Are there memory effects on greenhouse gas emissions (CO2, N2O and CH4) following grassland restoration?

Chapter 27 - Elevated CO2 and Warming Effects on Soil Carbon Sequestration and Greenhouse Gas Exchange in Agroecosystems: A Review

However, due to slow biogeochemical processes, the carbon accumulation in peatlands typically occurs at a low rate. In addition, peatlands that have been degraded or drained from land-use conversion become net carbon sources, as is the case with 82 % of the peatlands in Switzerland.Restoring drained peatlands is therefore widely recognised as a nature-based solution for carbon removal. With the development of carbon market, carbon credits from rewetting peatlands are often issued based on generalised estimates on avoided emissions. However, information on long-term greenhouse gas (GHG) exchanges in restored peatlands remains scarce, with limited spatiotemporal coverage and inconsistent outcomes likely stemming from varying climate conditions, initial states, vegetation types, and restoration approaches. Consequently, there is an urgent need to account for site-specific carbon dynamics and improve the methodologies for assessing the GHG balance when evaluating carbon credit schemes for peatland restoration.In this study, we use simulations of the well-established Terrestrial Biosphere Model, LPX-Bern, to investigate peatland carbon dynamics in Switzerland. Combined with empirical estimates based on field measurements, the modelled peatland carbon storage from the preindustrial period to the present day is analysed under natural processes, land use, and different restoration scenarios to assess the peatland restoration potentials. The results are compared with a carbon credit scheme using max.moor, a conservative estimate on avoided emissions following rewetting according to a generalised carbon density (56 kg C m-2) for a peatland in Niremont, Switzerland.We demonstrate that, over a 50-year timeframe, the dynamically simulated peatland carbon storage indicates a substantial overestimation, up to 43 %, of avoided emissions by the max.moor approach for the Niremont site. This highlights the necessity of incorporating approaches with increased accuracy on estimating peatland restoration impacts on carbon dynamics and GHG exchanges. Moreover, future climate change is expected to exacerbate the uncertainties associated with these estimates. This work thus contributes to advancing the understanding of peatland restoration impacts on GHG exchanges and feedback to climate change.

Energy, water, and greenhouse gas exchange in the permafrost zone play an important role in the regional and global climate system at multiple temporal and spatial scales. High-latitude warming in recent years has substantially altered ecosystem function, including biosphere–atmosphere interaction, which may amplify or dampen future high-latitude warming through a variety of feedback processes. In this chapter, we have reviewed the current state of energy, water, CO2, and CH4 exchange at the northern high-latitude permafrost zone, with synthesizing observed micrometeorological fluxes. Tundra has a higher summer and winter albedo with a longer snow period than boreal forests, resulting in that tundra less transfers sensible and latent energy to the atmosphere. Growing season length determines the spatial variability of the annual gross primary productivity and net growing season CO2 sink. In contrast, interannual variabilities of the annual CO2 budget at boreal forests are determined by ecosystem respiration, indicating an importance of ecosystem respiration in the boreal forests. The CO2 fertilization effect could be an important determinant of the long-term greenhouse gas budget at sites with a near neutral CO2 budget. In terms of annual greenhouse gas budget, CH4 emission is more important than CO2 budget for both boreal forest and Arctic wet tundra. Based on this synthesis, finally, we discuss future possible directions of study to reach a better understanding of changing high-latitude ecosystems, by synthesizing tower flux measurements in Alaska and Siberia and combining these in situ measurements with remote sensing data.

Seagrass meadows are important blue carbon habitats, due to their high productivity and large sedimentary carbon stocks. However, this can be biased when data from larger species, such as Posidonia oceanica, is extrapolated to global coverage. Carbon budgets of smaller seagrass species, such as Zostera noltei, have been seldom studied, despite these being one of the most common species found on North-West European coasts. In seagrass blue carbon studies, the inclusion of greenhouse gas (GHG) emissions is rare. This study addresses key knowledge gaps by analysing GHG (CO2 and CH4) exchange of the intertidal seagrass Z. noltei, in the southern North Sea, UK, across a full seasonal cycle. GHG exchange of Z. noltei meadows and adjacent non-vegetated mudflats (NVMF) were measured using novel in-situ closed-chambers, and the sedimentary microbial communities (notably, methanotrophs and methanogens) were characterised. Overall, CO2 fluxes of Z. noltei seagrass were not different to that of NVMFs. Seagrass respiration offsets a large proportion of the plant’s carbon sink capacity, and this was particularly noticeable in spring. When methane emissions were included in the carbon budget, both habitats have net zero carbon emissions. The inclusion of respiration and methane emissions, over multiple seasons, are highly important considerations that are often missed in GHG exchange studies, potentially causing overestimations of seagrass blue carbon, globally. This presentation will also discuss seasonal changes in carbon-cycling microbial communities and how they underpin the measured GHG flux. This pioneering research is of international importance with implications for blue carbon science and natural capital markets.

