Modeling the regional grazing impact on vegetation carbon sequestration ability in Temperate Eurasian Steppe

Anthropogenic activities profoundly influence carbon sequestration in the Eurasian Steppe. In particular, grazing has been identified as having a major effect on carbon sequestration. However, the extent to which grazing affects regional patterns or carbon sequestration is unknown. In this study, we evaluated the impact of regional grazing on grassland carbon seques- tration using the Boreal Ecosystem Productivity Simulator （BEPS） and the Shiyomi grazing model. Model performances were validated against the results from field measurements and eddy covariance （EC） sites. Model outputs showed that in 2008, the regional net primary productivity （NPP） was 79.5 g C m-2, and the net ecosystem productivity （NEP） was -6.5 g C m-2, characterizing the region as a Weak carbon source. The Mongol Steppe （MS） was identified as a carbon sink, whereas the Kazakh Steppe （KS） was either carbon neutral or a weak carbon source. The spatial patterns of grazing density are divergent between the MS and the KS. in the MS, livestock was mainly distributed in China with relatively good management, while in the KS livestock was mainly concentrated in the southern countries （especially Uzbekistan and Turkmenistan） with harsh environments and poor management. The consumption percentages of NPP in Turkmenistan, Tajikistan and Uzbekistan were 5.3, 3.3 and 1.2%, respectively, whereas the percentages in other countries were lower than 1%. Correspondingly, grazing consumption contributed to the carbon sources of Turkmenistan, Tajikistan and Uzbekistan by 11.6, 6.3 and 4.3%, respectively, while it weakened the carbon sink in Inner Mongolia, China and Mongolia by 1.6 and 0.5%. This regional pattern should be affected by different sub-regional characteristics, e.g., the continuous degradation of grassland in the southern part of the KS and the restoration of grassland in Inner Mongolia, China.

Grazing effects on herbage nutritive values depend on precipitation and growing season in Inner Mongolian grassland

Aims Grasslands are widely used for production of livestock,which depend on the nutritive value of herbage species.However,there are still large uncertainties as to how grazing,precipitation and growing season interactively affect herbage nutritive value.Methods Here,based on a grazing experiment with seven grazing intensities in an Inner Mongolian grassland since the year 2005,we analysed nutritive value of four dominant grass species from June to September in both a relatively wetter year(2008)and a drier year(2010).Herbage with high nutritive value is characterized by high concentration of crude protein,high concentration of cellulase digestible organic matter and low concentration of neutral detergent fibre.Important Findings We found that(i)grazing significantly increased the nutritive value of Leymus chinensis,Agropyron michnoi and Cleistogenes squarrosa but had minor effects on the nutritive value of Stipa grandis.(ii)For all species,nutritive values were greater in the wetter year than in the drier year and were greatest in the early growing season(June)and lowest at the end of the growing season(September).Inter-annual and inter-seasonal variations in nutritive value were much greater for L.chinensis and A.michnoi than for C.squarrosa and S.grandis,suggesting higher water use efficiency for the latter two species.(iii)Grazing significantly decreased the drought resistance of three species,but not of S.grandis.(iv)Grazing significantly increased inter-month variation in nutritive value for L.chinensis and A.michnoi but had relatively minor or no effects on that of C.squarrosa and S.grandis.Therefore,grazing effects on species nutritive values showed strong inter-annual and seasonal patterns,and species-specific responses might be related to species traits(i.e.water use efficiency,digestibility).Our results have important implications for ecosystem management of arid and semiarid grasslands under intense grazing and global climatic change.