| Located in arid and semi-arid regions,the Three North areas have been suffering from increasing land desertification and soil erosion since 1950s,which has severely constrained the socio-economic development of the region.In order to improve the environment of the region,several vegetation restoration projects have been implemented since 1978(e.g.,the Three-North Shelter Forest Program,the Project of Converting Farmland to Forestland and Grassland,etc.).Understanding the dynamic change patterns,the driving factors and their relationships of soil organic carbon and soil moisture during vegetation restoration is a prerequisite for evaluating the effectiveness of vegetation restoration projects and scientific management.In this study,the effects of vegetation restoration on soil organic carbon(119 papers)and water(98 papers)in the region were analysed based on the weighted and unweighted methods in Meta-analysis with artificial grasslands,artificial shrubs and artificial forests,and the main results are as follows:(1)The vertical distribution of soil organic carbon(0-100 cm)in the vegetation restoration area had an obvious "surface aggregation" characteristic,with the greatest soil organic carbon content(8.76 g/kg)in the 0-10 cm soil layer and the lowest(2.37 g/kg)in the 60-100 cm layer;In the 200 cm soil layer,there was no significant difference in the soil moisture content of each layer,showing a trend of first increasing and then decreasing,with values in the 7.77-9.31%,with the peak occurring in the 20-40 cm soil layer;Overall,in the 100 cm soil layer,soil moisture was significantly and positively correlated with soil organic carbon(R2=0.26,P<0.001),and for every 1%increase in soil moisture,the soil organic carbon would increase by 0.533 g/kg.In particular,soil organic carbon increased by 0.554 g/kg(R2=0.16,P<0.001)0.357 g/kg(R=0.22,P<0.001)and 0.456 g/kg(R2=0.51,P<0.001)for each 1%increase in soil moisture in forests,shrubs and grasslands,respectively,indicating that the highest water-carbon conversion efficiency was observed in forests.(2)Compared with farmland or sandy land,the vegetation restoration increased soil organic carbon in the 0-100 cm soil layer by 62.10%(P<0.05),with the largest increase of 129.08%in the 0-10 cm soil layer.In restoration areas where the initial land use was sandy,the increase in soil organic carbon was 3.5 times greater than in areas where the initial land use was farmland;In terms of the type of vegetation restoration and species planted,soil organic carbon increased more in forests than in shrubs,and the least in grasslands,with Pinus sylvestris plantation higher than sandy shrubs,and Pinus tabuliformis higher than Robinia pseudoacacia and Caragana korshinskii;The restoration period only affected soil organic carbon in the 0-60 cm soil layer,and it increased with the restoration period;The increase in soil organic carbon was higher in the low precipitation zone(<400 mm)than in the high precipitation zone(>400 mm);The higher the initial soil organic carbon content in the 0-40 cm soil layer,the lower the increase in soil organic carbon after vegetation restoration,while below 40 cm,there was no significant relationship between the two.(3)Compared with farmland or sandy land,soil moisture in the 0-200 cm soil layer in the vegetation restoration area decreased by 8.01%(P<0.05),mainly due to a larger decrease in soil moisture below 20 cm.The initial land use type only affected the surface soil moisture(0-20 cm),which increased after vegetation restoration in sandy areas,while the change in farmland was not significant;Grasslands consumed the least deep soil moisture(>20 cm),while both forests and shrubs consumed more,with Pinus tabuliformis having a slightly higher water retention capacity than Robinia pseudoacacia and Caragana korshinskii;With the increase of restoration period,the surface soil moisture fluctuated with the increase of restoration years,while deep soil moisture loss was greatest at 11-20 years and began to recover after 20 years,but was still below the pre-restoration level;Compared with other precipitation areas,vegetation restoration consumed the least deep soil water when precipitation>550 mm;Initial soil water content was negatively correlated with the rate of change of surface soil water,but not with the change of deep soil water;the higher the planting density in the restoration area,the greater the soil water consumption.The above results indicate that in the Three Northern Areas,vegetation restoration areas with precipitation less than 550 mm should be planted mainly with grasses and supplemented by shrubs and forests;In areas with precipitation greater than 550 mm,forests with low water consumption should be selected for restoration to improve carbon sequestration benefits.In addition,the planting density should be reasonably controlled in the vegetation restoration according to the carrying capacity of local water resources. |
