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

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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