Greater retention of manure-derived carbon under perennial than annual crops

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, the response of SOC stocks to animal manure may vary with management practices usually combined with manure application, such as tillage or cropping system including perennial forages or continuous cereals. The objective of our work was to determine the response of SOC stocks along a 0-50 cm soil profile to long-term (21 years) application of liquid dairy manure (LDM) in combination with two fall primary tillage practices (moldboard plowing vs. chisel plowing), and two cropping systems (cereal monoculture vs. cereal-perennial forage rotation). Along the entire soil profile, the effects of LDM application on SOC stocks were apparent only in the top 20 cm. The magnitude of the manure-induced changes in SOC was dependent on the cropping system, with a much greater effect of LDM application compared to mineral fertilization in SOC stocks in the cereal-perennial forage rotation than in the cereal monoculture. The retention of manure-derived C in SOC appeared to be much greater under the perennial-based rotation 72 than under the cereal monoculture. To the contrary, the primary tillage practice did not influence the accumulation of manure-induced SOC. It therefore appears that the lower frequency of tillage in the perennial-based rotation (every three years) than in the monoculture (yearly operation) favoured the accumulation of manure-derived C in soil, whereas primary tillage practice (moldboard vs. chisel plow) had little influence.