The content, composition, and influencing factors of organic carbon in the sediments of two types of constructed wetlands

Greenhouse gases emissions and carbon budget estimation in horizontal subsurface flow constructed wetlands with different plant species

Constructed wetlands (CWs) are pivotal for wastewater treatment due to their high efficiency and numerous advantages. The impact of plant species and diversity on greenhouse gas (GHG) emissions from CWs requires a more comprehensive evaluation. Moreover, controversial perspectives persist about whether CWs function as carbon sinks or sources. In this study, horizontal subsurface flow (HSSF) CWs vegetated with Cyperus alternifolius, Typhae latifolia, Acorus calamus, and the mixture of these three species were constructed to evaluate pollutant removal efficiencies and GHG emissions, and estimate carbon budgets. Polyculture CWs can stably remove COD (86.79 %), NH4+-N (97.41 %), NO3--N (98.55 %), and TP (98.48 %). They also mitigated global warming potential (GWP) by suppressing N2O emissions compared with monoculture CWs. The highest abundance of the Pseudogulbenkiania genus, crucial for denitrification, was observed in polyculture CWs, indicating that denitrification dominated in nitrogen removal. While the highest nosZ copy numbers were observed in CWs vegetated with Cyperus alternifolius, suggesting its facilitation of denitrification-related microbes. Selecting Cyperus alternifolius to increase species diversity is proposed for simultaneously maintaining the water purification capacity and reducing GHG emissions. Carbon budget estimations revealed that all four types of HSSF CWs were carbon sinks after six months of operation, with carbon accumulation capacity of 4.90 ± 1.50 (Cyperus alternifolius), 3.31 ± 2.01 (Typhae latifola), 1.78 ± 1.30 (Acorus calamus), and 2.12 ± 0.88 (polyculture) kg C/m2/yr. This study implies that under these operation conditions, CWs function as carbon sinks rather than sources, aligning with carbon peak and neutrality objectives and presenting significant potential for carbon reduction efforts.

Variations of sediment organic phosphorus and organic carbon during the outbreak and decline of algal blooms in Lake Taihu, China

In this study, sediment organic phosphorus (OP) and organic carbon (OC) in Lake Taihu, China, as well as their relationships, were analyzed during the outbreak and decline of algal blooms (ABs) over a five-month field study. The results showed synchronous temporal changes in the sediment OP and OC contents with the development of ABs. In addition, there was a significant positive correlation between the sediment OP and OC (p < 0.01), suggesting simultaneous deposition and consumption during the ABs outbreak. The sediment OP and OC contents decreased significantly at the early and last stages of the ABs outbreak and increased at the peak of the ABs outbreak and during the ABs decline. These temporal variation patterns suggest that the sediment OC and OP contents did not consistently increase during the ABs outbreak, even though algae are an important source of organic matter in sediments. The depletion or enrichment of OC and OP in sediments may also depend on the scale of the ABs outbreak. The obtained results revealed significant differences in the sediment OC and OP contents between the months (p < 0.05). In addition, OP in the sediments was dominated by orthophosphate diester (phospholipids and DNA-P) and orthophosphate monoester during the ABs outbreak and decline, respectively. The active OC contents and proportions in the sediments in the ABs outbreak were significantly lower than those observed in the ABs decline period, demonstrating the significant impacts of the ABs outbreak and decline on the sediment OC and OP in Lake Taihu.

Effect of salinity on carbon sequestration in constructed wetlands and its functional mechanisms

Currently, many constructed wetlands (CWs) are facing the threat of salinization, but its effect on the carbon sequestration function of CWs is still unclear. In this study, three CWs with different salinities (i.e., control: C-CW; low salinity: LS-CW; high salinity: HS-CW) were conducted. Increased salinity significantly reduced the carbon sequestration in CWs. The highest carbon sequestration was observed in C-CW (5.1 ± 0.2 kg C·m-2·y-1), and the carbon sequestration capacity of plants was identified as the major influencing factor. The substrate carbon pool decreased with salinity since it altered plant carbon inputs, enzyme activities, and microbial community structure. However, the decrement in the carbon pool management index with salinity indicated that salinity could enhance carbon pool stability and subsequently reduce carbon emissions of CWs. These findings improve the understanding in relationships between salinity and carbon sequestration in CWs and provide theoretical support for the proper management of CWs.

