Effects Of Afforestation Tree Species On The Storage And Stability Of Soil Organic Carbon On The Western Slope Of Wuyi Mountain

Forest soil organic carbon is of great importance to maintain terrestrial carbon balance and global climate system.In order to control soil erosion,China has launched the project of returning farmland to forest since 1999.Although returning farmland to forests changes soil organic carbon pool and its stability,the results are inconsistent,and the afforestation tree species may be a main factor causing such inconsistency.Therefore,this study selected20-year-old Cunninghamia lanceolata forest,Liquidambar formosana forest,and liquidambar formosana-Schima superba mixed forest in Jiangxi Matou Mountain National Nature Reserve on the west slope of Wuyi Mountain,and used abandoned farmland as the control.We measured the concentrations and related properties of soil organic carbon,dissolved organic carbon（DOC）,particulate organic carbon（POC）,and mineral-associated organic carbon（MAOC）.The main aim was to clarify the impact of afforestation tree species on soil organic carbon storage and its stability.The main research results are as follows:（1）In the soil depth of 0–30 cm,the organic carbon storage was 58.24 kg m^-2in abandoned farmland,88.72 kg m^-2in C.lanceolata forest,73.79 kg m^-2in L.formosana forest,and 67.92 kg m^-2in L.formosana-S.superba mixed forest.Compared with abandoned farmland,C.lanceolata,L.formosana,and L.formosana-S.superba mixed forests have higher soil organic carbon storage.Compared with the other three land use types,soil organic carbon in the C.lanceolata forests is more likely to each carbon saturation.Using FTIR analysis,C.lanceolata,L.formosana,and L.formosana-S.superba mixed forests had lower polysaccharide compounds and higher alcohol and phenolic substances than the abandoned farmland,showing the increases in proportions of labile soil organic carbon after returning farmland to forest.These results suggest that returning farmland to forests will increase soil organic carbon sequestration potential and change soil organic carbon chemical composition.（2）Among the four land use types,the DOC concentration of C.lanceolata forest was the highest at 116.8 mg kg^-1at 0–10 cm,while the DOC concentration at 10–30 cm of L.formosana-S.superba mixed forest was the lowest at 68.8 mg kg^-1.Soil DOC aromatic degree was higher in L.formosana forest than in the other three land use types in the 0–10cm depth,but was lower in the C.lanceolata forest than in the other three land use types in the 10–30 cm depth.Compared with abandoned farmland,L.formosana and L.formosana-S.superba mixed forests had lower DOC biodegradability in the 0–10 cm depth,while C.lanceolata forest had higher DOC biodegradability in the10–30 cm depth.Moreover,soil DOC biodegradability correlated negatively with the initial aromatic degree,but showed no significant relationship with C:N:P stoichiometric ratios.These findings suggest that afforestation tree species are the main factors affecting the quantity and quality of soil DOC,and shifted C quality drives the variations of DOC biodegradability after returning farmland to forest.（3）Compared with the abandoned farmland,L.formosana and L.formosana-S.superba mixed forests had higher soil MAOC concentration,while C.lanceolata forest had higher soil POC concentration.The content of POC and MAOC in abandoned farmland was 3.54 g kg^-1,12.48 g kg^-1,14.67 g kg^-1in L.formosana,14.36 g kg^-1in mixed forest,and 8.73 g kg^-1in C.lanceolata forest.Returning farmland to forest significantly decreased theδ¹³C value of POC in the 0–10 cm depth,but significantly increasedδ¹³C value of MAOC in both 0–10 and 10–30 cm depths.These observations indicate that returning farmland to forest only reduced the turnover of POC in 0–10 cm soil depth,but accelerated the turnover of MAOC in both 0–10 cm and 10–30 cm soil depths.Moreover,oxide minerals such as iron and soil nutrients were the main factors affecting soil organic carbon stability.These findings imply that afforestation with broad-leaved tree species enhances soil organic carbon stability,whereas afforestation with coniferous tree species reduces soil organic carbon stability in subtropical regions.In summary,on the western slope of Wuyi Mountain,returning farmland to forest will increase soil organic carbon storage and change the stability of soil organic carbon.Compared with coniferous tree species,afforestation with broad-leaved tree species can enhance soil organic carbon stability.Moreover,soil mineral elements and nutrient status are the main factors determining soil organic carbon accumulation.These research results provide scientific basis for improving the soil carbon sink function of subtropical plantations and evaluating the ecological benefits of the returning farmland to forest project on the west slope of Wuyi Mountain.