| Carbon element is not only an important component of green plants and microorganisms, but also a vital ecological factor in ecosystem. With the CDM (Clean Development Mechanism) implementation, it is well believd to increase carbon sequestration through afforestation and reforestation programme is of great significance in terms of mitigation of global climate change. Moreover, as compared to grassland or cropland, the ecological value of carbon sequestration of forest plantation is more prominent. The area of rubber plantation in China has already sprung up to about 100 million hectares in 2010, which has become an important plantation sector and an ecosystem in tropical China. The aim of the present study was to evaluate the soil carbon sink capability of rubber plantations at various ages and its related soil respiration factors. Therefore we calculated the carbon storage through measuring the soil carbon content at different soil depths in rubber plantation with different ages, calculated the annual returned carbon content and annual released carbon content by measuring litter production and its decomposition rates, and measured the soil respiration rate of respective soils in association with related environmental factors. It was expected that the outcome of the present study could be helpful to accurate and proper estimation of the carbon sink and source capability of rubber plantations, particularly the soil ecosystem part. The main results and conclusions are summared as follows:(1) The content of soil organic carbon (SOC), mineralized carbon (MC), microbial biomass carbon (MBC) were 3.14-8.15 g·kg-1,96.52-381.10 mg-kg-1 and 28.87-138.71 mg·kg-1 for rubber plantations at the age of 4 a,12 a,19 a,24 a and 29 a respectively. The three cabon components showed similar decreasing tendency with the increase in soil depths, but in different layers, the carbon contents of rubber plantations of different ages varied quite evidently. In respect of soil organic carbon content in different soil layers, the top soil layer (0-15 cm) of all age group except 29 a was significantly higher than all other layers. In mineralized carbon the top soil layer of all age group except 12 a was also much higher than other layers. However, no evident difference of soil mineralized carbon content of various soil layers between 4 a and 12 a and between 24 a and 29 a rubber plantations was recorded. In addition, the daily variation of soil mineralized carbon content represented a similar tendency with higher decomposision rate at the initial stage of our determination while slower at the later stage of our determination. This is because just in the first day of our culture trial the decomposition rate of soil organic carbon reached the peak after which a declining trend was observed. No prominent difference in soil microbial biomass carbon content was noted between the soil layers of 0-15cm and 15-30cm of all other three age groups except between 12 a and 24 a; while marked difference was recorded between the soil layer of 15-30cm and 30-45cm for all age groups expect 29 a.(2) The efficiency of soil mineralized carbon in rubber plantations at different ages was in the range of 2.20～5.55% and that of microbial biomass carbon in the range of 0.66～2.10%. There were significant positive linear correlations between soil organic carbon and mineralized carbon or microbial biomass carbon.(3) The carbon stock of soil organic carbon in 4 a,12 a,19 a,24 a and 29 a rubber plantations were 71.42 t·hm-2.82.01 t·hm-2,69.36 t·hm-2,84.46t·hm-2 and 78.94 t·hm-2 respectively;andthe carbon stock of mineralized carbon in the rubber plantations were 31.28 t·hm-2.29.00 t·hm-2,32.62 t·hm-2,19.61 t·hm-2 and 19.37 t·hm·2respectively whilethe carbon stock of soil microbial biomass carbon in the rubber plantations were 1.16 t·hm-2,0.98 t·hm-2,1.04 t·hm-2,1.11 t·hm-2 and 1.18 t·hm-2 respectively. These, added together, gave a total carbon stcocks of the 4 a,12 a,19 a,24 a and 29 a rubber plantations at 103.86t-hm-2,111.99 t·hm-2,103.02 t·hm-2,105.18 t·hm-2 and 99.49 t·hm-2 respectively.(4) The carbon content of litter collected from the 4 a,12 a,19 a,24 a and 29 a rubber plantations were determined and no significant difference was found between any two neighboring age group except for flower falling.However, the monthly variation of litter decomposition rate over the period of one year was obvious among the different age rubber plantations, while in the same month the variation was not so obvious. (5) The quantity of returned carbon in different age rubber plantations wwas 2.10 t·hm-2,2.91 t·hm-2,3.27 t·hm-2, 3.79 t·hm-2 and 3.85 t·hm-2 respectively. The quantity of the returned carbon from the litter was found to relate with plantation ages with an increasing tendency with ages. The percentage of the returned carbon from various litter components in the total carbon returned from the bulk of the litter of all age group rubber plantations was in the order of leaves>twigs>fruits>flowers although certain variation existed in the percentage.It was believed that the quantity of the returned cabon from various litter components mainly depended upon the biological characteristics of the rubber trees and climate factors. The annual releasing carbon from litter of the five age group rubber plantations was 0.12 t·hm-2,0.19 t·hm-2,0.21 t·hm-2,0.27 t·hm-2 and 0.26 t·hm-2 respectively. With the increase of tree age, more carbon was released from the litter as a whole.(6) During the observation period, diurnal variation of soil respiration rate in 12 a and 19 a rubber plantations was consistent, with the data recorded during 9:00～11:00 am close to the daily average. Monthly varations of soil respiration rate in 4 a,12 a,19 a,24 a, and 29 a rubber plantations were obvious, but not significant yet. And the values were 2.45μmol·m-2·s-1,2.63μmol·m-2·s-1,2.96μmol·m-2·s-1,2.85,μmol·m-2·s-1 and 2.64μmol·m-2·s-1. The soil respiration rate of various components in the rubber plantations was in the order of microbial respiration rate> root respiration rate> litter respiration rate. The relationship between soil temperature and soil respiration rate in the rubber plantations represented an exponential function and the correlation was significant (p<0.05), but its correlation with soil moisture was not significant (p> 0.05).(7) The annual released carbon from soil respiration in the five age group rubber plantations was calculated as 9.30 t·hm-2,10.00 t·hm-2,11.24 t·hm-2,10.81 t·hm-2 and 9.95 t·hm-2respectively. The maximum value appeared in August while the minimum in February or March.The releasing carbon content of each component of soil carbon respiration was 2.20-3.94 t·hm-2,1.83-4.73 t·hm-2,2.08-5.03 t·hm-2,2.58-5.54 t·hm-2 and 1.84-5.78 t·hm-2. respectively. In most cases the soil microorganism respiration rate was higher than that of root system while litter respiration rate was at the bottom of the list.It can be seen that the soil carbon stocks of rubber plantations at the age of 4 years old, 12 years old,19 years old,24 years old and 29 years old were generally higher than 100 t·hm-2. It is therefore concluded that the soil ecosystem of rubber plantations can be taken as an important carbon sink, and that the soil respiration rate of rubber plantations tended to be in a decreasing order of soil microorganisms respiration>root system respiration>litter respiration with prominent monthly variation. It is believed that the outcome of the present study is of certain significance to proper estimation of the carbon sink/source capability of rubber plantations, and is helpful to more accurate and systematic estimation of the carbon sink/source capability of rubber plantations with reference to theory and methodology, particularly to the soil ecosystem part. It can be of certain implications for the improvement of rubber soil management for sustainable purposes. |
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