Soil organic carbon dynamics, functions and management in West African agro-ecosystems Bationo A., Vanlauwe B., Kihara J. and Kimetu J.

Outline Introduction Variability of soil organic carbon content at agro- ecosystem and farm level. Effect of soil and crop management on soil organic carbon Role of organic amendments on land productivity Future research challenge with emphasis on organic matter quantity and quality Conclusion

Lack of Resources Lack of Knowledge Land Degradation Improved Knowledge Improved Soil Management Improved Livelihoods Virtuous cycle Vicious cycle The Vicious and Virtuous cycle

The growth rate for cereals grain yield is about 1% while population growth is about 3%. During the last 35 years, cereals production per capita has decreased from 150 to 130 kg/person, whereas in Asia and Latin America an increase from about 200 to 250 kg/person have been observed. Both labor and land productivity are among the lowest of the world. Per Capita food production declined by about 30% and cereal self-sufficiency from 85 to 65% Introduction…

 Annual cereal deficit in sub-Saharan Africa amounts to 100 million tons  Food imports increased by about 185% between 1974 and 1990, food aid by 295%  The food gap (requirements minus production) is widening  The average African consumes only about 87% of the calories needed for a healthy and productive life

Introduction … ¨16% of Africa’s current arable land base is so eroded that it cannot be useful any longer agriculturally ¨70% of deforestation is caused by farmers who in their quest for food have no incentive to ponder long- term environmental consequences ¨Increase in area under food crop in sub-Saharan Africa was mainly due to use of marginal lands hence further environmental degradation through soil erosion and nutrient mining

Increase in yield has been more due to land expansion than to crop improvement potential CropsAreaYieldProduction Cassava Maize Yam Cowpea Soybean Plantain Based on three-year average for and FAO database.

Growth rate of millet CountryArea (%)/year Yield (%)/ year Production (%)/year Production/ hbt (%)/year WA Mali Niger Nigeria B.F

Percentage chances in soil fertility parameters in farmers’ fields as a result of 50 years of cultivation in the Savannah zones Source: Balasubramanian et al ZonesExchangeable cations CaMgKpH Sudan N. Guinea S. Guinea

Macronutrient loss versus consumption in Africa

Biophysical and economic parameters related to household resource endowment (Shepherd & Soule, 1998)

Effect of depth of soil mechanical de-surfacing at Mbissiri, Cameroon

Influence of mulch cover on erosion

Influence of cumulated erosion for a three year banana mulch on grain yield of next maize crop

Variability of soil organic carbon content at agro-ecosystem and farm level

Total System C (t ha -1 ) Senegal River Valley Sahelian Shrubby Grassland (Sylvo-pastoral Zone) Sahalian-Sudanese Transition Drier Sudanese Woodland (Old Groundnut Basin) Wetter Sudanese Woodland (New Groundnut Basin) Sudanese-Guinean Transition (Casamance) forest cultivatedgrazed cultivateddegraded grazed parkland cultivated fallow woodlandgrazed cultivated forest parklandfallowcultivated Vegetation and Land Use 16.5 o N13.1 o N16.1 o N15.2 o N14.8 o N14.1 o N Source: Woomer 2003

Carbon stocks and other fertility indicators of granitic soils in different agro-ecological zones in West Africa Source:WindmeijerandAdriesse1993

Carbon stocks of different subsystems in a typical upland farm in the Sudan-savanna zone Home garden Bush field Village field Exchangeable K (mmol/kg) Available P mg/kg Total N g/kg OC g/kg pH H 2 O AEZ Source: Prudencio et al

Use of organic resources within a farm for various farmer typologies

Correlation (r) between selected soil (0-20 cm) fertility parameters and average annual rainfall ** and *** indicate statistical significance at the 0.05 and level, respectively. Source: Manu et al., 1991 Soil organic carbon losses are more related to clay and slit contents than rainfall

Effect of soil and crop management on soil organic carbon

Carbon losses (kg ha-1 yr-1) by erosion, runoff and leaching in the topsoil (30cm) in runoff plots (Adopted from Roose E and Barthes B, 2001)

Annual loss rates of soil organic carbon measured at farm level in WASAT SiteClay + silt (%) Annual losses (K) (%) Bambay37 Saria (non eroded)122 Saria eroded196

2:1 clays 1:1 clays Forest Grassland Cultivated Source: Six et al., 2002 Relationship between silt+clay content and silt+clay associated carbon for different systems

Organic carbon changes under continuous cropping and under fallow in an ultisol Adapted from Kang 1993

Evolution of carbon content in the 0-10cm horizon, as affected by time and treatment in runoff plots of Mbissiri station, Cameroon

Kg P/ha

Sorghum-groundnut rotation in Burkina Faso shows good crop as opposed to Continuous Sorghum crop (inset)

Role of organic amendments on land productivity

Phosphorus use efficiency (kg grain/kg P) in village (non degraded) and bush (degraded) fields for pearl millet production, Niger

Effect of cattle dung and urine on millet grain and total above ground bio-mass, Sadore Niger ManureDung+ Urine- Urine ApplicationGrainBiomassGrainBiomass Kg/ha Cattle s.e.m Adapted from Powell et al., 1998

Effect of fertilizer application and crop residue on maize grain yield Adapted from Kang 1993

Pearl millet total dry matter yield as affected by long-term application of crop residue and fertilizer Source: Bationo et al, 1998.

Incremental millet grain and stover yield due to fertilization in sadore, Niger Source: Bationo et al., 1995 GrainStover 1985CR-- Fertiliser67188 CR + Fertiliser CR-- Fertiliser57184 CR + Fertiliser YearTreatmentFertiliser effect kg per kg P applied ----

ORGANIC INPUTS FOR N MANAGEMENT Decision Guide

FEs for Different Organic Materials

Base saturation and pH (water) for soil experiments in Saria, Burkina Faso TreatmentBase saturation pH Control Chemical fertilizer Crop residues 5t ha (Source: Pichot et al 1981)

Soil pH as affected by soil depth and management practices. Sadoré, Niger, rainy season, 1996

Maximum phosphorus sorbed as affected by soil depth and management practices, Sadoré, Niger, 1999

Availability of crop residue Optimum rate : 2 t ha -1 Farmers field : 200kg ha -1 Farmers’ doses combined with the use of small quantities of P fertilizers can boost crop biomass. Limitations

Utilisation de micro-dose de P (4kg/ha) - P + P

Limitations – Manure use is part of internal flow and does not add always nutrients from outside the farm –Limited quantities, low nutrient content and often high labor demands for processing and application –Potential livestock transfer of nutrients in W. Africa is 2.5 kg N and 0.6 kg P per hectare of cropland

–5-20 tons recommended but less than 700kg is available in semi- arid W. Africa –Can only increase yield by 2% per year –Need between ha of grazing land to maintain yield on 1 ha of cropland Limitations

Future research challenge with emphasis on organic matter quantity and quality

Future research challenges Focus more on whether the organic resource quality concept is also useful for predicting different degrees of stabilization of applied organic C in one or more of the organic matter pools Increasing the dual purpose grain legume component for improvement of soil organic carbon and for a better integration of crop-livestock production systems Improvement of nutrient use efficiency in order to offer cost- effective mineral fertilizer recommendations to the small-scale farmers Use of decision support systems, modelling, and GIS for the extrapolation of research findings