Soil Ecosystem Responses to Climate Change and Land-Use Simulations and Estimation of Carbon Stocks in Steppe and Forest Ecosystems in Northern Mongolia

Northern Mongolia currently sequesters 31 Tg C yr-1 but it may become a carbon source if respiration rates increase due to climate change and overgrazing, or if projected boundary shifts between forest and steppe cause a change in the carbon storage of ecosystems. The objectives of the thesis are to study soil ecosystem response to simulated climate change and grazing, and to assess C stocks in the steppe and forest. Open-top chambers (OTCs) have been frequently used for simulating climate change. However, the pattern of temperature increase by OTCs contradicted the IPCC predictions. An alternative method, open-sided chambers (OSCs), was evaluated based on its effects on abiotic and biotic factors. The results indicated that OSCs manipulated air temperature in a pattern that was predicted by IPCC models, but the overall effect was too small, hence it is not an optimal device. In the subsequent study, OTCs were used to study soil respiration response to experimental warming in three ecosystems. Temperature increase by OTCs had no effect on soil respiration in the steppe but increased soil respiration in the forest (by 0.20 g CO2 m-2 h-1), demonstrating the importance of ecosystem setting. Although warming increased soil respiration, it decreased its temperature sensitivity as well (Q10 = 5.82 in control versus 2.22 in OTC). In addition to OTCs, watering and grazing effects on CO2 effluxes (ecosystem and soil respiration) were studied across the topographical gradients in the steppe. Our results show a robust, positive effect of soil moisture on CO2 effluxes across topography, and the contrasting effects of grazing on CO2 effluxes. Interactive effects of the treatments were minimal. Soil carbon of the forest was the same (8.3 kg C m-2) as the steppe (8.1 kg C m-2) but aboveground carbon in the forest (2.9 kg C m-2) was 3-7 times greater than that in the steppe. In summary, the results show that warming will slightly increase soil respiration in the forest, but in steppe precipitation will have stronger effect on CO2 flux than temperature change. The results also indicated that overgrazing and deforestation could trigger a greater loss of carbon