Soil CO₂ flux dynamics in the two main plantation forest types in subtropical China

Simulated net ecosystem productivity of subtropical forests and its response to climate change in Zhejiang Province, China

Net ecosystem productivity (NEP) is an important index that indicates the carbon sequestration capacity of forest ecosystems. However, the effect of climate change on the spatiotemporal variability in NEP is still unclear. Using the Integrated Terrestrial Ecosystem Carbon-budget (InTEC) model, this study takes the typical subtropical forests in the Zhejiang Province, China as an example, simulated the spatiotemporal patterns of forest NEP from 1979 to 2079 based on historically observed climate data (1979-2015) and data from three representative concentration pathway (RCP) scenarios (RCP2.6, RCP4.5, and RCP8.5) provided by the Coupled Model Intercomparison Project 5 (CMIP5). We analyzed the responses of NEP at different forest age classes to the variation in meteorological factors. The NEP of Zhejiang's forests decreased from 1979 to 1985 and then increased from 1985 to 2015, with an annual increase rate of 9.66 g C·m^-2·yr^-1 and a cumulative NEP of 364.99 Tg·C. Forest NEP decreased from 2016 to 2079; however, the cumulative NEP continued to increase. The simulated cumulative NEP under the RCP2.6, RCP4.5, and RCP8.5 scenarios was 750 Tg·C, 866 Tg·C, and 958 Tg·C, respectively, at the end of 2079. Partial correlation analysis between forest NEP at different age stages and meteorological factors showed that temperature is the key climatic factor that affects the carbon sequestration capacity of juvenile forests (1979-1999), while precipitation is the key climatic factor that affects middle-aged forests (2000-2015) and mature forests (2016-2079). Adopting appropriate management strategies for forests, such as selective cutting of different ages, is critical for the subtropical forests to adapt to climate change and maintain their high carbon sink capacity.

Metabolic profiles of moso bamboo in response to drought stress in a field investigation

An increasing number of moso bamboo habitats are suffering severe drought events. The improvement in our understanding of the mechanisms of drought-resistance in moso bamboo benefits their genetic improvement and maintenance of forest sustainability. Here, we investigated the metabolic changes across the annual growth cycle of moso bamboo in the field under drought stress using liquid chromatography coupled to mass spectrometry (LC-MS) based on untargeted metabolomic profiling. Our results showed that the metabolic profiles induced by drought stress were relatively consistent among the three growth stages. Specifically, most responsive metabolites exhibited enhanced accumulation under drought stress, including anthocyanins, glycosides, organic acids, amino acids, and sugars and sugar alcohols. The potential metabolism pathways involved in the response to drought stress were mainly included into amino acid metabolism and sugar metabolism pathways. Five common responsive metabolic pathways were found among three growth stages, including linoleic acid metabolism, ubiquinone and other terpenoid-quinone biosynthesis, tyrosine metabolism, starch and sucrose metabolism and isoquinoline alkaloid biosynthesis. Overall, our findings provide new insights into the responsive mechanisms of the moso bamboo under drought stress in terms of metabolic profiles.

Diurnal and Seasonal Variations in Soil Respiration of Four Plantation Forests in an Urban Park

Understanding the carbon dynamics of urban trees and forests is one of the key components for developing mitigation strategies for climate change in a fast-paced urbanized world. This study selected four plantation forests composed of poplar, black locust, Chinese pine and mixture of poplar and black locust, located in an urban forest park on a well-drained fluvial plain with same land-use history. The diurnal and seasonal changes in soil respiration (Rs) and biophysical factors were measured from April 2015 to March 2016. At the diurnal scale, Rs varied out of phase with soil temperature (Ts) and the time-lag occurred in May and July when Ts was relatively high and soil moisture (Ms) was low. Strong seasonal variations in Rs were mainly determined by Ts, while the growing-season mean Rs positively correlated with the fine root biomass (FRB), soil organic carbon content (SOC), and total nitrogen content (TN) for all the forests. FRB alone could explain 75% of the among-stand variability. This study concluded that urban forest plantations have similar soil respiration dynamics to forest ecosystems in non-urban settings.