Carbon and Water Fluxes in an Exotic Buffelgrass Savanna

<p>Forests are often considered to be able to play a significant role in tackling global warming. To fully understand their potential in mitigating climate change and to develop more accurate ecosystem GHG flux budgets and process-based models of forests, we require more knowledge of methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) exchange in forests, their underlying processes, environmental controls and responses to forest management. In recent years, it is becoming evident that not only soils but also the trees themselves may significantly contribute to CH<sub>4</sub> and N<sub>2</sub>O fluxes in forest ecosystems.</p><p>Our research mainly focussed on greenhouse gas (GHG) exchange in temperate UK forests on both mineral and organic soils. We will primarily concentrate on CH<sub>4</sub> fluxes as N<sub>2</sub>O fluxes were often relatively low in these forests and, by including CO<sub>2</sub> fluxes, we will put them into the context of the overall ecosystem GHG exchange. A range of flux methods at different scales were used in our field studies to be able to capture the often high temporal and spatial variability of the GHG exchange between the atmosphere and either soils, tree stems or entire trees aboveground, and to identify potential drivers of the fluxes. The impact of management practices including clear fell, drainage and the resulting micro-topography, and forest-to-bog restoration on CH<sub>4</sub> fluxes from organic soils following the first forest rotation will also be described. We regularly used novel automated and chamber approaches and technologies, and the advantages and limitations of the different flux approaches and their use to upscale fluxes to the landscape scale will be evaluated.</p>

The application of nitrogen fertilizers to Douglas fir forests is known to raise net ecosystem productivity (NEP), but also N2O emissions, the CO2 equivalent of which may offset gains in NEP when accounting for net greenhouse gas (GHG) exchange. However, total changes in NEP and N2O emissions caused by fertilizer between times of application and harvest, while needed for national GHG inventories, are difficult to quantify except through modeling. In this study, integrated hypotheses for soil and plant N processes within the ecosystem model ecosys were tested against changes in CO2 and N2O fluxes recorded with eddy covariance (EC) and surface flux chambers for 1 year after applying 20 g N m−2 of urea to a mature Douglas fir stand in British Columbia. Parameters from annual regressions of hourly modeled versus measured CO2 fluxes conducted before and after fertilization were unchanged (b = 1.0, R2 = 0.8, RMSD = 3.4 μmol m−2 s−1), indicating that model hypotheses for soil and plant N processes did not introduce bias into CO2 fluxes modeled after fertilization. These model hypotheses were then used to project changes in NEP and GHG exchange attributed to the fertilizer during the following 10 years until likely harvest of the Douglas fir stand. Increased CO2 uptake caused modeled and EC‐derived annual NEP to rise from 443 and 386 g C m−2 in the year before fertilization to 591 and 547 g C m−2 in the year after. These gains contributed to a sustained rise in modeled wood C production with fertilization, which was partly offset by a decline in soil C attributed in the model to reduced root C productivity and litterfall. Gains in net CO2 uptake were further offset in the model by a rise of 0.74 g N m−2 yr−1 in N2O emissions during the first year after fertilization, which was consistent with one of 1.05 g N m−2 yr−1 estimated from surface flux chamber measurements. Further N2O emissions were neither modeled nor measured after the first year. At the end of the 11 year model projection, a total C sequestration of 1045 g C m−2 was attributed to the 20 g N m−2 of fertilizer. However, only 119 g C m−2 of this was sequestered in stocks that would remain on site after harvest (foliage, root, litter, soil). The remainder was sequestered as harvested wood, the duration of which would depend on use of the wood product. The direct and indirect CO2‐equivalent costs of this application, including N2O emission, were estimated to offset almost all non‐harvested C sequestration attributed to the fertilizer.

