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Net Ecosystem CO2 Exchange (net + ecosystem_co2_exchange)
Selected AbstractsA review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emissionECOLOGY LETTERS, Issue 10 2009Lingli Liu Abstract Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO2, CH4 and N2O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO2 exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH4 emission by 97%, reduced CH4 uptake by 38% and increased N2O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO2 reduction could be largely offset (53,76%) by N stimulation of global CH4 and N2O emission from multiple ecosystems. [source] C3,C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxideFUNCTIONAL ECOLOGY, Issue 1 2007H. W. POLLEY Summary 1Plants usually respond to carbon dioxide (CO2) enrichment by increasing photosynthesis and reducing transpiration, but these initial responses to CO2 may not be sustained. 2During May, July and October 2000, we measured the effects of temporarily increasing or decreasing CO2 concentration by 150,200 µmol mol,1 on daytime net ecosystem CO2 exchange (NEE) and water flux (evapotranspiration, ET) of C3,C4 grassland in central Texas, USA that had been exposed for three growing seasons to a CO2 gradient from 200 to 560 µmol mol,1. Grassland grown at subambient CO2 (< 365 µmol mol,1) was exposed for 2 days to an elevated CO2 gradient (> 365 µmol mol,1). Grassland grown at elevated CO2 was exposed for 2 days to a subambient gradient. Our objective was to determine whether growth CO2 affected the amount by which grassland NEE and ET responded to CO2 switching (sensitivity to CO2). 3The NEE per unit of leaf area was greater (16,20%) and ET was smaller (9,20%), on average, at the higher CO2 concentration during CO2 switching in May and July. The amount by which NEE increased at the higher CO2 level was smaller at elevated than subambient growth concentrations on both dates, but relationships between NEE response and growth CO2 were weak. Conversely, the effect of temporary CO2 change on ET did not depend on growth CO2. 4The ratio of NEE at high CO2 to NEE at low CO2 during CO2 change in July increased from 1·0 to 1·26 as the contribution of C3 cover to total cover increased from 26% to 96%. Conversely, in May, temporary CO2 enrichment reduced ET more in C4 - than C3 -dominated grassland. 5For this mesic grassland, sensitivity of NEE and ET to brief change in CO2 depended as much on the C3,C4 composition of vegetation as on physiological adjustments related to prior CO2 exposure. [source] Atmospheric impact of bioenergy based on perennial crop (reed canary grass, Phalaris arundinaceae, L.) cultivation on a drained boreal organic soilGCB BIOENERGY, Issue 3 2010NARASINHA J. SHURPALI Abstract Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long-term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land-use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO2 -equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon-negative. An LCA sensitivity analysis revealed that net ecosystem CO2 exchange and crop yield are the major LCA components, while non-CO2 GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO2 uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long-term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies. [source] Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fenGLOBAL CHANGE BIOLOGY, Issue 5 2001Timothy J. Griffis Abstract This paper presents an empirical model of net ecosystem CO2 exchange (NEE) developed for a subarctic fen near Churchill, Manitoba. The model with observed data helps explain the interannual variability in growing season NEE. Five years of tower-flux data are used to test and examine the seasonal behaviour of the model simulations. Processes controlling the observed interannual variability of CO2 exchange at the fen are examined by exploring the sensitivity of the model to changes in air temperature, precipitation and leaf area index. Results indicate that the sensitivity of NEE to changing environmental controls is complex and varies interannually depending on the initial conditions of the wetland. Changes in air temperature and the timing of precipitation events have a strong influence on NEE, which is largely manifest in gross ecosystem photosynthesis (GEP). Climate change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit early in the growing season. Model simulations for this subarctic sedge fen indicate that carbon acquisition is greatest during wet and warm conditions. This suggests therefore that carbon accumulation was greatest at this subarctic fen during its early developmental stages when hydroclimatic conditions were relatively wet and warm at approximately 2500 years before present. [source] | ||||||||||