Trade-offs between crop yield, soil organic carbon and greenhouse gas emissions under reduced tillage and rainfall exclusion

Reduced tillage is often considered as an agroecological practice that promotes soil organic carbon (SOC) sequestration in the topsoil, offering potential for climate change mitigation. However, effective mitigation requires a comprehensive understanding of trade-offs among SOC stocks, greenhouse gas emissions, and crop yields. As climate change alters carbon and nitrogen cycling, these trade-offs must be evaluated under current and experimentally induced extreme conditions to assess the effectiveness of reduced tillage in a changing climate. In this study, we measured crop yields, soil carbon stocks and soil CO₂ and N₂O fluxes in a conventional tillage (CT) vs. reduced tillage (RT) field trial in central Germany. The long-term trial runs since 1970 in a field with Luvisol soil (73% silt, 15% clay, and 6.6 pH). The mean annual precipitation is 611±120 mm and the mean annual temperature is 9.6±0.7°C. The field trial follows a randomized block design and consists of 16 plots: eight under CT with inversion ploughing to a depth of 27-30 cm, and eight under RT with non-inversion harrowing to a depth of 7-10 cm. In 2022-23 and in 2023-24 we cultivated winter wheat and winter barley respectively. In February 2023, rain-out shelters (area =2 m × 2 m) designed to intercept 50% of the precipitation were installed in half of the plots, and we initiated the soil flux measurements with static chambers over permanently installed rings and portable gas analyzers. We measured crop yields in both years, and SOC in samples from 0-90 cm at 10 cm intervals sampled in August 2023. SOC traits were examined with by-size fractionation to particulate and mineral-associated organic matter and an incubation experiment with an automated respirometer. Winter wheat yield did not differ between tillage and precipitation treatments but, in the second year of our experiment, winter barley yield was lower under rainfall exclusion than ambient precipitation in the RT fields only (50% precipitation: 0.26±0.05 kg m^-2 vs. 100% precipitation: 0.52±0.02 kg m^-2). Regarding SOC, we found that fields under RT had higher stocks in the 0-10 cm depth than under CT (RT: 1.93±0.03 kg m^-2 vs. CT: 1.53±0.02 kg m^-2), but the opposite occurred in the 20-30 cm depth (RT: 1.16±0.04 kg m^-2 vs. CT: 1.58±0.06 kg m^-2). Comparing SOC stocks at 0-90 cm, there was no difference between the two tillage systems. Field soil N₂O fluxes did not differ significantly between tillage and precipitation treatments when considering block, plot and date as random effects. In contrast, field soil CO₂ fluxes were significantly lower in RT than CT fields under ambient precipitation but this did not result in higher SOC stocks under RT. Rainfall exclusion led to higher soil CO₂ fluxes both in the RT (in average, 50%: 32.0±1.0 mg CO₂-C m^-2 h^-1 vs. 100%: 30.6±0.9 mg CO₂-C m^-2 h^-1) and CT (in average, 50%: 30.1±1.1 mg CO₂-C m^-2 h^-1 vs. 100%: 24.2±0.7 mg CO₂-C m^-2 h^-1) fields. Based on the above, RT seems to have no climate change mitigation potential in a productive fine textured soil of temperate central Europe.