The Effects Of Grazing And Mowing On Plant Nutrient Resorption And Root Nutrient Accumulation In Autumn In A Typical Steppe

Plant nutrient resorption and root nutrient accumulation in autumn are important mechanisms for plants to improve nutrient use efficiency and adapt to nutrient-deficient environments.The typical steppe in Inner Mongolia is highly representative of temperate semiarid steppes on the Eurasian continent.Grazing and mowing are the main utilization types of grasslands,and have adverse effects on plant nutrient resorption and root nutrient accumulation,which is one of the important mechanisms for utilization-induced grassland ecosystem degradation.However,few studies have been done to explore the interrelations between plant nutrient resorption and root nutrient uptake and accumulation,and their responses to grassland use.Thus,we studied these processes in the typical steppe grassland in Inner Mongolia using a field experiment with grassland paddocks under moderate grazing,autumn mowing,or not used.We examined plant nutrient resorption and root nutrient accumulation at plant population level for dominant species Leymus chinensis,Stipa grandis and Cleistogenes squarrosa,at plant community patch level,the patches dominated by these three plant species,and at overall plant community level.By comparison among the treatments,we explored the effects of moderate grazing and autumn mowing on plant nutrient resorption and root nutrient accumulation,and quantified the respective contribution of plant aboveground nutrient resorption and root nutrient uptake to root nutrient accumulation in autumn.The main results are as follows:1.Compared to the grassland under no grazing nor mowing,under the moderate grazing treatment,plant leaf nitrogen（N）and phosphorus（P）concentrations of the three dominant species showed an increasing tendency,and plant community leaf N and P concentrations showed a significant increase.Under the mowing treatment,the leaf N and P concentrations of the three dominant species changed differently in different growth periods.At the plant biomass peak period,the aboveground N concentration of L.chinensis and C.squarrosa decreased significantly,while the aboveground P concentration of L.chinensis increased significantly;plant community aboveground N and P concentrations did not show a significant change;in the senescence period,the aboveground N or P concentrations of L.chinensis increased significantly,while that of S.grandis and C.squarrosa had no significant changes.2.Compared to the grassland under no grazing nor mowing,under the moderate grazing treatment,plant N and P resorption efficiencies of the three dominant species were significantly improved,except the N resorption efficiency of C.squarrosa.Under the mowing treatment,the N resorption efficiency of L.chinensis was significantly improved,while that of S.grandis and C.squarrosa did not change significantly.There was no significant change in N and P nutrient recovery efficiency of C.squarrosa;neither of plant community.3.During plant nutrient resorption in autumn,plant underground biomass of three dominant species did not change significantly,except that of L.chinensis and C.squarrosa under grazing which had a significant increase;the nutrient pool in plant roots of three dominant species also significantly increased,though the increase differs among species.Plant community underground biomass showed a significant increase under grazing,,but not under other treatments;and the nutrient pool in roots also increased significantly at community level.4.Compared to the grassland under no grazing nor mowing,moderate grazing significantly decreased plant underground biomass and N and P pools of L.chinensis,decreased plant underground biomass and N,but not P,pools of C.squarrosa,but not the underground biomass and N,P pools of S.grandis.Grazing also significantly reduced plant community underground biomass,but not N or P pools.No significant mowing effects were detected on the underground biomass and nutrient pools of the three dominant populations.5.Plant root nutrient accumulation depends more on root nutrient uptake under grazing and mowing.Compared to the grassland under no grazing nor mowing,moderate grazing reduced the contribution of plant community aboveground N ＆ P resorption to root N ＆ P accumulation by 23% and 20%,respectively;mowing reduced the contribution by 8% and 15%,respectively;In contrary,the contribution of plant nutrient uptake increased.This study helps to understand the important mechanisms of grassland utilization affecting nutrient cycling within plants and in grassland ecosystems.