Greater accumulation of soil organic carbon after liquid dairy manure application under cereal-forage rotation than cereal monoculture

As opposed to the well-known positive effect of solid animal manure on soil organic carbon (SOC) stocks, the effect of liquid animal manure is less clear. In addition, few long-term studies have measured the combined effects of animal manure application and management practices such as tillage or the inclusion of perennial forages in annual cropping systems on SOC stocks. The objective of this paper was to determine the response of SOC stocks along a 0-50 cm soil profile to 21 years of liquid dairy manure (LDM) application compared to mineral fertilizer in combination with two fall primary tillage practices (moldboard plowing vs. chisel plowing), and two crop rotations (cereal monoculture vs. cereal-perennial forage rotation) on a silty clay soil at Normandin, Quebec, Canada. The effects of LDM application on SOC stocks were apparent only in the top 20 cm of the 0-50 cm soil profile, with overall higher SOC stocks compared to mineral fertilizer. The magnitude of the LDM-induced change in SOC was also dependent on the crop rotation, with a 15% greater SOC accumulation in the 0-30 cm soil profile in the cereal-perennial forage rotation than in the cereal monoculture. The large retention of LDM-induced C under the cereal-perennial forage rotation could be explained by (1) slower C decomposition resulting from the lower tillage frequency in the cereal-forage rotation (every three years) compared to the cereal monoculture (yearly operation), and/or (2) greater C stabilization promoted by the presence of a high-quality labile litter (perennial legume) in the rotation. We conclude that LDM-induced changes in SOC can be strongly modulated by the inclusion of perennial forages in annual cropping systems, and may not be just the consequence of manure characteristics.