Spatial Distribution Of Nitrogen In Soil Aggregates In Pinus Yunnanensis With Different Restoration Models And Main Controlling Factors

In the fault basin area,the slope terrain is widely distributed and strongly affected by hydraulic erosion.As a key transition zone between karst plateaus and basins,the undulating terrain causes the shallow soil layer on the slope surface to be lost with surface runoff,rock outcrops,and rock desertification is aggravated.Vegetation restoration has a good effect on soil and water conservation on the topography of karst slopes.There are Pinus yunnanensis forests with complete succession sequences and different restoration models on the karst slopes of eastern Yunnan,but the co-evolution mechanism and influencing factors of Pinus yunnanensis vegetation restoration and stability of surface aggregates and nutrient accumulation in this area are not clear.In this paper,the aggregate structure and the aggregates?¹⁵N value are taken as the entry point,and the Castanopsis orthacantha and Lithocarpus dealbatus primary forest and the Myrsine africana shrub are used as reference vegetation,the distribution of soil aggregate stability,TN content and?¹⁵N value of natural secondary forest of Pinus yunnanensis,artificial mixed forest of Pinus yunnanensis and artificial pure forest of Pinus yunnanensis in the karst slopes of Eastern Yunnan province were compared and analyzed.The specific research method is:The distribution of macro-aggregates(>2mm),middle-aggregates(0.25-2mm)and micro-aggregates(<0.25mm)in soil layers of 0-10cm 10-20cm20-40cm and 40-60cm soils was obtained by wet screening method.Data analysis and one-way ANOVA were used to obtain differences in soil aggregate stability and aggregate N-element content of different vegetation types,the main control factor affecting the distribution of N elements of aggregates was obtained by redundant analysis,comprehensive evaluation of the ecological benefits of different vegetation restoration models in Pinus yunnanensis forests on the N-element cycle process of aggregate on karst slopes in Eastern Yunnan,which have a theoretical reference effect on ecological restoration and vegetation reconstruction of karst slopes.The main conclusions drawn are as follows:(1)The physical and chemical properties of the soil in the Pinus yunnanensis forest with different restoration models were different,and the structure and stability of the slope aggregates were significantly improved.The proportion of natural secondary forests of Pinus yunnanensis,artificial mixed forests of Pinus yunnanensis and artificial pure forests of Pinus yunnanensis in the soil layer of<0.25mm grain-level aggregates was significantly lower than that of Castanopsis orthacantha and Lithocarpus dealbatus primary forests and Myrsine africana shrub in the soil layer of 0-60cm.The mixed planting mode of Pinus yunnanensis natural secondary forest and Pinus yunnanensis artificial mixed forest has a better effect on improving the proportion of mass of large aggregates than that of Pinus yunnanensis artificial pure forest.The aggregate stability of the natural secondary forest of Pinus yunnanensis and the artificial mixed forest of Pinus yunnanensis was significantly better than that of the other three vegetation types,except for the artificial pure forest of Pinus yunnanensis,the stability of soil aggregates of each vegetation type decreased with the deepening of soil layer.(2)Different restoration models of Pinus yunnanensis forests had a poor effect on increasing the nutrient content of slope aggregates.The contents of inorganic nitrogen,aggregate SOC and TN in the soil layers of natural secondary forests,artificial mixed forests of Pinus yunnanensis and artificial pure forests of Pinus yunnanensis were significantly lower than those of Castanopsis orthacantha and Lithocarpus dealbatus primary forests and Myrsine africana shrub in the soil layers of 0-60cm.The contents of nitrate nitrogen,C/N and aggregate SOC and TN in the soil were reduced with the deepening of the soil layer,and the content of ammonium nitrogen in the soil was increased with the deepening of the soil layer.The SOC and TN contents of the five vegetation types aggregate were the most abundant in 0.25-2mm aggregates,followed by>2mm aggregates,and the lowest in<0.25mm aggregates.Among them,the TN content of each grain-level aggregate in the soil of artificial mixed forest of Pinus yunnanensis was significantly higher than that of natural secondary forest of Pinus yunnanensis and artificial pure forest of Pinus yunnanensis.In the soil of Pinus yunnanensis artificial pure forest in the early stage of Pinus yunnanensis vegetation restoration and in the soil of Pinus yunnanensis natural secondary forest in the late stage of vegetation restoration,it was severely N-restricted.(3)The aggregate structure of karst slopes is an important way for soil C and N to be adhered.There were differences in the amount of soil C and N retention of different particle level aggregates.Among the five quilt types,SOC and TN contents were highest in 0.25-2mm aggregates,compared with macro-aggregates of>2mm and micro-aggregates of<0.25mm,0.25-2mm aggregates contributed the most to the soil TN content,and the priority accumulation in the soil was also the largest,However,the TN content of 0.25-2mm aggregates was decreased the most with the deepening of the soil layer,and the nutrient leakage was more obvious,indicating that the retention of mesograin-level aggregates of 0.25-2mm in the soil environment was the key to promoting the improvement of soil TN reserves in this study area.Since the TN content of the five vegetation types in the study area was smaller,the effect of aggregate mass fraction on soil TN accumulation was more significant,and the TN contents of aggregates in topsoil is more likely to accumulate,while aggregate TN in deep soil is more likely to decompose.(4)The?¹⁵N values of soil aggregates of different vegetation types reflected the strength of N fractionation and the difference in cycle rate of soil aggregates of different vegetation types.The values of?¹⁵N of each particle-level aggregate in the five vegetation types increased with the deepening of the soil layer and increased with the decrease of the aggregate size.The vegetation soil of the natural secondary forest of Pinus yunnanensis showed the strongest fractionation effect of<0.25mm particle-level aggregates,among the other four vegetation types,the fractionation effect of>0.25mm particle-level aggregates was the strongest.The value of soil aggregate?¹⁵N of natural secondary forest of Pinus yunnanensis and artificial pure forest of Pinus yunnanensis showed negative values,the value of soil aggregate?¹⁵N of Castanopsis orthacantha and Lithocarpus dealbatus primary forest,Pinus yunnanensis artificial mixed forest and Myrsine africana shrub vegetation was all positive.The N cycle rate of soil aggregates in natural secondary forests and artificial pure forests of Pinus yunnanensis was lower than that of Castanopsis orthacantha and Lithocarpus dealbatus primary forest,artificial mixed forest of Pinus yunnanensis and Myrsine africana shrub.(5)The main controlling factors affecting the TN content of soil aggregates in the five vegetation types were soil nitrate nitrogen content and SOC content of each particle-level aggregate.The results show that the mixed planting of Pinus yunnanensis forest on the karst slope of eastern Yunnan has a significant effect on the improvement of soil structure.However,due to the influence of slow nutrient return,vegetation restoration is significantly affected by N restriction in the early and late stages.Measures such as planting nitrogen-fixing tree species,increasing nutrient return,and human control should be taken to promote ecological restoration.