Long-Term Tillage, Crop Rotation, and Nitrogen Rate Effects on Soil Carbon Stocks Over Time.

Soil organic carbon (SOC) level is controlled by different factors, but for given conditions it could be summarized as a balance between inputs and outputs. Consequently, the content of SOC in soils is related to the annual input of crop residue and the carbon (C) outputs from the soil. Cropping systems, fertilization and water availability determine productivity for a given environment, and therefore, regulate the amount of C returned to the soil. Tillage treatment on the other hand alters the decomposition rate of residues although it puts residue in intimate contact with soil particles. Given that annual changes in SOC are generally small compared to the SOC background, long-term experiments are necessary to observe C trends over time. Two long-term experiments (1985 and 1997) under contrasting management practices (tillage, crop rotation and nitrogen rate) were analyzed. Soil samples were taken at early stages of the experiment and used as a benchmark to analyze carbon trend over time. Carbon and nitrogen inputs (fertilization and crop residue) and outputs (grain harvest) were quantified annually. Results show no significant impact of carbon and nitrogen inputs on the rate of C change at either site in the last 12 years. Tillage treatment on the other hand has a significant impact on carbon change since more carbon is lost under moldboard plow than under no-till or disk. For these given conditions, SOC is decreasing under every tillage treatment analyzed, regardless of the annual C input.