Impact of legumes on soil microbial activity and C cycle functions in two contrasting Cameroonian agro-ecological zones

Unsustainable farming systems for a growing population in Sub-Saharan Africa stress natural resources and lead to soil degradation. Legume cultivation, however, promotes soil microbial communities and may help reverse soil degradation. The soil properties (pH, texture, C and nutrient contents) of a set of contrasting sites from two agro-ecological zones of Cameroon were determined. This study characterises the microbial activities and functioning in rhizosphere soils of the four legumes Phaseolus vulgaris L., Glycine max L., Arachis hypogea L. and Vigna subterranea L. in a laboratory incubation experiment. Ecological indicators (total respiration, microbial biomass determined as total phospholipid fatty acids, metabolic quotient) of soil health were quantified after microbial activation by 14C-glucose and related to site-specific parameters as well as to the effects of legume cultivation. Microbial activation after glucose addition was frequently site-specific. Cumulative glucose respiration increased over time, and glucose pulses repeatedly boosted glucose respiration and total respiration at all sites. The microbial biomass was lowest after the experiment in the soil from the High Guinean Savannah except for soils under soybean cultivation. Among the four legumes cultivated, only soybeans strongly increased microbial biomass in the High Guinean savannah reaching even the level of microbial biomass in Western Highland soils. The lowest metabolic quotient in soybean rhizosphere soil and lowest glucose respiration compared to the soils of other legumes suggests a high carbon use efficiency, presumably due to C limitation in the High Guinean Savannah. Abiotic soil properties (pH, clay content) strongly influenced microbial habitat properties and thus had a positive effect on the CO2 efflux, irrespective of glucose addition. Within the set of tested legumes, soybean cultivation can, therefore, be strongly recommended as a long-term sustainable crop to boost soil microbial functioning and probably also nutrient cycling, especially under low total organic carbon stocks.