The Microbial Mechanisms Involved In Soil Nutrient Cycling With Different Plant Species Applied As Cover Crops In South China Orchard

The overuse of chemical fertilizers and the adverse soil conditions in orchards are harmful to the sustainability of fruit production in South China.The“Reduced Application of Chemical Fertilizers”policy inaugurated by the Ministry of Agriculture in2015 has inspired to develop more scientific and sustainable managements in fruit production.Recently,low-input farming is the focus of sustainable agriculture.Cover cropping in orchards can stimulate plant-growth-promoting microorganisms and improve soil nutrient availability,representing an important management in low-input farming.Previous studies focused on the macroscopic aspects,such as the improved fruit production,orchard environment,soil fertilization and structure;however,we know little about the microbial mechanisms in cover cropping systems,which is the microscopic aspect.More importantly,how grasses and legumes,as two kinds of routinely used cover crops,differently mediate rhizosphere microbial community to affect soil nutrient cycling is far from understanding.Here,with bahiagrass?Paspalum notatum Flüggé,grass?and stylo?Stylosanthes guianensias,legume?as cover crops routinely used in orchards in South China,we established cover cropped orchard ecosystem both in field and in lab to study the microbial mechanisms.The main results are as follows:?1?Bahiagrass significantly increased soil organic matter?SOM?,total glomalin,soil total nitrogen?TN?and available nitrogen?AN?content because of its huge biomass input and deep root distribution after 3 years cover cropping in red soil orchard.However,the nutrient effect of stylo was not significant.The combined effect of bahiagrass and inoculation with arbuscular mycorrhizal fungi?AMF?was most significant.Cover cropping and symbiotic microbial inoculation strongly affected soil fungal community and the alpha diversity of soil microbial community,with bahiagrass combined with AMF inoculation the most effective.Bahiagrass significantly increased soil bacterial abundance.Soil TN and total potassium were the most correlated factors affecting soil bacterial community,with AN for soil fungal community,TN and total organic carbon?TOC?for soil function involved nutrient cycling in the cover cropping system.?2?Grasses and legumes affect soil nutrient contents with different degree in the 3-month lab microcosms,with TN and available phosphorate?AP?significantly higher in legumes than grasses.The alpha diversity of microbial community in legume soil was higher than that in grass soil.Both grasses and legumes increased soil bacterial and fungal abundances,with a higher magnitude in legume soil.Grasses enriched Bacteroidetes while legumes enriched Actinobacteria and Nitrospirae.Importantly,variation in soil microbial community structure associated with 3 different legume species was greater than that associated with 3 different grass species.?3?The soil metagenomic sequencing revealed the stronger effect of stylo than bahiagrass on taxonomic and functional profiles of soil microbial community.For KEGG,stylo increased the function of metabolism while bahiagrass increased the function of genetic information processing.For CAZy,stylo mainly increased the function of glycosyl hydrolases,while bahiagrass mainly increased the functions of carbohydrate-binding modules and glycosyl hydrolases.Moreover,soil chemical property showed much higher direct effect on microbial community structure and functional profiles than soil extracts,although the soil TN and some compounds in soil extracts were the best correlated with soil microbial community structure?nonanoic acid,xylulose,2-bromo dodecane,1-pentadecyne,3-heptadecene,heptadecane?and functional profiles?nonanoic acid,acetamide,cyclohexane,1-pentadecyne,heptadecane?.?4?The soil CH/C_enz?ratio of N-acquiring enzyme chitinase to C-acquiring enzymes?in stylo was higher than that of bahiagrass and control?no plant growth?.Metagenomic sequencing indicated that the potential activity of N assimilation in stylo soil was higher than that of bahiagrass.The diversity of N-cycling related functional genes in stylo soil was higher than that of bahiagrass and control,but a decreased diversity of N-cycling related microbial community and a higher abundance of generalists?microbial taxa with multiple N-cycling pathways?were observed in stylo soil.?5?Cover cropping did not affect the total contents of sparingly soluble inorganic phosphate;however,they significantly changed the transformation of P fractions.Both plants decreased aluminum phosphate?Al-P?content but increased occulded phosphorus?O-P?content.Stylo increased soil moderately resistant Po and total organic phosphorus contents,with the former significantly.When compared with control soil,the C/P ratio was significantly increased in stylo soil but not affected in bahiagrass soil,and phosphatase activity in stylo soil was significantly higher than that in bahiagrass and control soil.Soil metagenomic sequencing revealed that genes related to organic P mineralization from organic P mineralizing bacteria were more abundant but genes related to sparingly soluble inorganic P solubilization from phosphate solubilizing bacteria were less abundant in legume soil than in grass soil.Moreover,in legume soil,genes for bacterial taking up low-molecular-weight organic P were more abundant,but those taking up inorganic available P were less abundant.?6?The plant-derived C?PDC?content was less than 0.4%of soil organic carbon?SOC?,indicating that SOC was the main C for microorganism abundance.The PDC in bahiagrass was significantly higher than that in stylo.Stylo showed significantly positive priming effect on SOC decomposition,while bahiagrass only slightly decreased SOC content.PDC significantly affected bacterial community with some taxa significantly enriched in bahiagrass and stylo in sand culture system.Interestingly,the dissimilarity of bacterial communities and percentage of specific OTUs in planted sand substrate was significantly higher than that in soil substrate,indicating that soil-derived ancient C attenuated the strong regulation of plant-derived fresh C to bacterial community.?7?The presence of bahiagrass decreased the decomposition of bahiagrass and stylo root litter,while the presence of stylo did not affect root litter decomposition.The plant presence affected decomposer community and soil nutrient.The plant affected root litter decomposition mainly by affecting the decomposer community.The soil nutrient modification resulted from plant presence did not directly correlated with root litter decomposition,but indirectly affect decomposition by controlling the decomposer community.?8?Both plant species and distance affected soil microbial community function,with N-cycling the most extensively affected.Influence of distance on microbial community composition was greater than that of plant species.Plant species affected soil nitrate content,beta-xylosidase and leucine-aminopeptidase activity,and AOB-amoA abundance.Distance affected soil C?beta-glucosidase and beta-xylosidase activity?,N?chitinase activity,narG,nirS and nosZ abundance?,and P?phosphomonoesterase activity and alp abundance?cycling.Mantel test and Partial Least Squares Path Modeling?PLS-PM?analysis revealed that soil chemical property was significantly associated with microbial community function,with a higher coefficient than that of microbial community composition.These data suggest that legumes and grasses differently affect microbial community function,and community function is affected more strongly by soil chemical property than by community composition.In conclusion,as cover crops grown in the studied orchard of South China,grass and legume affected soil microbial community composition and potential function,and soil N-,P-cycling.The grass species bahiagrass increased soil organic matter and affected soil microbial community because of its huge biomass input and deep root distribution,however,the effect size of bahiagrass on microbial community was less strong than that of stylo;the leguminous stylo affected soil microbial community composition,soil N-and P-cycling.These results indicated the necessarity of choosing appropriate species and legume-grass intercropping in cover cropped orchards.