Characteristics Of Carbon And Nitrogen In Different Species Plantation Ecosystems In Southern Subtropical Area Of China

In the1980s, China’s tropical and subtropical regions launched a massive afforestationcampaign for ecological restoration of degraded slopes. In tropical and subtropical area ofChina, most of the plantations are composed of pure coniferous species. These pure coniferousspecies caused a series of problems, such as low ecosystem service and ecological instability,susceptible to pests and diseases attack.Many different functional hardwood species （such as fast-growing tree species and nativevaluable tree species, nitrogen-fixing and non-fixing tree species） are being advocated as aprospective silvicultural management to substitute large coniferous plantations in subtropicalChina. However, to different functional native hardwood species, people often pay moreattention to their timber income, little information is known about characteristics of ecosystemcarbon and nitrogen.In Tropical Forestry Experimental Center of Chinese Academy of Forestry Science, thethree adjacent monospecific plantations were selected to examine carbon and nitrogen storageunder different plantations, the effects of environmental factors on the transformation of carbonand nitrogen, the effects of soil microbial biomass and community composition on thetransformation of carbon and nitrogen, and the decomposition of leaf litter and fine root ofdifferent functional species, by using elemental analysis, barometric process separation （BaPS）,phospholipid fatty acid （PLFA）, and litter/root decomposition bags methods. One coniferousplantation was composed of Pinus massoniana, and the other broadleaf plantations wereErythrophleum fordii and Castanopsis hystrix. The main results are as follows:（1） The allocation of carbon in the organs of the different tree species had no significantcorrelation with their age, while that of nitrogen had closer relationship with the age. Thenitrogen concentration in young organs was higher than that in aged ones and the C/N ratio was higher in aged organs than that in young ones. It means that carbon is relatively stable, andnitrogen has an important role in the growth of young organs. Variations in soil organic carbonand total nitrogen among tree species could be primarily explained by the differentcharacteristics of tree species. The soil organic carbon and total nitrogen concentration werethe highest under the nitrogen-fixing species （E. fordii plantation）, followed by C. hystrixplantations, suggesting that indigenous broadleaf tree species in this study area are the mostpotential species for soil amelioration. The total ecosystem carbon storage of E. fordii, C.hystrix and P. massoniana plantations was236.22t·hm^-2,267.84t·hm^-2and200.57t·hm^-2andnitrogen storage was17.91t·hm^-2,12.38t·hm^-2and10.59t·hm^-2respectively. Carbon storageof tree layer occupied42.57%,36.31%and40.28%while0^-100cm soil occupied55.77%,62.52%and57.83%, respectively. Nitrogen storage of soil occupied92.00%,93.72%and95.53%. Our study indicated that the higher carbon and nitrogen sequestration ability in thenative broadleaf plantations.（2） Soil respiration and gross nitrification at all three plantations showed a pronouncedseasonal pattern with significantly higher rates during the wet versus dry season. Seasonalvariations of soil respiration and gross nitrification of the three plantations largely depended onsoil temperature and water content. Regression analysis showed that soil temperature explained76-86and54-79percent of the observed variance of soil respiration and gross nitrification,respectively, and soil water content explained31-56and14-32percent of the observedvariance of soil respiration and gross nitrification, respectively. The significant differences insoil respiration and gross nitrification rates between the three plantations could also beattributed to the influences of the tree species on the soil environment. Soil respiration andgross nitrification rates of the E. fordii plantation were much higher than those of the other twoplantations. These differences probably reflect the narrower C/N ratio and higher organic Ccontents in the soils of the E. fordii plantation.（3） The soil microbial community composition and the effects of soil microbialcommunity composition on the transformation of carbon and nitrogen were investigated in south subtropical plantations, in China. The results showed that the soil microbial biomass andsoil microbial community composition and C, N turnover rates were significantly influencedby plantation type. The soil organic carbon, total nitrogen, microbial biomass and total PLFAswere higher under the native broadleaf plantations, the highest turnover rates of C and N werefound under the soil of E. fordii plantation. Moreover, we found that the highest C and Nturnover rates coincided with the lowest fungal biomass could be primarily explained by thelower C/N ratio and pH value in E. fordii plantation. We also found the lowest microbialbiomass during the wet season, during the period of the lowest microbial biomass, however, weobserved the highest soil respiration and gross nitrification rates. These differences could bemostly attributed to the relatively greater nutrient demand by plants during the wet season （thepeak vegetative growth period） limited the availability of nutrients to soil microbes and therebyreduced their immobilization in microbial biomass. This also implies that the plant absorbnutrients for growth and soil microorganisms immobilize nutrients for microbial biomasswhich the two processes occur concurrently.（4） Decomposition parameter of leaf litter of E. fordii, C. hystrix and P. massoniana was0.98year^-1,0.88year^-1and0.62year^-1and decomposition parameter of fine root was0.65year^-1,0.59year^-1and0.47year^-1, respectively. Decomposition rates of leaf litter and fine root wereinfluenced not only by the chemical properties of the litter, but also by the environmentalfactors in different plantations. The leaf litter and fine root decomposition rates showedsignificant similarity among tree species. Leaf litter and fine root decomposition rates in threestands were positively correlated with nitrogen content and soil water content, negativelycorrelated with C/N ratio, lignin/N ratio. The significant correlation between leaf litter and fineroot decomposition rates could also be attributed to soil water content and chemical propertiesof the litter on decomposition rates of leaf litter and fine root were very similar.