Sustainable mangement of natural rangeland ecosystems

Abstract

Rangelands, a natural ecosystem widespread in Canada, are an important source of land for animal production. Around 26 million ha of rangelands are utilized for beef production and in Alberta, this ecosystem accounts for 6.6 million ha which provides up to 50% of total forage for livestock. To improve our understanding of how topography and grazing influence the nutrient cycling on a semiarid grassland, in the first part of my research I investigated the effect of those two factors on litter decomposition through a field experiment. The results showed that topography affected shoot litter decomposition -was faster down-slope, and associated with greater soil moisture-, and that grazing by altering litter chemical composition influenced its decomposition; the lignin content in the litter was linked with the remaining mass. I also investigated topography and grazing effect on plant and soil carbon stock through ecosys, a comprehensive mathematical model for natural and managed ecosystems. Ecosys modeled, as found in the field, a topographic effect on total aboveground plant carbon which was greater down slope, which was explained by better soil conditions at lower topographic location. Simulated carbon balance under current climate showed that uplands, independently of grazing, was net carbon source, and opposite to the condition found at lower locations. The average carbon balance without grazing from 2003 to 2005 showed that low and medium elevations were carbon sink of 132 and 12g C m-2 y-1 respectively, but adjacent uplands released 4 g C m-2 y-1. Simulated grazing reduced net biome productivity at all topographic locations (35, 37 and 51g of C m-2 y-1) and upper and middle elevations were carbon sources, but low elevation was still carbon sinks (81g m-2 y-1). Under climate change soil organic carbon increased more without (13, 9 and, 15 g C m-2 y-1) than with grazing (10, 5 and, 11g of C m-2 y-1 at upper, middle and, lower topographic locations), but all locations were carbon sink. There was conclude that, if properly set, grazing will not turn into carbon source this semiarid grassland ecosystem, which will be benefit by climate change.