In Vitro Simulated Dissolution Of Calcite And K-feldspar And Their Effects On Gut Microbial Community

Earthworms play an important role in soil ecosystem and are termed“ecosystem engineers”.They not only directly participate in the pedological mass circulation,but also indirectly affect the circulation of soil carbon and nitrogen by influencing the growth and development of vegetation.Earthworms have been widely concerned in soil minerals weathering,soil remediation and soil biological nitrogen fixation.But the biological mechanism and process of earthworms weathering soil mineral are still unclear.In this paper,soil minerals dissolution pcocess and kinetics,as well as the response of earthworm guts microflora to the environmental minerals composition by soil minerals dissolution in organic acids solution mimicking p H and concentration of earthworms gut fluid in vitro and earthworm guts microbes under different minerals feed analysis experiments,to reveal the earthworm guts microbial contribution and role in soil minerals weathering mechanism.The results of this study will help further understand the contribution of earthworms to soil minerals weathering and soil fertility improvement,and provide experimental basis for the study of elemental biogeochemical cycle with earthworms.This paper drawed the following conclusions:（1）When calcite and K-feldspar passed through earthworm guts,the mineral structure,morphology and surface groups showed no significant differences under the gut contents.XRD results showed that the crystal structure of calcite was maintained after passing through earthworm guts,while the crystal lattice of K-feldspar was partly damaged.Under SEM,the edges of calcite and K-feldspar particles were sharp,and no corrosion pits were found on the surface,indicating that no violent minerals dissolution occurred in earthworm guts.FTIR results showed that the absorption peaks of water and structural hydroxyl groups on the surface of K-feldspar were enhanced after passing through earthworms gut,suggesting that the metal cations on the surface of K-feldspar were dissolved.The above results indicated that the weathering dissolution of minerals in earthworm guts is limited to slight surface erosion and changes of surface groups.（2）In the simulation experiment of minerals dissolution,the dissolution concentration and rate of minerals in organic acids solution were about one order of magnitude higher than that in pure water.For calcite,there were many and deep dissolution pits on the surface of the residual solid,and the larger particles were gradually enriched during dissolution.The shrink core model showed that the dissolution of calcite was mainly affected by the internal diffusion rate of the reactants in solution.The Hixson-Crowell model fitted the dissolution process of calcite,calculating rate constants in organic acid and pure water were 7.18×10^-6±1.87×10^-7mg^1/3/min and 4.93×10^-7±1.21×10^-9 mg^1/3/min,respectively.For K-feldspar,there were few shallow pits on the surface of the residual solid,as well as the dominant particle size did not change during dissolution.But absorption peaks of water and structural hydroxyl group on the solid surface gradually increased.The dissolution rate of K⁺in organic acids and pure water is 0.258 mmol·L^-1·h^-1 and 0.022mmol·L^-1·h^-1 respectively by initial rate method.These results indicated that organic acids promote mineral dissolution.（3）Earthworm casts and environment soil were sampled separately and compared the differences of microflore between earthworm guts and environment soil under environmental media（original soil/sawdust）.The results showed that the earthworm guts has the similarity with the adjacent environment in the microbial community composition,but the bacteria diversity of environment is significantly higher than that in earthworm guts.Compared to the surrounding environment,the relative abundance of Proteobacteria increased in earthworm gut treated with original soil,while the relative abundance of Bacteroidetes,Planctomycetes and Gemmatimonadetes decreased.However,the relative abundance of Proteobacteria,Bacteroidetes and Firmicutes in earthworm guts treated with sawdust increased,and the relative abundance of Verrucomicrobia,Planctomycetes and Actinobacteria has declined.At the genus level,the relative abundance of dominant bacteria such as Aeromonas,Acinetobacter and Pseudomonas in earthworm guts was higher than in the environment.The environment of earthworm guts is special,which features undetectable oxygen,high moisture and abundant amino acids,causing environment bacteria cannot grow normally in earthworm guts.Therefore,the composition of the original microbial community in the environment changes after passing through earthworm guts.（4）The minerals composition in the environment can significantly change the diversity and community composition of microorganisms in earthworm guts.The results showed that theαdiversity index of bacteria community under calcite stress was significantly higher than that under K-feldspar stress,and theαdiversity index of bacteria community of earthworm guts under K-feldspar stress was significantly higher than that under sawdust treatment.At the genus level,the relative abundance of Aeromonas in earthworm guts treated with K-feldspar increased.In contrast,the relative abundance of Aeromonas and Acinetobacter in earthworm guts treated with calcite decreased,and the relative abundance of Aeromonas decreased more.