The impact of a pulsing groundwater table on greenhouse gas emissions in riparian grey alder stands

Functionality of methane cycling microbiome during methane flux hot moments from riparian buffer systems

Riparian buffer systems (RBS) are a common agroforestry practice that involves maintaining a forested boundary adjacent to water bodies to protect the aquatic ecosystems in agricultural landscapes. While RBS have potential for carbon sequestration, they also can be sources of methane emissions. Our study site at Washington Creek in Southern Ontario, includes a rehabilitated tree buffer (RH), a grassed buffer (GRB), an undisturbed deciduous forest (UNF), an undisturbed coniferous forest (CF), and an adjacent agricultural field (AGR). The objective of this study was to assess the diversity and activity of CH₄ cycling microbial communities in soils sampled during hot moments of methane fluxes (July 04 and August 15). We used qPCR and high-throughput amplicon sequencing from both DNA and cDNA to target methanogen and methanotroph communities. Methanogens, including the archaeal genera Methanosaeta, Methanosarcina, Methanomassiliicoccus, and Methanoreggula, were abundant in all RBSs, but they were significantly more active in UNF soils, where CH₄ emissions were highest. Methylocystis was the most prevalent taxon among methanotrophs in all the riparian sites, except for AGR soils where the methanotrophs community was composed primarily of members of rice paddy clusters (RPCs and RPC-1) and upland soil clusters (TUSC and USCα). The main factors influencing the composition and assembly of methane-cycling microbiomes were soil carbon and moisture content. We concluded that the differences in CH₄ fluxes observed between RBSs were primarily caused by differences in the presence and activity of methanogens, which were influenced by total soil carbon and water content. Overall, this study emphasizes the importance of understanding the microbial drivers of CH₄ fluxes in RBSs in order to maximize RBS environmental benefits.

Spatio-Temporal Variability of Methane Fluxes in Boreo-Nemoral Alder Swamp (European Russia)

In 1995–1998 and 2013–2016, we measured methane fluxes (1Q-median-3Q, mgC m−2 h−1) in the Petushikha black alder swamp of the boreo-nemoral zone of European Russia. At microelevations (EL sites), flat surfaces (FL), microdepressions (DEP), and water surfaces of streams and channels (STR) sites, the fluxes comprised 0.01–0.03–0.09, 0.02–0.06–0.19, 0.04–0.14–0.43, and 0.10–0.21–0.44, respectively. The biggest uncertainty of methane fluxes was caused by seasonal variability (the level of relative variability of fluxes is a nonparametric analogue of the coefficient of variation) which comprised 144%, then by spatial variability—105%, and the smallest by interannual variability—75%. Both spatial and temporal variability of methane fluxes at different elements of the microrelief is heterogeneous: the most variable are communities that are “unstable” in terms of hydrological conditions, such as FL and DEP, and the least variable are the most drained EL and the most moistened STR (“stable” in terms of hydrological conditions). The obtained data on the fluxes and their spatial and temporal variability are consistent with the literature data and can be used to optimize the process of planning studies of the methane budget of “sporadic methane sources”, such as waterlogged forests. This is especially relevant for an adequate assessment of the role of methane fluxes in the formation of the waterlogged forests carbon budget and a changing climate.

Soil Respiration in Alder Swamp (Alnus glutinosa) in Southern Taiga of European Russia Depending on Microrelief

Swamp forests have been insufficiently studied yet in comparison with thoroughly examined carbon pools and greenhouse gas fluxes of peat bogs. This is primarily since the GHGs in swamp forests have huge spatial (due to the developed microrelief) and temporal variations (due to strong fluctuations in the groundwater level (GWL)). This significantly complicates their study, producing ambiguous results, especially in short-term field research. From June to October 2013–2016, we measured soil respiration (Rsoil) in an alder swamp using the static chamber method at five microsites: depression (DEP), flat surface (FL), elevations (EL), tussocks (TUS), and near-stem tussocks (STUS). We carried out a computer simulation of the total Rsoil for the season based on Rsoil measurements, monitoring of GWL, and soil temperature. In 2013–2016, the average Rsoil values (mgC m−2 h−1 ± σ) on DEP, FL, EL, TUS and STUS comprised 54 ± 50, 94 ± 72, 146 ± 89, 193 ± 96, and 326 ± 183, respectively, whereas the total Rsoil values for the season (tC ha−1 season−1 ± σ) comprised 2.0 ± 0.5, 3.5 ± 0.5, 5.3 ± 1.6, 5.4 ± 2.7, and 12.6 ± 3.2. According to the results of observations, GWL was at the level of several cm below the soil surface for most of the season. In 2014 and 2015, there were extra dry periods that led to a drop in GWL to a mark of 30–40 cm below the soil surface. Despite their short duration (2–3 weeks), these dry periods can lead to an increase in the total Rsoil for the season from 9 to 45% in the TUS–EL–STUS–FL–DEP sequence.

