| Different types of vegetations have great differences in improving soil quality.To clarify the differences in the effect of different vegetation restoration on soil quality after large-scale return of farmland in the loess hilly region,the study systematically investigated the storages of soil C and N,fractals of soil aggregates,and their carbon and nitrogen distributions under typical vegetation restoration conditions in the Ansai Wuliwan Watershed.Through the study,the differences in the distributions of carbon and nitrogen in soil profiles,microbial biomass carbon,nitrogen,and different aggregates under typical vegetation restoration were initially ascertained;the effects of vegetation restoration on the fractal features of soil structures were clarified;the composition of soil aggregates and its influencing factors were explored under different vegetations restoring condition.The results of the research have great significance for understanding deeply the relationship between typical vegetation restoration and soil quality in the loess hilly region,accurately evaluating the benefits of ecological environment under different vegetation restoration modes,and the sustainable development of ecological restoration in the region.There are four conclusions in this study:(1)Vegetation restoration has effective promotion for the fixation of soil organic carbon.The range of soil carbon and nitrogen storage of the four typical types of vegetations in the study area varied from 3.476 t·hm-2 to 16.673 t·hm-2 and 0.366 t·hm-2 to 1.679 t·hm-2 respectively in the entire profile from 0 to 200 cm.With the increase of soil depth,it has a significant decrease in 0～40 cm soil layer,and basically stabilized in 40～200 cm soil depth.The organic carbon content and total nitrogen content of different vegetations is greater in abandoned grassland and shrubbery,and the smaller in farmland.The carbon and nitrogen storage of arbor forest as typical vegetation shows an upward trend with the increase of vegetation restoration period in the entire soil profile.(2)Vegetation restoration promotes the formation of large aggregates and effectively improves the structure of soil aggregates.The types of vegetation and its interaction with soil depth had extremely significant effects on the volume fractal dimension of soil particle and the mass fractal dimension of soil aggregate(P<0.01),while soil depth had a significant effect on the volume fractal dimension of soil particle(P<0.05),but the effect on the mass fractal dimension of soil aggregate is not clear.The study found that the composition of soil particles in the catchment was mainly silt while the content of clay was small;The range of volume fractal dimension of soil particle size distribution in the typical vegetation is from 2.512 to 2.599;it increases with the increase of soil depth,and reaches the maximum in 160～200 cm soil layer.The soil aggregates of four typical vegetations showed a "V" distribution pattern in the 0～40 cm soil profile and has the lowest percentage in the 0.25～0.5mm class.The mass fractal dimension of soil aggregaterange from 2.725 to 2.784,and it was negatively correlated with the content of macroaggregates.For tvolume fractal dimension of soil particle and the mass fractal dimension of soil aggregaterange of four typical vegetations in the same soil layer,the abandoned grassland shows the highest value while shrubbery forest shows the lowest value;and the average weight diameter of the soil aggregate in the shrubbery is the largest and abandoned grassland is smallest in the 0～40 cm soil layer,indicating that shrubs planted in the study area are more good at the formation of soil aggregates.(3)Vegetation restoration increases the retention of carbon and nitrogen in aggregates of different soil structures.The storage of organic carbon,total nitrogen,and carbon and nitrogen contents in large aggregates(>0.25 mm)of the four typical vegetation soils were significantly higher than that of microaggregates(<0.25 mm)in the study area.The total organic carbon content,total nitrogen content,and carbon and nitrogen storages in all the aggregates show a“∧”distribution,and reached a maximum at 0.25～0.5 mm.From the aspect of soil profile,the contents of organic carbon and total nitrogen in the soil aggregates of typical vegetation in the soil surface layer(0～10 cm)are higher than that of 10～20 cm and 20～40 cm soil layers,and all had a significant difference(P<0.05)except for the farmland.The organic carbon contents are positively correlated with the average weight diameter of agglomerate and the content of macroaggregates,but are negatively correlated with the fractal dimension of soil aggregates(P>0.05).In 0～10 cm soil layer,the organic carbon content and total nitrogen content of each grain size aggregate show the largest values in shrubs and the lowest values in farmland among different vegetations.(4)Vegetation types and restoration duration have significant effects on soil microbial carbon,nitrogen,the ratio of microbial carbon to soil organic carbon(MBC/SOC)and microbial nitrogen to total nitrogen(MBN/TN).Vegetation restoration has increased on soil microbial carbon and nitrogen content,and showed an increasing trend with the increase of vegetation restoration.The soil microbial biomass carbon,nitrogen,MBC/SOC,and MBN/TN of the four typical vegetations in the study area showed an decreasing trend with the increase of soil depth.The contents of microbial carbon and nitrogen in different vegetations on the same profile were as follows:Shrubs were larger,but the farmland was the lower;The values of MBC/SOC and MBN/TN among the four vegetations in the same soil layer all show the largest value in arbor forest and the lowest value in farmland.The contents of microbial carbon and nitrogen all increase with the increase of the vegetation restoration period;while the soil MBC/SOC and MBN/TN values showed a trend of first increasing,reached a maximum at 30 years and then decreasing with the increase of the restoration period.It indicated that microbial biomass carbon and nitrogen and the ratio of microbial biomass carbon(nitrogen)to soil organic carbon(nitrogen)were more effective in reflecting changes in soil quality,while the microbial carbon and nitrogen contents of the arbor forest and shrub forest improve better in the typical vegetation restoration in the study area. |
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