Grazing and heat stress protection of native grass by a sand-fixing shrub in the arid lands of northern China

Shrub species are used in restoration projects on dryland for their facilitation effects,which include environmental improvements and protection from herbivore feeding.Facilitation effects on forage grasses are potentially important in improving grazing capacity on rangelands.However,the morphology-dependent performance of benefactor plants in facilitating forage species growth and supplementation under moderate grazing intensity remains unclear.Here,our main purpose was to measure facilitation performance in terms of the survival of a native forage grass,Agropyron cristatum(L.)Gaertn.(Gramineae).,in accordance with the growth conditions of a sand-fixing benefactor shrub,Caragana microphylla Lam.,in the Hulun Buir Grassland,northern China.Six study sites with patches of A.cristatum and C.microphylla were established at the foot of fixed sand dunes.At each site,five quadrats were set in places where C.microphylla coverage was 100%and A.cristatum grew among the shrubs(shrub quadrats),and another five were set where A.cristatum grew alone without C.microphylla(grass quadrats).We measured the morphological traits of C.microphylla and A.cristatum in all 60 quadrats,along with the soil water content and soil temperature.The data were compared between the shrub and grass quadrats by generalized linear mixed-effect models to assess the shrub's facilitation effects.We also used such models to elucidate the relationship between the average height of C.microphylla and the morphological traits of A.cristatum in the shrub quadrats.The maximum height,average grazed height,and the number of seed heads of A.cristatum were greater in the shrub quadrats than in the grass quadrats.The soil surface temperature was lower in the shrub quadrats.The maximum height and seed head number of A.cristatum were positively associated with the average height of C.microphylla.These results suggest that the grazing impact and heat stress were smaller in shrub quadrats than in grass quadrats,and that the degree of this protective effect depended on the shrub height.The shrub canopy seemed to reduce the increase in soil temperature and keep the grass vigorous.Livestock likely avoided grazing grasses in the C.microphylla patches because of the shrub's spiny leaves;only the upper parts of the grass stems(including the seed heads)protruding from the shrub canopy were grazed.The sand-fixing shrub thus moderates the grazing impact and soil temperature,and contributes to vegetation restoration and grazing system sustainability.

用苯并噻冠乙醚和四溴荧光素萃取-荧光光谱法测定银离子

在阳离子萃取方面,苯并噻冠乙醚对银离子萃取的选择性比对另外一些重金属离子要高。银离子荧光光谱法的实现是使用一个离子对萃取体系,其中苯并噻冠乙醚作为配位体,四溴荧光素Y作为荧光阴离子。本法能测定2—10ppb的银离子。仅受到汞(Ⅱ)离子除外的碱金属,碱土金属和重金属离子的低干扰。本法成功地用于河水中微量银离子的测定。