| The improvement of soil quality is the fundamental guarantee to ensure the healthy and sustainable development of agricultural ecosystem.However,conventional tillage(CT)soil management pattern has been widely used in apple orchards in this area for a long time,which leads to soil structure destruction,soil erosion,soil organic carbon reduction,biodiversity reduction and a series of soil quality degradation problems.Implementing modern orchard sustainable soil management mode,such as cover crop(CC),cornstalk mulching(CS),and black ground fabric mulching(BF),making apple production and ecological environment coordinated development,has become a major practical demand to promote the healthy,efficient and sustainable development of apple industry in this region.Soil microorganism is an important index to measure soil quality,and it plays an important role in nutrient conservation and supply,erosion control,improving soil structure and water-holding capacity as well as efficient utilization of fertilizer and water.Therefore,revealing the composition,diversity and function of soil microbial community plays an important role in improving the regulation function of agricultural ecosystem internal circulation and soil quality.However,the study on the effect of mulching patterns on the improvement effect of soil quality and its microbiological mechanism in apple orchards on the Loess Plateau is rather weak.Therefore,this study based on the long-term positioning experiment,the physical and chemical properties and enzyme activities of soil under different mulching measures were determined,and the characteristics of soil microbial community and functional diversity under different mulching measures and the evolution of soil microorganisms under typical cover crop were analyzed by molecular biological methods such as high throughput(16Sr RNA and ITS)and metagenomic sequencing.The effect of soil quality improvement and the mechanism of microbiology under different mulching patterns were revealed.The main results are as follows:(1)Organic mulching improved soil quality more than inorganic mulching.Compared to CT treatment,ryegrass(RE,Lolium perenne L.)and CS mulching increased soil organic matter content by 11.8% and 36.5%,respectively.In addition,CS treatment increased soil available nitrogen content by 42.0%,while BF mulching did not increase soil nutrients.The heterogeneity of soil environment under different mulching methods resulted in the difference of dominant population and relationship of soil microbial community.Three mulching measures all significantly increased the abundance of the operational taxa(OUT)and alpha diversity of fungi,and increased the relative abundance of Actinobacteria and Firmicutes.Meanwhile,RE and CS mulching increased the abundance of saprophytic fungi: Ascomycota,CS mulching also increased the abundance of Chytridiomycot.In addition,RE mulching increased the cooperation between bacteria,BF mulching reduced the competition between bacteria,and CS mulching not only promoted the cooperation between fungi,but also increased the correlation between fungi and bacteria.Therefore,the contribution of three mulching measures to soil nutrients and the influence of microorganisms is CS > RE > BF.(2)Organic mulching was more beneficial to the increase of soil carbon and nitrogen cycle function than inorganic mulching.Due to RE and CS mulching input organic matter into the soil,these two mulching measures have higher carbon cycling enzymes: ?-1,4-glucosidase(BG),cellobiohydrolase(CBH)and ?-xylosidase(BXYL),and CS mulching measure increased the activity of nitrogen cycling enzymes: ?-1,4-N-acetylglucosaminidase(NAG)and urease(UR)activity.At the same time,metagenomic data showed that there were higher abundance carbon cycle pathway genes and cbb L carbon sequestration genes under RE and CS mulching than BF mulching,and there were higher abundance nitrogen metabolism pathway genes under CS mulching.While,BF mulching reduced the activity of carbon and nitrogen cycling enzymes and the abundance of genes in carbon and nitrogen metabolism pathways.This indicated that compared with inorganic mulching,organic mulching stimulated the increase of the carbon and nitrogen metabolism pathway genes of microorganisms and the activity of carbon and nitrogen cycling enzymes through the input of organic matter,thus improving the carbon and nitrogen level of soil.(3)Long-term cover crop can effectively improve soil quality and different populations of plants can lead to the differentiation of soil environment and microbial community structure and function.Compared CT treatment,13-year-old leguminous white clover(WC,Trifolium repens L.)and 13-year-old gramineous orchard grass(OG,Dactylis glomerata L.)increased the content of soil organic matter by 52.81% and 30.81%,and increased the activity of plant decomposition enzyme BG(20.65% and 26.07%)and CBH(61.57% and 69.48%),respectively.Both cover crops increased the abundance of decomposing bacteria: Actinobacteria,Firmicutes,Chloroflexi,Ascomycota,Bacillus and Blastococcus.This indicated that long term cover crop improves soil quality by increasing soil nutrient,enzyme activity and the abundance of dominant populations.In addition,due to the nitrogen fixation characteristics of white clover,WC mulching increased the content of total nitrogen and available nitrogen(72.55% and 100.91%),and the activity of nitrogen cycling enzymes NAG(174.76%)and UR(83.88%).The differences between the two grasses were also reflected in their effects on soil microbial community structure and function,the dominant populations in the soil under WC mulching were Proteobacteria,while the dominant populations in the soil under OG mulching were Actinobacteria.Carbon circulation pathway genes were enriched in both of the two cover crops,while nitrogen metabolism pathway genes were increased in WC cover crop.In addition,OG cover crop was more conducive to carbohydrate enzyme gene enrichment for cellulose and hemicellulose hydrolysis than WC cover crops.(4)As the number of years of cover crop increases,soil nutrients are accumulating,bacterial and fungal communities showed different evolution patterns.In the time gradient: 4-year white clover cover(WC-4)has little effect on soil nutrients,while 8-year(WC-8)and 13-year white clover cover crops(WC-13)can effectively increase the topsoil nutrients.The WC-13 treatment can also improve deep soil nutrients.Meanwhile,WC-13 effectively improved the alpha diversity of bacteria in the 0-20 cm soil layer and the relative abundance of Actinobacteria,Firmicutes and Ascomycota.At the genus level,the evolution of bacterial community structure became more stable with the years of grass growing,while the evolution of fungal community structure became more disorderly.Microbial evolution lags behind nutrient accumulation with the implementation of cover crops.In the space gradient: soil nutrient and alpha diversity of bacteria decreased with the deepening of soil layer,but the diversity of fungi has little changed with the deepening of the soil. |
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