Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Effects of rotational grazing intensity on the quantity and quality of soil organic carbon in Inner Mongolian grasslands

Auteur : Kok, Nora Hua Ly

Université de soutenance : Norwegian University of Life Sciences

Grade : Master Thesis 2021

Résumé partiel
The aim of this thesis is to assess the effects of rotational grazing at varying grazing intensity on soil organic carbon (SOC) quantity and quality in Inner Mongolia. People of the autonomous region of Inner Mongolia, China, have a long tradition of pastoralism. The semi-arid region contains one of the largest grassland ecosystems in the world and is severely affected by land degradation and the loss of soil organic matter due to overgrazing. The present study quantifies different SOC fractions down to one metre depth as an indicator of ecosystem health, at a rotational grazing site in Xilinhot, Inner Mongolia. Soil samples were collected from pastures under rotational grazing at three grazing intensities in addition to a reference plot, where grazing is excluded. Four grassland treatments were thus defined by the livestock densities 0 sheep/ha/year, 0.64 sheep/ha/year, 1.28 sheep/ha/year and 2.56 sheep/ha/year for non-grazed (NG), light grazing (LG), moderate grazing (MG) and high grazing (HG) intensity, respectively. Eight years of ongoing rotational grazing showed a tendency of highest SOC content in NG, and SOC depletion with increasing livestock density. HG was consistently an exception to this trend, where SOC content tended to increase in comparison to MG and reach concentrations close to NG. Changes in SOC content across treatments were not significant. However, the pattern was seen for both total SOC, particular organic carbon (POC), mineral associated organic carbon (MOC) and hot water extractable carbon (HWEC) in the upper 20 or 30 cm of the soil. HWEC was the only fraction that showed significant impact of treatment and was significantly higher in NG and LG compared to MG and HG. Following the general trend of total SOC, HWEC also tended to increase from MG to HG in the upper 20 cm. Results suggest that the upper 20 cm of the soil are most sensitive to grazing. In studies of continuous grazing, both MG and HG, which are representative for local practices of pastoralism in Inner Mongolia, are shown to reduce SOC levels. The present study shows that even rotational grazing with HG is below the threshold of sustainable grazing, i.e. the pressure that maintains SOC storage and overall ecosystem health. All three rotationally grazed treatments could thus be considered alternatives to grazing exclusion and continuous grazing. Overall, changes in SOC content and their stability were minimal and should not be used forcefully as an argument for choosing a particular management practice among the four treatments. However, tendencies in SOC fractions indicate possible future SOC enhancement under HG. As HG also more closely represents the generally preferred livestock densities, HG rotational grazing may be a viable management practice in Xilinhot

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