| Cryptomeria japonica is one of the important tree species for the construction of subtropical fast-growing timber forest plantation,playing an important role in regional economic development and ecological function.However,the development of artificial pure forest often leads to soil structure damage,soil erosion and loss of soil fertility.The stand density,canopy density,understory vegetation diversity,microbial community characteristics,and litter quantity and quality are different with different tree development stage,which have significant influence on soil physical and chemical properties.Soil aggregates are the basic units of soil structure formed by mineral particles and organic matter.The distribution characteristics and stability of soil aggregates are key factors in maintaining soil fertility.Moreover,the organic carbon,total nitrogen,and total phosphorus content,as well as their ecological stoichiometry ratios(C/N,C/P,N/P)of soil aggregates,are important indicators of soil ecosystem stability.However,the composition and stability of soil aggregates and their ecological stoichiometric characteristics of carbon,nitrogen and phosphorus in different ages of Cryptomeria japonica plantations are still unclear.Therefore,Cryptomeria japonica plantations at rainy area of west China with different ages(7 years,13 years,24 years,33 years,53 years)were taken as the research object in this study.Soils were sampled in the 0-30 cm soil layer.The soil physical and chemical properties,soil aggregates composition and stability,and carbon,nitrogen and phosphorus content and ecological stoichiometric of soil aggregates were measured to explore the main factors affecting the structure and stability of soil aggregates in the Cryptomeria japonica plantation along the forest age.The main results were showed as follow:(1)With the increase of forest age,soil bulk density and soil p H exhibited a trend of decreasing and then increasing,and reaching the minimum values in the 33-year-old and24-year-old of the Cryptomeria japonica plantations,respectively.The soil quality parameters,including water content,maximum water-holding capacity,capillary water-holding capacity,minimum water-holding capacity,lower limit of optimal water content,total porosity,capillary porosity,soil water storage capacity,clay content,organic carbon content,total nitrogen content,C/N,C/P,and N/P,all show a trend of increasing and then decreasing with the increase of plantation age,reaching the maximum values in the 33-year-old of the Cryptomeria japonica plantations.(2)Aggregates with a particle size of 5-10 mm and 2-5 mm are the main types,accounting for 44%-89% of the total aggregates.The percentage of soil aggregates decreases as the particle size decreases across the forest age.With the increase of forest age,the content of non-water-stable aggregates with a particle size of 0.5-5 mm,water-stable aggregates with a particle size of 2-10 mm and stability of soil aggregates first increased and then decreased,while the content of non-water-stable aggregates with a particle size of<0.25 mm and water-stable aggregates with a particle size of <1 mm first decreased and then increased,reaching maximum or minimum values in the 24-year-old or 33-year-old of the Cryptomeria japonica plantations.In addition,the clay content of non-water-stable aggregates and water-stable aggregates with a particle size of 0.053-10 mm first decreased and then increased with the increase of forest age,reaching the maximum value in the33-year-old of the Cryptomeria japonica plantations.(3)The organic carbon content,total nitrogen content,the water-stable aggregates’ total phosphorus content and the non-water-stable aggregates’ total phosphorus content in the 0.5-10 mm and 0.053-0.25 mm size fractions,C/P,N/P,C/N of water-stable aggregates<2 mm,as well as the C/N of non-water-stable aggregates 2-10 mm and 0.053-0.25 mm showed a trend of first increasing and then decreasing with the increase of forest age,reaching a maximum value in the 24 or 33-year-old of the Cryptomeria japonica plantations.However,the change of particle size did not significantly affect the ecological stoichiometry of carbon,nitrogen and phosphorus in soil aggregates.The organic carbon content,total nitrogen content,C/P,N/P,and C/N of non-water-stable aggregates 0.053-10 mm and water-stable aggregates 0.5-10 mm,were significantly higher than those in the15-30 cm layer.However,changes in particle size did not significantly affect the ecological stoichiometry of carbon,nitrogen,and phosphorus in soil aggregates.In addition,the carbon,nitrogen,and phosphorus storage and contribution rates of soil aggregates in the Cryptomeria japonica plantations decreased with decreasing particle size.(4)Pearson correlation analysis showed that the physical and chemical properties of soil and aggregates were correlated with the stability of soil aggregates.Stepwise regression analysis further showed that soil aggregate stability was positively correlated with soil total porosity,clay content and C/N,and negatively correlated with soil p H.The content of non-water-stable macroaggregates was positively correlated with the total nitrogen content of <0.053 mm aggregates and C/N of 0.25-0.5 mm aggregates,and negatively correlated with the sand content of 0.5-1 mm aggregates.The fractal dimension of non-water-stable aggregates was negatively correlated with total nitrogen content of0.053-0.25 mm aggregates,N/P of < 0.053 mm aggregates,and clay content of 0.5-1 mm aggregates.The stability of water-stable aggregates was positively correlated with the total phosphorus content of 1-2 mm aggregates,total nitrogen content of 0.25-0.5 mm aggregates,N/P of 5-10 mm aggregates,and negatively correlated with the sand content of5-10 mm aggregates.In conclusion,changes in forest age of the Cryptomeria japonica plantations in the rainy region of West China significantly affects soil physical and chemical properties,soil aggregate structure,and the ecological stoichiometry of carbon,nitrogen,and phosphorus in soil aggregates.Compared with other age stages,the 24-33 year-old of the Cryptomeria japonica plantations have a more stable soil aggregate structure and higher soil carbon,nitrogen,and phosphorus content and storage,which is a critical stage for maintaining soil fertility and sustainable management of the Cryptomeria japonica plantations in the region.These findings provide a reference for scientific management and sustainable development of plantation forests in the rainy region of West China. |
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