Abstract Despite the importance of nitrogen (N) deposition for soil biogeochemical cycle, how N addition affects the accumulation of humic substances in decomposing litter still remains poorly understood. A litterbag experiment was conducted to assess the potential effects of N addition (0 kg·N·ha −1 ·year −1 , 20 kg·N·ha −1 ·year −1 and 40 kg·N·ha −1 ·year −1 ) on mass remaining and humification of two leaf litter ( Michelia wilsonii and Camptotheca acuminata ) in a subtropical forest of southwestern China. After one year of decomposition, litter mass was lost by 38.1–46.5% for M . wilsonii and 61.7–74.5% for C . acuminata , respectively. Humic substances were declined by 12.1–23.8% in M . wilsonii and 29.1–35.5% in C . acuminata , respectively. Nitrogen additions tended to reduce mass loss over the experimental period. Moreover, N additions did not affect the concentrations of humic substances and humic acid in the early stage but often increased them in the late stage. The effect of N addition on the accumulation of humic substances was stronger for C . acuminate litter than in M . wilsonii litter. Litter N and P contents showed positive correlations with concentrations of humic substances and fulvic acid. Our results suggest that both litter quality and season-driven environmental changes interactively mediate N impacts on litter humification. Such findings have important implications for carbon sequestration via litter humification in the subtropical forest ecosystems experiencing significant N deposition.
Plant drought stress indicators such as crop water stress index (CWSI), plant motion in the form of covariance of top-projected canopy area (COVTPCA), leaf water content represented as equivalent water thickness (EWT), and their threshold values for drought stress detection were established from measurements. Performances of these indicators in detecting drought stress of New Guinea Impatiens plants in a controlled environment were evaluated. Analysis of variance (ANOVA) was conducted to compare the timing of drought stress detection by these indicators against the timing of incipient drought stress defined by evapotranspiration (ET) and timing of human visual detection. Statistical analysis was also performed to study the consistency of the threshold values of the indicators in different experiments. ANOVA results showed that the CWSI was the most reliable indicator for early plant drought stress detection. The timing of the drought stress detection from the earliest to the latest was CWSI, EWT, and COVTPCA. While COVTPCA and EWT were not able to detect drought stress as early as CWSI, ANOVA results indicated that these two indicators were able to detect drought stress no later than visual detection. ANOVA results also showed that there was no significant difference in threshold values of CWSI and COVTPCA in different experiments, but different cultivars used in the experiments resulted in significant differences in EWT threshold values.
To understand the differential effects of altitudinal gradient on soil inorganic nitrogen concentration and associated ammonia-oxidizingbacteria (AOB) and archaea (AOA), intact soil cores from a primary coniferous forest were in situ incubated in an alpine forest at a 3582-m altitude (A1) and transplanted to subalpine forests at a 3298-m altitude (A2) and 3023-m altitude (A3) on the eastern Tibetan Plateau. Transplant cooled the soil temperature of A2 but warmed the A3 soil temperature. Both AOA and AOB were found at the three altitudes. Compared to A1, A2 had greater AOA and AOB abundance, but A3 showed lower AOA abundance in organic soil. The AOA abundance was negatively correlated with ammonium concentration at all three altitudes, but AOB showed the reverse trend. Our results suggested that the soil nitrogen process responded differentially to soil core transplanting at different altitudes.
The effect of exogenously applied nitric oxide on the heat tolerance of Chrysanthemum morifolium was investigated by applying the NO donor sodium nitroprusside (SNP). We found that the SNP partially alleviated the heat stress by slowing down the reduction of photosynthetic pigment content and net photosynthetic rate. SNP treatment also lowered the increase in the non-photochemical quenching of fluorescence and malondialdehyde content and maintained higher activities of superoxide dismutase, peroxidase, catalase and ascorbate peroxidase.
