The Dynamic Of Soil Organic And Inorganic Carbon In Soil Under Different Vegetation Restoration-in Loess Hilly And Gully Region

In arid and semiarid areas,which are characterized by fragile ecological systems,deforestation and tillage have resulted in serious land degradation and net loss of soil carbon to the atmosphere.Vegetation restoration is considered to effectively curbland degradation,substantially improve the soil properties,and have great potential toalter the soil carbon stock.Although“Grain for Green Project”has been done on the Chinese Loess Plateau,how to assess and confirm the sustainability of vegetation restoration for soil carbon sequestration is still unclear.This study was based on the issues of the dynamics of soil organic carbon?SOC?,soil inorganic carbon?SIC?and soil water following vegetation restoration and paleoecological and paleoenvironmental reconstruction using?¹³C of SOC and SIC.We set the cropland,forestland,shrubland,and grassland in Zhifanggou watershed as study sites to study the influence of nitrogen fixation plants restoration on the dynamics of SOC in soil profile,the influence of vegetation restoration on the dynamics of SIC in soil profile,the sustainability of vegetation restoration based on soil water and soil carbon,and the contributing of pedogenic carbonate derived from atmospheric CO₂ to SIC;and set the loess-paleosol?S₀-S₁?of Luochuan profile as study sites to study contrasting paleoecological and paleoenvironmental reconstruction using?¹³C of SOC and SIC.Those results can provide scientific basis for the adaptive management of vegetation restoration on the Chinese Loess Plateau.The main results are as follows:1)Nitrogen fixation plants restoration have significant influence on the dynamics of SOC in soil profileSOC content at depths of 0-200 cm in forestland and shrubland were significantly higher than that in cropland.At depths of 0-20 cm,the proportions of new SOC of forestland and shrubland reach the proportions of old SOC in 30 and 20 years,respectively.Vegetation restoration increased new SOC content at depths of 0-200 cm,and contribution of new SOC accumulation at depths of 20-200 cm to whole soil profile in forestland and shrubland were more than 40%,indicating that new SOC accumulation in topsoil?0-20 cm?and subsoil?20-200 cm?both are important carbon sink after deep-rooted plant restoration.Vegetation restoration significantly decreased old SOC content at top 20 cm soil,and had no significant influence on old SOC at depths of 20-200 cm,indicating that nitrogen-fixing plants restoration accelerates the decomposition of old SOC in topsoil,but suppresses the priming effect and decrease the decomposition of old SOC in subsoil.2)Vegetation restoration have significant influence on the dynamics of SIC in soil profileSIC content at depths of 0-50 cm in grassland was significantly higher than that in cropland,while SIC content at depths of 0-200 cm in forestland had no significant difference with that in cropland.Because extra Ca²⁺derived from atmospheric deposition can be retained as SIC due to higher soil pH in grassland,and reforestation decrease soil pH,absorb Ca²⁺and then lead a portion of the SIC changed to CO₂ in forestland.Proportions of pedogenic carbonate in cropland showed increase trend with depth,but in forestland and grassland that increased in topsoil and then decreased with depth.Results can be due to the comprehensively effect of atmospheric deposition and pedogenic carbonate formation.Vegetation restoration significantly decreased parent carbonate content,and significantly increased pedogneic carbonate content in top soil?0-100 cm?and subsoil?below 100 cm?of forestland and grassland.It illustrated that conversion from parent carbonate to pedogenic carbonate occurs in topsoil and subsoil.The per decensum model may be the main reason for the formation and redistribution of pedogenic carbonate in topsoil,and deep roots may play important role in the formation and distribution of pedogenic carbonate in subsoil.3)According to soil water and soil carbon,the sustainability of vegetation restoration is different among different vegetation restoration typesAfter 10 years vegetation restoration,SOC content at topsoil in forestland,shrubland and grassland increased significantly,and SIC content at subsoil in shrubland and grassland increased significantly due to more pedogenic carbonate formed by Ca²⁺derived from the decomposed litter and biogenic CO₂.The absolute values of the slops of the linear regression patterns between SOC and SIC was in the order grassland>forestland and shrubland and indicates that under the grassland the increment in SIC is larger per unit decrement in SOC.After 20 years vegetation restoration,the soil water content under forestland and grassland decreased to 4.74%-6.16%and 4.08%-5.21%which are close to the wilting coefficient?5%?for the sandy loam soil in Zhifanggou watershed,resulting the obstacle to sustainable land use.The conversion from cropland to natural grassland kept the relatively high level of soil water and may be the sustainable vegetation restoration approach to increase soil carbon.4)Significant amounts of pedogenic carbonate derived from atmospheric CO₂ are included in SICPC_{respired CO2}?%?is the contribution of pedogneic carbonate derived from respired CO₂to SIC calculated by?¹³C_sic and?¹³C_soc.PC_total?%?is the contribution of pedogneic carbonate derived from respired CO₂ and atmospheric CO₂ to SIC calculated by Mn/Ca of SIC.The difference between PC_{respired CO2}?%?and PC_total?%?is the contribution of pedogenic carbonate derived from atmospheric CO₂ to SIC(PC _{atmospheric CO2}?%?)and the average value of PC_{atmospheric CO2}?%?in cropland,foresland and grassland were 12.59%,25.59%and 22.48%,indicating that significant amounts of pedogenic carbonate derived from atmospheric CO₂ are included in SIC.The formation of pedogenic carbonate derived from atmospheric CO₂ may occur in the presence of vegetation because of low biologicalactivity in winter resulting in the more influence of diffused atmospheric CO₂ on soil CO₂.5)parent carbonate and pedogenic carbonate derived from atmospheric CO₂ account for the contrasting paleoecological and paleoenvironmental reconstruction using?¹³C of SOC and SICProportions of parent carbonate to SIC ranged from 3%to 25%in loess-paleosol layers.?¹³C_{pc-respired CO2} is the?¹³C of the pedogenic carbonate derived from respired CO₂and?¹³C_pc-total is the pedogenic carbonate derived from respired CO₂ and atmospheric CO₂.The values of?¹³C_{pc-respired CO2}c-respired CO₂ minus?¹³C_pc-total???in L₁LL₁,L₁SS₁ and L₁LL₂ layers were0.38‰,0.90‰and 0.37‰which were less than 1‰indicating there are contribution of PC_{atmospheric CO2}tmospheric CO₂ to SIC.The least proportions of PC_{atmospheric CO2}tmospheric CO₂ were 6%,1%and 6%in L₁LL₁,L₁SS₁ and L₁LL₂ layers,and proportions of PC_{atmospheric CO2}tmospheric CO₂ in paleosol layer S₀ and S₁were both 0%.Moreover,?value in paleosol layer S₀ and S₁?3.31‰and 2.42‰?may be equal to the value of SOC isotopic fractionation,providing a method to directly calculate the value of SOC isotopic fractionation in paleosol layers.