The Dominance of Temperature in Governing Residue Decay at Diverse Sites Across Canada.

The rate of plant litter decay affects the functioning of ecosystems in several fundamental ways: it governs the amount of carbon stored in the soil, it reflects the rate of solar energy transfer to soil organisms, and it controls the rate of nutrient release. Understanding the factors that dictate residue decay is especially important in light of impending global changes, including those related to climate. We established a long-term experiment to measure the decay of plant residues at sites across the agricultural regions of Canada. Barley was grown to maturity in a greenhouse and labelled with ¹³C to an abundance of about 11 atom%. This residue was applied, in the fall of 2007, to microcosms at ten sites, selected to span a range of representative climates and soil types. After about 0, 0.5, 1, 2, 3, and 5 years, four replicates of microcosms were destructively sampled at each site, and analyzed for recovery of remaining residue-derived ¹³C. The results showed rapid initial decay of the residue – about 50 to 70% was lost after 1 year – followed by gradually-diminishing rates of loss, so that about 10 to 20% remained after 5 years. Temporal patterns of decay varied among sites, but a single two-pool exponential model effectively described the kinetics of decay for all sites when time was adjusted for temperature, using the concept of ‘thermal time’, defined as cumulative degree-years (calculated from measured soil or air temperature above a baseline of 0 degrees C). Soil properties, such as clay or sand content had minimal discernible influence on decay kinetics. These findings demonstrate that temperature is the primary driving variable for residue decay in cool climates like those in the agricultural regions of Canada. They show, further, that projected increases in temperature under various climate change scenarios may exert a strong and lasting influence on residue decay, microbial activity, and storage of carbon in soil. Our experiment is intended to continue for at least another decade, to measure the fate and stabilization of the remaining ¹³C applied in the residue. Findings from the Canadian sites will be melded with those from similarly-established experiments in other countries to extend the range of climate and soil types.