The sediment characterisation of wetlands belonging to the Northeastern Region of India particularly regarding the assessment of sediment carbon stock is very scanty. The presently available literature on the wetlands cannot be employed as a common model for managing the wetlands of the Northeastern Region of India as wetlands are a sensitive ecosystem with a different origin or endogenous interventions. Thereby, this research was conducted on Deepor Beel for investigating the spatial and seasonal variation of sediment parameters, the relationship between the parameters and pollution status of the wetland. Results revealed that the study area is of an acidic nature with a sandy clay loam type texture. Organic carbon, total nitrogen and available nitrogen were higher in sediments in the monsoon period. The mean stock of the sediment carbon pool of Deepor Beel is estimated to be 2.5 ± 0.7 kg m−2. The average non-residual fraction percentage (63.2%) of Pb was higher than the residual fraction. Zn content ∼490 mg kg−1 exceeding its effect range medium (ERM) was determined to suggest frequent biological adverse effects. Highest metal enrichment factor (EF) values were shown by Zn and Pb, which ranged between 78 and 255. Risk assessment code (RAC) values of Pb between 21 and 29% indicated its high bio-accessibility risk. Pearson's coefficient matrix revealed a low degree of positive correlation between organic carbon content and metal concentration. Principal component analysis revealed that the first component comprising of EC, basic cations and metals accounted for 62.3% of variance while the second component (OM, OC, TN, AN, AP) and the third component (pH) accounted for 21.8% and 7.0% of the variance, respectively. The present study revealed the adverse impact of human inputs on the Deepor Beel quality status.

Nitrogen (N) deposition in China has increased greatly, but the general impact of elevated N deposition on carbon (C) dynamics in Chinese terrestrial ecosystems is not well documented. In this study we used a meta-analysis method to compile 88 studies on the effects of N deposition C cycling on Chinese terrestrial ecosystems. Our results showed that N addition did not change soil C pools but increased above-ground plant C pool. A large decrease in below-ground plant C pool was observed. Our result also showed that the impacts of N addition on ecosystem C dynamics depend on ecosystem type and rate of N addition. Overall, our findings suggest that 1) decreased below-ground plant C pool may limit long-term soil C sequestration; and 2) it is better to treat N-rich and N-limited ecosystems differently in modeling effects of N deposition on ecosystem C cycle.

Although urbanisation is a major cause of land-use change worldwide, towns and cities remain relatively understudied ecosystems. Research into urban ecosystem service provision is still an emerging field, yet evidence is accumulating rapidly to suggest that the biological carbon stores in cities are more substantial than previously assumed. However, as more vegetation carbon densities are derived, substantial variability between these estimates is becoming apparent. Here, we review procedural differences evident in the literature, which may be drivers of variation in carbon storage assessments. Additionally, we quantify the impact that some of these different approaches may have when extrapolating carbon figures derived from surveys up to a city-wide scale. To understand how/why carbon stocks vary within and between cities, researchers need to use more uniform methods to estimate stores and relate this quantitatively to standardised ‘urbanisation’ metrics, in order to facilitate comparisons.

This study compares the effectiveness of two different thickness of green roof substrate with respect to nutrient and heavy metal retention and release. To understand and evaluate the long term behaviour of green roofs, substrate columns with the same structure and composition as the green roofs, were exposed in laboratory to artificial rain. The roofs act as a sink for C, N, P, zinc and copper for small rain events if the previous period was principally dry. Otherwise the roofs may behave as a source of pollutants, principally for carbon and phosphorus. Both field and column studies showed an important retention for Zn and Cu. The column showed, however, lower SS, DOC and metal concentrations in the percolate than could be observed in the field even if corrected for run-off. This is most probably due to the difference in exposition history and weathering processes.

Polycyclic aromatic hydrocarbons (PAHs) are formed by the incomplete combustion of fossil fuels and forest or biomass burning. PAHs undergo long-range atmospheric transport, as evidenced by in situ observations across the Arctic. However, monitored atmospheric concentrations of PAHs indicate that ambient PAH levels in the Arctic do not follow the declining trend of worldwide anthropogenic PAH emissions since the 2000s, suggesting missing sources of PAHs in the Arctic or other places across the Northern Hemisphere. To trace origins and causes for the increasing trend of PAHs in the Arctic, the present study reconstructed PAH emissions from forest fires in the northern boreal forest derived by combining forest carbon stocks and MODIS burned area. We examined the statistical relationships of forest biomass, MODIS burned area, emission factors, and combustion efficiency with different PAH congeners. These relationships were then employed to construct PAH emission inventories from forest biomass burning. We show that for some PAH congeners, for example, benzo[a]pyrene (BaP)—the forest-fire-induced air emissions are almost one order of magnitude higher than previous emission inventories in the Arctic. A global-scale atmospheric chemistry model, GEOS-Chem, was used to simulate air concentrations of BaP, a representative PAH congener primarily emitted from biomass burning, and to quantify the response of BaP to wildfires in the northern boreal forest. The results showed that BaP emissions from wildfires across the northern boreal forest region played a significant role in the contamination and interannual fluctuations of BaP in Arctic air. A source-tagging technique was applied in tracking the origins of BaP pollution from different northern boreal forest regions. We also show that the response of BaP pollution at different Arctic monitoring sites depends on the intensity of human activities.

