Content and distribution of organic matter in natural and managed ecosystems in the mixed prairie grassland region in Saskatchewan

Abstract

Weight of organic matter aboveground and underground in ungrazed natural grassland in sand-, loam-and clay-textured soil and the effects of grazing and cultivation in modifying the weight and distribution of organic matter in loam-textured soil were studied near Saskatoon, Saskatchewan. Mean weight of organic matter in ungrazed sand, loam and clay sites was, respectively, 213, 358 and 262 g/m2 canopy; 274, 299 and 263 g/m2 mulch; and to to a depth of 35 cm, 1 004, 1 480 and 1 218 g/m2 underground plant parts; and 14.2, 24.5 and 28.3 kg/m2 soil organic matter. The proportion of total system organic matter aboveground and underground as plant parts was respectively, 3.1 and 6.4 percent in sand soil, 2.4 and 5.6 percent in loam soil and 1.7 and 4.1 percent in clay soil, the remaining proportion in each case being soil organic matter. Heavy grazing in loam soil transformed a Festuca, Agropyron, Stipa community to a Stipa, Agropyron, Festuca community, and reduced organic weight of the canopy from 258 to 82 g/m2, mulch from 299 to 186 g/m2 and soil organic matter in the upper 15 cm by 16.7 percent in comparison to ungrazed natural grassland. Weight of underground plant parts was not affected by grazing. Replacement of the dominant grasses with shallow-rooted species such as Koeleria cristata, Carex spp., Bouteloua gracilis and Phlox hoodii apparently maintained under-ground biomass. Aboveground biomass accounted for 1.1 percent and underground biomass accounted for 6.2 percent of the total system organic matter in grazed sites. Cultivation of loam soil for a period averaging 21 years reduced soil organic matter weight by 26 percent in the plough layer. The decline in soil organic matter content in grazed and cropped land is attributed to a reduction in the amount of plant material returned to the soil under grazing and cropping and the increased rate of decomposition as a result of tillage. The significance of the reduction in organic matter content is discussed.