Effects of reindeer grazing on thermal stability of organic matter in topsoil in Arctic tundra

Reindeer grazing is a geochemical factor of carbon (C) and nutrient cycling in high-latitude ecosystems. Reindeer consume and trample aboveground biomass, and return nutrients by urine and faces on soil surface. Degradation of vegetation cover due to overgrazing induces soil warming, decreases soil organic matter (SOM) content, and affects its stability. We investigated the effects of reindeer grazing on the thermal stability and temperature sensitivity (Q10) of SOM decomposition in the Gydan Arctic tundra. Thermogravimetric analysis of the topsoil (0–5 cm) from grazed and ungrazed tundra was used to divide SOM into three thermal pools – labile, stable, and persistent. The thermally labile pool contributed 62 % to SOM, reflecting large portion of partly decomposed plant litter. The thermal stability of SOM was evaluated by the temperature, at which the half of the organic matter was lost (T50), and by the activation energy of SOM combustion (Ea TG). Reindeer grazing increased the thermal stability of SOM in tundra by decrease in the thermally labile SOM content, as well as increase in T50 and Ea TG. The decrease in the labile SOM content in grazed compared to ungrazed tundra explained the reduction of soil microbial respiration at sites under reindeer grazing. The CO2 efflux from the ungrazed and grazed soils at increasing temperatures was used to investigate the Q10 of SOM decomposition. A 10 °C increase in soil temperature accelerated microbial SOM decomposition by 3.1 and 2.9 times in ungrazed and grazed tundra soils, respectively. Concluding, reindeer grazing in the Arctic tundra decreased SOM content in topsoil and increased the thermal stability of the remaining organic matter.