| Vegetation restoration is the fundamental management to construct ecological environment,which plays an important role in controlling soil erosion and promoting soil function and ecosystem service value on the Loess Plateau.The plantation of shrub such as Caragana korshinskii Kom.is one of the main measures of artificial vegetation restoration in loess hilly region.However,the long-term effects of C.korshinskii plantation on soil structure and its relationship with soil moisture and organic carbon on slope land still unclear.In this study,three stand ages of C.korshinskii with 15-,25-,and 35-year-old were selected,and a natural grassland(>40 years old)served as the control.Based on field investigation and laboratory analysis,the distributions and stabilities of soil aggregate were studied at four plots and three slope positions(upper,middle,and lower).The distributions and sequestration potentials of soil organic carbon(SOC)and its fractions in bulk soil and aggregates were measured.Soil moisture distributions and its deficit effects were revealed.Finally,the relationships among soil structure,organic carbon and moisture were analyzed.The results were expected to provide scientific basis for the evaluation of soil environmental effects of vegetation restoration on the Loess Plateau.The main conclusions of this study were shown as following:(1)Aggregate stability at surface soil improved with the increase of C.korshinskii stand ages,and these effects in lower slope were better than that in upper and middle slope.In all cases,the proportion of soil macro-aggregates(>0.25 mm)and mean weight diameter(MWD)of soil aggregates increased with increasing stand age,indicating that older stands better facilitated the formation of macro-aggregates.The results showed that plantations of C.korshinskii mainly affected aggregate stability at the shallower depths(0-20cm).On the upper slope,the highest MWD was found in 35-year-old(3.08 mm),whereas on the lower slope,the highest MWD was in the control(3.27 mm).The valves of MWD at different slope positions followed the order:lower slope>upper slope>middle slope.Based on the relative slaking index(RSI)and relative mechanical breakdown index(RMI),soil aggregates were sensitive to both slaking and mechanical breakdown on the middle and upper slopes,whereas soil aggregates at the deeper depths(20-40 cm)were more sensitive to mechanical breakdown on the lower slope.(2)The content of SOC and carbon sequestration capacity were significantly increased after 25 years old stands.The contents of total SOC and easily oxidized organic carbon(EOC)within bulk soil and aggregate were increased significantly by planting C.korshinskii(P<0.05),especially for 2-0.25 mm aggregates,and its contribution rate to SOC reached 40%.On the upper and middle slopes,the highest soil organic carbon stock(SOCS)was found in 35-year-old plot(26.7Mg·ha-1),whereas on the lower slope,the highest in the control(23.6 Mg·ha-1).The SOCS among various plots and slope positions were as follows:lower slope>upper slope>middle slope in 0-100 cm depth.Compared with the control grassland,the SOCS in the 0-100 cm soil layer showed an initial decrease in 15-year-old plot(-0.3 Mg·ha-1),and then an increase to net C gains in 25-(1.7 Mg·ha-1)and 35-year-old plot(3.55 Mg·ha-1).Moreover,the differences resulted from the accumulation of SOCS in deep soil layer(>40 cm)increased significantly.The amount of SOC sequestration in the upper and middle slopes were higher than that in the lower slope,which showed a positive carbon sequestration effect.(3)Long-term plantation of C.korshinskii had been generated the deep soil moisture deficit effect,but this effect varied with slope position.The soil moisture stock(SMS)was the lowest in May(growing season)and the highest in October in 0-500 cm soil depth.The SMS decreased with the increase of stand age of C.korshinskii.In terms of each sample plot,soil moisture deficit was most severe in 25-year-old plot,and in the upper was more serious.SMS deficit in the growing season was positive accumulation except for the negative deficit of natural grassland and 35-year-old plot at 200-300 cm depth.In the 0-200 cm soil depth,SMS in each sample plot was significantly higher in 2018 than in 2009,but the opposite was true in more than 15-year-old plot below 200 cm soil depths(4)There was a significant coupling relationships among soil structure,moisture and SOC.Redundancy analysis(RDA)indicated that soil moisture was the dominant factor for the aggregate stability,accounting for 8.5%of the variation.Soil depth significantly affected the SOC balance and soil moisture environment,with a positive correlation.Its contribution were45.8%and 7.8%,respectively.Stand age can explain the variation of SOC by 10.7%,slope position can explain the variation of SMS by 4%.Based on structural equation modeling(SEM)analysis,SOC was the main factor for controlling the changes of soil structure stability,with a path coefficient 0.86.Soil moisture had a direct negative effect on SOC balance,while it influenced on aggregate indirectly through soc with path coefficients of-0.35.In conclusion,with increasing stand ages of C.korshinskii,soil aggregate stability improved and SOC sequestration increased in the loess hilly region,.Meanwhile,long-term plantation of shrub also caused a deficit for deep soil moisture(>300 cm).In overall,the positive carbon sequestration by afforestation of leguminous shrub in short term may be further weakened by water shortage,which will affect the soil structure.These effects varied with slope positions.Therefore,the comprehensive effects of slope position and stand age should be considered when the effects of C.korshinskii on soil environment were evaluated in Loess Plateau. |
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