Response Mechanisms Of Three Typical Plants Nitrogen And Phosphorus Cycling To Nitrogen Deposition In Temperate Meadow Grasslands

The recycling of nutrient elements is one of the main functional processes of terrestrial ecosystem.The nutrient cycle and balance directly affect.the productivity and the stability of ecosystem.Atmospheric nitrogen deposition is a critical part of nutrient cycle and its impact on ecosystem structure and function has become the focus of global change ecology.As one of the important ecosystem types in the world,temperate meadow steppe provides valuable ecosystem services and functions for human society.The increase of nitrogen deposition and the diversity of its components lead to significant changes in the structure and function of temperate meadow steppe,and affect plant nutrient uptake,nutrient resorption and litter decomposition,thus affecting the biogeochemical cycle process.The distribution and metabolism of nitrogen and phosphorus in plants determine the growth process and productivity of plants.Therefore,exploring the response of plant nitrogen and phosphorus nutrient cycling to nitrogen deposition has become a hot ecological issue.Plant nutrient uptake,nutrient resorption and litter decomposition play an important role in the nutrient cycling process of ecosystem.This study closely combined these three processes to carry out experiments with different nitrogen dosages and types,and systematically explored the response of nitrogen and phosphorus nutrient cycling to nitrogen deposition.In this study,three nitrogen compounds(NH4HCO3,NH4NO3,(NH4)2SO4)and six nitrogen supplemental dosages(0,2,5,10,20,50 g N m-2yr-1)were selected to study three common species(Leymus chinensis,Thermopsis lanceolate and Potentilla bifurca)in temperate meadow steppes.Response of nutrient uptake and nutrient recovery to nitrogen deposition to explore the effects of different nitrogen dosage and types on nutrient uptake and nutrient recovery of stems and leaves of different species.Meanwhile,stem and leaf litters of Leymus chinensis,the dominant species in temperate meadow steppe,were selected to explore the effects of nitrogen deposition on the decomposition of litters.In addition,manganese is a key resource to regulate the decomposition of litters.Therefore,in the late stage of litters decomposition,the effect of manganese addition on the decomposition rate of litters was discussed through comparative experiments.This paper mainly draws the following conclusions:(1)Nitrogen deposition has a significant effect on plant nutrient uptake.As for nitrogen nutrient uptake,nitrogen nutrient uptake in mature stems and leaves of the three species increased with the increase of nitrogen supplemental level.Nitrogen addition type had no significant effect on nitrogen content of mature leaves,but had significant effect on nitrogen content of mature stems:the stem nitrogen content was lower when applying NH4HCO3,while the stem nitrogen content was higher when applying(NH4)2SO4.As for the nutrient uptake of phosphorus,the phosphorus uptake of stems and leaves of the three species showed a downward trend or no significant difference with the increase of nitrogen supplemental level.Nitrogen addition type had no effect on the content of phosphorus in mature leaves,but had significant effect on the content of phosphorus in mature stems:the content of phosphorus in stems was higher when NH4HCO3 was applied,and the content of phosphorus in stems was lowestt when(NH4)2SO4 was applied.(2)Nitrogen deposition has a significant effect on plant nutrient resorption.With the increase of nitrogen supplemental level,the leaf nitrogen nutrient resorption efficiency of Leymus chinensis and Potentilla bifurca showed a downward trend,while the leaf nitrogen nutrient resorption efficiency of Thermopsis lanceolate showed a trend of first increasing and then decreasing.The stem nitrogen nutrient resorption efficiency of the three plants increased first and then decreased.In addition,the nitrogen resorption efficiency of leaves was higher than that of stems.Nitrogen compound type had no significant effect on nitrogen nutrient resorption efficiency of leaves and stems.Besides,the efficiency of phosphorus resorption in leaves of the three plants decreased with the increase of nitrogen supplemental level.The stem phosphorus resorption efficiency of Leymus chinensis had no significant difference with the increase of nitrogen supplemental level,but the stem phosphorus resorption efficiency of Thermopsis lanceolate and Potentilla bifurca showed a trend of decreasing first and then increasing.Nitrogen compound type had no significant effect on phosphorus nutrient resorption efficiency of leaves and stems.(3)Nitrogen deposition has a significant effect on the decomposition of plant litter.With the increase of nitrogen application rate,the initial nitrogen content of litters increased and the decomposition rate of litters accelerated.Nitrogen application type had no significant effect on the decomposition of leaf litters,but had a significant effect on the decomposition of stem litters:the decomposition of stem litters was slower when NH4HCO3 was applied.while the decomposition of stem litters was faster after NH4NO3 treatment.The addition of manganese had no significant effect on the decomposition of leaf litters.but had a significant effect on the decomposition of stem litters.In this study,by combining field experiments with laboratory experiments,three important nutrient cycles,including plant nutrient uptake,nutrient resorption and litter decomposition.were combined to comprehensively explore the response of nitrogen and phosphorus nutrient cycles of common species in temperate meadow steppe to nitrogen deposition.It will not only elucidate the adaptation strategies of plant nutrients to nitrogen deposition,but also help to predict the change direction of plant nutrient cycling under the change of nitrogen deposition,and provide theoretical support and scientific basis for the adaptive management of temperate meadow grassland ecosystem.