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To explore potential responses of ecosystem carbon density to changes of community structure during natural regeneration of woody plants, we analyzed the relationships between ecosystem carbon density and its com ponents, tree species diversity, structural diversity (CVDBH) and spatial structure parameters (mingling, aggrega tion, dominance, crowding) of Cunninghamia lanceolata forests with different sprouting densities (1154, 847 an< 465 individuals • hm-2) at the early stage of succession in Baishanzu National Park. The results showed that tree species diversity (species richness index and Shannon diversity index) increased with the decrease of sproutin density of C. lanceolata. Among the stand structural parameters, CVDBH, stand density, and mingling increased with the decrease of sprouting density of C. lanceolata. The stand distribution pattern of different C. lanceolata densities was uniform, with sub-dominant stand growth status and relatively dense status. The carbon density of tree layer under high, medium, and low sprouting densities of C. lanceolata were 57.56, 56.12 and 46.54 t • hm-2, soil carbon density were 104.35, 122.71 and 142.00 t • hm-2, and the total carbon density of ecosystem were 164.59 182.41 and 190.13 t • hm-2, respectively. There was little variation in carbon density of understory layer and litte layer among different treatments. The carbon density distribution characteristics of different C. lanceolata densities were following the order of soil layer (63.4%-74.7%) > tree layer (24.5%-35.0%) > understory layer and litter layer (0.8% -2.0%). The results of variance partitioning analysis indicated that the change of tree layer carbon density was mainly influenced by stand structure diversity, soil layer carbon density was influenced by both tree spe¬cies diversity and stand structure diversity, while ecosystem carbon density was mainly influenced by tree species diversity. Stand spatial structure parameters had a relatively little effect on ecosystem carbon density and its compo¬nents. The sprouting density of C. lanceolata significantly affected ecosystem carbon accumulation during the con¬version from C. lanceolata plantations to natural forests. A lower remaining density of C. lanceolata (about 500 indi¬viduals • hm-2) was more conducive to forest carbon sequestration. [ABSTRACT FROM AUTHOR]

Background: Continuous nitrogen (N) deposition interferes with soil N cycling in forests, which highly impacts soil nitrous oxide (N2O) emissions and accelerates global warming. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is one of the most widely planted species in southern China, and is usually located in areas with high N deposition rates. However, the impact of N deposition on soil N2O emissions in subtropical Chinese fir plantations and the potential risk of increasing N input remain elusive. Methods: Here, we conducted an in situ study in a subtropical Chinese fir plantation at Fengyang Mountain Nature Reserve, China, from 2019 to 2020 with four N addition rates: control (CK: ambient N deposition), low-N (LN: 50 kg N ha−1 yr−1), medium-N (MN: 100 kg N ha−1 yr−1), and high-N (HN: 200 kg N ha−1 yr−1). Results: We found that soil N2O emission rates increased with N addition rates by 71%, 176%, and 241% under LN, MN, and HN treatment compared to CK, respectively, and reached a significant level only under HN. Soil moisture was significantly reduced together with increased leaf N concentrations under N addition. Meanwhile, the microbial biomass in the middle of the growing season was significantly lower than at the end of the growing season. These results may suggest that N deposition stimulated plants to take up more N and water, which intensified plant-microbe competition and therefore alleviated further increases in N2O emissions under N deposition, especially under low N inputs. N deposition enhanced the abundance of ammonia oxidizing archaea and bacteria and the accumulation of NO3−-N in the soil but did not affect the abundance of nitrate-reducing bacteria (nirS and nirK). The results likely support that nitrification processes act as the major source of enhanced N2O emissions under N fertilization. Conclusions: Our study advances our understanding of the impacts of N deposition on the soil N2O emissions in the Chinese fir plantations and highlights that plant N acquisition needs to be incorporated as an important explanatory variable when predicting N2O fluxes under global increases in N deposition. [ABSTRACT FROM AUTHOR]