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Steppe Soil Respiration and Decomposition Rates

Avirmed Otgonsuren, Clyde E. Goulden and Danzan-Osor Avaadorj

Soil respiration composes almost half of the carbon released from ecosystems to the atmosphere. Carbon dioxide is released from the soil as a result of the decomposition of organic matter primarily plants, respiration by the decomposer microorganisms and plant root respiration. Within the Hovsgol project studies our objective has been to understand characteristics of the soil C cycle, and to determine how soil respiration and decomposition processes are modified by intense pasture grazing and climate warming. A second objective has been to measure decomposition rates of steppe vegetation to better understand the role of the accumulation of dead organic matter as mulch in insulating soils, reducing soil temperature and the loss of soil moisture in steppe habitats. Conservation of soil moisture is essential to plant growth in the semi-arid climates of northern Mongolia and Central Asia.

Soil respiration has been measured with an infrared gas analyzer equipped with soil respiration chamber. Soil respiration rates using this method were measured in 7 plots for each steppe transect of the six valleys during 2003, 2004 and in 2005. The soil respiration rate data indicate that during cold and wet years, soil respiration depends on air or soil temperature, with higher rates on south facing slopes and grazed sites associated with the higher temperatures of these slopes. However, during warm years; soil respiration is higher on north facing slopes, riparian zones and ungrazed sites. Soil microclimate and substrate quality are likely the major proximal controls over respiration; temperature appears to be the major limitation to respiration for much of the landscape. Similarly, though the influence of grazing was small, the effects may be interpreted as the response to changes in soil microclimate, with higher soil temperatures under heavy grazing that resulted in higher respiration rates during the coolest year.

Decomposition rates of steppe vegetation were measured using litterbags made of polypropelene at the bottom and 1 mm mesh netting on the top. A grazed south-facing slope was selected to compare with an ungrazed south-facing slope to determine if grazing had an impact on decomposition rates of steppe vegetation. In addition, we selected a grazed north-facing slope to make comparisons between north- and south-facing slopes. From each site, decomposition rates for three dominant plant species’ were measured between July 2003 and June 2004. Decomposition rate data of Potentilla acaulis and Poa attenuata placed in both south- and north-facing slopes suggests that decomposition rates are higher in north-facing slope. Decomposition rate data of the three dominant plant species of each site shows that forbs and sub-shrubs of the genus Artemisia decay relatively rapidly compared with grass in each of the sites. Therefore, because grasses dominate in ungrazed sites, dead plant tissue or mulch tends to stay on the ground surface longer than forbs and Artemisia shrubs that are the dominants in the heavily grazed slopes. Decomposition rates are higher in ungrazed sites.

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