Seasonal Dynamics Of Plant Nitrogen Allocation,nitrogen And Phosphorus Concentrations And Their Responses To Grazing In A Typical Steppe

Grassland plays a key role in the development of animal husbandry andmaintains regional ecological security.However,grazing-induced grassland degradation has provided challenges the sustainable development of grassland ecosystems.The plant nutrient content in grassland may reflect the nutrient status of plant health and its habitat,and plant’s adaptability to the environment.It is also an important parameter characterizing the nutrient cycling of ecosystems.Exploring the seasonal dynamics of plants’nutrient uptake and allocation in grassland and its response to grazing may provide a theoretical basis for rational utilization of grassland ecosystems.Most studies on plant nutrients in temperate grasslands focus on the nutrient dynamics of aboveground plant parts and the process of nutrient recycling in autumn,while the dynamic processes of plant nutrient uptake,allocation and dynamics during the complete growth cycle are less explored.This study was conducted on an experimental platform for grassland grazing in a typical steppe region of Inner Mongolia.The experiment included two treatments,comparing the plant nutrient dynamic characteristics under-grazing versus no-grazing condition（replicated by 4blocks）.In each block,plant-soil micro-areas were established for different community patches with different dominant species.and the N content and ¹⁵N abundance in soil and different organs of these micro-area were determined by using ¹⁵N(K¹⁵NO₃)isotope tracer techniques at the four key plant growth stages within a full year（i.e.,the‘before regreening’,‘after regreening’‘peak-growth’and‘autumn senescent’stages）,to understand the dynamics in plant N absorption,allocation and N content and their its response to grazing.The main results were as follows:（1）Grazing has no effect on the seasonal dynamics patterns of plant community aboveground and belowground biomass,but significantly reduced belowground biomass at all growth stage and aboveground biomass at peak-growth and autumn senescent stage.Grazing also promoted plant regeneration and nutrient uptake to compensate for the loss in the plant’s aboveground nutrient pool caused by defoliation and maintain the relative stability of the plant’s nutrient pool.（2）Grazing had an effect on the stoichiometry of plant community.Grazing significantly increased the N content of and reduced C/N at all plant growth stage,and increased the P content and reduced C/P at peak-growth and autumn senescent stage.The N and P content of the plant community was highest in aboveground tissues after re-growth and lowest during senescence,while the N and P content in belowground tissues was highest during senescence。（3）Grazing and season had effect on plant community N uptake and allocation between aboveground（shoots）and belowground parts（roots）.After plant regreening and during senescence stage,the plant community allocated more N to below-ground tissues;while allocated more N to aboveground tissues during peak-growth stage.Grazing significantly enhanced the allocation of N to aboveground tissues during peak plant growth（4）Grazing did not change the seasonal dynamics of plant available N and P in soil however it is significantly increased plant available N in soil between March and May and plant available P in soil after regreening.At the peak growth,grazing had no significant effect on plant available N and P in soil.Grazing enhances the effectiveness of soil nutrients through livestock excreta.However,with the interaction of defoliation and the return of manure and urine,the boosting effect of livestock excreta on soil nutrients is offset by the negative effect of defoliation on soil nutrients.This study is important for understanding the effects of grazing on the nutrient uptake,allocation and utilization of grassland plants,and also provides scientific data and a theoretical basis for the precise management of nutrients in grassland ecosystems.