Impacts Of Afforestation On Soil Organic Carbon Stability And Temperature Sensitivity In Danjiangkou Reservoir Area

Afforestation,as one main component of land use change,significantly impacts vegetation type and soil microenvironments(i.g.,soil moisture,pH,temperature),and also affects the soil microbial community and activity,and ultimately leading to changes in soil organic carbon stability and temperature sensitivity.However,the effects of afforestation on soil organic carbon stability and temperature sensitivity are not fully understood.In this study,we selected four types of land use(woodland,shrubland,cropland and uncultivated land)to explore the impact of afforestation on soil organic carbon stability and temperature sensitivity in Danjiangkou reservoir area,by investigating soil organic carbon physical and chemical protection,biochemical protection,soil organic carbon mineralization rate and temperature sensitivity.The main conclusions are as follows:(1)Impacts of afforestation on soil organic carbon contentAfforestation significantly increased soil organic carbon content,with the order of woodland(12.5 g kg^-1)>shrubland(7.4 g kg^-1)>cropland(2.7 g kg^-1)>uncultivated land(2.0 g kg^-1).And afforestation also increased the content of organic carbon in macroaggregates(Mac),microaggregates(Mic),clay and silt(C+S).The increased soil organic carbon content was dominated by macroaggregates(more than 70%)under conversion from uncultivated land to the afforested land(shrubland and woodland),while the increased soil organic carbon content was mainly caused by microaggregates(more than 60%)under conversion from uncultivated land to cropland.The content of organic carbon significantly differed between soil depths with lower level of the content of bulk organic carbon and each aggregate in deep soil than in top soil(except uncultivated land).These results suggest that afforestation significantly increased soil organic carbon storage,especially in top soil of woodland.(2)Impacts of afforestation on soil organic carbon stabilityAfforestation decreased the ratio of fungi to bacteria,and increased the carbon and nitrogen imbalance of substrate and C-acquisition and N-acquisition enzyme activities imbalance of microorganism,which could possibly lead to the decrease of carbon use efficiency of soil microorganisms.Low carbon use efficiency promoted the reduction of microbial anabolism,and caused the reduction of microbial necromass,which could hinder the formation of stable mechanism with mineral particles.Thus,afforestation could result in decreases in physical protection(microaggregates protection)and chemical protection(calcium,iron,clay and silt protection)of soil organic carbon.In addition,the recalcitrant carbon index(RIC)was higher in uncultivated land(0.78)and cropland(0.72)compared woodland(0.62)and shrubland(0.58),so afforestation led to the decrease of biochemical protection.Soil layer also greatly affected the stability of soil organic carbon with higher level of carbon use efficiency in deep soil than in top soil,and hence the physical and chemical protection intensity in deep soil was stronger than top soil.With the development of afforestation,the organic carbon content of clay and silt,microaggregates in top soil did not significantly increase with the increase of the bulk organic carbon content of the soil,whereas the soil organic carbon of the microaggregates,clay and silt in deep soil increased with the increase of the total organic carbon content.This finding indicated that the carbon saturation could occur in top soil in woodland.Overall,the biochemical,physical and chemical protection of soil organic carbon could decline with the development of afforestation,with lower degrees in deep soil than in top soil.Thus,afforestation could hinder the stability of soil organic carbon in long-term,especially in the top soil.(3)Impacts of afforestation on soil organic carbon mineralization rate and temperature sensitivityAfforestation significantly increased the mineralization rate of soil organic carbon,with the order of the woodland(57.7 mg CO₂-C g^-1OC)>shrubland(42.7 mg CO₂-C g^-1OC)>cropland(28.0 mg CO₂-C g^-1OC)>uncultivated land(24.3 mg CO₂-C g^-1OC).The mineralization rate of soil organic carbon was significantly higher in top soil than deep soil except in the uncultivated land.There were three most important factors in affecting the mineralization rate of organic carbon,which were microbial properties(F/B and PLFAs),recalcitrant carbon index(RIC)and physical and chemical protection of organic carbon(Fe,CA,Mic and C+S).Among them,microbial attributes were the most critical factor,and.higher F:B ratio led to lower decomposition rate.The second factor was physical and chemical protection.The stronger of Fe,Ca,microaggregates,clay and silt protection led to the lower decomposition rate.Finally,soil RIC was used as an indicator of the biochemically difficult decomposition of soil organic carbon with higher value of RIC led to the lower the decomposition rate of organic carbon.Afforestation significantly increased the temperature sensitivity(Q₁₀)of soil organic carbon,and the Q₁₀ of woodland(1.98)and shrubland(1.79)was significantly higher than that of cropland(1.50)and uncultivated land(1.43).The Q₁₀ in top soil was significantly higher than deep soil in woodland and shrubland.The higher F:B ratio could cause lower temperature sensitivity,and the weaker protection of substrate could possibly lead to higher temperature sensitivity.The RIC did not significantly affect temperature sensitivity,and further indicated that the quality of the substrate was not a key factor affecting temperature sensitivity.In summary,although afforestation increased soil organic carbon content,but it significantly reduced the biochemical,physical and chemical protection of soil organic carbon,which possibly hinder the stability of soil organic carbon at long-term.Meanwhile,afforestation significantly enhanced the rate of soil organic carbon mineralization and temperature sensitivity,which could be more sensitive to climate changes.Taken together,our results suggested that the forest management should focus on improvement on soil microbial properties and functions,and enhanced the protection intensity of woodland soil organic carbon,which could be benefical to soil organic carbon sequestration at long-term,mitigating the positive feedback to climate warming.