| Terrestrial ecosystems are an important carbon(C)sink,and play an important role in regulating ecosystem C cycling.Reducing the CO2 emissions and increasing the carbon sink function of terrestrial ecosystems are effective way to achieve the C neutrality.Both water and nitrogen are limiting resources in semi-arid grassland ecosystems.Atmospheric nitrogen(N)deposition has dramatically increased,which mainly related to fertilizer application and fuel combustion.Besides,global climate models have predicted that precipitation regimes will altered with precipitation amount and frequency of extremity precipitation events(drought and wet)especially in semi-arid areas.Such variations in N deposition and precipitation regimes are expected to affect ecosystem C cycling due to the limitation of N and water in semiarid grasslands,which further feedback to climate change.The agro-pastoral ecotone in northern China covers about 6.5 × 105 km2 and is a transitional region from farmland to natural grassland.The grassland in this area has suffered different degrees of salinization,which then impacted plant growth and microbial activities.However,the response of ecosystem CO2 fluxes to N addition and altered precipitation in agro-ecotone saline-alkaline grasslands remains unclear.This study consists of two parts: one is the network experiment platform of N addition and precipitation change in Youyu Loess Plateau Grassland Ecosystem Research Station,and the other is a long-term N addition experiment platform set up in Duolun County,Inner Mongolia.The response of ecosystem C fluxes to N addition and precipitation alteration and its regulation mechanisms were studied.In the part 1,the response of soil total respiration(soil total respiration(SRTOT),soil heterotrophic l respiration(SRH),soil autotrophic respiration(SRA))and ecosystem C exchange(ecosystem respiration(ER),net ecosystem CO2 exchange(NEE),gross ecosystem productivity(GEP))to N addition and precipitation alteration were studied to explore the regulation mechanisms of ecosystem C cycling.In part2,the effects of long term N addition on plant N use efficiency(NUE)and water use efficiency(WUE)were studied.Through the above analysis,we aimed to quantified the different responses of each component of ecosystem carbon flux to N addition and precipitation alteration,and revealed the main controlling factors of C flux.The influence of resource supply and utilization on grassland ecosystem C flux and its regulating mechanisms were also explored.The main research results are as follows:1)Firstly,the effect of N addition or altered precipitation alone on the plant productivity was not significant.While N addition and increased precipitation synergistically affects plant productivity,especially on ANPP.Compared with plants,soil microorganisms was insensitive to N addition and precipitation change.The effects of N addition and altered precipitation had no effect on soil exchangeable base cations and soil p H,which plays crucial role in regulating plant productivity and microbial biomass.2)Secondly,N addition or precipitation aleration alone had no significant effect on SRTOT,SRH,or SRA.However,N addition with increased precipitation significantly stimulated SRTOT through primarily promotion in SRA due to the increased of BNPP.This results showed that SRH was insensitive to short-term N addition and precipitation alteration.The effect of N addition on SRA was stronger under wet than that under drought conditions,indicating that increased in precipitation will enhanced the effects of N deposition on SRTOT by accelerating SRA in agro-pastoral ecoton saline-alkaline grassland of Northern China.The response of soil p H and base cations to N addition and changed precipitation regulated SRTOT and its components through plants and soil microorganisms.3)Thirdly,single N addition or precipitation alteration showed no significant effect on ecosystem CO2 fluxes(GEP,ER and NEE)during three growing seasons(2018-2020).These results suggest that there are a common constraints of N and water in our study sites,and that the utilization of one nutrient is limited by the other.There is an interaction between N addition and precipitation changes on NEE,and the effects of N addition on NEE are inconsistent under drought and wet conditions.The effects of N addition on GEP and NEE were increased with precipitation.Under increased precipitation,N addition significantly stimulated NEE,while N addition reduced NEE under decreased precipitation in drought year.Hence,seasonal precipitation regulates the effects of N addition on carbon exchange.4)The ecosystem resource use efficiency was calculated,and we found that N addition alone had no significant effect CUE,while significantly increased WUE across three years;decreased precipitation showed no effect on CUE and WUE,while increased precipitation significantly stimulated WUE and CUE across three years.The effect of N addition on resource efficiency of dominant plants was also studied,and results showed that N addition did not affect plant water use efficiency,but significantly reduced N use efficiency,and then indirectly affect productivity.Based on the study on the effects of long-term nitrogen addition on plant water and nitrogen use efficiency in Inner Mongoli,we found that plants can adjust their WUE to adapt to the changes of soil water and N conditions;soil water content regulates the effects of N addition on plant resource utilization efficiency and productivity.Hence,our results showed that N deposition and precipitation alteration jointly regulated resource use efficiency in ecosystem and individual level.5)N addition with increased precipitation significantly increased ecosystem C fluxes(C uptake and release).N addition with increased precipitation significantly increased NEE,while the CUE was unchanged,indicating that the effects of long-term N addition and changing precipitation on C sequestration is still uncertain.While N addition with decreased precipitation inhibited NEE and CUE in drought year,which suggested that plants will consume more energy(increase respiration)to use N under drought.The ecosystem CO2 fluxes is regulated by the supply,demands and utilization of resources in saline-alkaline grassland.Soil p H and base cations play an important role in regulating the carbon flux of grassland ecosystem based on the results of SEM.Above all,N addition and precipitation co-regulate ecosystem CO2 fluxes due to the common limitations of N and water,and the use of one nutrient is often limited by another in this saline-alkaline grasslands.Hence,the effects of various environmental factors(N deposition and precipitation alteration)on C fluxes should be considered simultaneously.The saline-alkaline grassland in the agro-pastoral ecoton is a C sink,and the increased of N deposition and water will promote ecosystem carbon flux,and thus increased the function of C sink,which is mainly regulated by plant growth.However,soil microorganisms and activities(SRH,MBC)were insensitivity to N addition and precipitation alteration in saline-alkaline grassland.We also found that N addition could inhibited NEE under decreased precipitation,which related to the reduced in leaf physiological characteristics and ecosystem C use efficiency.According to the results of SEM,the ecosystem C fluxes of saline-alkaline grassland is regulated by the supply,demand and utilization of resources.In addition,the response of soil p H and base cations to the N deposition and precipitation changes plays an important role in regulating the utilization of resources and ecosystem carbon fluxes. |
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