| Arbuscular mycorrhizal fungi(AM fungi)are considered as ecosystem engineers to improve plant resilience to stress and provide a new option for improving agricultural productivity and soil quality.Previous studies have shown that AM fungi have different water-saving effects on plants/crops under different planting densities and different soil moisture conditions,and some results are even completely opposite.Moreover,their effects on major soil nutrients are also inconsistent.The relationship between roots and soil and the regulation mechanism of AM fungi need to be further explored.Based on a review of the existing literature,we hypothesized that the effects of AM fungus on the root-soil relationship and yield formation of upland wheat may be affected by both planting density and soil available water,and the positive promoting effect of AM fungi on wheat growth is not universal.The effects of AM fungi on yield formation,rhizosphere process and soil quality of upland wheat are highly uncertain and need to be systematically observed,demonstrated and explored.This study conducted a four-year(2017-2020)field and pot experiments at the State Key Laboratory of Grassland Agroecosystems,Lanzhou University,Lanzhou,Gansu Province.By collecting climatic factors,wheat growth and development,grain yield,water use efficiency(WUE),wheat yield formation factors,gas exchange parameters,mycorrhizal colonization rate,soil extramatrical hyphal length(EMH),soil water storage,and soil microbial biomass carbon(MBC),soil microbial biomass nitrogen(MBN),soil nutrients,the index of the crop physiological ecology,and AM fungal community diversity indicators,explore the impact of AM fungi inoculation on the productivity of wheat(Triticum aestivum L.)and soil quality in the semi-arid Loess Plateau,and analyze the water use efficiency and the allometric relationship of reproduction.The main results are as follows:1.Effects of AM fungi on wheat yield and reproductive allocation under different planting densities and soil moisture treatmentIn 2017,we carried out a pot experiment of inoculating AM fungus(Funneliformis mosseae)with wheat.We set up two water gradient treatments(CK(FWC80),two drought stress(FWC40),80%and 40%field water holding capacity)and four planting densities(low density is 6 and 12 plants pot-1,high density is 24 and48 plants pot-1).At low density,AM fungus inoculation had no significant effect on wheat aboveground biomass,grain yield and WUE.However,at high density,AM fungus inoculation significantly reduced the aboveground biomass,grain yield and WUE of wheat in FWC80 treatment.AM fungus inoculation in the FWC40 treatment significantly increased the net photosynthetic rate(Pn),grain yield,leaf area and WUE of wheat,and reduced the root:shoot ratio of wheat.Similarly,in FWC40,the relationship between reproductive biomass(R)and vegetative biomass(V)(R-V relationship)and the relationship between grain biomass(sink)and leaf biomass(source)belongs to the allometric distribution pattern(?>1,P<0.001),AM fungus inoculation significantly increased?.But in FWC80,the relationships turned to be isokinetic distribution(??1,P<0.001)and the inoculation of AM fungi had no significant effect on?.The above results indicated that the effects of AM fungi on wheat yield,WUE,reproduction and distribution were density-and moisture-dependent.2.AM fungi optimize the root-soil interaction and reproductive yield effects of plant density and water dependence in wheatIn 2018,we conducted a pot experiment to analyze the rhizosphere water,nutrient status and the root-soil interaction of dryland wheat after inoculation with AM fungus(F.mosseae)under different planting densities and water treatments.We set up two water gradient treatments(CK(FWC80)and two drought stress(FWC40),80%and 40%field water holding capacity)and four planting densities:1 plants pot-1(no competition,NC),2 plants pot-1(low competition,LC),9 plants pot-1(medium competition,MC)and 32 plants pot-1(high competition,HC).Inoculation with AM fungi significantly increased mycorrhizal colonization rate and EMH.Soil drought and the intensity of competition significantly reduced wheat biomass,reproductive distribution,rhizosphere soil water content(SWC),WUE and soil easily oxidizable organic carbon(EOC).Inoculation of AM fungi increased wheat biomass,WUE,EOC,MBC,MBN,MBC/MBN,and promoted wheat's absorption of soil inorganic nitrogen under drought stress.For example,in water stress conditions,compared with non-AM fungus treatments,inoculation with AM fungus increased the soil EOC content of MC and HC by 58.5%and 55.6%,respectively.Inoculation with AM fungi significantly improved wheat productivity and soil quality under drought stress.These effects were obtained by optimizing rhizosphere WUE and promoting roots'absorption of soil nutrients.However,under sufficient water conditions,the addition of AM fungi had no significant effect on wheat yield and soil quality,and even under highly competitive treatment,AM fungi inoculation had a negative effect on wheat yield and soil quality.3.AM fungi improve wheat productivity and soil quality by planting density and water-dependent effectsTo verify the results of the pot experiment,we conducted a two-year(2017-2018)field experiment to test the effects of AM fungi on wheat productivity and soil quality in the Loess Plateau with two irrigation treatments(irrigated and non-irrigated).In non-irrigation treatment,AM fungal inoculation greatly increased dryland wheat aboveground biomass,crop productivity,harvest index,water use efficiency,soil particulate organic carbon(POC)and EOC(P<0.05)compared with non-inoculated treatment,but it significantly reduced soil inorganic nitrogen content(P<0.05).For example,compared with non-inoculated plants,the grain yield of plants without irrigation but inoculated with AM fungi increased by 59%in 2017 and 36%in 2018,and the increase of the water use efficiency of the grains in 2017 and 2018 was 53%and 25%,respectively.However,different results were observed under irrigation conditions.In the irrigation treatment,AM fungus inoculation had a little effect on grain yield and soil quality at low plant density,but had a negative effect at higher density.Here,we demonstrated for the first time that inoculation with AM fungi