【Objective】This study aimed to examine seasonal variations in soil respiration rates during secondary tropical forest succession, which could provide a reference basis for an accurate assessment of the effects of tropical forest restoration on soil carbon and nitrogen pool changes.【Method】The LI-6400-09 respiration chamber connected to a portable infrared gas analyzer was employed for continuous observation of soil respiration rates in Mallotus paniculatus and Syzygium oblatum communities. We also assessed the effects of changes in soil microbial biomass carbon, bulk density, pH, and concentrations of carbon and nitrogen pools on soil respiration rates using the statistical methods of correlation and principal component analyses. 【Result】There were significant seasonal variations in respiration rates in the two secondary forests. Soil respiration exhibited a similar fluctuation pattern, with a single peak occurring during the wet season (June). Soil respiration rates were significantly higher in the S. oblatum community[3.80~6.19 μmol/(m2·s)]than in the M. paniculatus community[2.40~4.35 μmol/(m2·s)]. However, the variations in soil respiration rates were significantly higher in the early restoration stage (1.81 times) than in the late restoration stage(1.63 times). Soil respiration rates increased nonlinearly with seasonal variations in soil temperature and water values (P<0.01, or P<0.05). The contributions of soil temperature (49.00%~65.30%) and water content (2.96%~53.00%) to soil respiration variations were higher in the S. oblatum community than in the M. paniculatus community (6.40%~49.10%, 2.48%~43.70%). Soil respiration rates in the M. paniculatus and S. oblatum communities were positively correlated with the concentrations of the carbon pool (e.g., total organic carbon and microbial biomass carbon) and nitrogen pool (e.g., total nitrate and ammonium nitrogen)(P<0.01, or P<0.05). In contrast, there was a negative correlation with soil pH (P<0.01). Furthermore, the values of easily oxidized carbon, nitrate nitrogen, and water content contributed the most to variations in soil respiration rates, whereas the contributions of soil temperature, microbial carbon, total nitrogen, ammonium nitrogen, and hydrolyzable nitrogen to soil respiration dynamics ranked second. 【Conclusion】The secondary succession of Xishuangbanna tropical forests significantly promoted the soil respiration rates. The temporal variations in soil respiration rates were mainly controlled by the soil microclimate (e.g., water content), the concentrations of the carbon (e.g., easily oxidized carbon)and nitrogen (e.g., nitrate nitrogen) pools.
【Objective】The aim of this study was to reveal the impact of the unique hydrological characteristics of Hongze Lake, which has a low water level in summer and a high water level in winter, on the net ecosystem exchange carbon flux (NEE) of poplar plantations at the confluence of Hongze Lake and Huai River. 【Method】In this study, eddy covariance and soil moisture monitoring systems were used to observe the NEE and environmental factors of typical poplar plantations in the river-lake intersection area for three consecutive years(2016-2018) to analyze the variations in NEE and its relationship with environmental factors on a monthly basis. 【Result】(1) The NEE values of the poplar plantations in this area displayed an obvious V-shaped curve, showing carbon absorption during the daytime and carbon release at night, with an average daily carbon sink time of 10 h. (2) The highest value of NEE during the annual 10 days study was 7.117 g/m2 in early February 2018, and the lowest NEE was -212.256 g/m2 in mid-July 2017. The average annual NEE value in the three years was -1 413.403 g/m2. (3) The water level of Hongze Lake was low during the sluice-opening period (from May to August), and soil moisture content and wind speed were the main factors affecting NEE, whereas the water level was high during the sluice-closing period (September to April of the next year), and NEE was mainly affected by air temperature and saturated vapor pressure differences. 【Conclusion】Opening the sluices and releasing water in summer were advantageous for the growth of poplar trees and increased the carbon sink. In winter, the groundwater level increased, and the adverse effects on the poplar growth could be eliminated by the ditching and ridge construction, which would be beneficial for improving the annual carbon sink function of poplar plantations in this region.
