Is soil quality improvement by legume cover crops a function of the initial soil chemical characteristics?

The aim of this study, which was conducted in a humid savannah zone of central Cote d'Ivoire, was to examine changes in the quality of soil cultivated with herbaceous legume cover crops as a function of initial soil characteristics. Mucuna pruriens var utilis and Pueraria phaseoloides were used in a two side-by-side location experiment: a shrubby savannah (the savannah site or "SAV") and a natural fallow dominated by Chromolaena odorata (the fallow site or "FAL"). The latter was mainly characterized by higher organic matter [organic carbon (C) 10 vs. 7.5 mg kg(-1); total nitrogen (N) 0.8 vs. 0.5 mg kg(-1)) and total phosphorus (P) (282.3 vs. 168.3 mg kg(-1)) contents in the upper soil layer (0-10 cm). After 8 months of growth, biomass production by M. pruriens was found to be 6.5 and 4.9 t dry matter (DM) ha(-1) at FAL and SAV, respectively. For P. phaseoloides, the values were 7.2 and 6.4 t DM ha(-1), respectively, in approximately the same period. The quantities of nutrients released by decomposing legume litter were higher at FAL than at SAV. Between-site differences in soil quality improvement were most noticeable in terms of available P, microbial biomass carbon (MBC) and MBC:total carbon (TC) ratio. The FAL site experienced a faster improvement of soil parameters under both legume species: available P increased from 18 to 58 mg kg(-1) under M. pruriens, and from 19 to 52 mg kg(-1) under P. phaseoloides; MBC increased from 88 to 185 mg kg(-1) under M. pruriens, and from 127 to 192 mg kg(-1) under P. phaseoloides. In contrast, the parameters remained constant over time at SAV. Soil C and N contents as well as C mineralization showed similar trends at both sites. Based on these results, we conclude that soil quality improvement under cover crops appears to be faster when the initial soil organic C, total N and P contents are adequate. These findings will be useful in assisting governmental decision-making on approaches to be taken for restoring soil fertility in low-input agricultural systems in West Africa.