in the field can significantly increase crop productivity and soil quality,and its impact depends on the planting density and water conditions.Therefore,optimizing the amount of AM fungus is an effective measure to promote the sustainable development of agriculture.4.Inoculation of exogenous AM fungi on the composition and diversity of AM fungal communities in rhizosphere soilField test results showed that inoculation with AM fungus(F.mosseae)significantly increased the mycorrhizal colonization rate and EMH,which was consistent with the potted results.In addition,inoculation with F.mosseae significantly increased the relative abundance of Glomeraceae related to this species,and the richness of Glomeraceae was the highest.For example,in 2017 and 2018,compared with non-AM fungi and irrigation conditions(WW),inoculation of AM fungi(WW+A)increased the relative abundance of Glomeraceae from 70%and 66%to 93%and 97%,respectively.The relative abundance of Glomeraceae increased from 77%and 74%to 93%and 98%without inoculation of AM fungus(WS)compared with inoculation of AM fungus(WS+A)without irrigation.The above results indicated that F.mosseae has been successfully colonized in the soil under field test conditions.However,AM fungi richness and Shannon index were not affected by soil moisture and AM fungus inoculation,indicating that AM fungus inoculation did not affect the diversity of native AM fungal communities.5.Effects of AM fungi on planting density and water dependence of wheat grain and leaf carbon and nitrogen ecological stoichiometryField test results showed that water stress significantly inhibited the carbon and nitrogen content of wheat grains and leaves,and the inoculation of AM fungi significantly promoted the accumulation of carbon and nitrogen content in wheat grains and leaves.The effect of AM fungi decreased with increasing density,but under irrigation conditions,AM fungus inoculation has no significant effect on the carbon and nitrogen content of wheat grains and leaves.This is because the inoculation of AM fungi under water stress conditions promoted the photosynthesis of wheat,and also improved the ability of wheat roots to absorb soil N,thereby increasing the nutrient content of the plant.Water stress treatment limited the absorption of nitrogen by crops.Inoculation with AM fungi promoted the crop absorption of nitrogen,thereby reducing the C:N ratio of wheat grains and leaves,improving leaf carbon assimilation and nitrogen absorption capacity,and was beneficial to fix and absorb more CO2and N,thereby improving photosynthetic rate,and ultimately promoting the accumulation of grain yield.Therefore,inoculation of AM fungi in water stress conditions can optimize the stoichiometric relationship between wheat C and N and reduce the nitrogen limitation of wheat growth.It is an effective measure to further tap the production potential of dryland spring wheat,and it provides the possibility for AM fungi to increase the stability of the ecosystem.6.Effects of AM fungi on planting density and water dependence of soil microbial biomass carbon and nitrogen ecological stoichiometryOur results show that soil MBC,MBN and MBC:MBN ratio were increased when AM fungi were inoculated.However,at high planting density,AM fungi inoculation had no significant effect on the soil MBC and MBN in irrigated treatment.The moisture and density dependence of the AM fungi effect was verified again.The inoculation of AM fungi increased MBC:MBN,thereby promoting the proliferation of fungi in the soil.Higher fungal biomass will reduce the leaching potential of N and increase the carbon sequestration capacity of the host,which is conducive to the sequestration of soil organic matter.Based on the above,our field experiments showed that the application of mycorrhizal inoculation in agriculture was feasible.Inoculation of AM fungi could not only increase crop yields,but also reduce the use of chemical fertilizers and soil carbon emissions.7.Effects of AM fungi on the productivity,reproduction and distribution of primitive wheat and modern wheatThe results showed that inoculated with F.mosseae could successfully colonize the roots of different ploidy wheat,and AM fungus could significantly promote the growth of different ploidy wheat and the accumulation of grain yield.With the increase of wheat ploidy,wheat yield,the biomass of various organs and yield components decreased gradually under the inhibition of drought,indicating that the growth,and yield accumulation of diploid wheat were the most affected by drought,followed by tetraploid,and the most stable performance of hexaploid wheat,which also indicated that hexaploid wheat had stronger drought tolerance.In drought stress conditions,AM fungus inoculation promoted grain yield,the biomass of various organs and yield components of different ploidy wheat.Moreover,inoculation of AM fungi could increase the photosynthetic rate,stomatal conductance and instantaneous water use efficiency of different ploidy wheat,which was caused by the improvement of water absorption capacity of wheat by AM fungi,indicating that the symbiosis between AM fungi and crops could improve the drought tolerance of crops.Both water conditions and host species affected the effect size of AM fungi.Inoculation of AM fungi was beneficial to promote more reproductive allocation of wheat biomass,but the effect was not significant.In summary,AM fungi inoculation in the semi-arid Loess Plateau significantly improved crop productivity and soil quality,and thus promoted the service function and sustainable development level of farmland ecosystem.Water control and field trials from AM fungal inoculation on wheat yield composition,WUE,gas exchange parameters,reproduction distribution,soil microbial carbon,microbial nitrogen,soil quality and other aspects,reveals the AM fungus inoculation can significantly improve wheat productivity and soil quality,but its effect is related to planting density,soil moisture conditions and wheat varieties.Therefore,optimizing AM fungus addition is an effective measure for sustainable agricultural development in semi-arid areas of the Loess Plateau,and provides a new solution for soil degradation management in the semi-arid environment. |
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