| The study of underground parts of forest tends to occupy a primary place of all levels of ecosystem.However,there is relatively inadequate research on the root system of the underground part(fine root in particular)when compared with that of the aboveground part.Critically,fine root is not only an important "pool" of forest net primary productivity,but also the main "source" of soil C and nutrients,acting as the key link for the study of energy flow and material cycle of forest ecosystem.In terms of the study of fine root dynamics(mainly including fine root biomass,production,distribution,turnover,respiration and decomposition,etc.),there is a lack of in-depth elucidation of rules and mechanisms of fine root decomposition owing to the restriction of the strength and weakness of different methods for fine root classification and the difficulty in the unification of these methods.Furthermore,temperate and boreal coniferous forests are the most common objects of study in prior researches on fine root biomass,production and turnover of forest ecosystems at home and abroad.Few studies are emphasized on tropical and subtropical forests.Meanwhile,there are abundant data of root biomass measurement in the forest ecosystem,but less data measured simultaneously with soil respiration.In addition,owing to the limitation of methods for the separation of root respiration and microbial respiration of forest trees,it has become a focus of research concerning accurate measurement and estimation of the root,fine root and microbial respiration in soil respiration at the same time of reducing the interference of the research system.With respect to the above,systematic study on the dynamics of forest fine root is of great significance for better understanding the spatial distribution pattern of forest fine root biomass and response to the spatial heterogeneity of soil nutrients,which may provide scientific reference for the selection of silvicultural measure,as well as for the energy flow and material circulation of forest ecosystem.Pinus massoniana is a native and widely distributed tree species in the subtropical area of China.The Three Gorges Reservoir area is located in the middle reaches of the Yangtze River,which is the key area of ecological environment protection in China.P.massoniana is one of the largest forest vegetation types in the Three Gorges Reservoir area.This study was performed in the following three aspect:(1)By using decomposing-bag method,the present study explored the decomposition dynamics and nutrient release rate(C and N)of fine roots with different diameter classes and orders within a 360-day decomposition period.Further study was also performed on the correlation of decomposition rate with the initial carbon component content and the initial element content of fine roots.Meanwhile,on the basis of the comparison between root-diameter method and root-order method,fine root decomposition of P.massoniana was studied to explore the similarities and differences between the two classification methods in the law of fine root decomposition and the dynamics of nutrient release.(2)Subsequent discussion was carried out on the growth law of fine roots with different diameter classes and soil layers with seasonal changes by using root sampling method and dynamic chamber method,aiming at comparing and discussing factors affecting the production and turnover of fine roots of P.massoniana.The correlation between fine root biomass and soil respiration was studied at the same time.(3)On the basis of the correlation between root biomass and soil respiration,our study further deduced the results such as the proportion of root respiration and fine root respiration to total soil respiration.Furthermore,the study determined the relationship of soil microbial respiration with total number of microorganisms,microbial population,soil respiration and forest parameters.The main results are described as follows:(1)There was significant difference in the decomposition rates of fine roots with different diameter classes.In the decomposition process,a significant negative correlation of the decomposition rate of fine roots with different diameter classes was found with the initial N concentration and AUR content.In the early stage of decomposition,the initial C:N ratio of fine root ranged between 45.47 and 59.42,showing a continuous release of N element and gradual decrease of C:N ratio.Besides,there was an obvious continuous enrichment in N element and gradual decrease of C:N ratio when the initial C:N ratio of fine root was much higher than 59.42.Furthermore,in the later stage of decomposition,the C:N ratio of fine roots with different diameter classes decreased gradually and tended to 37.37,followed by an upward trend of the C:N ratio.In addition,an enrichment was observed in N element of fine root with initial C:N ratio ranging between 45.47 and 59.42,while there was a release in N element of fine root when initial C:N ratio was higher than 59.42.Moreover,the turnover rate of fine roots of different diameter classes decreased with the increase of the diameter of fine roots.The turnover rate of fine root with a diameter of 0.00?0.45 mm was 1.35 times and 2.16 times of that of fine root with the diameter of 0.45?0.98 mm and 0.98?2.00 mm,respectively.Furthermore,compared with fine roots of larger diameter classes,fine roots of smaller diameter classes had slower decomposition rate,faster annual yield and turnover rate of fine roots,and increased release of C and N elements to the soil,which was conducive to soil organic carbon and nutrient N cycles.The release or enrichment of N occurred when C:N ratio reached its critical value during the decomposition of fine roots with different diameter classes.Additionally,compared with fine roots of larger diameter classes,fine roots of smaller diameter classes showed higher initial N concentration and lower C mass,which inhibited the decomposition rate of fine roots,suggesting a combined effect of C and N quality of roots on the differential decomposition of fine roots.(2)By using two classification methods of fine roots,there was no difference in the dynamic changes of N,P,K,Ca and Mg concentrations as well as C:N ratio of fine roots in the process of fine root decomposition(0?240 days);while differences were observed in the dynamic changes of nutrient element concentration and C:N ratio in fine root tissue during the decomposition process from the 240 th day to the 360 th day.Nevertheless,there was a consistency in the law of fine root decomposition by applying different classification methods.(3)There were significant seasonal changes in the biomass of fine roots of three different diameter classes in 0?10cm and 10?20cm soil layers.Seasonal changes of soil temperature,humidity and fine root biomass were the main influencing factors of soil respiration,in which soil temperature and fine root biomass exerted promotional effects while soil humidity had an inhibitory effect on soil respiration.Furthermore,separation of root respiration by root extrapolation accounted for 52.85% of total soil respiration,while microbial respiration made up 11.34% of total soil respiration;and there was a slight difference in the result by applying trench method in the same area,but basically the same.In accordance with the above results,it was speculated to be feasible by applying root extrapolation for separation of root respiration through root biomass measurement of P.massoniana with different diameters at breast height and in different directions and subsequent establishment a correlation with corresponding soil respiration.Simultaneously,estimation of soil heterotrophic respiration was realized on the premise of minimizing the interference to the research system.(4)The litter respiration rate in summer and fine root respiration accounted for 35.81%and 41.2% of total soil respiration,respectively.The ratio of the square of diameter at breast height of P.massoniana to the distance from sampling point explained the estimated soil respiration rate of 11.6?31.2%,which was basically consistent with that of 15.3%?30.7%explained by fine root biomass.The diurnal changes of soil respiration and microbial respiration both showed uni-peak type.Meanwhile,microorganism DNA copy exhibited a positive correlation with soil heterotrophic respiration.Microbial population change was one of the driving factors of heterotrophic respiration,besides,species diversity and richness were positively correlated with heterotrophic respiration.To sum up,it is feasible to study fine root decomposition law of P.massoniana and its underlying mechanism using the adjusted diameter class method.Meanwhile,the different decomposition rate of fine roots with different diameter classes or orders is mainly attributed to the combined effect of C and N masses.Furthermore,our study confirms the feasibility of root extrapolation in the separation of root respiration through measurement of root biomass of P.massoniana with different diameters at breast height and in different directions and subsequent establishment of a correlation with corresponding soil respiration.In addition,it remains to be further explored and verified in terms of correlation establishment based on forest parameters and soil respiration,as well as subsequent separation and estimation of fine root respiration. |
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