Soil acidification has been expanding in many areas of Asia due to increasing reactive nitrogen (N) inputs and industrial activities. While the detrimental effects of acidification on forests have been extensively studied, less attention has been paid to grasslands, particularly alpine grasslands. In a soil pH manipulation experiment in the Qinghai-Tibet Plateau, we examined the effects of soil acidification on plant roots, which account for the major part of alpine plants.After three years of manipulation, soil pH decreased from 6.0 to 4.7 with the acid-addition gradient, accompanied by significant changes in the availability of soil nitrogen, phosphorus and cations. Plant composition shifted with the soil acidity, with graminoids replacing forbs. Differing from findings in forests, soil acidification in the alpine grassland increased root biomass by increasing the fraction of coarse roots and the production of fine roots, corresponding to enhanced sedge and grass biomass, respectively. In addition, litter decomposability decreased with altered root morphological and chemical traits, and soil acidification slowed root decomposition by reducing soil microbial activity and litter quality.Our results showed that acidification effect on root dynamics in our alpine grassland was significantly different from that in forests, and supported similar results obtained in limited studies in other grassland ecosystems. These results suggest an important role of root morphology in mediating root dynamics, and imply that soil acidification may lead to transient increase in soil carbon stock as root standing biomass and undecomposed root litter. These changes may reduce nutrient cycling and further constrain ecosystem productivity in nutrient-limiting alpine systems.

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

A conversion of the global terrestrial carbon sink to a source is critically dependent on the microbially mediated decomposition of soil organic matter (SOM). We have developed a detailed, process-based, mechanistic model for simulating SOM decomposition and its associated processes, based on Microbial Kinetics and Thermodynamics, called the MKT model. We formulated the sequential oxidation-reduction potential (ORP) and chemical reactions undergoing at the soil-water zone using dual Michaelis-Menten kinetics. Soil environmental variables, as required in the MKT model, are simulated using one of the most widely used watershed-scale models - the soil water assessment tool (SWAT). The MKT model was calibrated and validated using field-scale data of soil temperature, soil moisture, and N₂O emissions from three locations in the province of Saskatchewan, Canada. The model evaluation statistics show good performance of the MKT model for daily soil N₂O simulations. The results show that the proposed MKT model can perform better than the more widely used process-based and SWAT-based models for soil N₂O simulations. This is because the multiple processes of microbial activities and environmental constraints, which govern the availability of substrates to enzymes were explicitly represented. Most importantly, the MKT model represents a step forward from conceptual carbon pools at varying rates.

Nitrogen (N) deposition and biological N fixation (BNF) are main external N inputs into terrestrial ecosystems. However, few studies have simultaneously quantified the contribution of these two external N inputs to global NPP and consequent C sequestration. Based on literature analysis, we estimated new net primary production (NPP) due to external N inputs from BNF and N deposition and the consequent C sinks in global boreal, temperate and tropical forest biomes via a stoichiometric scaling approach. Nitrogen-induced new NPP is estimated to be 3.48 Pg C yr⁻¹ in global established forests and contributes to a C sink of 1.83 Pg C yr⁻¹. More specifically, the aboveground and belowground new NPP are estimated to be 2.36 and 1.12 Pg C yr⁻¹, while the external N-induced C sinks in wood and soil are estimated to be 1.51 and 0.32 Pg C yr⁻¹, respectively. BNF contributes to a major proportion of N-induced new NPP (3.07 Pg C yr⁻¹) in global forest, and accounts for a C sink of 1.58 Pg C yr⁻¹. Compared with BNF, N deposition only makes a minor contribution to new NPP (0.41 Pg C yr⁻¹) and C sinks (0.25 Pg C yr⁻¹) in global forests. At the biome scale, rates of N-induced new NPP and C sink show an increase from boreal forest towards tropical forest, as mainly driven by an increase of BNF. In contrast, N deposition leads to a larger C sink in temperate forest (0.11 Pg C yr⁻¹) than boreal (0.06 Pg C yr⁻¹) and tropical forest (0.08 Pg C yr⁻¹). Our estimate of total C sink due to N-induced new NPP approximately matches an independent assessment of total C sink in global established forests, suggesting that external N inputs by BNF and atmospheric deposition are key drivers of C sinks in global forests.

Agriculture-based climate change mitigation may occur through enhancing the carbon sink or through reducing greenhouse gases (GHGs) emissions from agricultural residue treatment, as open burning of agricultural residues produces millions of tons of GHGs and air pollutants annually worldwide. Charring slashed biomass, termed as slash-and-char, has been considered as a promising alternative to open burning in dealing with agricultural residues such as rice straw. Previous studies, however, focused on relatively sophisticated slash-and-char systems, which could not be practiced easily by smallholder farmers in developing countries. Here we introduce a simple slash-and-char system to mitigate the environmental problems associated with open burning of rice straw. This system could convert 30.7% of the initial carbon in rice straw into biochar, much higher than that retained in the ash generated by open burning (3.95%). It could also cut GHGs, particulate matters and polycyclic aromatic hydrocarbons (PAHs) emissions by 26.9%, 99.0% and 99.4%, respectively. If open burning of rice straw was replaced by the slash-and-char, the annual emissions of GHGs, particulate matters and PAHs in China would decrease by at least 15.4 Tg, 1.51 Tg and 1.27 Gg, correspondingly. This decrease is nearly twice the size of China's estimated forest C sink (8.81 Tg).