Changes In Nutrient Status Of Grasses And Legumes Along An Aridity Gradient In Grasslands Of Northern China

Temperate steppe occupies about 78% of China’s total grassland area,mainly distributed in arid and semi-arid areas in northern China,which is an important part of the Eurasian steppe.In the north of China,from west to east,the grassland types were desert steppe,typical steppe,and meadow steppe.Since the temperate steppe in northern China is located in arid and semi-arid regions,the changes of precipitation is an important climatic factor affecting ecosystem structure and function in this region.Gramineae are the most widely distributed plant family in the temperate grasslands of northern China.They are the dominant species and the building species of many grasslands.Legume is an important plant functional group in grassland ecosystems,affecting nitrogen input and soil nutrient,which is essential for maintaining grassland sustainability and ecosystem functions such as biomass production and nutrient cycling.In this study,grasses and legumes were selected as representative plant group in the northern temperate steppe.Based on plant and soil samples collected from a large-scale survey transect across an aridity gradient in the temperate steppe of northern China,I examined the variations of key mineral nutrient concentrations and stocks in both grasses and legumes along the aridity gradient.At the same time,the mineral nutrition provided the basis for analyzing the differences of forage quality in different climatic regions.The results obtained in this study are as follows:(1)From west to east,species richness and aboveground biomass of grasses gradually increased along the transect.Species richness and aboveground biomass of grasses were positively correlated with mean annual precipitation(P<0.001),but negatively correlated with mean annual temperature and altitude(P<0.001).The species richness and aboveground biomass of grasses were higher in the eastern region and lower in the western region.From west to east,species richness and aboveground biomass of legumes decreased gradually,showing a trend of higher in the west and lower in the east.There was no significant correlation between species richness and aboveground biomass of legumes with mean annual precipitation,mean annual temperature,and altitude(P>0.05).(2)The concentrations of Ca,Mg and Na in grasses of each site increased with the increases of aridity(P<0.001),while K concentration decreased with the increases of aridity(P<0.01).There was a significant positive correlation between grass Ca,K,and Na concentrations and the corresponding element concentration in soil(P<0.01 or P<0.001).There was no significant correlation between grass magnesium concentration and soil magnesium concentration(P>0.05).There was no significant correlation between the concentrations of legume Ca,Mg,Na and aridity(P>0.05),but K concentration decreased with the increases of aridity(P<0.05).There was no significant correlation of calcium,magnesium,potassium,and sodium between legumes and soil(P>0.05).(3)There was a significant negative correlation between the stocks of Ca,Mg,K,Na and aridity(P<0.001 or P<0.01).The stocks of calcium,magnesium,potassium and sodium in grasses were lower in the western region and higher in the eastern region.There was no significant correlation between the stocks of all the four nutrients in legumes and aridity(P>0.05).(4)Fe and Mn concentrations in grasses increased with the increases of aridity(P<0.001).Zn concentration in grasses decreased with the increase of drought index(P<0.01),and there was no significant correlation between grass Cu concentration and aridity(P>0.05).There were significant negative correlations between grass and soil nutrient concentrations for Fe and Mn(P<0.01 or P<0.001).There was no significant correlation between grass and soil nutrient concentrations for Cu and Zn(P>0.05).There was no significant correlation between legume Fe,Cu,Zn concentration and aridity(P>0.05).Legume Mn concentration increased with the increases of aridity(P<0.05).There was no significant correlation between the concentrations of Fe,Mn,Cu,and Zn in legume and their corresponding elements in soil(P>0.05).(5)There were significant negative correlations between the stocks of Fe,Mn,Cu and Zn in grasses and aridity(P<0.001),but no significant correlation between the stocks of Fe and the aridity of legumes(P>0.05).The stocks of calcium,magnesium,potassium and sodium in grasses were lower in the western region and higher in the eastern region,while the stocks of iron,manganese,copper and zinc in the legumes were higher in the western region and lower in the eastern region.The results indicated that iron stocks were closely related to aridity and regional habitat with different herbaceous species.Results from this study shed light on the responses of species diversity and aboveground biomass of both grasses and legumes across the aridity gradient in the temperate steppe of northern China.More importantly,this work reveals the variations of mineral nutrition status of both grasses and legumes across the natural aridity gradient,and thus would help improve our understanding of regional biogeochemical cycles and evaluate the nutritional quality of herbage of key plant groups.