Background Revealing the variations in soil aggregate-related organic carbon (OC) and labile organic carbon (LOC) fractions in a chronosequence of Chinese fir plantations plays an important role in better understanding the impact of soil carbon sink or source on the Chinese fir plantation ecosystem. In this study, soil samples in a depth of 0–20 cm were collected from Chinese fir plantations at different stand ages (0, 9, 17, and 26 years old) in Guangxi, China. With the optimal moisture sieving method adopted, the soil aggregates of 4 different sizes were obtained, including > 2-mm, 2–1-mm, 1–0.25-mm, and < 0.25-mm aggregates. Soil OC and LOC fractions were measured in the aggregates of different sizes. The LOC fractions included readily oxidizable carbon (ROC), particulate organic carbon (POC), microbial biomass carbon (MBC), water-soluble organic carbon (WOC), and mineralized organic carbon (MOC). Results Soil aggregate stability, as indicated by the mean weight diameter (MWD), was the highest in the 17-year-old Chinese fir plantations and was significantly positively related (p < 0.05) to the concentrations of OC and LOC fractions (except for the ROC and MOC), with the POC in particular. As for all stand ages of Chinese fir plantations, the concentrations of soil OC and LOC fractions were significantly increased as the aggregate size decreased. Consequently, there were more OC and LOC fractions distributed in the < 0.25-mm aggregates. During the stand development, the concentrations of soil OC and LOC fractions first increased and then decreased, with the highest levels detected in the 17-year-old Chinese fir plantations, indicating that the 17-year-old Chinese fir plantations were conducive to the accumulation of soil OC and LOC fractions. Conclusion After 17 years of planting, promoted soil carbon (especially for the POC) accumulation contributes significantly to enhancing soil aggregate stability for the Chinese fir plantations in Guangxi, China.

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Background The mixing of Eucalyptus with N2-fixing trees species (NFTs) is a frequently successful and sustainable cropping practice. In this study, we evaluated nitrogen (N) transfer and conducted a proteomic analysis of the seedlings of Eucalyptus urophylla × E. grandis (Eucalyptus) and an NFT, Dalbergia (D.) odorifera, from intercropping and monocropping systems to elucidate the physiological effects and molecular mechanisms of N transfer in mixed Eucalyptus and D. odorifera systems. Results N transfer occurred from D. odorifera to Eucalyptus at a rate of 14.61% in the intercropping system, which increased N uptake and growth in Eucalyptus but inhibited growth in D. odorifera. There were 285 and 288 differentially expressed proteins by greater than 1.5-fold in Eucalyptus and D. odorifera roots with intercropping vs monoculture, respectively. Introduction of D. odorifera increased the stress resistance ability of Eucalyptus, while D. odorifera stress resistance was increased by increasing levels of jasmonic acid (JA). Additionally, the differentially expressed proteins of N metabolism, such as glutamine synthetase nodule isozyme (GS), were upregulated to enhance N competition in Eucalyptus. Importantly, more proteins were involved in synthetic pathways than in metabolic pathways in Eucalyptus because of the benefit of N transfer, and the two groups of N compound transporters were found in Eucalyptus; however, more functional proteins were involved in metabolic degradation in D. odorifera; specifically, the molecular mechanism of the transfer of N from D. odorifera to Eucalyptus was explained by proteomics. Conclusions Our study suggests that N transfer occurred from D. odorifera to Eucalyptus and was affected by the variations in the differentially expressed proteins. We anticipate that these results can be verified in field experiments for the sustainable development of Eucalyptus plantations.

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