Management Impacts on Organic Carbon Under Continuous Perennial Grass, Perennial Grass-Legume Mixture, And Annual Cereals On A Thick Black Chernozemic Soil

Impacts of annual and perennial pasture management on soil organic carbon (SOC) and equivalent SOC stocks (equal soil mass basis) were investigated in two trials [(CAESA (1994 to 1997) and BMP (2008 to 2012) trials] conducted on the same experimental paddocks at Lacombe, Alberta. The original site was broken from perennial grass in 1992 and the CAESA trial established in 1993. Between 1994 and 1997, half of the paddocks included winter triticale and a mixture of triticale and spring barley; half included smooth and meadow bromegrass; and each paddock was light, medium, or heavily grazed. The BMP trial (2008 to 2012) on the same paddocks included fertilized, direct seeded barley as silage; grazing and haying of unfertilized meadow bromegrass, fertilized meadow bromegrass, and meadow bromegrass and alfalfa mixture; and unfertilized oldgrass that was continuous since 1994. Between trials (1998 to 2007) all paddocks received no fertilizer. In the 0-15 cm depth SOC under oldgrass was constant between 1994 and 2012 and averaged 88 Mg C ha-1. Under barley silage SOC decreased from 89 to 72 Mg C ha-1 by 2012. Between 1994 and 2012 SOC decreased in all treatments re-established on original annual forage (1994 to 1998) but not to the level of barley silage. Light fraction carbon (LFC) was highest under oldgrass and lowest under barley silage. Overall, oldgrass with no fertilizer inputs maintained a constant SOC, although annuals reduced SOC stocks. Re-establishment of perennial grass with grazing may therefore reduce SOC loss whereas haying perennial grass may not.