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University of Tsukuba (2014)

Effects of Large-Scale Agriculture and Livestock Industry on the Vegetation and Soil Properties in the Steppe Region of Kazakhstan and Inner Mongolia, China

Shinchilelt

Titre : Effects of Large-Scale Agriculture and Livestock Industry on the Vegetation and Soil Properties in the Steppe Region of Kazakhstan and Inner Mongolia, China

Auteur : Shinchilelt

Université de soutenance : University of Tsukuba

Grade : Doctor of Philosophy in Agricultural Science 2014

Résumé
The grasslands in the semi-arid regions of the Eurasian Continent are known as the steppe. In recent years there has been great concern about increases in steppe soil degradation, vegetation degeneration, and soil erosion due to the effects of grazing and increasing cropland coverage driven by population growth. A range of measures have been introduced in different countries experiencing these pressures in order to protect grassland ecosystems, with varying levels of application and enforcement. These include banning or deferring grazing to control overgrazing and forcing the abandonment of croplands to mitigate the effects of excessive cultivation. These measures are expected to promote the recovery of grassland ecosystems. However, they do not always lead to the recovery of vegetation and soil at the targeted sites. We therefore need a better understanding of the recovery of vegetation and soil at sites where these policies are enforced. This thesis describes field studies conducted in abandoned grazing areas and croplands in Kazakhstan, and abandoned croplands of Inner Mongolia to characterize their vegetation and soil properties. The objectives were to investigate the recovery patterns for soil and vegetation in the area and to clarify the effects of deferred grazing, which has been one of the preferred policies for promoting vegetation recovery in steppe ecosystems. In the steppe regions of Kazakhstan, data were collected from a total of 35 plots across six study sites using phytosociological methods. It was found that the vegetation and soil at abandoned grazing sites and croplands took at least 13 years to start recovering. In some abandoned grazing areas, the recovery of the vegetation was incomplete even after 27 years, and there was no evidence of soil recovery. In abandoned croplands, vegetation recovery was observed in sites abandoned for 50 years but soil recovery was not achieved. The recovery of vegetation was found to depend on precipitation, and soil recovery was poor in dry regions. This demonstrates that the speed of vegetation and soil recovery depend on land use patterns and precipitation levels. The main factors that hinder grassland recovery are the slow soil recovery observed across the study area as a whole and the particularly slow recovery of soil and vegetation in the dry regions.

It has been demonstrated that the level of soil erosion in abandoned croplands is much higher than in natural grassland, which will have significant effects on vegetation and soil recovery at these sites. To quantify this erosion, 137Cs levels were measured at sites in Inner Mongolia. In wet regions where cultivation had been abandoned for 20 years, partial recovery of vegetation was observed but there was no such recovery of soil microorganisms or soil physicochemical properties. In dry regions where cultivation had been abandoned for 20 years, no recovery of vegetation or soil was observed. Soil erosion was identified as a key limiting factor in the inhibition of recovery. The rate of soil erosion increased with the length of time for which the plot was cultivated and was higher at recently abandoned sites and those with low precipitation. One key aim of this work was to determine whether deferred grazing policies are effective at promoting grassland recovery under different precipitation conditions in over-grazed areas of Inner Mongolia. Based on a two-way ANOVA, vegetation recovery will deferred grazing will promote vegetation recovery in wetter regions but not dry ones. That is to say, the capacity for recovery after deferred grazing depends on precipitation. The results obtained suggest that recovery in dry regions will not occur for at least 8 years after site abandonment. Overall, the results obtained clearly show that over-grazing and cultivation are the main reasons for steppe desertification. The rates of soil erosion in abandoned croplands were higher than in untouched steppe sites. It was clear that the extent of recovery for both vegetation and soil nutrient content increased with the time since abandonment, but no clear trend towards recovery was observed at low precipitation sites. This demonstrates the impact of the desertification crisis. In order to protect grassland ecosystems, it will be necessary to completely prohibit inappropriate cultivation, and to impose the use of grazing systems that are appropriate for the prevailing conditions, particularly the region’s level of precipitation.

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