Effects Of Vegetation Restoration On Labile Carbon And Soil Respiration In Degraded Red Soil

The research about carbon cycle has been a hotspot in global scales because there bas been a continual increase in the atmospheric concentration of carbon dioxide (CO₂) since the industrial revolution. Soil carbon pool plays key role in the global carbon cycle. But soil erosion leads to the severe decline of soil quality and depletion of soil organic carbon pool, therefore, there are higher potential of carbon sequestration through adoption of available restoration practices. There is severe soil erosion in the mountainous and hilly red soil region of subtropical China, and many ecological restoration and rehabilitation practices of soil and water conservation had been applied, but the function of soil carbon sequestration was not to attain enough attention and the central role of soil carbon in the degraded red soil quality was not to be emphasized. In this study, the effect of vegetation restoration on soil organic carbon (SOC), light fraction organic carbon (LFOC), microbial biomass carbon (MBC), dissolved organic carbon (DOC) and their seasonal dynamics, distribution of organic carbon in aggregate, and soil respiration and it's seasonal dynamics has been studied in Hetian town, Changting county, Fujian province by fixing position measurement in field and analyzing in laboratory. Three regeneration ecosystems viz. Pinus massoniana forest (planted in 1981 on severely eroded land), Castanea mollissima plantation (planted in 1999 on severely eroded land) and Paspalum notatum Flugge. grassland (planted in 2000 on severely eroded land), and severely eroded land (bare land) as a control ecosystem before restoration, the secondary forest conserved best in local as the control ecosystem after restoration were involved in this study. The objective of this study is that: (i) carbon sequestration characteristic and potential in eroded red soil;(ii) soil labile carbon fractions characteristic and their relation to soil organic carbon;(iii) effects of vegetation restoration, soil carbon and labile carbon fractions on the stability of soil aggregate;and (iv) illuminates the mechanism of soil respiration affected by vegetation restoration in eroded red soil.Major conclusions were summarized as follows:(1) There was a distinct increase of SOC content and storage after vegetation restoration in eroded degraded land and significant difference was found in the 0-5 cmdepth, where SOC content in Pinus massoniana forest, Castanea mollissima plantation and Paspalum notatum Flugge grassland were 13.3, 4.2 and 12.8 times higher than that in bare land respectively. SOC content and storage were little affected by vegetation restoration under the depth of 40cm in regeneration ecosystem's soil. Comparing with secondary forest, carbon sequestration in bare land was about 56 t C?hm"2o(2) In bare land average content of light fraction organic carbon (LFOC) was low (< 0.10 g?kg"') and the proportion of LFOC to SOC was from 1.2 to 1.3%. Also, there were no distinct seasonal changes of LFOC in bare land. After vegetation restoration, LFOC increased significantly and distince seasonal pattern was found. Content of LFOC and the proportion of organic C present as LFOC both decreased with depth, corresponding to the distribution of roots. There was significant linear relatinhsip between LFOC and SOC (P < 0.01). In addtion, accumulation rate of LFOC was faster than that of SOC, which was important for soil sustainability and carbon managemnet.(3) Microbial biomass carbon (MBC) in bare land was lower than 83 mg'kg"1. After vegetation was restored, average content of soil MBC was 2.3-7.8 times higher than that of bare land and seasonal pattern of soil MBC content was highest in summer and lowest in winter respectively. MBC had significantly positive correlation to contents of SOC, total N, total P, water soluble N, available P, total K and available K, but negative correlation with soil bulk density and depth. Vegetation restoration increased mean flux of soil MBC significantly, but not for turnover rate and time of MBC.(4) Vegetation restoration increased the content of soil DOC due to restoration of source of soil DOC. Similar to SOC, the content of DOC also decreased with depth. Seasonal dynamics of soil DOC in restored ecosystems was complex because of synthetical impacts of litterfall, precipitation, temperature, moisture and microbe. The effect of soil microorganism on DOC was greater than that of total SOC and significantly positive relation between DOC and MBC was found (P < 0.05).(5) Vegetation restoration facilitated the formation of aggregate and enhanced its stability, imposed significant impact on SOC in the different size-class aggregate, and increased the proportion of SOC in the macroaggregate. In the process of vegetation restoration, the acumulative rate of carbon in the macro-aggregate was higher than that in the micro-aggregate. Stability of soil aggregate was related to SOC, MBC, LFOC and DOC. Further, microbe played the most important role in the formation of aggregate and greatly affected its stability.(6) There was power function relationhip between soil respiration rate measured by IRGA and by alkali absorption method. Overestimation of 77% was for alkali absorptionmethod when soil respiration rate was low, whereas alkali absorption method underestimated 41% when soil respiration rate was high.There was no effect of vegetation restoration on daily soil respiration, while significant seasonal change was observed, generally maximum in summer and minimum in winter. The increase in soil respiration rate after vegetation restoration may be due to increase of litterfall, root biomass, contents of SOC and labile organic C, and quantity of soil microorganism, as well as change in soil temperature and moisture. Soil respiraiton could be better simulated by an exponential funciton with soil temperature as the driving variable, especially after vegetation restoration. On the other hand, whether significant increase in mean anuual flux of soil respiration after vegetation restoration would not benefit atmosphere CO2 sequestration, it needed to further analyze ecosystem carbon balance.In summary, vegetation restoration increased content and storage of SOC and its labile fractions. Also, accumulation rates of labile fractions were significantly higher than that of total SOC, which could provide nutrients for plant growth. In addtion, vegetation restoration promoted the generation of soil aggregate and increased its stability, which could protect SOC from erosion. Soil respiration also increased in restored ecosystems and was more sensitive to change of soil temperature. The question whether restored ecosystems functioned as C source or C sink will need further study.