<p>Riparian forest ecosystems have been considered to be a natural source of nitrous oxide (N<sub>2</sub>O) and a natural sink of methane (CH<sub>4</sub>), both of which are important greenhouse gases (GHG) originating from microbiological processes. Wetland trees may also contribute to the GHG exchange by the release of both gases to the atmosphere or uptake therefrom. Recent studies have investigated the role of tree stems, underlining their importance in understanding forest GHG dynamics, focussing on various tree species, soil conditions or seasonal dynamics. However, knowledge about the short-termed day and night-time distributed GHG exchange of tree stems with the atmosphere is still scarce. We studied stem fluxes in a riparian forest ecosystem aiming to investigate the diurnal pattern and predict the potential influence of solar radiation.</p><p>The diurnal flux measurements were performed at 40-year-old grey alder (Alnus incana) forest stand in Estonia with 12-hour interval during July-September 2017 and May-September 2018 (n=16). The exchange of N<sub>2</sub>O and CH<sub>4</sub> was measured from 12 trees at profile height up to 5 m (0.1, 0.8, 1.7, 2.5, 5.0 m) using non-steady state stem chamber systems and gas chromatography. Simultaneously, soil fluxes were automatically quantified using a dynamic chamber system (Picarro 2508); piezometers, automatic groundwater level wells, soil temperature and moisture sensors were installed to determine coherent soil conditions.</p><p>Our preliminary results showed N<sub>2</sub>O and CH<sub>4</sub> emissions from alder tree stems during daytime (4.91 ± 0.15 µg m<sup>-2</sup> h<sup>-1</sup> and 66.38 ± 16.02 µg m<sup>-2</sup> h<sup>-1</sup>, mean ± s.e.) and lower during nighttime (3.65 ± 0.22 µg m<sup>-2</sup> h<sup>-1</sup> and 51.49 ± 13.83 µg m<sup>-2</sup> h<sup>-1</sup>, mean ± s.e.) at 0.1 m stem height, revealing a likely link to solar-driven physiological tree activity. Further, with increasing stem height, the relation of night to daytime fluxes diminished. However, the day-wise variation, including a minor GHG uptake indicates a fast response to changing micro-spatial environmental conditions like water regime in the soil and temperature.</p><p>Our study demonstrates the GHG exchange between tree stems and atmosphere occurs both in day- and night-time, showing slightly higher values in day-time, probably due to the trees’ physiological activities. Furthermore, our findings provide the potential to predict reaction kinetics in future modelling of flux pathways in forest ecosystems.</p><p>Acknowledgement</p><p>This research was supported by the Ministry of Education and Science of Estonia (SF0180127s08 grant), the Estonian Research Council (IUT2-16, PRG-352, and MOBERC20), the Czech Science Foundation (17-18112Y), the Czech National Sustainability Program I (LO1415), and the EcolChange Centre of Excellence, Estonia.</p>

Climate change and atmospheric nitrogen (N) deposition are two of the most important global change drivers. However, the interactions of these drivers have not been well studied. We aimed to assess how the combined effect of soil N additions and more frequent soil drying-rewetting events affects carbon (C) and N cycling, soil:atmosphere greenhouse gas (GHG) exchange, and functional microbial diversity. We manipulated the frequency of soil drying-rewetting events in soils from ambient and N-treated plots in a temperate forest and calculated the Orwin & Wardle Resistance index to compare the response of the different treatments. Increases in drying-rewetting cycles led to reductions in soil NO3- levels, potential net nitrification rate, and soil:atmosphere GHG exchange, and increases in NH4+ and total soil inorganic N levels. N-treated soils were more resistant to changes in the frequency of drying-rewetting cycles, and this resistance was stronger for C- than for N-related variables. Both the long-term N addition and the drying-rewetting treatment altered the functionality of the soil microbial population and its functional diversity. Our results suggest that increasing the frequency of drying-rewetting cycles can affect the ability of soil to cycle C and N and soil:atmosphere GHG exchange and that the response to this increase is modulated by soil N enrichment.

