conservation agriculture Journée des doctorants de l’ABIES – 14 et 15 avril 2016 Increasing soil organic carbon storage with alternative cropping systems: results from a 16-year field experiment Contact: Bénédicte Autret, benedicte.autret@laon.inra.fr Bénédicte Autret1, Bruno Mary1 , Claire Chenu2 , May Balabane3 , Cyril Girardin3 , Michel Bertrand2 , Gilles Grandeau2 , Nicolas Beaudoin1 1 INRA, UR 1158 AgroImpact, Site de Laon, F-02000 Barenton-Bugny, 2 INRA, UMR Agronomie, AgroParisTech, Université Paris-Saclay, F-78850 Thiverval-Grignon, 3 INRA, UR Ecosys, AgroParisTech, Université Paris-Saclay, F-78026 Versailles Introduction Alternative cropping systems are expected to decrease negative impacts of agriculture on the environment through sequestration of soil organic carbon (SOC) and mitigation of greenhouse gas emissions. We studied SOC dynamics in the long term field experiment of “La Cage” (Versailles, France) which compares four arable cropping systems free from manure application. Materials and methods Field experiment of “La Cage”: Measurements between 1998 and 2014: Crops yields Carbon inputs via crop residues SOC content SOC stocks at equivalent soil Bulk densities mass over the 0-30 cm depth 4 cropping systems 1998 627 mm 11.3°C AMG model: CON conventional LI low input CA conservation agriculture ORG organic ploughing each year every year no till kg N ha-1 yr-1 143 114 104 10 Temperature, humidity Humified organic matter Crop residues Ca active SOC Cs stable SOC CO2 h k m Objectives To compare SOC stocks in alternative cropping systems To predict the dynamics of SOC stocks with AMG model To understand the drivers of C storage with modelling cover crop (fescue or alfalfa) m : annual aerial and belowground carbon inputs (t C ha-1 yr-1 ) h : humification coefficient of crop residues Ca and Cs : active and stable carbon content (t C ha-1 ) k : mineraliszation rate of active carbon (yr-1) catch crop Modelling SOC stocks evolution: With default parameters (S0) With optimized m and k for each or all of the cropping systems (S3 and S6) main crop auxiliary crop Figure 1. Cumulative frequencies of occurrence of main and auxiliary crops, tillage and nitrogen fertilization between 1998 and 2014. Results Carbon inputs: Soil organic carbon contents: C content (g kg-1) Depth (cm) Figure 3. Mean annual C inputs from crop residues over the period 1998-2014. Figure 4. Distribution of carbon concentration in the soil profile in 1998 and 2014. The depths correspond to fixed equivalent soil masses. Asterisks indicate significant evolution between 1998 and 2014 (* p<0.05; ** p<0.01; *** p<0.001). In short Carbon inputs ranked as (t C ha-1 yr-1): CA (5.41) > CON (4.09 ) > LI (3.81 ) > ORG (2.87) Evolution of SOC stocks between 1998 and 2014: +24% in CA, +12% in ORG, +3% in CON and +1% in LI Simulations were optimized with a common k of 0.041 yr-1 and a higher alfalfa and fescue belowground inputs. Soil organic carbon stocks evolution and modelling: Figure 5. Evolution of SOC stocks in the 0-30 cm layer from 1998 to 2014 in block 2: observed (symbols) and simulated values (lines). S0 = simulation with original parameters; S6 = simulation with 6 parameters optimized; S3 = simulation with 3 parameters optimized. Conclusion Outlook The assessment of the environmental impacts of these cropping systems is not achieved regarding greenhouse gas emissions. Further research will address the N budget, including N2O emission. A high SOC storage was observed only in CA and was slighter in ORG Modelling SOC stocks evolution was possible with a single mineralization rate for the 4 cropping systems High carbon inputs derived from alfalfa and auxiliary crops belowground residues explained SOC storage, rather than a reduced soil diturbance Acknowledgements “La Cage” experiment is coordinated by INRA Versailles. We acknowledge P. Saulas, D. Le Floch and C. Montagnier for managing the experiment, J.P. Pétraud, F. Mahu and E. Venet for their technical assistance, C. Dominiarczyk and A. Teixeira for processing samples and O. Delfosse for carbon analyses. Reference Saffih-Hdadi, K., Mary, B., 2008. Modeling consequences of straw residues export on soil organic carbon. Soil Biol. Biochem. 40, 594–607. doi:10.1016/j.soilbio.2007.08.022