Reduced tillage and cover crops as a strategy for mitigating atmospheric CO2 increase through soil organic carbon sequestration in dry Mediterranean agroecosystems María Almagro, Noelia Garcia-Franco, Joris de Vente, Carolina Boix-Fayos, Elvira Díaz, María Martínez-Mena Soil and Water Conservation Research Group Centre for Applied Soil Science and Biology of the Segura Spanish National Research Council

Soil physical properties Background Soil physical properties Soil C cycle ↓ Soil loss by erosion Yield Productivity Runoff control & ↑ SWC ↑ C inputs SUSTAINABLE MANAGEMENT PRACTICES Prevent land degradation Biodiversity & Ecosystem services CC mitigation & adaptation No Tillage (NT) Reduced Tillage (RT) Green Manure (RTG)

Study areas & Experimental design RTG RT RT CT RTG CT RT 370 mm 16 ºC 284 mm 15 ºC 3

General soil characteristics of the study areas Soil properties “Burete” farm “Alhagüeces” farm Soil depth (cm) 0-30 0-45 Clay (%) 15.5 ± 4. 2 17.76 ± 3.7 Silt (%) 41.5 ± 9.0 48.9 ± 11.5 Sand (%) 43.0 ± 12.0 33.2 ± 14.8 SOC (%) 1.2 ± 0.3 2.30 ± 0.6 Total N (%) 0.12 ± 0.04 0.20 ± 0.04 Polsen (ppm) 15.60 ± 5.8 41.8 ± 18.7 CaCO3 (%) 54. 2 ± 11.5 44 ± 9.2 pH (water, 1:25) 8.7 ± 0.2 8.8 ± 0.14 EC (siems cm-1) 168.3 ± 23.3 90.8 ± 14.7

Implemented SLM practices No tillage (NT) Conventional tillage (CT) chisel ploughing (3-5 times yr-1) 15 cm depth No tillage (NT) Reduced tillage (RT) chisel ploughing (2 times yr-1) 15 cm depth Reduced tillage+green manure (RTG) chisel ploughing (2 times yr-1) at 15 cm depth Avena sativa & Vicia sativa 1:3 at 150 kg ha-1 Reduced tillage (RT) chisel ploughing (2 times yr-1) 15 cm depth 5

Green manure: wet vs. dry growing season (Nov-May) 2009 (P=345 mm) 2011 (P=230 mm) ‘Alhagüeces’ & ‘Burete’ farms Vicia sativa + Avena sativa (3:1) at 150 kg ha-1 Vicia sativa + barley (3:1) at 150 kg ha-1

Monitoring soil CO2 efflux and OC sequestration

Monitoring soil CO2 efflux and OC sequestration Monthly measurements of soil CO2, T & WC Soil sampling (0-15 cm; autumn 2012) Carbon inputs derived from plant residues Water-stable soil aggregates and OC ↑ C inputs

Results: Soil CO2 efflux dynamics and after tillage 61% 58% P <0.05 P >0.05 82% 53% P >0.05 Almagro et al (2009) P <0.05

Results: Annual balance of soil CO2 emissions 20% 11% P >0.05 Annual soil CO2 emissions were not significantly increased by tillage! The slight increase in annual soil CO2 emissions at RT & RTG compared to CT was off-set by increments in the SOC stocks driven by plant biomass incorporation to the soil. Tillage suppression decreased annual soil CO2 emissions but did not increased the SOC stock. 162 g m-2 138 g m-2 183 g m-2 114 g m-2 182 g m-2 CT RT RTG NT

Results: Soil aggregation and carbon sequestration SLM & soil aggregates size distribution at Alhagüeces (left) and Burete (right) farms + 74% + 61- 88% ↑ C inputs soil aggregates formation!

Results: Soil aggregation and carbon sequestration Organic C in soil aggregate sizes at Alhagüeces (left) and Burete (right) farms Garcia-Franco et al (2015) ~60% ~35% +44% +32% +24% +36% ~130% +125% +112% CT RT RTG NT

Conclusions Reducing tillage frequency and incorporating plant residues promoted soil aggregation and the physico-chemical OC protection without increasing soil CO2 emissions to the atmosphere. Suppressing tillage slightly reduced soil CO2 emissions but did not increase the amount of carbon stored in the soil. ↑ C inputs Longer term results and studies at larger spatial scales are needed to fully understand the mitigation capacity of SLM practices in dry agroecosystems under climate change.

Thank you very much! mbonmati@cebas.csic.es