The role of closed ecological systems in carbon cycle modelling

Acquiring a mechanistic understanding of the role of the biotic feedbacks on the links between atmospheric CO2 concentrations and temperature is essential for trustworthy climate predictions. Currently, computer based simulations are the only available tool to estimate the global impact of the biotic feedbacks on future atmospheric CO2 and temperatures. Here we propose an alternative and complementary approaches by using materially closed and energetically open analogue/physical models of the carbon cycle. We argue that there is potential in using a materially closed approach to improve our understanding of the magnitude and sign of many biotic feedbacks, and that recent technological advance make this feasible. We also suggest how such systems could be designed and discuss the advantages and limitations of establishing physical models of the global carbon cycle

COMBINED USE OF OPEN-AIR AND INDOOR FUMIGATION SYSTEMS TO STUDY EFFECTS OF SO-2 ON LEACHING PROCESSES IN SCOTS PINE LITTER

Both an open-air fumigation system and a laboratory-based system were used to expose decomposing Scots pine (Pinus sylvestris L.) needles to controlled concentrations of SO2 (arithmetric mean less-than-or-equal-to 48 nl litre-1) during a period, in total, of 301 days. The experimental design involved reciprocal litter transplants from 'clean' to 'polluted' air and vice versa, using the two fumigation systems. The objectives were (1) to observe the effects of SO2 on leachate and litter chemistry, (2) to assess whether pollution-induced changes are reversible in clean air, and (3) to test the suitability of small-scale fumigation chambers (litter microcosms) compared with open-air systems in soil studies.Through the formation of SO4(2-) ions, dry-deposited SO2 exhibited a marked capacity to remove 'base' cations (Ca2+, Mg2+ and K+) from decomposing pine needles, and also to acidify litter leachates (as indicated by proton fluxes from the litter). When litter was transferred from polluted air (48 nl litre-1 SO2, in the open-air system) to either clean or polluted air in the laboratory, the effects of prior exposure to SO2 on leachate composition were still evident even after 86 days: the role of base cation depletion within the litter, caused by SO42- -induced leaching, is discussed.Data for SO42- fluxes in leachates collected from the small-scale chambers indicated that dry deposition velocities for SO2 were not anomalously high within this fumigation system. It is therefore concluded that microcosm studies can provide information complementary to the open-air fumigation approach in soils research.</p

Do volcanic emissions affect carbon gas fluxes in peatlands?

Recently, a link has been suggested between volcanic deposition of SO4 and the suppression of CH4 emissions in northern peatlands (Gauci et al., 2008). This link stems from the widely accepted idea that acid rain SO4 additions to peatlands can cause a shift in microbial communities as SO4 reducing bacteria out-compete methanogens for substrates, which results in a suppression of CH4 emission. However, volcanic emissions contain besides S other chemically reactive species that are potentially harmful to the environment. In particular, gaseous and particulate F emissions from volcanoes constitute a steady or intermittent source of F emission and deposition into the environment both close to the source and within fallout range of large eruptions. The objective of this study was to investigate the effect of volcanic depositions of SO4, both alone and in combination with F, on CH4 emission in peatlands. Peat mesocosms collected from Pennine uplands in the UK were treated with weekly pulses of Na2SO4 and NaF over 20 weeks in doses of 74 kg SO4/ ha and 13.5 and 135 kg F /ha. CH4 emissions were measured at regular intervals by taking headspace samples, which were analysed by GC-FID. CO2 fluxes were also measured using a portable Infra Red Gas Analyser (IRGA). No significant differences in CH4 and CO2 emissions were observed for any of the treatments when compared to the controls, which had only received deionised water. These findings are in contrast with previous studies where SO4 reduces CH4 emission in peatlands. The reason for this is unclear but may be due to the heterogeneous nature of peat soils. An alternative explanation relates to the previous history of the soils used in the mesocosms which are known to have been previously exposed to large volumes of anthropogenic S pollution. This may have caused microbial communities to evolve and become acclimatised to high levels of S addition. In either case, the assumption that CH4 suppression in peatlands occurs upon exposure to volcanic depositions is questionable. Gauci, V., S. Blake, et al. (2008). Halving of the northern wetland methane source by a large icelandic volcanic eruption. JGR, doi:10.1029/2007JG00049