Short-term flooding increases CH4 and N2O emissions from trees in a riparian forest soil-stem continuum

One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder (Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N₂O and CH₄ turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N₂O and CH₄ during the flooding. Contrary to N₂O, the increase in CH₄ fluxes delayed in response to flooding. Stem N₂O fluxes were lower although stem CH₄ emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N₂O fluxes. The stems contributed up to 88% of CH₄ emissions to the stem-soil continuum during the investigated period but soil N₂O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH₄ and N₂O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.

Plant Diversity and Agroecosystem Function in Riparian Agroforests: Providing Ecosystem Services and Land-Use Transition

Achieving biologically diverse agricultural systems requires a commitment to changes in land use. While in-field agrobiodiversity is a critical route to such a transition, riparian systems remain an important, yet understudied, pathway to achieve key diversity and ecosystem services and targets. Notably, at the interface of agricultural landscapes and aquatic systems, the diversification of riparian buffers with trees reduces the non-point source pollution in waterways. However, in riparian agroforestry systems, little is known about herbaceous community patterns and, importantly, the herbaceous community’s role in governing carbon (C) and nitrogen (N) cycling. Our study investigated herbaceous community taxonomic and phylogenetic diversity patterns in riparian (i) grasslands (GRASSLAND), (ii) rehabilitated agroforests (AGROFOREST-REHAB), and (iii) remnant forests (AGROFOREST-NATURAL). We then determined the biodiversity-ecosystem function relationships between community functional diversity metrics, C and N cycling, and greenhouse gas fluxes. We observed significant differences in taxonomic and phylogenetic diversity among riparian buffer types. We found that herbaceous plant communities in riparian agroforestry systems expressed plant trait syndromes associated with fast-growing, resource acquiring strategies, while grassland buffer plants exhibited slow-growing, resource conserving strategies. Herbaceous communities with high functional diversity and resource acquiring trait syndromes, such as those in the agroforestry riparian systems, were significantly correlated with lower rates of soil CO2 efflux and N mineralization, both of which are key fluxes related to ecosystem service delivery. Our findings provide further evidence that functionally diverse, and not necessarily taxonomically diverse, plant communities are strongly correlated to positive ecosystem processes in riparian agroforestry systems, and that these communities contribute to the transition of agricultural lands toward biologically and functionally diverse landscapes.

Effects of plant diversity on carbon dioxide emissions and carbon removal in laboratory-scale constructed wetland

Previous studies have shown that plant diversity can enhance methane (CH₄) emission and nitrogen purification efficiency in constructed wetlands (CWs), but effect of plant diversity on carbon dioxide (CO₂) flux and carbon removal efficiency in CWs is unknown. Therefore, we established four plant diversity levels (each level containing 4, 3, 2, and 1 species, respectively) in laboratory-scale wetland microcosms fed with simulated wastewater. Results showed that plant species richness enhanced CO₂ emissions (84.7-124.7 mg CO₂ m^-2 h^-1, P < 0.01), carbon fixation rate (P < 0.05), and microbial biomass carbon (P < 0.001), but did not improve carbon removal (P > 0.05). The presence of Pontederia cordata increased CO₂ emissions, carbon fixation rate of belowground, and microbial biomass carbon (P < 0.05), whereas the presence of Phragmites australis only enhanced CO₂ emission (P < 0.05). However, the presence of Typha orientalis or Lythrum salicaria did not show an influence on CO₂ emissions and carbon removal (P > 0.05). Hence, our study highlights the importance of plant diversity in mediating CO₂ emission intensity and carbon processes but not carbon removal in CWs.

Plant species diversity reduces N2O but not CH4 emissions from constructed wetlands under high nitrogen levels

Constructed wetlands (CWs) have been widely used for treating wastewater. CWs also are the sources of greenhouse gas (GHG) due to high pollutant load. It has been reported that plant species diversity can enhance nitrogen (N) removal efficiency in CWs for treating wastewater. However, the influence of plant species diversity on GHG emissions from CWs in habitats with high N levels still lack research. This study established four species richness levels (1, 2, 3, 4) and 15 species compositions by using 75 simulated vertical flow CWs microcosms to investigate the effects of plant species diversity on the GHG emissions and N removal efficiency of CWs with a high N level. Results showed plant species richness reduced nitrous oxide (N₂O) emission and N (NO₃^--N, NH₄⁺-N, and TIN) concentrations in wastewater, but had no effect on methane (CH₄) emission. Especially, among the 15 compositions of plant species, the four-species mixture emitted the lowest N₂O and had under-depletion of N (D_minTIN < 0). The presence of Oenanthe javanica had a significantly negative effect on the N₂O emission but had no effect on N removal efficiency. The presence of Rumex japonicus significantly reduced the N (NO₃^--N and TIN) concentrations in wastewater but had no effect on the N₂O and CH₄ emissions. The N concentrations and GHG emissions in the community of R. japonicus × Phalaris arundinacea were as low as those in the four-species mixture. Assembling plant communities with relatively high species richness (four-species mixture) or particular composition (R. japonicus × P. arundinacea) could enhance the N removal efficiency and reduce the GHG emissions from CWs for treating wastewater with a high N level.