Study On The Characteristics And Driving Mechanisms Of Soil Microbial Nutrient Limitation

Soil is the place of material circulation and energy flow,and it is the basis of ecosystem food chains;it also stores a lot of organic carbon(SOC),which regulates the concentration of CO₂ in the atmosphere.Microbial metabolisms directly determine soil material circulation and energy flow,and its processes are also limited by the soil resources supply.However,the characteristics,mechanisms and key drivers of soil microbial metabolic limitation under vegetation restoration and global change are still unclear.In this study,the characteristics of soil microbial metabolic limitation under different vegetation restoration types in the Loess Plateau,the dynamics of microbial metabolic limitation during vegetation succession,the spatial patterns of soil microbial metabolic limitation under typical vegetation and the responses to global change such as precipitation and atmospheric nitrogen(N)deposition were studied by field sampling investigation.The effects and its mechanisms of microbial metabolic limitation on soil C cycles and the responses of microbial community to nutrient limitation were further revealed through laboratory simulation experiments with isotope tracer and high-throughput sequencing technology.In addition,several new ecoenzymatic stoichiometry models for quantifying microbial metabolic limitation were proposed through theoretical deduction and data verification.The main results of this study are as follows:(1)It was found that soil microorganisms were generally limited by soil C and phosphorus(P)under different vegetation restoration types in the Loess Plateau.C and P limitations in the microbial communities were the lowest in the natural grassland,indicating that natural grassland can provide optimal nutrient environment for plants and microorganisms.Therefore,the restoration of natural grassland should be recommended as the preferred option for ecosystem restoration in these arid and semi-arid regions.(2)The spatial patterns of microbial metabolic limitation with mean annual precipitation(MAP)in grassland soils in arid and semi-arid regions were investigeted.Microbial C and P limitations decreased with the increase of MAP(300-900 mm),and lowest limitation in the areas with MAPs of 700-900 mm,whereas N limitation occurred in the areas with MAPs>700mm.The results of a variation-partitioning analysis and partial least squares pathmodeling indicated that the microbial C and N/P limitations on regional scales were mainly determined by climate factors(MAP and mean annual temperature(MAT)),followed by vegetation biomass and soil properties.(3)Microbes maintained stoichiometric homeostasis in all successional stages,but plants did not during the vegetation succession.Microbial metabolism was limited by low soil P availability throughout the succession,whereas plants were limited by low soil P during the late successional stages(from 60 to 150 years)only.An increase in soil moisture during succession was associated with greater P limitation in microbes and plants.There was less microbial P limitation at the 35-year successional stage,and the greatest microbial P limitation occurred at the 130-year successional stage.This coincided with the lowest microbial CUE at 100 years of succession(CUE was from0.24 to 0.41),suggesting a change in the physiological responses from microbes(such as enzyme synthesis and the priming effect),that tended to reduce soil C sequestration.(4)In Chinese forest,microbial metabolism was generally limited by soil P availability across five climate zones and only limited by soil C in about 25%of cases.Across the entire latitudinal and profile range of this study(latitude 19 to 54°and profile O-,A-and C-horizons),P limitation was lowest in the warm-temperate zone(34-39°)and O-horizon.Geographical patterns of microbial P limitation were positively related to MAP,MAT and anthropogenic N deposition.A strong coupling of microbial P and C limitations through organic matter turnover suggests that the decomposition of SOC is a key mechanism alleviating microbial P limitation,reflected by the opposite patterns of microbial P and C limitations both in soil horizons and across the geographical scale.Moreover,the positive geographic correlation of microbial and plant P limitations across broad climatic zones suggested that soil P deficiency is a universal phenomenon in these forest systems and could be an important control on forest C loss by stimulating litter decay in an effort to“mine”organic P while simultaneously suppressing primary productivity.Our estimates of future climate change scenarios(SSP126,SSP370 and SSP585)did not change the spatial patterns or magnitudes of microbial P and C limitations to the year 2040.This consistency suggested temporal stability and persistence of resource limits to microbial metabolism under changing environments,implying long-term constraints of soil P on C sinks across forests.(5)Based on the simulation experiments of different C sources and P addition with isotope tracer technology,we found that the regulation mechanisms of microbial metabolic limitation on soil C turnover are that the metabolic limitation ultimately regulates the soil C release by affecting the rate of organic C decomposition(represented by q CO2)and microbial C use efficiency(CUE).These results suggest that microbial metabolic limitation is a key driver of soil nutrient cycling and resource redistribution.(6)Combining with high-throughput sequencing technology,it was found that metabolic stress caused by nutrient limitation was not conducive to the maintenance of microbial community diversity.Moreover,nutrient limitation could lead to the transformation of the microbial community assembly from stochastic process to deterministic process.Specially,fungal communities are more sensitive to nutrient limitation than bacteria.Their key species,community interaction and assembly were significantly affected by soil nutrient limitation.These results indicate that resource constraints regulate heterotrophic microbial metabolism by affecting microbial community structure and its assembly process.(7)Based on ecological stoichiometry and ecological metabolic theories,we constructed serveal new ecoenzymatic stoichiometry models.Our new models showed more accuracy and sensitivity than previous models by predicting the metabolic limitation of soil microorganisms for different ecosystems on a global scale.The predictions indicated that microorganisms were mainly limited by P in forest soils,limited by N in grassland soils,whereas the limitation characteristics in agricultural ecosystems depended on the input of nutrients.These models provide important theoretical basis for the interpretation of higher order ecological phenomena and the development of mechanism models of terrestrial C cycling.