【Objective】To offer a theoretical basis for improving the supply of carbon sinks and reducing the cost of carbon sequestration, the dynamics of the optimal rotation periods and carbon sequestration costs under different carbon payment mechanisms were explored. 【Method】Based on a survey of management practices for Chinese fir plantations in Yangkou State-owned Forest Farm, Fujian Province, models for the optimal rotation period and carbon sequestration cost of Chinese fir under different carbon subsidy scenarios were derived, and the effects of site conditions, carbon prices and discount rates on the optimal rotation periods and carbon sequestration cost were investigated. 【Result】Under an ideal scenario, when the discount rate increased from 2% to 7%, the carbon sequestration cost of the fertile, general and barren land decreased by 31.42%,32.03% and 33.15%, respectively, the optimal rotation period was shortened by four years. When the carbon price increased from 20 to 350 yuan/t, the carbon sequestration cost of fertile, general and barren land increased by 19.24, 19.43 and 19.38 times, and the optimal rotation period was extended by 8, 9 and 10 years, respectively. Under the annual payment scenario, when the discount rate increased from 2% to 7%, the carbon sequestration cost of fertile, general, and barren land decreased by 70.35%, 70.66% and 71.14%, respectively, and the optimal rotation period was shortened by four years. When the carbon price increased from 20 to 350 yuan/t, the carbon sequestration cost of fertile, general and barren land increased by 15.79, 15.82 and 16.50 times, respectively. The optimal rotation periods for both general and fertile land extended for one year; but remained unchanged for the barren land. 【Conclusion】Under the two scenarios of carbon subsidy, both carbon sequestration costs and optimal rotation periods were negatively correlated with the discount rate and positively correlated with the carbon price. The carbon sequestration cost of general land is the most sensitive to the change in the carbon price for the ideal scenario, but the barren land is the most sensitive under the annual payment scenario. In short, the ideal scenario is more conducive to the development of forest ecological functions and the realization of forest ecological benefits.
【Objective】The aim of analyzing the spatial distribution characteristics and driving factors of soil organic carbon density on a large spatial scale in the water source area of the Middle Route of China South-to-North Water Diversion Project is to reveal its spatial differences and the changes with the altitude, soil types, land use, and forest types. This work should provide a scientific basis for the rational utilization of land resources and soil organic carbon (SOC) management in the area. The spatial distribution and driving factors of soil organic carbon density (SOCD) were explored to provide a scientific basis for the accurate assessment of the ecosystem carbon cycle in the climate transition zone.【Method】Based on a comprehensive assessment of spatial factors such as the elevation, soil types, and land use, the spatial variations of SOCD in this region were evaluated by the field sampling and indoor analysis with the help of a geographic information system (GIS). In addition, the main driving factors affecting the spatial distribution of SOCD in the study area were determined by analyzing the explanatory power of various influencing factors on soil organic carbon density and the difference in the degree of interaction among various factors using the geographic detector model. 【Result】(1)The results showed that the average SOCD in 0-20 cm and ≥20-40 cm soil layers was 4.18 kg/m2 and 2.67 kg/m2 respectively, in which the SOCD of 0-20 cm soil layer was 56.55% higher than the national average level (2.67 kg/m2).(2)Over the entire region, the SOCD was higher in forests in the northern and southern areas and lower in farmlands and grasslands in the middle area. The soil organic carbon density greater than 10 kg/m2 is concentrated in the forest land with an altitude of ≥1 500-2 000 m, and less than 1 kg/m2 is concentrated in grasslands with altitudes <500 m. The SOCD first increased and then decreased with an increasing elevation. The SOCD of 0-20 cm and ≥20-40 cm soil layers showed peak values (7.32 kg/m2 and 4.94 kg/m2) at an altitude of ≥1 500-2 000 m. In terms of soil types, the average SOCD of limestone soil is the greatest and that of cinnamon soils is the smallest. The SOCD storage of yellow-brown earths and brown earths in the 0-20 cm soil layer was the highest, 2.005 Pg and 0.815 Pg, respectively, accounting for 72.83% of the total storage. In terms of land-use types, the forest and farmland were the main land-use types. The SOCD of 0-20 cm and ≥20-40 cm soil layers in the forest land were 4.87 kg/m2 and 3.05 kg/m2, respectively, and that of farmlands were 2.75 kg/m2 and 2.00 kg/m2, respectively. These levels were 77.09% and 52.50% lower than that of the forest land, and the SOC storage of the forest land accounted for 87.48% of the total C storage, followed by farmlands (12.02%).(3)The most powerful explanation for the spatial distribution of SOCD was the altitude (0.25) and land use (0.20), followed by soil clay (0.11). The explanatory power of the different driving factors under interaction was significantly higher than that of a single factor. 【Conclusion】The altitude and land use were the main driving factors affecting the spatial patterns of SOCD in this region. After the interaction of different driving factors, there was a synergistic effect of the two factors. The SOCD of farmlands was significantly lower than that of the forest land. Therefore, the ecological engineering construction in the study area should be strengthened, the forest protection and vegetation restoration should be carried out, and the soil carbon storage capacity of the water source in the Middle Route of China South-to-North Water Diversion Project should be improved.
