Effects Of Plantation Of Different Tree Species On The Composition Of Soil Aggregates And The Stoichiometry Of Carbon,nitrogen And Phosphorus

Soil aggregates are the basic units of soil structure and an important carrier for material circulation and energy flow in soil,which is of great significance for maintaining the physical characteristics,fertility and ecosystem stability of soils.Previous studies have shown that the differences in organic matter input and turnover,root activities and soil biological communities in different tree species composition,vegetation types and community succession stages might profoundly affect the composition,stability of soil aggregates and the ecostoichiometry characteristics of carbon,nitrogen and phosphorus of soil aggregates.Therefore,the rational selection of afforestation tree species is not only related to the sustainability of aboveground stand productivity,but also affects the stability of soil fertility quality underground.In this study,the native tree species homogenous garden,established in 2015 at the Modern Agriculture Research and Development Base of Sichuan Agricultural University,was selected as the research object.Seven tree species,including Cinnamomum austrosinense,Cinnamomum longipaniculatum,Cinnamomum camphora,Cinnamomum japonicum,Toona sinensis,Toona ciliata,and Alnus cremastogyne,as well as fallow land(control),were chosen as study sites to investigate the effects of tree species changes on the physicochemical properties,aggregate composition,and ecological stoichiometry of carbon,nitrogen,and phosphorus in the 0-10 cm,10-20 cm,and 20-30 cm soil layers.The main results are showed as follows:(1)The change of tree species significantly affected soil p H,bulk density,aeration,firmness and particle composition,while soil layer significantly affected soil water holding capacity,porosity and shear strength of soil.Soil p H varied from 6.53 to 7.74,with the highest value in Cinnamomum austrosinense and the lowest value in Cinnamomum longipaniculatum.Compared with fallow land(near neutral),the soil p H of Cinnamomum austrosinense,Cinnamomum camphora,Cinnamomum japonicum,Toona sinensis,and Toona ciliata was higher and weakly alkaline,while the soil p H of Alnus cremastogyne and Cinnamomum longipaniculatum was lower and weakly acidic.The soil bulk density was highest in Toona sinensis and lowest in Toona ciliata.Compared with fallow land,the soil bulk density increased in Toona sinensis and Cinnamomum camphora,while it decreased in Cinnamomum austrosinense,Cinnamomum longipaniculatum,Cinnamomum japonicum,Toona ciliata,and Alnus cremastogyne.The total porosity and water-holding capacity of each tree species soil(maximum water holding capacity,minimum water holding capacity,and capillary water holding capacity)decreased with the increase of soil layer,while the soil solidity and shear strength increased with the increase of soil layer.The soil aeration of Cinnamomum longipaniculatum,Cinnamomum japonicum,Toona ciliata,and Alnus cremastogyne soil increased with the increase of soil layer,while that of Cinnamomum camphora,Toona sinensis,and fallow land showed the opposite trend.(2)The content of soil aggregates of different tree species changed significantly with particle size,and the fractal dimension of soil aggregates varied significantly with tree species.The soil aggregates of each tree species and fallow land were mainly composed of aggregates >5 mm and 2-5 mm in size,and the sum of their contents is greater than 90.11%,which decreases with decreasing particle size in the soil depth of 0-10 cm,10-20 cm and 20-30 cm.Among the different soil layers(0-10 cm,10-20 cm,and 20-30 cm),the content of aggregates >5 mm is highest for the Alnus cremastogyne,ranging from 84.57%-85.45%,and the content of 2-5 mm aggregates is highest for the Cinnamomum camphora,ranging from12.35%-13.17%.The different tree species in the homogeneous garden did not affect the mean weight diameter and geometric mean weight diameter of soil aggregates,but had a significant impact on the fractal dimension of soil aggregates.In the soil depth of 0-10 cm and 10-20 cm,the fractal dimension of soil aggregates is lowest for the Cinnamomum japonicum(1.76-1.79)and highest for the Cinnamomum austrosinense(1.96-1.98).(3)The ranges of soil aggregate carbon,nitrogen,and phosphorus contents for different tree species were 7.85-74.00 g/kg,0.12-1.44 g/kg,and 0.12-2.47 g/kg,respectively.Tree species,soil layer,and particle size significantly influenced the content and storage of carbon,nitrogen,and phosphorus in soil aggregates.Except for Cinnamomum camphora,the content of carbon,nitrogen,and phosphorus in soil aggregates for each tree species and particle size decreased with increasing soil depth.Across all soil layers(0-10 cm,10-20 cm,and 20-30cm),the highest organic carbon and total nitrogen contents of soil aggregates were found in Cinnamomum japonicum,while the lowest were found in Alnus cremastogyne.The highest total phosphorus content was found in Alnus cremastogyne,while the lowest was found in Cinnamomum japonicum.The carbon,nitrogen,and phosphorus storage of soil aggregates for different tree species and soil layers were highest in the >5 mm particle size and lowest in the 0.25-0.5 mm particle size.In the 0-10 cm,10-20 cm and 20-30 cm soil layer,the contribution of soil aggregate carbon,nitrogen,and phosphorus contents to the total soil carbon,nitrogen,and phosphorus contents decreased as the particle size decreased,with >5mm aggregates being the highest and 0.25-0.5 mm aggregates being the lowest,respectively.(4)The ranges of soil aggregate carbon-to-nitrogen ratio,carbon-to-phosphorus ratio,and nitrogen-to-phosphorus ratio for different tree species were 7.27-374.65,3.36-562.04,and 0.15-9.11,respectively.Tree species,soil layer,and particle size all significantly influenced the ecological stoichiometry of carbon,nitrogen,and phosphorus in soil aggregates.In the 0-10 cm,10-20 cm and 20-30 cm soil layer,the carbon-to-nitrogen ratio,carbon-to-phosphorus ratio,and nitrogen-to-phosphorus ratio for particle sizes >5 mm,2-5mm,1-2 mm,0.5-1 mm,and 0.25-0.5 mm were highest in Cinnamomum japonicum and lowest in Alnus cremastogyne.Pearson correlation analysis showed that the carbon-tonitrogen ratio of soil aggregates was significantly positively correlated with the carbon-tophosphorus ratio,carbon content,and carbon storage of soil aggregates,and significantly negatively correlated with the nitrogen-to-phosphorus ratio and nitrogen content of soil aggregates.However,there was no correlation between soil aggregate composition and carbon-to-nitrogen ratio,carbon-to-phosphorus ratio,or nitrogen-to-phosphorus ratio.In summary,compared with open areas,native tree species in homogeneous gardens have richer water sources and nutrient content,The change of tree species had significant effects on soil physicochemical properties,soil aggregate stability and ecostoichiometry characteristics of carbon,nitrogen and phosphorus.In general,compared with other tree species,the evergreen broad-leaved species Cinnamomum japonicum and deciduous broadleaved species Alnus cremastogyne and Toona ciliata in homogeneous gardens,they can more effectively promote the stability of soil aggregate structure,and optimize the soil carbon,nitrogen and phosphorus ecological stoichiometric ratio characteristics.It may be an excellent tree species for afforestation in the study area.