Influence of Soil Organic Carbon on Greenhouse Gas Emission Potential After Application of Biogas Residues or Cattle Slurry: Results from a Pot Experiment

<p>Organic soil amendments are proposed to mitigate climate change and support soil fertility by introducing recalcitrant carbon into soil. However, the full impact of recycled biosolids on soil greenhouse gas (GHG) dynamics are still unknown. We conducted a laboratory incubation to assess the climatic effects of two biochars (willow and spruce) and two ligneous biosolids on GHG emissions in controlled moisture conditions. The soil used in the incubation was collected from a soil-amendment field experiment on a clay cropland in South-West of Finland. The soil was sieved, air-dried and then individual samples were adjusted to 25%, 50%, 80% and 120% of water filled pore space (WFPS) before being incubated for 32 days in laboratory conditions. Soil GHG fluxes were measured after 1, 5, 12, 20 and 33 days.  </p><p>The application of 20–40 Mg ha<sup>-1</sup> of ligneous amendments, two years prior to our experiment, had increased soil pH, soil organic carbon content and plant available water content. The carbon dioxide (CO<sub>2</sub>) fluxes were unaffected by the amendment treatments and correlated mainly with soil moisture and microbial biomass. Nitrous oxide (N<sub>2</sub>O) emissions were reduced by all amendments compared to the un-amended control. Methane (CH<sub>4</sub>) exchange consisted mostly of slight uptake by the soil but played only a minor role in the total GHG budget overall. </p><p>The sum of CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub> emissions, calculated as CO<sub>2</sub>-equivalents, exhibited a strong linear relationship with soil moisture. Where the GHG budget was dominated by CO<sub>2</sub>, it was accompanied by significant N<sub>2</sub>O emissions at 120% WFPS. The results indicate that soil moisture critically affects the GHG emissions and that while organic soil amendments may have persisting effects on GHG exchange, they primarily occur in water-saturated conditions through N<sub>2</sub>O dynamics.</p>

Water management reduces greenhouse gas emissions in a Mediterranean rice paddy field

Climate change (extreme rainfall) and water management activities have led to variation in hydrological regimes, especially inundation, which may alter the function and structure of wetlands as well as wetland-atmosphere carbon (C) exchange. However, the degree to which different inundation depths (standing water depth above the soil surface) affect ecosystem CH4 fluxes, ecosystem respiration (Reco) and net ecosystem CO2 exchange (NEE) remains uncertain in wetland ecosystems. We conducted a field inundation depth manipulation experiment (no inundation, i.e. only natural precipitation; 0, water-saturated; 5, 10, 20, 30 and 40 cm inundation depth) in a freshwater wetland of the Yellow River Delta, China. The CH4 fluxes, Reco and NEE were measured with a static chamber technique during the growing seasons (May–October) of 2018 and 2019. Inundation depth significantly increased plant shoot density, above-water level leaf area index (WLAI), above-water level plant shoot height (WHeight), aboveground and belowground biomass of the dominant grass Phragmites australis in both years. Meanwhile, inundation depth increased the CH4 fluxes, Reco (except for 0 cm) and NEE compared to no inundation, which could be attributed partly to the increased plant productivity (shoot density, WLAI, WHeight, biomass). Additionally, the CH4 fluxes, Reco or NEE exhibited parabolic responses to inundation depth. Furthermore, global warming potential (GWP) was significantly decreased under different inundation depths during the growing season, especially from 5 to 40 cm inundation depth in 2019. NEE was the largest contributor to the seasonal GWP, which indicates that the inundated wetlands are a net sink of C and have a cooling climate effect in the Yellow River Delta. Inundation depth substantially affects the magnitude of CH4 fluxes, Reco and NEE, which were correlated with altered plant traits in wetland ecosystems. Inundation depth could mitigate greenhouse gas emissions in the P. australis wetlands during the growing season. Inundation depth-induced ecosystem C exchange should be considered when estimating C sequestration capacity of wetlands due to climate change and water management activities, which will assist to accurately predict the impact of hydrological regimes on C cycles in future climate change scenarios.

