Plant-earthworm partnership to increase crop productivity in the tropics

Tropical soils are often nutrient-depleted in their mineral form, which constrains crop growth. Therefore any process leading to the mineralization of N and P trapped into the soil organic matter need to be studied in order to developsustainable agricultural tools in a near future. Priming effect is defined as a stimulation of the soil organic matter mineralization (SOM) by the amendment of fresh organic matter (FOM). This microbially-mediated process is not completely understood. In theory, organic amendments could stimulate nutrient recycling from the SOM stock, provided that they will be managed under the right conditions. Endogeicearthworms are ecosystem engineers known to influence the dynamics of SOM as they ingest high amount of soil and assimilate one part of the organic carbonin relation with microbial activity stimulation. During the gut transit, microorganisms are awaked and their activities are stimulated by the small molecules contained in the mucus and by the meeting with fresh residues. During a first laboratory study we observed that earthworms could increase the Priming Effect intensity resulted from a fresh residue amendment*. Priming effect stimulation could be linked to specific bacterial phylogenetic groups* and its intensity was dependent on the type of soil and the earthworm species. Dichogaster saliensis a small size earthworm species living at the root vicinity of Bracharia sp. We observed that this species was able to generate an important priming effect in his gut, and therefore could mineralize nutrients liberated close to the roots. In a second experiment rice plants cultivated in outdoor pots were inoculated with increasing number of D.saliens specimens. At the rice maturity, a positive relationship between the number of earthworms and the number of grains per panicle was measured. As plant-fungi partnership, earthworm can also be a good partner, providing nutrients to the plant when most of nutrients are in the organic matter pool.