Exploring the “overflow tap” theory: linking forest soil CO2 fluxes and individual mycorrhizosphere components to photosynthesis

Quantifying soil organic carbon stocks (SOC) and their dynamics accurately is crucial for better predictions of climate change feedbacks within the atmosphere-vegetation soil system. However, the components, environmental responses and controls of the soil CO2 efflux (Rs) are still unclear and limited by field data availability. The objectives of this study were (1) to quantify the contribution of the various Rs components, specifically its mycorrhizal component, (2) to determine their temporal variability, and (3) to establish their environmental responses and dependence on gross primary productivity (GPP). In a temperate deciduous oak forest in south east England hourly soil and ecosystem CO2 fluxes over four years were measured using automated soil chambers and eddy covariance techniques. Mesh-bag and steel collar soil chamber treatments prevented root or both root and mycorrhizal hyphal in-growth, respectively, to allow separation of heterotrophic (Rh) and autotrophic (Ra) soil CO2 fluxes and the Ra components, roots (Rr) and mycorrhizal hyphae (Rm). Annual cumulative Rs values were very similar between years (740±43 g Cm−2 yr−1) with an average flux of 2.0±0.3 μmol CO2 m−2 s−1, but Rs components varied. On average, annual Rr, Rm and Rh fluxes contributed 38, 18 and 44 %, respectively, showing a large Ra contribution (56 %) with a considerable Rm component varying seasonally. Soil temperature largely explained the daily variation of Rs (R2 = 0.81), mostly because of strong responses by Rh (R2 = 0.65) and less so for Rr (R2 = 0.41) and Rm (R2 = 0.18). Time series analysis revealed strong daily periodicities for Rs and Rr, whilst Rm was dominated by seasonal ( 150 days), and Rh by annual periodicities. Wavelet coherence analysis revealed that Rr and Rm were related to short-term (daily) GPP changes, but for Rm there was a strong relationship with GPP over much longer (weekly to monthly) periods and notably during periods of low Rr. The need to include individual Rs components in C flux models is discussed, in particular, the need to represent the linkage between GPP and Ra components, in addition to temperature responses for each component. The potential consequences of these findings for understanding the limitations for long-term forest C sequestration are highlighted, as GPP via root-derived C including Rm seems to function as a C “overflow tap”, with implications on the turnover of SOC

Carbon Dioxide and Methane Flux Response and Recovery From Drought in a Hemiboreal Ombrotrophic Fen

Globally peatlands store 500 Gt carbon (C), with northern blanket bogs accumulating 23 g C m−2 y−1 due to cool wet conditions. As a sink of carbon dioxide (CO2) peat bogs slow anthropogenic climate change, but warming climate increases the likelihood of drought which may reduce net ecosystem exchange (NEE) and increase soil respiration, tipping C sinks to sources. High water tables make bogs a globally important source of methane (CH4), another greenhouse gas (GHG) with a global warming potential (GWP) 34 times that of CO2. Warming may increase CH4 emissions, but drying may cause a reduction. Predicted species composition changes may also influence GHG balance, due to different traits such as erenchyma, e.g., Eriophorum vaginatum (eriophorum) and non-aerenchymatous species, e.g., Calluna vulgaris (heather). To understand how these ecosystems will respond to climate change, it is vital to measure GHG responses to drought at the species level. An automated chamber system, SkyLine2D, measured NEE and CH4 fluxes near-continuously from an ombrotrophic fen from August 2017 to September 2019. Four ecotypes were identified: sphagnum (Sphagnum spp), eriophorum, heather and water, hypothesizing that fluxes would significantly differ between ecotypes. The 2018 drought allowed comparison of fluxes between drought and non-drought years (May to September), and their recovery the following year. Methane emissions differed between ecotypes (p sphagnum > water > heather, ranging from 23 to 8 mg CH4-C m−2 d−1. Daily NEE was similar between ecotypes (p > 0.7), but under 2018 drought conditions all ecotypes were greater sources of CO2 compared to 2019, losing 1.14 g and 0.24 g CO2-C m−2 d−1 respectively (p < 0.001). CH4 emissions were ca. 40% higher during 2018 than 2019, 17 mg compared to 12 mg CH4-C m−2 d−1 (p < 0.0001), and fluxes exhibited hysteresis with water table depth. A lag of 84–88 days was observed between rising water table and increased CH4 emissions. A significant interaction between ecotype and year showed fluxes from open water did not return to pre-drought levels. Our findings suggest that short-term drought may lead to a net increase in C emissions from northern wetlands

