Characteristics Of Soil Carbon And Iron Changes And Their Contribution To Aggregate Formation And Stability Following Returning Farmland To Grassland

The Loess Plateau is located in the arid and semi-arid area of China with serious soil erosion and is a typical ecologically fragile area.Returning farmland to forest(grass)is considered to be an effective means of vegetation restoration and reconstruction in ecologically fragile areas,which can not only promote the virtuous cycle of soil nutrients,maintain the stability of soil structure,but also increase vegetation coverage and promote plant nutrient absorption.The stability of soil structure is the basis of the virtuous cycle of soil nutrients and the growth of vegetation,and its stability depends on the stability of soil aggregates.Aggregate is the basic unit of soil structure,which plays an important role in the regulation of soil water,fertilizer,gas and heat,and is an important indicator of soil structure stability.Therefore,the stability of soil aggregates is crucial to the stability of soil structure in terrestrial ecosystems.Soil aggregates are aggregates formed by the combination of soil particles and binding agents in the soil.However,the contribution of the change of binding agent contents to the stability of aggregates in the process of natural restoration of grassland in the semi-arid region of the Loess Plateau is not clear.In this study,the soil samples with different years of restoration of grassland(farmland,6 years,15 years,25 years and 45 years)in Zhifanggou watershed in Ansai,Shaanxi Province were taken as the research object,the vertical distribution characteristics of different soil binding agents(soil organic carbon,soil inorganic carbon and iron oxide)in 0-50cm soil depths were analyzed,and the change characteristics of binding agent content in the process of vegetation restoration were clarified.The distribution characteristics of binding agents in different particle size aggregates and their contribution to the formation and stability of aggregates were investigated in order to provide scientific basis for vegetation ecological restoration on the Loess Plateau.The main results are as follows:(1)In the process of the vegetation restoration,the content of organic carbon(SOC)in surface(0-20cm)soil increased following the vegetation restoration,and reached the maximum in 45 years,with an increase of 43.5%(0-10 cm soil layer)and 80%(10-20 cm soil layer),respectively,compared with the farmland,while the SOC content in 20-50 cm soil depths did not change significantly following vegetation restoration.The SIC content in the bulk soil increased following vegetation restoration,but the SIC content in 0-30 cm soil depths after vegetation restoration was lower than that in farmland.And the SIC content in the 30-50 cm soil layers in 45 years was not significantly different from that in the farmland,but the SIC content in other vegetation restoration samples were still lower than that in farmland.With increasing soil depth,the SOC content in the bulk soil decreased and the content of SIC increased.In addition,the correlation between SOC and SIC increased following vegetation restoration,indicating that vegetation restoration favors the increase of SOC and SIC contents and makes the relationship between organic and inorganic carbon stronger.(2)In the process of the vegetation restoration,the contents of free iron oxide(Fe_d),amorphous iron oxide(Fe_o)and complex iron oxide(Fe_p)in bulk soil reached the maximum in 25 years with the vegetation restoration,and then decreased.The content of Fe_d and Fe_o increased with increasing soil depth,and there was no significant trend in Fe_p overall.The activation(Fe_o/Fe_d)and complexation(Fe_p/Fe_d)of soil iron oxide increased with the vegetation restoration.The results showed that the restoration of grassland vegetation was beneficial to the increase of soil iron oxide content,and the activation degree and complexation degree of iron oxide were also improved.(3)The SOC content of each particle size of aggregates increased with the vegetation restoration and decreased with the increase of soil depth.The content of SIC increased with the vegetation restoration,which was lower than that of farmland as a whole,a nd increased with the increase of soil depth.The content of SIC was the highest in microaggregates(0.25-0.053 mm)and the lowest in silty+clay(<0.053 mm).Fed and Feo increased at first and then decreased with the vegetation restoration.Fe p had no obvious change trend with the vegetation restoration.With the increase of soil depth,Fe o as a whole showed an increasing trend,while Fed and Fep had no obvious change trend.(4)The stability index of soil aggregates(MWD)and the number of aggregates larger than 0.25 mm particle size(R0.25)increased following the vegetation restoration,while MWD decreased with the increase of soil depth,and the surface soil layer(0-10 cm)was significantly higher than that of other soil layers,and the value of R0.25 in 0-30 cm soil depth is the maximum in 45 years.In the process of vegetation restoration,the content of macroaggregate(>2 mm)increases gradually,while the content of silt+clay(<0.053 mm)decreases gradually.It shows that the promoting effe ct of vegetation restoration on the stability of aggregates is concentrated in the 0-20cm soil depths,while the vegetation restoration has no obvious effect on the stability of aggregates in 30-50cm soil depths.(5)The stability of aggregates gradually improved with the vegetation restoration,and the contribution of the bonding effect of the binding agents in the0-50 cm soil depths to the stability of the aggregates reached 51.3%.Among them,the contribution of binding agents in the 0-10 cm and 10-20 cm soil depth to the stability of the aggregates was 68.6%and 81.8%,respectively.While the binding agents contribution rate of 20-50cm soil depths to aggregate stability is relatively low,which is 23.2%-33.3%.Moreover,the contribution of Fe o to aggregate stability was the largest in the 0-10 cm soil layer and the contribution of SOC to aggregate stability was the largest in the 10-20 cm soil layer.(6)Different soil binding agents determine the formation and stability of aggregates at different parti cle size.SOC and Fe o mainly increase the stability of large macroaggregates(>2 mm),SIC mainly increases the stability of small macroaggregates(0.25-2 mm),and Fep makes the greatest contribution to the stability of microaggregates(0.053-0.25 mm).In the process of vegetation restoration,Fed indirectly contributes to the stability of aggregates by transforming into Feo.The increase of Fe d will promote the formation of large aggregates.