| At present,many ecosystems are affected by changes in environment factors such as extreme drought caused by nitrogen and phosphorus nutrient deposition and changes in precipitation patterns.Soil nitrogen and phosphorus nutrients,extreme drought,and complex interactions between them may affect alpine grasses in the context of global change.The carbon flux and plant nutrient characteristics of the Eden ecosystem have significant effects.Regarding these impacts,there are still few studies,especially in alpine meadow ecosystems,which also limits to some extent our ability to predict global changes in response and feedback to many ecological processes in alpine meadow ecosystems Ability.In order to study the nitrogen use efficiency of ecosystem plants,generally only the characteristics or processes of nitrogen use efficiency of plants in a single ecosystem are considered.Even when studies have looked at processes of plant nitrogen efficiency in multiple ecosystems,these processes have been isolated(without a uniform approach to data acquisition)for analysis.Based on the research background of nitrogen utilization efficiency of plants in alpine meadows on nitrogen and phosphorus nutrient addition and drought response,we take the regional nitrogen use efficiency of plant leaves and roots in Inner Mongolia and the Qinghai-Tibet Plateau grassland ecosystems as the research object on a regional scale,aiming to reveal plant nitrogen Regional pattern of utilization efficiency and further analysis of how nitrogen use efficiency of grassland plants is affected by climate,plant functional groups,soil nitrogen content,and ecological types.In this paper,the alpine meadow ecosystem of the Qinghai-Tibet Plateau is taken as the research object,and the specific design is nitrogen addition(10 g N m-2yr-1),phosphorus addition(10 g P m-2yr-1),and rainfall reduction by 50%(drought)Three-factor interactive control experiment,adopting random block test design.There are 7 experimental treatments involved in this study,which are Control,Nitrogen(+N),Phosphorus(+P),Drought(D),Nitrogen and Phosphorus(N+P),Nitrogen and 50%rain reduction(N+D),nitrogen and phosphorus addition and50%rain reduction(N+P+D),each treatment was repeated 4 times.Measure the ecosystem carbon flux,plant stoichiometry,and plant nutrient use efficiency to study the response of alpine meadow ecosystems to nitrogen and phosphorus nutrient addition and drought,and study the spatial pattern and influencing factors of plant nitrogen use efficiency on a regional scale To improve the response of plant nutrient traits to biological abiotic factors in ecosystems.The main research conclusions obtained are as follows:(1)During the two years of the experiment,the effects of each treatment on ecosystem carbon flux(ecosystem net CO2 exchange NEE,ecosystem respiration ER,and total ecosystem productivity GEP)in 2018were not significant.The carbon flux of the ecosystem in 2019 is mainly affected by nitrogen addition and drought treatment.Both nitrogen addition and nitrogen and phosphorus addition significantly increase the ecosystem NEE,ER,GEP,and Rabove,(by 10.8%,11.2%,15.6%,63.2%respectively)And the drought treatment significantly reduced the ecosystem NEE,ER,GEP,and Rabove(reduction of 17.6%,11.7%,15.2%,8.9%respectively),other treatments have no significant effect on them,and there is no significant interaction effect between them;nitrogen and phosphorus addition,drought and their interaction treatments significantly reduce soil respiration SR,but between treatments There is no significant interaction effect on soil respiration SR;it indicates that the carbon flux of this ecosystem is mainly limited by nitrogen nutrition elements.Nitrogen addition eases the restriction of nitrogen nutrition on plant growth,improves the above-ground biomass of plants,and is conducive to ecosystem carbon fixation.However,drought treatment leads to a decrease in soil water content,inhibits plant growth,reduces ecosystem carbon flux,and is disadvantageous.Fixed,phosphorus nutrients ecosystem carbon currently is not a major limitation of nutrient elements of the ecosystem.(2)The leaf and root system C:N,C:P and N:P of dominant plants in alpine meadow are mainly affected by soil nitrogen and phosphorus availability.Nitrogen addition significantly reduces plant leaf and root system C:N and significantly increases plant leaf and root system.Addition of N:P,phosphorus significantly reduced plant leaves and roots C:P,plant leaves and roots C:N and N:P were greatly affected by soil moisture caused by short-term drought treatment,and soil available nitrogen affected plant leaves and roots N:The effect of P is more sensitive than the effect of soil available phosphorus;the response of C:N,C:P,and N:P to the soil moisture content of grasses is more sensitive than that of weeds;the soil moisture content and the leaves and roots of the soil plant C:The effect of N and N:P is more sensitive than that of C:P;nitrogen addition and drought have a negative interaction on C:N and a positive interaction on N:P in the grasses of Poaceae in 2018.Nitrogen and phosphorus addition and drought had no interaction effects on the leaf and root C:N,C:P and N:P of three plant species in 2018 and 2019.