Effects of native and tame grassland species reintroduction on carbon sequestration potential on the Canadian Prairies

Abstract Text:

Rising concentration of carbon dioxide in the atmosphere has prompted interest in implementing improved grassland management practices that could lead to net accumulation of carbon in grassland soils.  Converting cropland into native or tame perennial grasslands may result in substantial increase in soil C sequestration.  Two studies were started in Southern Saskatchewan where semi-arid cropland was converted to perennial grasslands: Study 1 (2000-2014) seeded two different native pasture mixes (Simple: 7 species and Diverse: 12 species) and, Study 2 (2006-2011) seeded four different pasture types [meadow bromegrass + alfalfa (A), native grass mix (NG), NG + A, and NG + native legume]. The objective of the studies was to determine the change in soil organic carbon (SOC) levels as affected by type of forage pasture mix and form of disturbance (grazing and non-grazing).  Study 1: the disturbance treatments were continuous, rotational and non-grazing, and the stocking rates were 0.8 and 1.9 animal unit (AU) ha^-1, respectively.  Study 2: continuous grazing occurred, and the stocking rate ranged from 2.0 to 4.0 AU ha^-1 depending on which pasture treatment was used.  All pastures were grazed to a utilization rate of 50 to 60%.   Soil samples from each pasture were collected from three locations and at each location a five radial (star pattern) sampling pattern occurred.  From each of the five micro-sites, core samples were taken at five depths (0-7.5, 7.5-15, 15-30, 30-45 and 45-60 cm).  Soil sampling for study 1 occurred in 2000, 2004, 2008, 2011 and 2014, while in study 2, it occurred in 2008 and 2011.  In study 1, no SOC level (0-15 cm) differences were observed between disturbance and pasture mix combinations and interaction after 14 production years.  SOC levels were affected by year (P<0.0001) which was expected with the different environmental conditions experienced among the different soil sampling years.  In study 2, no SOC level (0-15 cm) differences were observed for interaction or main effects after three production years.  Our studies did not support our hypothesis that a more diverse native mix (higher species richness) and tame grass + alfalfa would have higher SOC level than other treatments.  Detecting small SOC change is difficult due to spatial heterogeneity in initial SOC, soil texture, bulk density, and plant productivity. Using our results we develop criteria for measurement systems to do a detect changes design to detect SOC change.

Keywords: grazing, soil organic carbon, native and tame forages