Carbon Dynamics in Response to the Tillage of a Long-Term Forage Soil.

Forage soils contain large C stocks and attention should be given to management systems that can affect this C pool. We looked at the response of soil respiration and carbon fractions to the tillage of a long-term hay field. In fall 2007, two plots, one with history of fertilization with pig slurry and the other non-amended, were divided into sub-plots and treated with herbicide. They were then either left undisturbed, or turned by mouldboard plough in fall or spring. Static chambers and soil probes were installed to measure gases at the soil surface (CO₂, O₂) and throughout the soil profile. Soil samples were taken to monitor labile carbon (soluble at 80^oC). The plots were left fallow for one year.  Emissions of CO₂ from untilled subplots were 55% higher than the fall tillage plots on the fertilized soils. During the summer of 2008, CO₂ emissions from the spring tilled plots also decreased by 75% on plots not tilled. Multiple regression analysis indicated that, overall, emissions from non-tilled plots were strongly related to the temperature of the soil surface, while emissions from tilled plots in the fall were linked to temperature at depth (40 cm) with a negative effect related to the amount of oxygen soil profile. Emissions from tilled plots in the spring were also related to the amount of oxygen in the soil profile, and temperature of the soil surface. Monitoring of soluble C showed significantly lower values (20-30%) in the ploughed soils, compared to the non-ploughed soils, suggesting a possible adsorption of the C placed at depth. On these soils, respiration was reduced by full inversion tillage due to the placement of soil carbon at depth where the soil is cooler and there is less exposure to oxygen and immobilization of C at depth on fresh mineral surfaces.