Variability of carbon stored in inland freshwater wetland in Northeast India

Inland freshwater wetland ecosystems are among the largest sink of carbon (C) in the biosphere. However, improved scientific understanding of the C stability and sequestration potential is required to predict response of C pool under environmental change and to identify priorities for lacustrine C sink management. This study analyses the concentration of organic C fractions based on their stability and estimates C stock along with depth and eco-zones of the Rudrasagar lake in Northeast India. Sediment samples up to 100 cm depth were collected from littoral, sub-littoral and deep layers, and analysed for organic C concentrations. Results showed that C concentration decreases with depth in the littoral layer but increases with depth in sub-littoral and deep layers. Two-way analysis of variance showed that concentrations of soil organic C (SOC) fractions were significantly different among the eco-zones but not between the soil depth. Average SOC stock was significantly higher in the deep layer (334.9 Mg C ha^-1) followed by sub-littoral (248.4 Mg C ha^-1) and littoral layer (106.1 Mg C ha^-1). Overall, we show that substantial spatial variability in SOC stock exists among the eco-zones and depth that may be driven by water inundation in deep layer and fluctuating hydrological conditions at the edges of the lacustrine ecosystem. Our study demonstrates that inland freshwater wetland is a major sink of organic C and if disturbed it can act as a carbon dioxide source.

Deepened snow loosens temporal coupling between plant and microbial N utilization and induces ecosystem N losses

Seasonal differences in plant and microbial nitrogen (N) acquisition are believed to be a major mechanism that maximizes ecosystem N retention. There is also a concern that climate change may interrupt the delicate balance in N allocation between plants and microbes. Yet, convincing experimental evidence is still lacking. Using a ¹⁵ N tracer, we assessed how deepened snow affects the temporal coupling between plant and microbial N utilization in a temperate Mongolian grassland. We found that microbial ¹⁵ N recovery peaked in winter, accounting for 22% of the total ecosystem ¹⁵ N recovery, and then rapidly declined during the spring thaw. By stimulating N loss via N₂ O emission and leaching, deepened snow reduced the total ecosystem ¹⁵ N recovery by 42% during the spring thaw. As the growing season progresses, the ¹⁵ N released from microbial biomass was taken up by plants, and the competitive advantage for N shifted from microbes to plants. Plant ¹⁵ N recovery reached its peak in August, accounting for 17% of the total ecosystem ¹⁵ N recovery. The Granger causality test showed that the temporal dynamics of plant ¹⁵ N recovery can be predicted by microbial ¹⁵ N recovery under ambient snow but not under deepened snow. In addition, plant ¹⁵ N recovery in August was positively correlated with and best explained by microbial ¹⁵ N recovery in March. The lower microbial ¹⁵ N recovery under deepened snow in March reduced plant ¹⁵ N recovery by 73% in August. Together, our results provide direct evidence of seasonal differences in plant and microbial N utilization that are conducive to ecosystem N retention; however, deepened snow disrupted the temporal coupling between plant-microbial N use and turnover. These findings suggest that changes in snowfall patterns may significantly alter ecosystem N cycling and N-based greenhouse gas emissions under future climate change. We highlight the importance of better representing winter processes and their response to winter climate change in biogeochemical models when assessing N cycling under global change.

Evaluation of ecosystem carbon storage in major forest types of Eastern Himalaya: Implications for carbon sink management

Forest's ecosystem is changing at an alarming rate and anthropogenic alteration of forests to other land use is a major driver of carbon (C) emission and biodiversity loss. We estimated ecosystem-level C stock and factors affecting C stock in six major forest types; tropical wet evergreen forest, montane subtropical forest, temperate forest, bamboo forest, quercus forest, and jhum land of the eastern Himalayan region (India). We determined ecosystem structure, biodiversity, and plant and soil C stock by laying random plots in each forest site. The average C stock was estimated in the range of 79.0-373.4 Mg C ha^-1 and found significantly different among the forest types. Partitioning ecosystem C stocks in plant (24-55%), soils (43-75%), deadwood (1-4.8%) and litter (0.20-1.25%) components varied largely. Pearson correlation analysis shows a significant positive relation of basal area with species diversity, tree density, and ecosystem C stock. Linear mixed-effect model demonstrates the high influence of species density and soil moisture content on the ecosystem C stock. We recommend the inclusion of forest structural attributes and pedological characteristics while predicting synergies between C stock and future climatic conditions. Additionally, conversion of natural forests to jhum land should be minimized because they stored lesser ecosystem C stocks thus plays a minimum role in C accumulation and cycling. The study provides estimates of C stocks in major forests that can be useful in suggesting a path forward to partially fulfill India's commitments to REDD + policy.