Soil respiration of a Moso bamboo forest significantly affected by gross ecosystem productivity and leaf area index in an extreme drought event

Moso bamboo has large potential to alleviate global warming through carbon sequestration. Since soil respiration (R s ) is a major source of CO2 emissions, we analyzed the dynamics of soil respiration (R s ) and its relation to environmental factors in a Moso bamboo (Phllostachys heterocycla cv. pubescens) forest to identify the relative importance of biotic and abiotic drivers of respiration. Annual average R s was 44.07 t CO2 ha-1 a-1. R s correlated significantly with soil temperature (P < 0.01), which explained 69.7% of the variation in R s at a diurnal scale. Soil moisture was correlated significantly with R s on a daily scale except not during winter, indicating it affected R s . A model including both soil temperature and soil moisture explained 93.6% of seasonal variations in R s . The relationship between R s and soil temperature during a day showed a clear hysteresis. R s was significantly and positively (P < 0.01) related to gross ecosystem productivity and leaf area index, demonstrating the significance of biotic factors as crucial drivers of R s .

Temperature sensitivity of total soil respiration and its heterotrophic and autotrophic components in six vegetation types of subtropical China

The temperature sensitivity of soil respiration (Q₁₀) is a key parameter for estimating the feedback of soil respiration to global warming. The Q₁₀ of total soil respiration (R_t) has been reported to have high variability at both local and global scales, and vegetation type is one of the most important drivers. However, little is known about how vegetation types affect the Q₁₀ of soil heterotrophic (R_h) and autotrophic (R_a) respirations, despite their contrasting roles in soil carbon sequestration and ecosystem carbon cycles. In the present study, five typical plantation forests and a naturally developed shrub and herb land in subtropical China were selected for investigation of soil respiration. Trenching was conducted to separate R_h and R_a in each vegetation type. The results showed that both R_t and R_h were significantly correlated with soil temperature in all vegetation types, whereas R_a was significantly correlated with soil temperature in only four vegetation types. Moreover, on average, soil temperature explained only 15.0% of the variation in R_a in the six vegetation types. These results indicate that soil temperature may be not a primary factor affecting R_a. Therefore, modeling of R_a based on its temperature sensitivity may not always be valid. The Q₁₀ of R_h was significantly affected by vegetation types, which indicates that the response of the soil carbon pool to climate warming may vary with vegetation type. In contrast, differences in neither the Q₁₀ of R_t nor that of R_a among these vegetation types were significant. Additionally, variation in the Q₁₀ of R_t among vegetation types was negatively related to fine root biomass, whereas the Q₁₀ of R_h was mostly related to total soil nitrogen. However, the Q₁₀ of R_a was not correlated with any of the environmental variables monitored in this study. These results emphasize the importance of independently studying the temperature sensitivity of R_t and its heterotrophic and autotrophic components.

Assessment of carbon sequestration potential of revegetated coal mine overburden dumps: A chronosequence study from dry tropical climate

Development of secondary forest as post-mining land use in the surface coal mining degraded sites is of high research interest due to its potential to sequester atmospheric carbon (C). The objectives of this study were to assess the improvement in mine soil quality and C sequestration potential of the post-mining reclaimed land with time. Hence, this study was conducted in reclaimed chronosequence sites (young, intermediate and old) of a large open cast coal project (Central Coal Fields Limited, Jharkhand, India) and results were compared to a reference forest site (Sal forest, Shorea robusta). Mine soil quality was assessed in terms of accretion of soil organic carbon (SOC), available nitrogen (N) and soil CO₂ flux along with the age of revegetation. After 14 years of revegetation, SOC and N concentrations increased three and five-fold, respectively and found equivalent to the reference site. Accretion of SOC stock was estimated to be 1.9 Mg C ha^-1year^-1. Total ecosystem C sequestered after 2-14 years of revegetation increased from 8 Mg C ha^-1 to 90 Mg C ha^-1 (30-333 Mg CO₂ ha^-1) with an average rate of 6.4 Mg C ha^-1year^-1. Above ground biomass contributes maximum C sequestrate (50%) in revegetated site. CO₂ flux increased with age of revegetation and found 11, 33 and 42 Mg CO₂ ha^-1year^-1 in younger, intermediate and older dumps, respectively. Soil respiration in revegetated site is more influenced by the temperature than soil moisture. Results of the study also showed that trees like, Dalbergia sissoo and Heterophragma adenophyllum should be preferred for revegetation of mine degraded sites.

