Relationships between plant communities and soil carbon in the prairie ecozone of Saskatchewan

Abstract

Accumulation of CO2 in the atmosphere has triggered research on topics related to causes, effects, and solutions to potential problems associated with global warming. The present research was conducted to determine if grassland plant communities can be managed to promote sequestration of carbon in the soil, potentially mitigating the effects of increasing atmospheric CO2. The effects of shrub invasion or heavy livestock grazing on peak standing crop of phytomass, root mass and soil organic carbon content were therefore studied. These studies were complimented by a study of the decomposition rates of leaves and roots of snowberry and grasses. The effects of snowberry encroachment on peak standing crop of aboveground phytomass, and soil organic carbon content (SOC) were also studied. Total aboveground phytomass in the snowberry community was more than triple that of the ecotone and was 6-times greater than that of the grassland community. Similarly, the mass of large roots was greatest in the snowberry community (1.2 kg m-2, SE= 0.19), intermediate in the ecotone (0.5 kg m-2, SE= 0.08), and least in the grassland (0.1 kg m-2, SE= 0.04). Conversely, the mass of fine and medium roots was not different (P>0.05) among the three communities, averaging 0.7 kg m-2 in all communities (SE= 0.03, 0.07, 0.49 in snowberry, ecotone and grassland, respectively). Greater aboveground phytomass did not correspond with greater SOC in the snowberry community. Soil organic carbon in the upper 50 cm averaged 8.3 (SE= 0.7), 7.9 (SE= 1.0), and 7.9 (SE= 0.7) kg m-2 in snowberry, ecotone, and grassland communities, respectively. Peak standing crop of aboveground phytomass averaged 157 g m-2 (SE= 27) and 488 g m-2 (SE= 48) in grazed and ungrazed grassland, respectively. Conversely, grazing had no affect on root mass. The mass of fine roots averaged 0.9 kg m-2 (SE= 0.04) and 0.8 kg m-2 (SE= 0.06) in grazed and ungrazed grassland, respectively, while that of medium roots averaged 0.6 kg m-2 (SE= 0.07) in both grazing treatments. Total SOC in the upper 50 cm of soil was not affected (P>0.05) by livestock grazing, averaging 5.5 kg m-2 (SE= 0.7) in grazed and 6.8 kg m-2 (SE= 0.9) in ungrazed grassland. Livestock grazing also had no effect (P>0.05) on SOC at the 0-3, 3-10, 10-20, 20-30, and 30-40 cm depths. The SOC in fine- and coarse-textured soils averaged 7.6 kg m-2 (SE= 0.8) and 5.1 kg m-2 (SE=0.7), respectively. Differences existed between decomposition of roots and leaves for graminoids and snowberry. On a monthly basis decomposition was 0.6 to 0.8 % greater in leaves than roots. The decomposition of roots and leaves ranged from 2.2 to 5.0 % month-1. Decay rate constants for leaves ranged from 0.45 yr-1 (SE= 0.03) to 0.71 yr-1 (SE= 0.02) while those of roots ranged from 0.34 yr-1 (SE= 0.03) to 0.47 yr-1 (SE= 0.04). The decomposition of roots and leaves did not correspond with macroclimatic or regional climate data nor with initial C:N content of the plant material. In summary, invasion of snowberry into grassland does not appear to conflict with goals related to maintenance of SOC in Mixed Prairie. Current grazing management regimes also appear to be consistent with goals related to maintenance of existing SOC. Soil texture had a greater effect on SOC than management of the plant community. Decomposition of leaves and roots appeared to be controlled by many interacting factors such as plant organ type, collection year, study year (climate) and physical and/or chemical characteristics of the site.