Effects Of Vegetation Restoration On Micro Aggregates And Their Cementing Substances In Degraded Red Soil

As a basic component of soil aggregates,soil microaggregates play an important role in regulating soil fertility and improving soil quality.To better understand impacts of vegetation restoration on soil microaggregates in erosion zones of the red soil region,three types of restored forests(i.e.,different vegetation models)including the broad-leaved forest(BF;dominated by Liquidambar formosana),the coniferous forest(CF;dominated by Pinus massoninana)and the mixed broadleaf-conifer forest(MF;consisted of Schima superba and Pinus massoninana),along with the naturally restored grassland(GL)as the normal control,were selected in this research for studying distribution patterns of soil microaggregates and dynamics of organic carbon,iron-aluminum oxides,and bound-organic carbon in soils of the degraded red soil region after vegetation recovery.The main results are:(1)For the three restored forests,microaggregates with diameter < 0.02 mm all accounted for the majority of soil microaggregates,amounting to 62.91%-71.82% of total microaggregates,higher than the GL where the percentage was only about 42.99%.The averaged contents of soil microaggregates with diameter < 0.02 mm in different vegetation restoration modes were characterized as: BF > MF > CF > GL.(2)Vegetation restoration tended to increase contents of organic carbon in soil microaggregates,which generally decreased with the size of microaggregates and the depth of soils.Contents of organic carbon in soil microaggregates among different vegetation restoration modes also followed the order of BF > MF > CF > GL.Specifically,contents of organic carbon in different sizes of soil microaggregates were all higher in BF(1 – 3.2 times),CF(3.9 – 4.1 times)and MF(2.3 – 2.7 times)when compared with the GL.(3)All forms of iron and aluminum oxides in soils of different vegetation restoration models showed enrichments to fine grains.Fed and Fep content in different sizes of soil micro aggregates were higher than that of Ald and Alp,compared to the ot-her grassland,the whole contenst of iron and aluminum oxides in various particle sizes of soil micro aggregates in the other three kinds of restoration mode has been significantly improved: the average contents of Fed in BF and CF and mixed conifer-broadleaf forest were 1.3-3.63 times of the meadow at the size of 0.02-0.05 mm and <0.02 mm,the contents of Feo in BF,CF and MF were 2.8-4.7,1.9-3.9,2.9-5.2 times to grass-land in different sizes(from large to small),the change rule of Fep content in the soil micro aggregates of each restoration was the same as the organic carbon,which showed decreases with the increase of particle size,and decreases with the increase of soil depth,the order of the Fep average content in soil microaggregates of each restoration models BF > BF > MF > GL.The average contents of Ald and Alo in BF,CF and MF were 4.7-7.2 and 5-5.6 times of GL,the average content of Alp was 17.1-18.9 times of GL.Vegetation restoration on soil iron oxide activation degree also has a great influence.The grass activation degree of iron oxide was relatively stable in different sizes of each soil layer,and the other recovery modes between degree of activation of iron oxide changed significantly,iron oxide activation of other recovery modes change greatly,the order of the average iron oxide activation degree was BF > MF > CF > GL.(4)In different restoration modes,the regularities of Ca-SOC and Fe(Al)-SOC distribution in soil micro aggregate showed a trend of increasing with the decrease of particle size,and the content of Fe(Al)-SOC was higher than Ca-SOC at each size of soil microaggregates.Vegetation restoration on soil showed little influence on Ca-SOC,and Ca-SOC of soil micro aggregates had no significant difference in the same size under different restoration modes on the whole,the contents of Fe(Al)-SOC showed a trend of increasing with the soil depth decreased on the whole.The order of the averaged content of Fe(Al)-SOC under different restoration modes was mixed conifer-broadleaf forest > Broad-leaved forest > Coniferous forest > grassland.based on the related analysis of Ca-SOC,Fe(Al)-SOC and SOC,the correlation of Fe(Al)-SOC and SOC was better than Ca-SOC and SOC,the correlation of Ca-SOC,Fe(Al)-SOC and SOC showed the the correlation increases with the decrease of particle size,compared to the Ca-SOC,Fe(Al)-SOC was more conducive to soil organic carbon fixation in Southern Red Soil to a certain extent.Conclusion: Vegetation restoration has effectively increased the contents of organic carbon in aggregates,conducive to the formation of soil.In addition,vegetation restoration also promoted the transformation of iron and aluminum oxides in microaggregates,improving the activation degree of iron oxides in microaggregates.Therefore,in the restoration of the degraded red soil,broad-leaved forests and coniferous forests should be given priority at the early stage,and broadleaf trees should be replanted within coniferous forests as they play an important role in the ecological restoration and the improvement of soil quality of the degraded red soil region.