| Planted forest plays a significant role in carbon(C)sequestration and climate change mitigation and may serve as an alternative for ecological services provided by natural forests and thus maintain C balance at the local,regional and global scale.C sequestration in planted forest is influenced by a number of environmental and biological factors such as stand age,soil type,management history,geology,terrain,etc.However,little information has been available on the changes in C stock with stand age in Pinus massoniana(Masson pine)plantations.Using the Masson pine relative growth equation,biomass C and nitrogen(N)were obtained in a chronosequence(1-year,3-year,5-year,7-year,9-year,15-year,21-year,27-year,33-year and 39-year)stands.In addition,soil organic C and total N(SOC and TN),dissolved organic C and N(DOC and DON)and soil microbial biomass C and N(MBC and MBN)were estimated at the 0-10,10-30 and 30-100 cm depths along the chronosequence.In each stand,100 m*100 m main plot was established and three sampling plots of 20 m*20 m were further laid out for sampling the tree and soil layers,five 25 m2(5*5 m)and five 1 m2(1*1m)quadrat sub-plots were further established for sampling of shrub species and litter,grasses and herbs(LHGs)on the forest floor.Results of our study showed that;Biomass stock varied from 0.45 to 265.3 t·ha-1,5.01 to7.67 t·ha-1 and 2.18 to 5.09 t·ha-1 in the tree,shrub and herb layers.C concentrations in plant tissues were 49.3%,39.7%and 41.6%in the tree,shrub and herb layers.The aboveground biomass C contributed 80.2%and 67.7%in the tree and shrub layers and the root to shoot(R/S)of the tree and shrub layer biomass averaged 0.22 and 0.59.Biomass C and N stock increased significantly(p<0.05)with forest age whereas the changes in biomass C and N accumulation rate decreased significantly(p<0.05)in the tree,shrub and herb layers.The annual net C sequestration rate increased significantly(p<0.05)with stand age in the tree layer but decreased significantly(p<0.05)in the shrub and herb layers.The SOC and TN contents decreased with soil depth but increased with forest age whereas the stock of SOC and TN increased with soil depth and forest age.The total ecosystem C storage increased significantly with forest age,it ranged from 189.1 t·ha-1 in the 1-year plantation to 391.7 t·ha-1 in the 27-year plantation of which 79.5%,19.9%and 0.6%comes from the mineral soil,vegetation and litter layer.The ratio of the ecosystem C sequestration by the 39-year to the 1-year stand was 2.0,implying substantial amounts of carbon accumulation during the transition period from saplings to mature stands.The forest ecosystem had the capacity of storing up to 282.9 t·ha-1 C,assisting in mitigating climate change by sequestrating 1037.3 t·ha-1 of carbon-dioxide(CO2)equivalents which indicates that;the forest is an important C sink.Concentrations of DOC and DON varied with stand age and decreased significantly(p<0.05)with soil depth from approximately 484.4 mg/kg,353.5 mg/kg,and 264.6 mg/kg in the 0-10,10-30 and 30-100 cm soil layers.DOC and DON were significantly correlated,the ratios of DOC/TOC and DON/TN increased significantly(p<0.05)from 0.14,0.13 to 0.26 and 0.34,0.37 to 0.62 in the upper,middle and lower soil layers of 0-10 cm,10-30 cm and 30-100 cm.Concentrations of MBC increased with forest age while the MBN decreased along the chronosequence.The C/N ratio of soil microbial biomass(SMB)increased with forest age and soil depth,and the microbial quotient increased from 1.72%,1.94%and 2.61%for C(MBC-C)and 3.31%,3.93%and 4.32%for N(MBN-N)in the upper,middle and lower soil layers of 0-10 cm,10-30 cm and 30-100 cm upper to the lower soil layer.Stand age and soil moisture content are the dominant biological and environmental factors influencing SMB whereas the C/N ratio and stocking density are the main chemical and abiotic factors influencing SMB.The findings of our studies suggest that;1)forest age is the controlling factor of photosynthetic and biological processes and thus changes in biomass accumulation and C sequestration in the Masson pine plantations,2)the Mature stands have facilitated the process of immobilization whereas the younger stands have facilitated the process of mineralization,3)the soil microbes have played a greater role in re-cycling nutrients and participated effectively in mitigating climate change by sequestrating 4.1 t·ha-1 C equivalent to 15.2 t·ha-1 of the atmospheric CO2,4)soil pH and moisture content affect dissolved organic matter(DOM)accumulation,and the DOM concentrations are influenced by export of organic matter from litter and the forest floor surface.Therefore,the results of this study validate the importance of considering forest age in assessing and monitoring nutrients and elements cycling in terrestrial ecosystems and thus global C management. |
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