Land-Use and Climate effects on Soil Organic Carbon at Hovsgol Lake, Mongolia: The Role of Grazing and Permafrost

Undertaken as Senior Honors Thesis through the Hamburg Lab at the Center for Environmental Studies, Brown University. Support through the Academy of Natural Sciences, Philadelphia, and the Hovsgol Global Environment Facility, Mongolia.

(Read my whole thesis or view the presentation)

Approximately 15% of global soil carbon is stored in permafrost soils. In Central Asia, the melting of the southernmost belt of permafrost soils combined with increasing use of land for pasture has the potential to accellerate the decomposition of C in these soils and create feedbacks to the global C cycle.

I studied the amount, vertical distribution, and potential mineralization of soil organic carbon in four sites at the Hovsgol Global Environmental Facility, Mongolia. Two sites were lightly grazed and two heavily grazed, and one in each grazing level was underlain by permafrost. In the site without permafrost, soil C (0-110 cm) was 7.40 \(\pm\) 0.8 kg m2 and there was no difference between the lightly and heavily grazed sites without permafrost. In sites with permafrost, the permafrost table was deeper where there was heavy grazing than where there was light grazing, and C storage was more than four times greater (30 \(\pm\) 15 kg m2) in soil above the permafrost at the light grazing site than to 110 cm at the heavy grazing site (7.1 \(\pm\) 0.7 kg m2).

Except for the permafrost underlain, lightly grazed site, C mineralization from soils in 90 day incubations was not statistically different among sites. The lightly grazed permafrost site had the greatest total mineralized C per unit area, but less than the other sites as a fraction of total soil C.

Grazing may lead to the loss of large amounts of soil C through erosion of peat layers and indirectly by soil warming due to the loss of the insulating peat layer, and consequent lowering of the permafrost table. However, the limited extent of lightly grazed grasslands underlain by permafrost suggest that any positive feedback from global warming will be limited.

The observed patterns of C distribution and mineralized C among sites suggests a relationship between grazing, permafrost, and C cycling that requires further research.

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