| Subtropical China has more than 60 % of the total plantation area in China and over 70 % of these subtropical plantations are composed of pure coniferous species. In view of low ecosystem services and ecological instability of pure coniferous plantations, indigenous broadleaf plantations are being advocated as a prospective silvicultural management for substituting in place of large coniferous plantations in subtropical China. However, little information is known about the effects of tree species conversion on stock and stability of soil organic carbon(SOC). Also, little information is known about the effects of tree species conversion on soil-atmosphere greenhouse gas(GHG)exchanges. Elucidating the processes of leaf litter and fine root decomposition has been a major research focus, while the underlying correlation between leaf litter and fine root decomposition is unclear.The four adjacent monospecific plantations were selected to examine the effects of tree species on the stock and chemical composition of SOC, the effects of tree species on soil-atmosphere exchanges of N2O, CH4 and CO2, and the in situ decomposition and N dynamics of leaf litter and fine root of four subtropical tree species to determine whether leaf litter and fine root decomposition is correlated across species as well as to identify key factors influencing decomposition above versus below ground, by using elemental analysis, solid-state 13C nuclear magnetic resonance ( NMR ) spectroscopy, the static chamber and gas chromatography techniques, and litter/root decomposition bags methods. One coniferous plantation was composed of Pinus massoniana(PM), and the three broadleaf plantations were Castanopsis hystrix(CH), Michelia macclurei(MM), and Mytilaria laosensis(ML). The main results are as follows:(1)SOC stock differed significantly among the four plantations in the upper(0 ~ 10 cm)layer, but not in the underneath(10 ~ 30 cm)layer. SOC stocks in the upper(0 ~ 10 cm)layer were 11 %, 19 % and 18 % higher in the CH, MM and ML plantations than in the PM plantation. The differences in SOC stock among the four plantations were largely attributed to fine root input rather than aboveground litterfall input. However, the soils in the broadleaf plantations contained more decomposable C proportion, indicated by lower percentage of alkyl C, higher percentage of O-alkyl C and lower alkyl C/O-alkyl C ratio compared to those in the PM plantation. Our findings highlight that future strategy of tree species selection for substituting in place of large coniferous plantations in south subtropical China needs to consider the potential effects of tree species on the chemical composition of SOC in addition to the quantity of SOC stock.(2)The mean soil N2O and CO2 emissions in the PM plantation were 4.3μg N m-2 h-1 and 43.3 mg C m-2 h-1, respectively, lower than those in the broadleaf plantations. The mean CH4 uptake in the PM plantation soil was higher than that in the broadleaf plantation. Variations in soil N2O emissions among tree species could be primarily explained by the differences in litter C:N ratio and soil total N stock. Differences in soil CH4 uptake among tree species could be mostly attributed to the differences in mean soil CO2 flux and water filled pore space(WFPS). Litter C:N ratio could largely accounted for variations in soil CO2 emissions among tree species. This study indicated that there was no GHG benefit of converting PM plantation to broadleaf plantations in south subtropical China. Therefore, the future strategy of tree species selection for substituting in place of large coniferous plantations in south subtropical China needs to consider the potential effects of tree species on soil-atmosphere GHG exchanges.(3)Decomposition rate of leaf litter was related to that of fine root across species. The strong correlation between leaf litter and fine root decomposition rates accounted largely for the following several reasons. First, soil moisture had the similar influences on both leaf litter and fine root decomposition rates. Second, traits(i.e., initial Ca concentration)important to both leaf litter and fine root decomposition rates showed significant similarity among species. Third, initial P, N and aromatic C concentrations, and C/N ratio were uniquely important for fine root decomposition rate, while no unique traits for leaf litter decomposition rate. This also could account for the strong correlation between leaf litter and fine root decomposition rates. Our study suggested that among these south subtropical trees, species effects on in situ decomposition rates of leaf litter and fine root were very similar. Thus, species differences in decomposition rates may be as large as they would be if faster decomposition of leaf litter was correlated with faster decomposition of fine root. N immobilization rate of leaf litter was not related to that of fine root across species. Our results contributed to the understanding of some important mechanisms by which tree species influence litter in situ decomposition.
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