(3)Nitrogen use efficiency of leaves and roots of dominant plants in alpine meadows is mainly affected by soil available nitrogen.Nitrogen addition significantly reduces nitrogen use efficiency of plant leaves and roots.Drought treatment only partially affects nitrogen use efficiency of plants,which significantly reduces the leaves of grasses.Nitrogen use efficiency implies that the nitrogen nutrients in this ecosystem are more important than phosphorus nutrients and soil water content;leaf nitrogen use efficiency(60.58 g·g-1)is lower than root nitrogen use efficiency(74.85 g·g-1),Leaf nitrogen use efficiency is about 80.94%of the root system;among different plant functional groups,the nitrogen use efficiency of grasses(65.86 g·g-1)and the leaf nitrogen use efficiency of sedges(65.94 g·g-1)),And weed plants have the lowest nitrogen use efficiency(52.63g·g-1);the nitrogen use efficiency of plant leaves and roots on biological(aboveground and underground biomass)abiotic factors(effective soil nitrogen and phosphorus content and soil water content)There are differences in the responses of plant leaves.The nitrogen use efficiency of plant leaves is more sensitive to the response of soil available nitrogen and aboveground biomass than the nitrogen use efficiency of roots.Nitrogen use efficiency is more sensitive to soil moisture than weed leaves and roots.Nitrogen use efficiency in leaves and roots of grasses and sedges is more sensitive to underground biomass than nitrogen use efficiency in weed leaves.Drought has a negative interaction on the nitrogen efficiency and efficiency of leaves of grasses,and has no interaction on the nitrogen use efficiency of sedges,weeds and roots.Simultaneous addition of nitrogen and phosphorus and nitrogen and phosphorus addition and drought have a negative effect on nitrogen use of all plant leaves and roots.There are no interaction effects of efficiency.(4)On a regional scale,this paper analyzes the nitrogen use efficiency of leaves and roots of 139 plant species and their relationships with environmental factors and plant functional groups at 82 survey sites in Inner Mongolia and the Qinghai-Tibet Plateau grassland.The experimental results show that:1)The nitrogen use efficiency of meadow grassland plant leaves was 53 g·g-1,which was significantly higher than that of alpine meadow(46 g·g-1),desert grassland(41 g·g-1),and typical grassland(39 g·g-1).The root nitrogen use efficiency of alpine meadow was 108 g·g-1,which was significantly higher than that of other ecosystems.2)Leaf nitrogen use efficiency is more sensitive to temperature than the root system,but with the increase of the drought index,both showed a significant decrease.3)Leaf and root nitrogen use efficiency of weeds is lower than that of Cyperaceae and Poaceae plants,and leaf and root nitrogen use efficiency of legumes is 48%and 60%lower than that of non-legume plants,respectively.4)There is no significant relationship between plant nitrogen use efficiency and soil nitrogen content.The above results indicate that the main and interaction effects of nitrogen and phosphorus nutrient addition and drought and their interactions on the alpine meadow ecosystem are different among the carbon flux of the ecosystem,the stoichiometry of plant leaves and roots and the nitrogen utilization efficiency.In this paper,the responses of major processes of the carbon cycle,dominant plant stoichiometric ratio and nitrogen utilization efficiency to nutrient addition and drought treatment and their interactions are studied,and the key processes that affect them and the main factors that affect them are nitrogen and drought treatment.The results provide experimental evidence and theoretical basis for predicting and simulating the response of alpine meadow ecosystem to nitrogen and phosphorus nutrient deposition and extreme drought.In addition,the spatial pattern and influencing factors of nitrogen utilization efficiency in leaves and roots of plants in Inner Mongolia and Qinghai-Tibet plateau were analyzed on a regional scale,and the main influencing factors were plant functional groups and drought index.All the above studies are helpful to understand the grassland productivity maintenance mechanism in China under the background of global change and provide scientific basis for grassland ecosystem management. |
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