Conclusion : quelles possibilités d’accroissement des stocks de carbone organique du sol par l’apport au sol de nouvelles sources de carbone exogène ?

La quasi-totalité des effluents d’élevage produits en France retournent au sol et il n’y a donc pas de possibilité d’accroitre les stocks de C organique des sols par une mobilisation supplémentaire de ce gisement. On observe cependant qu’une part croissante de ces effluents subit des transformations avant épandage, allant d’une simple séparation de phase à une digestion anaérobie, dont les conséquences sur la composition des produits et le stockage de C après retour au sol doivent être étudiés. D’autres produits organiques comme les boues d’épuration ne reviennent que partiellement au sol, le reste étant incinéré. Sous réserve que l’épandage puisse être fait sans risque de contamination des sols, il y a là un gisement supplémentaire de carbone qui pourrait être mobilisé pour accroitre le stockage de C dans les sols. L’analyse bibliographique a par ailleurs confirmé l’intérêt, en termes de stockage additionnel de C, d’autres produits organiques comme les composts et les digestats. Ces produits sont actuellement presque toujours recyclés en agriculture mais les volumes produits sont faibles. La mobilisation de nouvelles ressources comme les biodéchets et les déchets verts et leur transformation par compostage ou méthanisation permettraient d’augmenter les volumes produits et donc le retour au sol. Un chiffrage des volumes mobilisables est nécessaire mais difficile à faire compte tenu de la multiplicité des étapes et facteurs à considérer (réseau de collecte, type de tri, mode de transformation…). L’acceptabilité sociale des scénarios envisagés doit cependant être soigneusement considérée. Enfin l’analyse tout juste esquissée dans le paragraphe précédent montre que la mobilisation de nouvelles ressources organiques pour accroitre le retour au sol et le stockage de C a plusieurs conséquences susceptibles d’impacter le bilan gaz à effet de serre global des scénarios envisagées (émissions liées à la collecte et au transport des matières, effets de substitution par éventuelle production d’énergie, modification des apports d’engrais minéraux et des émissions associées, etc.). En termes d’atténuation du changement climatique le chiffrage du stockage additionnel de carbone lié à la mobilisation de ces nouvelles

ressources organiques doit donc être complété par des informations sur les autres postes du bilan gaz à effet de serre global.

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3.8.6. Autres pratiques : fertilisation minérale, irrigation, chaulage