Effects of Long-Term (17 Years) Nitrogen Input on Soil Bacterial Community in Sanjiang Plain: The Largest Marsh Wetland in China

Increased nitrogen (N) input from natural factors and human activities may negatively impact the health of marsh wetlands. However, the understanding of how exogenous N affects the ecosystem remains limited. We selected the soil bacterial community as the index of ecosystem health and performed a long-term N input experiment, including four N levels of 0, 6, 12, and 24 gN·m-2·a-1 (denoted as CK, C1, C2, and C3, respectively). The results showed that a high-level N (24 gN·m-2·a-1) input could significantly reduce the Chao index and ACE index for the bacterial community and inhibit some dominant microorganisms. The RDA results indicated that TN and NH4+ were the critical factors influencing the soil microbial community under the long-term N input. Moreover, the long-term N input was found to significantly reduce the abundance of Azospirillum and Desulfovibrio, which were typical N-fixing microorganisms. Conversely, the long-term N input was found to significantly increase the abundance of Nitrosospira and Clostridium_sensu_stricto_1, which were typical nitrifying and denitrifying microorganisms. Increased soil N content has been suggested to inhibit the N fixation function of the wetland and exert a positive effect on the processes of nitrification and denitrification in the wetland ecosystem. Our research can be used to improve strategies to protect wetland health.

Effects of plants and soil microorganisms on organic carbon and the relationship between carbon and nitrogen in constructed wetlands

Constructed wetland is an ideal place for studying the effects of plants and microorganisms on the nutrient cycling and carbon-nitrogen coupling in wetland for their clear background. This study examined both bare plots and others with plants (Phragmites australis or Typha angustifolia) in constructed wetlands and vegetation and soil samples were collected to investigate the effects of plants and soil microorganisms on carbon and nitrogen content. Results showed that the soil organic carbon content was high in plots with high plant biomass, and the increase of soil organic carbon driven by plant biomass was mainly from light fraction organic carbon (LFOC). Correlation analysis and redundancy analysis (RDA) suggested that plants play an important role in the cycle of carbon and nitrogen elements in constructed wetland soils, and that plant nitrogen components were key factors influencing wetland soil carbon and nitrogen. In addition, this study found that most of the main microbial taxa were significantly correlated with dissolved organic carbon (DOC), ammonium nitrogen (NH₄⁺), and nitrate and nitrite nitrogen (NO_x^-) indicating that microorganisms might play an important role in regulating soil element cycles in constructed wetlands by affecting the metabolism of activated carbon and reactive nitrogen. This study has implications for increasing the carbon sink of constructed wetlands to mitigate the effects of global warming.

Habitats generated by the restoration of coal mining subsidence land differentially alter the content and composition of soil organic carbon

The content and composition of soil organic carbon (SOC) can characterize soil carbon storage capacity, which varies significantly between habitats. Ecological restoration in coal mining subsidence land forms a variety of habitats, which are ideal to study the effects of habitats on SOC storage capacity. Based on the analysis of the content and composition of SOC in three habitats (farmland, wetland and lakeside grassland) generated by different restoration time of the farmland which was destroyed by coal mining subsidence, we found that farmland had the highest SOC storage capacity among the three habitats. Both dissolved organic carbon (DOC) and heavy fraction organic carbon (HFOC) exhibited higher concentrations in the farmland (20.29 mg/kg, 6.96 mg/g) than in the wetland (19.62 mg/kg, 2.47 mg/g) or lakeside grassland (5.68 mg/kg, 2.31 mg/g), and the concentrations increased significantly over time, owing to the higher content of nitrogen in the farmland. The wetland and lakeside grassland needed more time than the farmland to recover the SOC storage capacity. The findings illustrate that the SOC storage capacity of farmland destroyed by coal mining subsidence could be restored through ecological restoration and indicate that the recovery rate depends on the reconstructed habitat types, among which farmland shows great advantages mainly due to the nitrogen addition.

Compositions of sequestrated soil carbon in constructed wetlands of Taiwan

Constructed wetlands are an ecological engineering technology that has been widely applied to treat anthropogenic wastewater. Until now, few studies have focused on soil carbon (C) in the constructed treatment wetlands in tropical regions. Therefore, this study provides insight into the changes in soil C composition of tropically constructed wetlands at different ages. Five constructed wetlands were investigated in northern Kaohsiung, Taiwan. Soil C was analyzed at three different depths using an acid-hydrolysable method. The results showed that soil TOC content was highest on the soil surface (0-2 cm) and decreased at greater soil depths (2-5 and 5-10 cm) in all the studied constructed wetlands. There was more soil acid-hydrolysable C in the older constructed wetlands than in the younger ones at all depths. On the contrary, the soil recalcitrant carbon (RP-C) did not vary much across the wetland soils. In addition, the RP-C to TOC ratios were higher in the younger than older constructed wetlands, implying that the soil bioavailable C sources for microbial growth increased with the wetland's age. As a result, the compositions of organotrophic microbes, such as methanogens (mcrA copies), appeared to increase with wetlands' ages (i.e., negatively correlated with RP-C/TOC), while the total microbial abundance (16S rDNA) and abundance of lithotrophic microbes, such as methanotrophs (pmoA copies), were not correlated with RP-C/TOC or AHPI-C/TOC ratios, based on the results of our canonical correspondence analysis. Furthermore, the constructed wetlands accumulated soil RP-C from 2.33 to 0.08 g C m^-2 day^-1 in the constructed wetlands 1 to 30 years old, respectively.