The Formation And Stabilization Mechanisms Of Soil Aggregates Driven By Arbuscular Mycorrhizal Fungi

The Loess Plateau located in middle north of China is characterized by arid climate,concentrated rainfall and severe soil erosion.The implementation of the Grain for Green Project improved soil quality and reduced soil erosion.Soil structure plays an important role in improving soil quality and controlling soil erosion.Soil aggregates are progressively formed through various organic and inorganic cements binding soil particles.The structure of soil aggregates are in hierarchy.Soil aggregate stability correlates tightly with the type and distribution of cementing agents.Arbuscular mycorrhizal(AM)fungi can form mutualistic symbiosis with more than 80% of terrestrial plant families,which can relieve drought stress to plants,furthermore,AM fungal hyphae and glomalin are extensively related with soil aggregation.However,the potential mechanisms of AM fungi on the formation and stability of soil aggregates are not clear.The effect of AM fungi on soil aggregate hierarchy are unkown.In this paper,the effect of AM fungi on the formation and stability of soil aggregates under different vegetation restorations were determined.Field investigation,compartment culture,batch adsorption experiment were conducted.The stability and hierchary of soil aggregate were evaluated by wet-sieving and ultrasonic agitation.The contribution of AM fungi on soil aggregation were analyzed by structural equation modelling,partial methods and Monte Carlo permutation test.The mechanisms of AM fungi on soil aggregation were revealed by In situ ATR-FTIR,synchronous fluorescence,static light scattering,and ultrasonic agitation.These studies will provide guidance for ecological restoration and soil erosion control in the process of returning farmland to forest on the Loess Plateau.The main results are as follows: 1)The contribution characteristics of AM fungi to soil aggregate stability under different vegetation restorationsThe soil aggregate stability and the distribution of AM fungi were investigated by field survey and laboratory analysis under different vegetation restorations.The effect of vegetation restorations and AM fungi on soil aggregation were evaluated by structural equation modelling.The results showed that returning cultivated land to Robinia pseudoacacia and Caragana korshinskii forest were more conducive to the growth of AM fungal hyphae and soil aggregation.Malus pumila forest as an artificial economic forest was often disturbed by human action,where AM fungal growth was inhibited and thus soil water-stable aggrenate stability decreased.With the increase duration of returning farmland to forest,hyphal length and glomalin content increased and soil aggregate stability improved.AM fungal hyphae were more in soil water-stable magroaggregates.AM fungal hyphal length,easilyextracted glomalin-related soil protein(EE-GRSP)and total glomalin-related soil protein(T-GRSP)content in 0-10 cm soil layer were significantly higher than that in 0-20 cm.Under different vegetation restorations,AM fungal mycelium and glomalin were important factors in promoting the formation and stability of soil water-stable aggregates by structural equation modeling analyses.The direct effect coefficients of AM fungal hyphae and EE-GRSP in 0.053-0.25 mm soil waterstable aggregates were 0.30 and 0.67,respecitively.The direct effect coefficients of AM fungal hyphae in 0.25-2 mm soil water-stable aggregates was 0.34.The direct effect coefficients of EE-GRSP in >2 mm soil water-stable aggregates was 0.44.2)The mechanism of AM fungi on the stability of soil aggregatesA three-compartment system was set up under well-watered and drought stress conditions.The stability and hierarchy of soil aggregate were evaluated by ultrasonic agitation and aggregate disruption characteristic curve.The contribution characteristics of AM fungi on the formation and stability of soil aggregates were analyzed by partial methods and Monte Carlo permutation test.These results showed that AM fungi could form good symbiosis with plant under both wellwatered and drought stress conditions.Inoculation with AM fungi significantly increased the percentage of >2 mm soil water-stable aggregates and mean weight diameter.Furthermore,inoculation with R.intraradices and F.mosseae significantly enhanced the dispersive energy of 0.25-2 mm soil water-stabe aggregates by 42.3% and 15.4%,respectively.Compared with the non-inoculated,the disruption rate of 0.25-2 mm soil water-stable aggregates decreased in the inoculated treatments,and soil aggregate hierarchy improved under drought stress condition.Partial methods and Monte Carlo permutation test indicated that the contribution of AM fungal hypha,EE-GRSP and T-GRSP to soil aggregion was 71.48%.And the determination coefficients of AM fungal hyphae,EE-GRSP and T-GRSP were 0.28,0.23 and 0.21,respectively.3)The network characteristics and aggregation effect of AM fungal mycelium on quartz sandA five-compartment system was set up with four fertilizer treatments((control treatments(CK),chemical NPK fertilizaes(NPK),manure(M),chemical NPK fertilizaes plus manure(NPKM)).The characteristics and aggregation mechanisms of AM fungal network were determined by ethanol-water solutions and ultrasonic agitation.These results showed that AM fungi preferred to form larger mycelium network in <0.053 mm and 0.053-0.25 mm quartz sands,and the quartz sand aggregates was formed only in <0.053 mm and 0.053-0.25 mm quartz sands.Inoculation with AM fungi significantly increased the dispersive energy of quartz sand aggregates by 98.6% under NPKM.The mycelial network effect and hydrophobicity of AM fungi were the main contributors to aggregation.4)The surface chemical properties of GRSP and bonding mechanism with goethiteThe surface chemical properties of GRSP were determined by highperformance size-exclusion chromatography(HPSEC)and flourier transformed infrared spectroscopy(FTIR)and Zeta potential.And the adsorption and aggregation characteristics of GRSP on goethite were eluciated by in situ attenuated transform infrared spectroscopy,synchronous fluorescence spectroscopy,static light scattering and ultrasonic agitation under different inoic strength and p H.The results showed that the purified GRSP extracted from soil contained polysaccharide-like,amide and aliphatic hydroxyl functional groups.The point zero charge and molecular weight were 2.46 and 8.79 k Da,respectively.GRSP could adsorb onto goethite by ligand exchange or electrostatic interactions.Protein-like macromolecular substances in GRSP preferentially adsorbed by amide functional groups.GRSP could adsorb onto goethite by ligand exchange or electrostatic interactions.Protein-like macromolecular substances in GRSP preferentially adsorbed by amide functional groups.On the other hand,GRSP increased the hydrophobicity of the Goethite-GRSP complexs by aliphatic hydroxyl groups.The results demonstrated the "super glue" role of GRSP.