Changes in ecosystem carbon pool and soil CO2 flux following post-mine reclamation in dry tropical environment, India

Open strip mining of coal results in loss of natural carbon (C) sink and increased emission of CO₂ into the atmosphere. A field study was carried out at five revegetated coal mine lands (7, 8, 9, 10 and 11years) to assess the impact of the reclamation on soil properties, accretion of soil organic C (SOC) and nitrogen (N) stock, changes in ecosystem C pool and soil CO₂ flux. We estimated the presence of C in the tree biomass, soils, litter and microbial biomass to determine the total C sequestration potential of the post mining reclaimed land. To determine the C sequestration of the reclaimed ecosystem, soil CO₂ flux was measured along with the CO₂ sequestration. Reclaimed mine soil (RMS) fertility increased along the age of reclamation and decreases with the soil depths that may be attributed to the change in mine soils characteristics and plant growth. After 7 to 11years of reclamation, SOC and N stocks increased two times. SOC sequestration (1.71MgCha^-1year^-1) and total ecosystem C pool (3.72MgCha^-1year^-1) increased with the age of reclamation (CO₂ equivalent: 13.63MgCO₂ha^-1year^-1). After 11years of reclamation, soil CO₂ flux (2.36±0.95μmolm^-2s^-1) was found four times higher than the natural forest soils (Shorea robusta Gaertn. F). The study shows that reclaimed mine land can act as a source/sink of CO₂ in the terrestrial ecosystem and plays an important role to offset increased emission of CO₂ in the atmosphere.

Invasion of moso bamboo into a Japanese cedar plantation affects the chemical composition and humification of soil organic matter

Bamboo, which has dense culms and root rhizome systems, can alter soil properties when it invades adjacent forests. Therefore, this study investigated whether bamboo invasions can cause changes in soil organic matter (SOM) composition and soil humification. We combined solid-state ¹³C NMR spectroscopy and chemical analysis to examine the SOM in a Japanese cedar (Cryptomeria japonica) and adjacent bamboo (Phyllostachys edulis) plantation. Bamboo reduced soil organic C (SOC) content, compared to the cedar plantation. The value of ∆logK (ratio of absorbance of humic acids at 400 and 600 nm) was cedar > transition zone > bamboo soils. Our results indicated that bamboo increased SOM humification, which could be due to the fast decomposition of bamboo litter with the high labile C. Furthermore, intensive management in the bamboo plantation could enhance the humification as well. Overall, litter type can control an ecosystem’s SOC nature, as reflected by the finding that higher labile C in bamboo litter contributed the higher ratios of labile C to SOC and lower ratios of recalcitrant C to SOC in bamboo soils compared with cedar soils. The invasion of bamboo into the Japanese cedar plantation accelerated the degradation of SOM.

Winter soil CO2 flux from different mid-latitude sites from Middle Taihang Mountain in north China

Winter soil respiration is a very important component of the annual soil carbon flux in some ecosystems. We hypothesized that, with all other factors being equal, shorter winter SR result in reduced contribution to annual soil C flux. In this study, the contribution of winter soil respiration to annual soil respiration was measured for three sites (grassland: dominated by Artemisia sacrorum, Bothriochloa ischaemum and Themeda japonica; shrubland: dominated by Vitex negundo var. heterophylla; plantation: dominated by Populus tomatosa) in a mountainous area of north China. Diurnal and intra-annual soil CO₂ flux patterns were consistent among different sites, with the maximum soil respiration rates at 12∶00 or 14∶00, and in July or August. The lowest respiration rates were seen in February. Mean soil respiration rates ranged from 0.26 to 0.45 µmol m⁻² s⁻¹ in the winter (December to February), and between 2.38 to 3.16 µmol m⁻² s⁻¹ during the growing season (May-September). The winter soil carbon flux was 24.6 to 42.8 g C m⁻², which contributed 4.8 to 7.1% of the annual soil carbon flux. Based on exponential functions, soil temperature explained 73.8 to 91.8% of the within year variability in soil respiration rates. The Q₁₀ values of SR against ST at 10 cm ranged from 3.60 to 4.90 among different sites. In addition, the equation between soil respiration and soil temperature for the growing season was used to calculate the “modeled” annual soil carbon flux based on the actual measured soil temperature. The “measured” annual value was significantly higher than the “modeled” annual value. Our results suggest that winter soil respiration plays a significant role in annual soil carbon balance, and should not be neglected when soil ecosystems are assessed as either sinks or sources of atmospheric CO₂.