Impact of 34-Year Tillage on Soil CO2 Emissions and Carbon Sequestration in Monoculture and Rotational Cropping Systems.

Soil processes, such as soil respiration play a crucial role in the carbon cycle and the evolution of greenhouse gases into the atmosphere. The soil carbon storage in cropping systems is affected by intensive tillage practices that modify soil conditions. Soil physical properties control several processes including the storage and flow of water and gases from the soil, oxidation of organic matter and microbial activity that contribute to soil respiration. The objective of this study was to investigate the effect of soil moisture and temperature on soil respiration in a long-term cropping systems experiment site (33-years) at the Texas A&M University Research Farm in College Station, Texas. Soil carbon dioxide (CO₂) emissions were measured in monoculture soybean and winter-wheat/soybean systems in 2016 using four automated chambers and compared between tillage practices. Soil temperature and volumetric water content (θ_v) were continuously monitored at 5cm, 10cm, 20cm and 30 cm depths during the measurement period. Similar soil temperature profiles were observed in winter-wheat plots under conventional and no-till management. However, no-till plots showed the higher θ_v that could have reduced soil CO₂ emissions compared to conventional plots. Average soil CO₂ flux from the conventionally managed winter wheat plot was 16% higher (2.46 ± 1 µmol m^-2 s^-1) compared to no-tillage plot (2.06 ± 0.97 µmol m^-2 s^-1). Residues accumulated on the surface under no-till decreased the temperature-dependency of CO₂ emissions that resulted in lower emissions. The opposite trend was observed in the monoculture soybean, in which high soil temperatures coupled with lower soil moisture in conventional till suppressed microbial activity and soil CO₂ emissions. The optimum soil temperature range in no-till increased carbon losses that resulted in high emissions. This information could be used to improve our understanding of soil CO₂ fluxes and soil organic carbon measures as indicators of carbon sequestration.