Patterns and driving factors of biomass carbon and soil organic carbon stock in the Indian Himalayan region

Tree-based ecosystems are critical to climate change mitigation. The study analysed carbon (C) stock patterns and examined the importance of environmental variables in predicting carbon stock in biomass and soils of the Indian Himalayan Region (IHR). We conducted a synthesis of 100 studies reporting biomass carbon stock and 67 studies on soil organic carbon (SOC) stock from four land-uses: forests, plantation, agroforest, and herbaceous ecosystem from the IHR. Machine learning techniques were used to examine the importance of various environmental variables in predicting carbon stock in biomass and soils. Despite large variations in biomass C and SOC stock (mean ± SD) within the land-uses, natural forests have the highest biomass C stock (138.5 ± 87.3 Mg C ha^-1), and plantation forests exhibited the highest SOC stock (168.8 ± 74.4 Mg C ha^-1) in the top 1-m of soils. The relationship between the environmental variables (altitude, latitude, precipitation, and temperature) and carbon stock was not significantly correlated. The prediction of biomass carbon and SOC stock using different machine learning techniques (Adaboost, Bagging, Random Forest, and XGBoost) shows that the XGBoost model can predict the carbon stock for the IHR closely. Our study confirms that the carbon stock in the IHR vary on a large scale due to a diverse range of land-use and ecosystems within the region. Therefore, predicting the driver of carbon stock on a single environmental variable is impossible for the entire IHR. The IHR possesses a prominent carbon sink and biodiversity pool. Therefore, its protection is essential in fulfilling India's commitment to nationally determined contributions (NDC). Our data synthesis may also provide a baseline for the precise estimation of carbon stock, which will be vital for India's National Mission for Sustaining the Himalayan Ecosystem (NMSHE).

Plant biodiversity and carbon sequestration potential of the planted forest in Brahmaputra flood plains

Globally, while experts debated whether planted forests (PF) restore biodiversity or create biological deserts, their potential role in mitigating climate change is mostly overlooked. In this study, we investigated the long-term impact of PF on the species composition, plant diversity, biomass stock, and carbon (C) storage potential in the Brahmaputra flood plain of North-East India. The phytosociological study was conducted using a modified Gentry plot method and species-specific allometric models were used to estimate biomass stock in the 39-year old PF and equivalent age of natural forest (NF). We identified 57 trees, 22 shrubs, and 23 herb species in the PF, and 54 trees, 17 shrubs, and 8 herb species in the NF. Species richness and biodiversity indices showed greater values in PF whereas species dominance and evenness were higher in NF. After 39-year of plantation, total biomass C was estimated at 165 Mg C ha^-1 in PF and 197 Mg C ha^-1 in equivalent age of NF. Bombax ceiba, Dalbergia sissoo, Samanea saman, Tetrameles nodiflora, and Gmelina arborea were the dominant tree species that contribute 56% of the total biomass C in the PF. The ecosystem carbon pool (plant biomass + deadwood + litter + SOC) was 17% higher in NF and showed the greater potential of carbon dioxide sequestration (959 Mg CO₂ ha^-1) compared to the PF (818 Mg CO₂ ha^-1). Our study suggested PF in flood plain degraded lands can act as a major C sink and stored a substantial amount of carbon dioxide after 39-year of the plantation. It is concluded that PF can be a preferable ecosystem management tool to fulfill the objectives of biodiversity conservation and provisioning climate services like C sequestration.

Reclamation of coal mine spoil and its effect on Technosol quality and carbon sequestration: a case study from India

A field study was carried out to assess the impact of revegetation on Technosol quality in the post-mining sites (Central Coalfield Limited, India). The study evaluated community structure, biodiversity, Technosol quality, and carbon (C) dynamics in the post-mining ecosystem (PME). The multivariate statistical tool was used to identify the key soil properties, and soil quality was evaluated by using Technosol quality index (TQI). One unreclaimed site (0 years) and four chronosequences revegetated coal mine sites (3, 7, 10, and 15 years) were studied and compared with an undisturbed forest as a reference site. Plant biodiversity indices [Shannon index of diversity (2.42) and Pielou's evenness (0.97) and Patric richness (12)] were highest in 15-year-old revegetated sites. Soil physicochemical and biological properties were recovered with the revegetation age. Soil organic C (SOC) stock significantly increased from 0.75 Mg C ha^-1 in 3 years to 7.60 Mg C ha^-1 after 15 years of revegetation in top 15 cm of soils. Ecosystem C pool increased at a rate of 5.38 Mg C ha^-1 year^-1. Soil CO₂ flux was significantly increased from 0.27 μmol CO₂ m^-2 s^-1 in unreclaimed sites to 3.19 μmol CO₂ m^-2 s^-1 in 15-year-old revegetated site. Principal component analysis (PCA) showed that dehydrogenase activity (DHA), available nitrogen (N), and silt content were the key soil parameters that were affected by reclamation. A 15-year-old Technosol had a greater TQI (0.78) compared to the control forest soils (0.64) that indicated the suitability of revegetation to recuperate soil quality in mining-degraded land and to increase C sequestration potential.

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.