To characterize the effects of Cd supplies on the accumulation efficiency of Cd, Pb, Zn, and Cu in an ornamental plant (osmanthus, Osmanthus fragrans var. thunbergii ), a pot experiment using current-year osmanthus in field was carried out in western China. Biomass and its components showed a decreasing tendency as Cd supply increased, though insignificant differences were observed between treatments with a low and no Cd supply. Cd supplies increased the concentrations of Cd and Pb in plants, but the concentrations of Zn and Cu in plant showed a decreasing tendency with the increase of Cd supplies. Cd supplies also increased Cd accumulation for the plant, although the highest Cd accumulation was observed at a low Cd supply (T1). A higher Pb accumulation in the osmanthus was also detected in T1 than the other treatments. However, Zn and Cu accumulations decreased with the increase of Cd supplies. The examined osmanthus showed relative higher Cd and Pb transfer efficiencies in the presence of Cd supplies, but Cd supplies can limit the transfer of Zn and Cu. The results suggested that the examined osmanthus has potential for use in metal-contaminated environments due to phytoremediation application in the soil.
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Dissolved carbon (DC) in forest streams plays a crucial role in maintaining the structure and productivity of adjoining aquatic ecosystems as well as informing biogeochemical links between mountain forests and adjoining rivers. Nevertheless, the functions of forest stream DC dynamics are rarely incorporated into river management. To better understand the biogeochemical links between subalpine forests and adjoining streams, the seasonal dynamics of DC in 15 representative forest streams were investigated in a geographically fragile subalpine-gorge catchment in the upper reaches of the Yangtze River. Depending on stream characteristics and critical periods, the DC stocks in the streams ranged from 0.22 to 2.35 mg m−2 for total DC, from 0.10 to 1.66 mg m−2 for dissolved inorganic carbon (DIC), and from 0.12 to 1.27 mg m−2 for dissolved organic carbon (DOC). Moreover, the annual stocks of DC, DIC, and DOC were 1.01, 0.56, and 0.45 mg m−2, respectively. Correspondingly, the averaged export rates for DC, DIC, and DOC from the forest streams ranged from 0.27 to 1.98 mg s−1, from 0.24 to 1.48 mg s−1, and from 0.18 to 0.90 mg s−1, respectively, in the subalpine forest catchment. The annual export rates of total DC, DIC, and DOC were 1.06, 0.75, and 0.31 mg C s−1, respectively. In particular, the highest rates of export were 4.67, 3.53, and 1.34 mg s−1 for DC, DIC, and DOC, respectively, in the snowmelt period. The average ratios of DOC to DIC stock in the export water ranged from 0.23–2.41 for the 15 streams, and the average value was 0.85 during this one-year investigation. In addition, the maximum and minimum values of the DC stocks, their exports, and the DIC:DOC ratio were consistently observed during the snowmelt season and the late growing period. In summary, precipitation, temperature, water discharge rate, and sediment depth regulated the stocks and export rates of DC and its components. In general, forest streams are important links between the carbon biogeochemical cycle of subalpine forests and adjoining streams.
By using forest inventory data in combination with plot measurement, the characteristics of carbon density, stock, and partitioning in artificial forest ecosystem in Sichuan Province of China were studied. The results showed that the carbon density in this forest ecosystem was averagely 161.16 Mg C x hm(-2), being ranked in the order of soil layer (141.64 Mg C x hm(-2)) >tree layer (17.95 Mg C x hm(-2)) >litter layer (1.06 Mg C x hm(-2)) >shrub layer (0.52 Mg C x hm(-2)), and the total carbon stock was 573.57 Tg C, with 63.88 Tg C, 1.836 Tg C, 3.764 Tg C, and 504.09 Tg C, accounting for 11.14%, 0.32%, 0.66%, and 87.88% of the total in tree layer, shrub layer, litter layer, and soil layer, respectively. The carbon density and stock in different artificial forest ecosystems varied from 75.50 Mg C x hm(-2) to 251.74 Mg C x hm(-2) and from 1.21 Tg C to 99.44 Tg C, with the highest and lowest values observed in soil layer and shrub layer, respectively. Comparing with other regions in China, Sichuan Province had a lower carbon density in the tree layer of artificial forest ecosystem, due to the higher proportion of young and middle age forest stands, which implied that a proper management of artificial forest could increase the carbon sequestration in forest ecosystem of Sichuan. To monitor the carbon stock in artificial forest ecosystem at ecosystem level could be helpful to the improvement of the precision of forest carbon sequestration evaluation.
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