Abstract. Wetlands can either be net sinks or net sources of greenhouse gases (GHGs), depending on the mean annual water level and other factors like average annual temperature, vegetation development, and land use. Whereas drained and agriculturally used peatlands tend to be carbon dioxide (CO2) and nitrous oxide (N2O) sources but methane (CH4) sinks, restored (i.e. rewetted) peatlands rather incorporate CO2, tend to be N2O neutral and release CH4. One of the aims of peatland restoration is to decrease their global warming potential (GWP) by reducing GHG emissions. We estimated the greenhouse gas exchange of a peat bog restoration sequence over a period of 2 yr (1 July 2007–30 June 2009) in an Atlantic raised bog in northwest Germany. We set up three study sites representing different land use intensities: intensive grassland (deeply drained, mineral fertilizer, cattle manure and 4–5 cuts per year); extensive grassland (rewetted, no fertilizer or manure, up to 1 cutting per year); near-natural peat bog (almost no anthropogenic influence). Daily and annual greenhouse gas exchange was estimated based on closed-chamber measurements. CH4 and N2O fluxes were recorded bi-weekly, and net ecosystem exchange (NEE) measurements were carried out every 3–4 weeks. Annual sums of CH4 and N2O fluxes were estimated by linear interpolation while NEE was modelled. Regarding GWP, the intensive grassland site emitted 564 ± 255 g CO2–C equivalents m−2 yr−1 and 850 ± 238 g CO2–C equivalents m−2 yr−1 in the first (2007/2008) and the second (2008/2009) measuring year, respectively. The GWP of the extensive grassland amounted to −129 ± 231 g CO2–C equivalents m−2 yr−1 and 94 ± 200 g CO2–C equivalents m−2 yr−1, while it added up to 45 ± 117 g CO2–C equivalents m−2 yr−1 and −101 ± 93 g CO2–C equivalents m−2 yr−1 in 2007/08 and 2008/09 for the near-natural site. In contrast, in calendar year 2008 GWP aggregated to 441 ± 201 g CO2–C equivalents m−2 yr−1, 14 ± 162 g CO2–C equivalents m−2 yr−1 and 31 ± 75 g CO2–C equivalents m−2 yr−1 for the intensive grassland, extensive grassland, and near-natural site, respectively. Despite inter-annual variability, rewetting contributes considerably to mitigating GHG emission from formerly drained peatlands. Extensively used grassland on moderately drained peat approaches the carbon sequestration potential of near-natural sites, although it may oscillate between being a small sink and being a small source depending on inter-annual climatic variability.

Climate change is expected to alter the Arctic’s carbon (C) balance and changes in these C-rich ecosystems may contribute to a positive feedback on global climate change. Low-center mudboils, a form of patterned ground in the Arctic, are distinct landforms in which the exchange of greenhouse gases between the atmosphere and soil has not been fully characterized, but which may have an important influence on the overall C balance of tundra ecosystems. Chamber systems were used to sample net ecosystem exchange of CO2 (NEE) and CO2 and CH4 effluxes along a 35-m transect intersecting two mudboils in a wet sedge fen in Canada’s Southern Arctic (lat. 64°52′N, long. 111°34′W) during the summer months in 2008. Mudboil features gave rise to dramatic variations in vegetation, soil temperature and thaw depth, and soil organic matter content along this transect. Variations in NEE were driven by variations in the amount of vascular vegetation, while CO2 and CH4 effluxes were remarkably similar among the two mudboil (CO2 effluxes: 1.1 ± 0.9 and 1.4 ± 0.7 µmol m-2 s-1; CH4 effluxes: 83.1 ± 189.4 and 23.1 ± 9.4 nmol m-2 s-1, ± 1 standard deviation) and the sedge fen (CO2 effluxes: 1.6 ± 0.7 mol m-2 s-1 ; CH4 effluxes: 28.0 ± 62.0 nmol m-2 s-1) sampling areas. Vegetation appeared to play an important role in limiting temporal variations in CH4 effluxes through plant mediated transport in both mudboil and sedge fen sampling areas. One of the mudboils had negligible vascular plant colonization presumably due to more active frost heave processes. The relatively high CO2 and CH4 efflux in this mudboil area was speculated to be a result of growth and decomposition of cryptogamic organisms, inflow of dissolved organic C, and warmer soil temperatures. Key words: Patterned ground, nonsorted circle, tundra, net ecosystem exchange, methane, carbon dioxide