Organic matter inputs and earthworm inoculation enhance C storage in tropical soils following application of the FBO technology

We evaluated the potential of the FBO (Fertilisation Bio Organique ®) technology to store carbon provided in the form of organic fertilizer in a Colombian plantain banana plantation. FBO is an agroecological technique which consists of adding low- and high-quality organic materials and endogeic earthworms in a specific design in 1.0 × 0.4 × 0.3 m deep trenches where perennial plants are further planted. Trenches may also be organized in inter rows in established plantations. The overall surface occupied by trenches was limited to 2.2 % of total area in our experiment. In the experimental setup, the two sources of organic materials were either separated (FBO1) or mixed (FBO2). In control plots, trenches were dug and refilled with no addition of organic matter nor earthworms and chemical fertilization was applied. We postulated that earthworm casts would be further stabilized as large soil macro aggregates, within which organic matter would be physically protected from decomposition and mineralization. Total macroinvertebrate population densities were 76.6 to 85.3 % higher than in the control in the FBO treatments 2 and 1 respectively. Populations of Isopoda increased markedly (from 402 ± 133 m−2 in Control to 2586 ± 458 in FBO1 and 2274 ± 280 in FBO2), in the same way as Diplopoda (180 ± 40 m−2 in C to 1453 ± 324 in FBO1 and 1259 ± 214 in FBO2) and earthworms (from 68 ± 24 m−2 in C to 322 ± 39 m−2 in FBO1 and 261 ± 40 in FBO2). Soil macro aggregation was greatly enhanced in the FBO treatments with significant increases in the percentages of large biogenic (+48.1 %), medium sized biogenic (+66.9 %), and small biogenic (+454.3 %) macro aggregates created at expenses of the physical aggregates and non macroaggregated residual soil that were almost absent. The organic matter content of aggregates was lowest in physical and large biogenic macro aggregates, intermediate in medium sized biogenic macro aggregates and highest in residual soil and small biogenic macro aggregates. On a hectare scale and to a depth of 30 cm, the organic C stored in the FBO treatments represented between 6.5 and 7.8 ‰ of the total initial stock of organic carbon. Respirometric activity declined by two thirds in the large and medium sized biogenic macro aggregates as compared with the non-macroaggregated residual soil, emphasizing their potential for C sequestration. Our results illustrate the substantial effect of macroinvertebrate activities on the progressive incorporation of organic