addition of manure and other organic fertilisers improves
nutrient efficiency and enhances biomass yields (Nyamangara et al.,
2003). Increasing biomass can improve soil organic carbon (SOC)
storage therefore becomes a major focal point for climate change
mitigation through accumulation of significant quantities of organic C.
This study evaluated the effects of tillage practices and fertility
amendments on SOC storage in sandy and clayey soils of Zimbabwe.
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
m.a.s.l. with annual unimodal rainfall of 750-1000 mm. Soil samples
were collected at 0-10 and 10-30 cm depths in three tillage systems;
(conventional tillage (CT), Minimum tillage (MT) with a ripper, No
tillage (NT) using a direct seeder in Haplic Arenosols (sandy) in
Shamva and Rhodic Ferralsols (clayey) in Bindura. Minimum and no
tillage treatments received 2.5-3.0 Mg ha-1 organic inputs and the
three treatments received equal amounts of inorganic fertiliser. To
assess effects of agricultural land use on SOC, irrespective of
treatment, soil samples were also collected from adjacent natural
forests. In another experiment cattle manure and nitrogen fertiliser
were added to conventionally tilled soils and SOC was assessed.
RESULTS & DISCUSSION:
CONCLUSIONS
When conventional tillage is the only available option, application of nitrogen fertiliser can be more beneficial for increasing C stocks in sandy soils
whereas application of organic fertiliser (cattle manure) has greater C benefits in clayey soils. Increased SOC improves crop production thus,
ensuring climate change mitigation and food security. Residue retention strategies need to be developed to improve environmental and productive
capacity of cropping systems in smallholder farming systems in arid and semi-arid areas where communal grazing rights are common.
Carbon storage under fertility treatments in
conventionally tilled soils Application of N fertiliser plus cattle manure significantly increased
the SOC stocks in soil compared to application of N Fertiliser alone at
all depths on clayey soils. On sandy soil, application of N Fertiliser
resulted in greater SOC than N Fertiliser + manure and control at all
depths except the 10-20 cm depth
Figure 3: Carbon storage in three density fractions (a) free light
fraction (fLF) (b) Occluded light fraction (oLF) (c) Mineral associated
heavy fraction (MaHF).
Figure 1: Carbon storage in bulk soils in three tillage systems and natural
forests on two contrasting soil types
Caborn storage in tillage systems On clayey soils, C storage was higher in minimum tillage (32 Mg ha-1)
than no tillage and conventional tillage which had similar C stocks (31
Mg ha-1) at 0-30 cm. There were no significant difference in SOC stocks
among tillage systems in clayey soils. Sandy soils however, showed
more C under no tillage (11 Mg ha-1) than minimum tillage (10 Mg ha-1)
and conventional tillage (8 Mg ha-1). Lack of significantly different C
gains under conservation tillage practices (MT and NT) could be
attributed to limited residue cover which makes soils more vulnerable to
agents such as wind erosion compared to conventionally ploughed soils,
where the roughness created by tillage can reduce wind and water
erosion.
Figure 2
CT NT MT
Depth distribution of soil organic carbon in tillage treatments Depth distribution showed significantly higher (F= 22.98; p<0.01) C
stocks at 0-10 cm than at 10-30 cm in all land use systems (Figure 2).
On sandy soils, at 0-10 cm depth C was higher under NT (6.9 Mg ha-1)
than MT (6.8 Mg ha-1) and CT (5.2 Mg ha-1) whilst on clayey soils there
was more C under CT (19.1 Mg ha-1) than MT and NT although the
differences were not significant. At lower depths C storage in clayey soils
was not significantly different from natural forest.
Literature cited: Chigonda, T. (2008). Conservation Tillage among Communal Farmers in Kawere Ward of Mutoko District: Current Practices, Constraints and
Prospects. Zimbabwe Journal of Geographical Research, 2, 15-23.
Food and Agriculture Organisation (FAO) 2010. Agriculture, Food Security and soil carbon. Rome.
Nyamangara, J. Bergström, L. F. Piha, M. I. & Giller, K. E. (2003). Fertilizer use efficiency and nitrate leaching in a tropical sandy soil. Journal of Environmental Quality 32(2), 599-
606.
