Decomposition Of Leaf Litter And Fine Root Of Different Quality Along The Elevation Gradient Of Wuyi Mountain And Their Fates In Soil Organic Carbon Fraction

Litter decomposition is a key process in the biogeochemical cycle of terrestrial ecosystems,which controls soil organic carbon storage and nutrient availability,thus affecting vegetation productivity.As the main pathway of plant input,the decomposition of leaves and roots is an important process of ecosystem nutrient turnover.Due to the obvious differences in chemical composition and decomposition environment,the response of their decomposition to climate change may not be consistent.As the main part of forest organic carbon input,whether the difference in litter quality will affect the decomposition and formation of soil organic matter（SOM）needs to be further solved.Therefore,this study selected mountain forests with different elevations located in Wuyi Mountains as the research platform for climate change.The litter and fine roots decomposition of Cunninghamia lanceolata,Pinus taiwanensis,Phoebe zhennan,Schima superba,and Castanopsis carlesii with different quality（C/N and lignin/N）along the elevation gradient were explored by decomposition bag method.The different trends of litter and fine roots decomposition of the same tree species in response to elevation changes were revealed,and the response mechanism of litter and fine roots decomposition with different quality to elevation changes was clarified.At the same time,the effects of the input of different quality litter(¹³C labeled leaves and roots of Cunninghamia lanceolata and Castanopsis carlesii)on the soil organic carbon components at different elevations,and their fates were studied by in situ culture experiments and combined with stable isotope ¹³C labeling technology.The main results are as follows:（1）After 3.5 years of decomposition,the average mass loss rates of leaf litter and fine roots at three elevations were 86.22%and 71.91%,respectively.There are significant effects on the decomposition rate of leaf litter in elevation,tree species,and their interactions.There is the highest decomposition rate of the leaf litter of Schima superba,with an average k value of 0.69,while the decomposition rates of Cunninghamia lanceolata leaves and Phoebe zhennan leaves are lower,with an average k value of 0.47and 0.48.The effects of elevation on leaf litter decomposition varied with tree species.The decomposition rate of low-quality Cunninghamia lanceolata and Pinus taiwanensis leaves slows down with increasing elevation,and the decomposition rate of Phoebe zhennan and Schima superba leaves have no significant differences in elevation,while that of high-quality Castanopsis carlesii leaves speeds up with increasing elevation.The decomposition rate of fine roots of all tree species was lower than that of leaf litter,and the effect of elevation on the decomposition of fine roots was consistent,which means the decomposition rate of fine roots slowed down with the elevation increasing.In addition,the partial least squares path model（PLS-PM）analysis found that climate,soil,microbial characteristics,and leaf litter characteristics along the elevation gradient collectively accounted for 45.2%of the decomposition rate of litter,with leaf litter characteristics having the most vital impact on the decomposition rate（-0.614）;In terms of fine roots,the overall effect of climate factors is the strongest（-0.758）,soil properties also have a significant impact on the decomposition rate of fine roots.（2）Both leaf litter and fine roots showed a direct release mode of C.The C release rate of Schima superba leaves was the highest（92.37%）,while that of Cunninghamia lanceolata leaves was the least（83.77%）.There was an elevation difference in the C release rate of Cunninghamia lanceolata and Pinus taiwanensis leaves at the late decomposition stage.The elevation had significant effect on the release of fine root C of all tree species,and the release of fine root C of Cunninghamia lanceolata was the least（61.27%）.There are differences in the release rates of N and P elements in the litter at different decomposition stages,and the release rates of N and P elements in the leaf litter and fine roots of Cunninghamia lanceolata are the lowest.Elevations,tree species,decomposition time,and their interactions all had significant effects on element losses in leaf litter and fine roots.Litter C/N decreased with decomposition,while N/P increased continuously and was significantly affected by elevation.The higher the substrate quality of litter and fine roots,the stronger the coupling of their C N losses.The C P losses and N P losses of leaf litter and fine roots are linearly correlated,and the coupling is strong.（3）In terms of chemical composition,the extractable residues of leaf litter and fine roots lost 58.12%～84.857%and 58.16%～70.30%,respectively;after 990 days of decomposition,there were elevation differences in Cunninghamia lanceolata,Pinus taiwanensis leaves,and most fine roots at the late stage of decomposition.The cellulose degradation of leaf litter was first fast and then slow,and the average degradation rate of various tree species ranged from 71.36%to 91.53%,and the cellulose degradation rate at700 m elevation was faster than the rest of elevations（except for Schima superba leaves）in the first year of decomposition.The cellulose of fine roots degraded from 37.26%to85.93%,among which the cellulose of fine roots of Castanopsis carlesii degraded the fastest,and the cellulose of fine roots at 200 m elevation degraded significantly faster than other elevations.In the first 180 days of decomposition,lignin degradation of low-quality litters was slower,and the whole process was significantly affected by elevation.However,the lignin degradation of fine roots was slower than that of litter,with the average degradation rate of each tree species at elevation ranging from 31.42%to73.41%.（4）To investigate the fates of leaf litter and fine roots of different qualities in soil organic carbon components after decomposition,¹³C labeled leaves and fine roots of Castanopsis carlesii and Cunninghamia lanceolata were added to soil at different elevations for in situ culture.The results showed that after 8 months of litter addition,high-quality Castanopsis carlesii litters added more SOM and formed more MAOC than low-quality litters of Cunninghamia lanceolata.However,in the long run,the contribution of Cunninghamia lanceolata leaves to SOM and MAOM was greater than that of Castanopsis carlesii leaves at middle and low altitudes with suitable temperature and water conditions,while the decomposition of leaf litter and organic matter was relatively slow at high elevations.Therefore,the supplementation of Castanopsis carlesii leaves（65.8%）was still higher than that of Cunninghamia lanceolata leaves（24.8%）.At each elevation,the contribution of Cunninghamia lanceolata fine roots to SOM and MAOM was lower than that of Castanopsis carlesii fine roots at 24 months.After long-term litter addition,the POC formed by fine roots of the corresponding tree species was higher than by litter,and the MAOC content of fine roots was higher than that of leaf litter due to the influence of placement location.From the aspect of elevation,the ratio of POC and MAOC of litter had a strong negative correlation with soil temperature,and the change of soil clay content on the elevation gradient also affected the distribution of MAOC of litter.In summary,the response of litter decomposition rate to climate change depends on litter traits,and climate is the main factor controlling the decomposition rate of fine roots on the elevation gradient.Elevation,tree species,decomposition time and interaction had significant effects on the element loss of leaf litter and fine roots,and there was a significant coupling effect between different elements.The degradation of chemical composition of low quality leaf litters and most fine roots was significantly affected by elevation.In addition,in situ incubation experiments found that the length of litter input affected the contribution of different quality litter to soil SOM on the elevation gradient.