Study On Soil Aggregate Stability And Stoichiometric Characteristics Of Carbon,Nitrogen And Phosphorus In Reclaimed Forestland In The Loess Region Of Northern Shaanxi Province

The loess area of northern Shaanxi is one of the most serious areas of soil erosion in China.In recent years,with the construction and implementation of forestry ecological projects such as returning farmland to forest,the local ecological environment has been significantly improved.Through the study of soil aggregate composition,stability and carbon,nitrogen and phosphorus stoichiometry characteristics of typical returning farmland forest land formed after returning farmland to forest in this area,it has important theoretical and practical significance for the prevention and control of soil erosion,soil nutrient loss and the optimization of vegetation types in soil and water conservation forest in this area in the future.Therefore,in this study,nine typical vegetation types in the loess area of northern Shaanxi were selected as the research objects.The soil aggregate fractions(>2 mm,2-0.25 mm and <0.25 mm)were separated by wet sieve method,and the contents of carbon,nitrogen and phosphorus in soil aggregates of different particle sizes were determined at the same time,in order to clarify the stability of soil aggregates and the differences in stoichiometric characteristics of carbon,nitrogen and phosphorus in different vegetation types,and to explore the influencing factors leading to the variability of soil aggregate stability and stoichiometric ratio of carbon,nitrogen and phosphorus.The main results are as follows:(1)There were significant differences in the mass fraction of soil aggregates among different vegetation types(P<0.05),with grassland having significantly higher mass fractions of >2 mm-sized aggregates in the 10-20 cm soil layer than those of 2-0.25 mm and <0.25 mm-sized aggregates,while the other vegetation types of soil aggregates were dominated by <0.25 mm-sized aggregates in the 0-40 cm soil layer.(2)Vegetation restoration had a significant impact on the stability of soil aggregates.Grassland,Hippophae rhamnoides,Populus davidiana,and Pinus tabuliformis forest had higher percentages of water-stable aggregates(WSA),mean weight diameter(MWD),and geometric mean diameter(GMD),and lower percentages of aggregate destruction(PAD),and showed the strong soil stability of these vegetation types.(3)There were significant differences in the carbon,nitrogen,and phosphorus contents of soil aggregates under different vegetation types(P<0.05),the carbon content of soil aggregates in grassland and Pinus tabuliformis was higher,the nitrogen content of aggregates in Hippophae rhamnoides forests was higher,and the phosphorus content of aggregates in Robinia pseudoacacia × Hippophae rhamnoides forests was higher.In general,the carbon,nitrogen,and phosphorus contents of soil aggregates were mostly concentrated in the larger aggregates(>0.25 mm).(4)The C: N and C: P ratios of soil aggregates from grassland and Pinus tabuliformis were relatively high,while the N: P ratio of soil aggregates from Hippophae rhamnoides was relatively high.The stoichiometric values of carbon,nitrogen,and phosphorus in soil aggregates of different vegetation types were higher in larger aggregates.(5)There was significant correlation between the stability and stoichiometry ratios of soil aggregates under different vegetation types and soil physical and chemical properties(P<0.05).Soil bulk density and organic carbon content were significantly correlated with MWD,GMD,and PAD.The maximum soil water-holding capacity,capillary water-holding capacity and different-sized soil aggregate organic carbon content were significantly correlated with aggregate C:N ratios.The maximum soil water-holding capacity,capillary water-holding capacity,total porosity,soil organic carbon content and different-sized soil aggregate organic carbon content were significantly correlated with aggregate C:P ratios,while soil total nitrogen,electrical conductivity,and organic carbon,and different-sized soil aggregate nitrogen content were significantly correlated with aggregate N:P ratios.In summary,the soil aggregates of Grassland,Hippophae rhamnoides,Populus davidiana,and Pinus tabuliformis forest were more stable.The soil aggregates under grassland and Pinus tabuliformis forest had higher carbon content and C:N and C:P ratios,and Hippophae rhamnoides forest had higher nitrogen content.Therefore,in the future,during the construction of forestry ecological projects to improve soil water fixation and soil retention in the study area,forest vegetation such as Pinus tabuliformis,Hippophae rhamnoides and grassland can be planted on the slope,and some Populus davidiana forest can be configured at the bottom of the gullies.