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Close-to-nature forest management of multi-layer mixing is one of the most common forest conversion patterns in subtropical regions of China, but its effects on soil organic carbon(SOC)accumulation and stability are still unclear. In this study, the close-to-nature plantations of masson pine(Pinus massoniana)(CP)by multi-layer mixed management in the southern subtropical region were selected as the research object, and the adjacent pure plantations of masson pine(PP)without modification were taken as the control(CK). And then we investigated the stability, organic carbon component, microbial biomass and enzyme activity of soil aggregates with different particle sizes and their relationships. The results were as follows:(1)Mean weight diameter(MWD), which was used to characterize the stability of soil aggregates, increased significantly after near-natural modification. Compared to PP, SOC, labile organic carbon(LOC), intermediate organic carbon(IOC), recalcitrant organic carbon(ROC), soil C/N, microbial biomass carbon and nitrogen(MBC and MBN), microbial respiration(MR), β-glucosidase(BG), cellobiohydrolase(CB), N-acetyl-glucosaminidase(NAG)and phenol oxidase(PO)were significantly increased, but the MBC/MBN and microbial metabolic quotient(qCO2)were significantly decreased.(2)The SOC, total organic carbon(LOC+IOC), ROC and ROC index(RCI)of in soils or most soil aggregates were significantly positively correlated with MWD.(3)The change of SOC content and its components mainly resulted from the interaction among soil physical and chemical, microbial and plant properties. Furthermore, the significant changes of litter carbon to nitrogen ratio(C/NLF)and soil total nitrogen(TN)caused by masson pine with multi-layer mixed forest management were the primary drivers of differences in SOC content and its components. Our findings indicate that that the close-to-natural forest management model of multi-layer mixing is an effective management measure to improve SOC and its stability components, and so as to promote masson pine plantations to sequester more and more stabilized SOC in the subtropical region of China. [ABSTRACT FROM AUTHOR]

Soil enzyme activity and its stoichiometric ratio are important indicators reflecting soil nutrient availability and microbial growth and metabolic nutrient requirements. However, the effects of nitrogen-fixing tree species on soil aggregate-associated enzyme activities and ecoenzymatic stoichiometric characteristics in Eucalyptus plantations are still unclear. In this study, an 11-year-old pure E. urophylla plantation(PP)and an 11-year-old mixed E. urophylla and Dalbergia odorifera plantation(MP)in the Shaoping Experimental Field of the Tropical Forestry Experimental Centre of Chinese Academy of Forestry were selected as the research objects. Soil aggregates which were collected from 0-10 cm soil layer were classified into >2 mm, 1-2 mm, 0.25-1 mm, and <0.25 mm fractions by the improved dry-sieving procedure. And then the soil physicochemical properties and the activities of hydrolase [β -glucosidase(BG), N-acetyl-glucosidase(NAG), leucine aminopeptidase(LAP)and acid phosphatase(ACP)] related to soil carbon(C), nitrogen(N)and phosphorus(P)cycling in soil aggregates were measured and the effects of mixing nitrogen-fixing tree species with Eucalyptus on soil enzyme activities and its stoichiometry were examined. The results were as follows:(1)The dominant soil aggregates were >2 mm fractions which were higher than other aggregates. The contents of SOC, TN, NO3--N, AP, pH and activities of BG, NAG, ACP were significantly increased on each aggregate fractions except for LAP, respectively, in MP than in PP.(2)pH, TN, AP, NO3--N, NH4+-N had significant correlation with the ecoenzymatic stoichiometry. The redundancy analysis(RDA)found that the contents of NO3--N, SOC, and AP were key factors affecting the soil hydrolase enzyme activities and its stoichiometry.(3)The ecoenzymatic stoichiometry of C, N, P in this area was 1:0.86:0.74 [deviated from the global pattern(1:1:1)] which indicated that the soil microorganisms in this area were easily limited by C source. The C:N and C:P ratio of enzyme stoichiometric in most of the soil aggregates in MP were lower than those in PP, and significant differences were found in <0.25 mm fractions(P<0.05). Our findings suggest that the introduction of nitrogen-fixing tree species into eucalyptus plantations can not only alleviate the C source limitation of soil microbial growth and metabolism, but also effectively alleviate the N and P limitation of soil in this region. Findings from this study provide a scientific basis for the application of nitrogen-fixing tree species in improving soil quality and productivity of eucalyptus plantations. [ABSTRACT FROM AUTHOR]

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