Influences Of Soil Biota Mediated By Agricultural Practices And Pest On The Crop Growth-Defense Trade-off Relationship

With the increase of crop yields,the agricultural practices have resulted a lot of environmental and ecological problems(such as soil acidification,loss of biodiversity,and the damage of ecosystem functionality).The loss of biodiversity and the damage of ecosystem functionality will further hinder the development of sustainable agriculture.Recently,with the development of ecological intensive agriculture,increased attention has been paid to the research on the agroecosystem processes based on the theory of soil ecology.However,less attention has been paid to the effects of soil biota on key ecological functions such as plant growth performance.It is urgent to understand whether and how soil community mediated by agricultural managements could regulate crop growth and resistance to herbivores.Such knowledge would help comprehensively understand the mechanisms of ecosystem process as well as the potential of soil biota to reduce the use of chemical fertilizers and pesticides and further optimize crop production.Our study combined the long-term field experiment and manipulative experiment in laboratory to investigate the mechnisms of soil biota mediated plant growth performance in a subtropical rice field ecosystem.Soils influenced by different agricultural practices(fertilizer management and rice cultivar)and aboveground herbivores(Nilaparvata lugens)were collected for exploring the effects of soil biota affected by agricultural practices on rice growth and resistance to herbivores.First of all,soil biota as influenced by chemical or organic amendments were inoculated to sterilized soils to investigate the pure effects of soil biota on rice growth and resistance to herbivores.Then,a reciprocal transplant experiment was established to disentangle effects of soil abiotic and biotic properties on plant growth and performance of the planthopper.Moreover,soil biota that affected by crop cultivar and elevated CO2 were introduced to sterilized soils to explore feedbacks of soil biota on rice growth performance.Last,a multi-generational approach was used to observe soil biota mediated mechanisms of rice growth and resistance to Nilaparvata lugens.The main results are as follows:1.The presence of soil biota increased rice root-to-shoot ratio and the concentrations of root nitrogen and soluble sugar(p<0.05),but decreased rice shoot sugar and nitrogen contents and the biomass of Nilaparvata lugens,suggesting that soil biota could promote rice resistance traits by altering the nutrient allocation of rice between aboveground and belowground.Meanwhile,soil biota that derived from organic-amended soils promoted the synthesis of root phenolics(p<0.05),indicating that soil biota as influenced by organic amendments could facilitate plant resistance against aboveground herbivores.2.Microbial abundance and diversity was positively correlated with rice plant total biomass but negatively correlated with plant nutrients(concentrations of shoot amino acids and sugars)(p<0.001),which revealed that diverse and active soil community can enhance plant tolerance(i.e.,increasing plant total biomass)and resistance(i.e.,decreasing platn nutrients)against herbivores.Plant tolerance was more related to their associated soil biota than to soil chemical properties(F=42.3,p<0.01),while plant resistance was more strongly affected by interactions of soil biota and soil chemical properties(F=17.7,p<0.05),suggesting the vital role of soil biota in regulating plant growth performance.3.Soil biota as influenced by the strongly or weakly responsive cultivars in turn negatively or positively affected plant growth,respectively(p<0.05).Plant-soil feedback effects were negatively correlated with the relative abundance of herbivores(p<0.05),and the later was identified as the best predictor of plant-soil feedback effects(p<0.01).Under eCO2,when plants were grown in the soils that inoculated with their native soil biota,a growth-defense trade-off was existed in the strongly responsive cultivars,but on the contrary,the weakly responsive cultivars simultaneously increased plant biomass and defense compounds(p<0.05),suggesting that the weakly responsive cultivars might be a potential candidate for enhancing both crop yield under future CO2 scenarios.4.The presence of Nilaparvata lugens changed soil community composition and increased the Shannon diversity of nematode,but had no significant effects on the Shannon diversity of microbes and protists.Responses of rice to aboveground pest led to a significant increased accumulation of palmitic acid and low-molecular weight compounds in rhizodeposition(p<0.05),and the abundance of Pseudomonas and Sphingomonas,which promoted the rice plant growth and resistance to herbivores.The presence of Nilaparvata lugens increased the average degree of networks by 18.95%,indicating an increase of network complexity.In addition,the multiple plantings of rice could enrich for beneficial biota that enhance plant resistance to herbivores in successive generations,but narrowed from the fifth generation,as a result of the complexed plant-herbivore interactions.Together,soil biota as mediated by aboveground pest had a positive feedback effect to plant resistance against herbivory.Taken together,crop growth performance mediated by soil biota was strongly depended on agricultural practices.Specifically,soil biota development in response to organic amendments could increase plant biomass and decrease the abundance of aboveground herbivores.Soil biota as influenced by the strongly responsive cultivars in turn negatively affected plant growth,mainly due to the high abundance of root herbivores.Responses of rice to aboveground pest increased the abundance of beneficial microbes and the network complexity through increasing the quantity and quality of rhizodeposition,which further positively feed back to plant growth performance(i.e.enhancing plant growth and resistance to herbivores).The current study provides a foundation for developing the potential functions of soil biota on plant aboveground performance,which in turn will facilitate the sustainable development of ecologically intensive agriculture.