How does agroecology practices impact soil carbon stock and fluxes in a maize field?

Soil plays a major role on carbon cycle, through both carbon stock which is one of the most important carbon terrestrial pool and soil CO₂ efflux which represents one of the largest amounts of natural carbon emissions. It is known that soil respiration, through roots respiration and carbon mineralisation by microorganisms, is mainly controlled by temperature and humidity but the impact of crop management practices still needs to be investigated. Previous studies have demonstrated that crop management and more particularly reduced or no-tillage (NT) as well as cover-crops (CC) play a key role to mitigate soil respiration and increase soil organic carbon (SOC) content, but the impacts of the synergy of these practices are still unclear. Our study aims at better understanding the effect of sustainable agriculture through agroecological crop management practices on soil carbon dynamics.

Soil respiration was measured in south-west of France on two distinct sites, CAS in 2018 and ABA in 2019, characterized by different initial soil carbon content, 106.9 % higher in CAS than in ABA. Each site included two joint maize fields using agroecological (NT and CC, named Agroeco) and conventional (tillage and bare soil, named Conv) practises. Agroeco have been settled for 12 and 19 years at CAS and ABA, respectively, at the time of experiment. Soil respiration chamber as well as temperature and moisture sensors were used to collect data twice a month, while pedoclimatic variables were monitored continuously on each field. Soil samples were collected in the fields before the experiment to define SOC and nutrient content as well as physical properties, through the entire soil profile.

Mean soil respiration rate was higher on ABA-Agroeco (0.86 g CO₂ m^-² h^-1) than on ABA-Conv (0.50 g CO₂ m^-² h^-1) and was significantly correlated with soil temperature and humidity at Conv and only with soil temperature at Agroeco. Similar relations were found at CAS but with lower soil respiration rates. SOC concentration for ABA in the top 0-15 cm was higher at Agroeco (13.4 g kg^-1) than at Conv (8.0 g kg^-1) but little difference was found at CAS where SOC was high. These results suggest that soil respiration rates depend less on soil humidity on Agroeco than on Conv because agroecology management practices both keep more water at the surface and store additional soil organic carbon in soils, inducing more activity through the carbon cycle with higher soil respiration rate. For both sites, agroecological practices induced higher SOC content compared to conventional ones, however, only for ABA site, soil respiration was higher for agroecological field while SOC content was higher. This study supports the idea that agroecological management practices can increase carbon cycle activity by increasing soil carbon stocks thus allowing the mitigation of greenhouse gases emissions and climate change, even by increasing soil CO₂ efflux.