ELUM Year 2 report for Work Package 3 - Network of field sites to measure soil C dynamics and GHG emissions. Report V2

This report describes the second year of Work Package 3 (WP3) activities within the ETI’s Ecosystem Land Use Modelling Project (“ELUM”). It expands upon information reported in the first year and provides a forward look to WP3 activities for the remainder of the project. The soil C (carbon) and GHG (Greenhouse Gas) measurements recorded as part of WP3 are required to help reduce the uncertainty associated with the sustainability of bioenergy crop deployment across the UK. This data will be used to parameterise and test the underlying process models in the WP4 modelling work, as part of the development of the over-arching meta-model. A full review of all the data collected across the WP3 network sites will be reported in the D3.5 deliverable due in May 2014. Progress with the development and testing of novel methods for GHG measurement is also included in this report; these could offer means of improving monitoring resolution, thereby enhancing the collection of GHG flux data. A complete review of this work will follow in May 2014 with the D3.4 deliverable

Responses of six coniferous forest soils to increased (NH4)2SO4 inputs

Defining Critical N Loads for forests remains a major challenge, as our understanding of processes determining forest ecosystem responses to increased N deposition is still poor. Acidification will mainly depend on the nitrification capacity, and coupled nitrate leaching, of the forest soil; yet factors controlling nitrification are not clearly established. As part of the CORE project (CEC), investigating nutrient dynamics in European coniferous forest soils, we studied the effects of increased (NH4)2SO4 deposition on soil solution chemistry at six sites situated across a climatic and pollution gradient. Roofed tension lysimeters were treated every two weeks with 2.88 kg N ha-1 (75 kg N ha-1 a-1) above ambient throughfall and monitored at the same interval for a total of 18 months. Responses to increased (NH4)2SO4 inputs varied from high ammonium retention, to highly increased ammonium leaching at the different sites. The important links between N cycle transformations and cation leaching were confirmed. The response of the nitrifiers to increased external ammonium supply was slow at all sites. The different responses of these six soils to increased (NH4)2SO4 deposition emphasizes that the effects of N loads have to be discussed in relation to soil N storage and nitrification capacity

Environmental factors controlling NO3- leaching, N2O emissions and numbers of NH4+ oxidisers in a coniferous forest soil

Main and interactive effects of temperature, throughfall volume and NH4+ deposition on soil solution NO3- concentrations, N2O emissions and numbers of NH4+ oxidisers were investigated in a controlled laboratory experiment. Large intact soil cores from a Picea abies (L.) Karat. stand were incubated according to an 'incomplete factorial design' at 4, 12 or 20 degrees C and watered every 2 weeks with 300, 500 or 700 ml (442, 737 and 1032 mm yr(-1)) of a natural throughfall solution enriched with 0, 37.5 or 75 kg NH4+-N ha(-1) yr(-1). Watering and sampling were performed every 2 weeks, during a 112 d period. At d 112, a temperature optimum for NO3--N concentrations in the leachate, NO3--N fluxes and numbers of NH4+ oxidisers in the mineral soil layer was determined at ca. 11 degrees C. NO3--N concentrations also decreased with throughfall volume, towards a minimum at 590 ml, with temperature however contributing most to modelling NO3--N concentrations and the two factors acting independently. The model explained 59% of the variability in the data, and the regression between observed and predicted concentrations was highly significant (P < 0.0001, r(2) = 0.93). NO3--N fluxes increased quadratically with throughfall volume, and throughfall volume and NH4+ deposition interacted significantly in determining the numbers of NH4+ oxidisers in the mineral soil layer. Numbers of NH4+ oxidisers were higher in the humus layer and decreased with increasing temperatures. N2O fluxes increased quadratically with temperature, and the linear and quadratic effects of throughfall volume (maximum at 500 ml). Results suggest that optimum temperatures for net nitrification may have been overestimated in previous studies by the use of disturbed soils