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Potential of improved tropical legume fallows and zero tillage practices for soil organic carbon sequestration

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Agroforestry systems such as improved fallows (IF) have been suggested as promising management practices to reduce net C02 emissions, hence increasing soil carbon (C) storage, a process known as soil C sequestration. However, no process level studies have been done to illustrate such a potential fo IF. Experiments with IF planted in sequential arrangement with maize crops were established in different sites in the sub-humid highlands of western Kenya. Soils were analyzed for C content and fractionated for water stable aggregates (WSA, 20 mm -2 mm), microaggregates (MI, 0-20 mm) and free organic matter (fOM) by wet sieving. Properties of aggregate fractions such as their C protective capacity, resistance to microbial degradation and composition were studied. Introducing IF increased soil C stocks in the top 0-20 em depth in clayey and sandy soils progressively in 3 years, with an annual C sequestration rate of 2-3 Mg C ha-1yr-1, and was not significant in the subsoil. Soil C increases were largely associated with litterfall and root activity, and partly with returned fallow biomass. Soil C increase was reflected in increased macroaggregate (212-2000 mm)-C. During aggregation, 35-70% of native C was included in WSA largely as soil fine fractions (20 mm). Tillage practices did not alter soil C stocks and C distribution in aggregates in the 0-20 cm soil depth. Mineralization coefficients of aggregates after 500 days of incubation were on average 8% and 15% for clayey and sandy soils respectively, with about 40% of potentially labile C being physically protected in WSA. Less C-saturated soil fine fractions had lower C mineralization and higher efficiency of preserving newly added C compared to those more saturated with C. Near infrared spectroscopy, a rapid and non-destructive method provided good prediction of chemical and C mineralization data in soils/ aggregates and fOM fractions. A developed aggregate submodule satisfactorily predicted C dynamics in aggregates, but there is room to expand its applicability by linking it to a larger dynamic model such as WaNuLCAS. Thus, it is evident that there is potential for IF to/sequester soil organic carbon (SOC) in the top 0-20 cm soil depth, especially in clay-rich soils. However, there is still a likelihood for SOC sequestration in the 20-100 em soil depth in the long-term due to the deep-rootedness and large root biomass of fast growing legumes. While embarking on adoption of the IF technology by farmers, a bioeconomic analysis of direct and indirect benefits including C sequestration is desirable to ascertain overall sustainability.
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    Mutuo P K




    agroecosystems, agroforestry, carbon, fallow, legumes, organic compounds

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