Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its delta ¹³C signature. Leaching of biogenic DIC was 8.3 +/- 4.9 g m-2 yr-1 for forests, 24.1 +/- 7.2 g m-2 yr-1 for grasslands, and 14.6 +/- 4.8 g m-2 yr-1 for croplands. DOC leaching equalled 3.5 +/- 1.3 g m-2 yr-1 for forests, 5.3 +/- 2.0 g m-2 yr-1 for grasslands, and 4.1 +/- 1.3 g m-2 yr-1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4 +/- 4.0 g C m-2 yr-1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5-98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24-105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems

Bioenergy from crops is expected to make a considerable contribution to climate change mitigation. However, bioenergy is not necessarily carbon neutral because emissions of CO 2 , N 2 O and CH 4 during crop production may reduce or completely counterbalance CO 2 savings of the substituted fossil fuels. These greenhouse gases (GHGs) need to be included into the carbon footprint calculation of different bioenergy crops under a range of soil conditions and management practices. This review compiles existing knowledge on agronomic and environmental constraints and GHG balances of the major European bioenergy crops, although it focuses on dedicated perennial crops such as Miscanthus and short rotation coppice species. Such second-generation crops account for only 3% of the current European bioenergy production, but field data suggest they emit 40% to >99% less N 2 O than conventional annual crops. This is a result of lower fertilizer requirements as well as a higher N-use efficiency, due to effective N-recycling. Perennial energy crops have the potential to sequester additional carbon in soil biomass if established on former cropland (0.44 Mg soil C ha À1 yr À1 for poplar and willow and 0.66 Mg soil C ha À1 yr À1 for Miscanthus). However, there was no positive or even negative effects on the C balance if energy crops are established on former grassland. Increased bioenergy production may also result in direct and indirect land-use changes with potential high C losses when native vegetation is converted to annual crops. Although dedicated perennial energy crops have a high potential to improve the GHG balance of bioenergy production, several agronomic and economic constraints still have to be overcome.Keywords: biofuel, carbon debt, carbon footprint, land management, methane, Miscanthus, nitrous oxide, short rotation coppice, soil organic carbon Greenhouse gas saving with bioenergy -a European perspectiveThe European Union has committed to increase the proportion of renewable energy from 9% in 2010 to 20% of Correspondence: Axel Don,

Invasions by alien plants provide a unique opportunity to examine competitive interactions among plants. While resource competition has long been regarded as a major mechanism responsible for successful invasions, given a well-known capacity for many invaders to become dominant and reduce plant diversity in the invaded communities, few studies have measured resource competition directly or have assessed its importance relative to that of other mechanisms, at different stages of an invasion process. Here, we review evidence comparing the competitive ability of invasive species vs. that of co-occurring native plants, along a range of environmental gradients, showing that many invasive species have a superior competitive ability over native species, although invasive congeners are not necessarily competitively superior over native congeners, nor are alien dominants are better competitors than native dominants. We discuss how the outcomes of competition depend on a number of factors, such as the heterogeneous distribution of resources, the stage of the invasion process, as well as phenotypic plasticity and evolutionary adaptation, which may result in increased or decreased competitive ability in both invasive and native species. Competitive advantages of invasive species over natives are often transient and only important at the early stages of an invasion process. It remains unclear how important resource competition is relative to other mechanisms (competition avoidance via phenological differences, niche differentiation in space associated with phylogenetic distance, recruitment and dispersal limitation, indirect competition, and allelopathy). Finally, we identify the conceptual and methodological issues characterizing competition studies in plant invasions, and we discuss future research needs, including examination of resource competition dynamics and the impact of global environmental change on competitive interactions between invasive and native species.

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Hiomass-specifnc, dark resiiiration rates of niicroalfJiao vary by almost two orders ot niaijnitiidc' from OOl lo ()(i d '. In general, dark respiration rates increase witli growth rates in botli lnlraspeeitic and interspeeific comparisons, hut the precise relationship between respiration and growth rate varies. L'nder optimal conditions, respiration rates are aliout 20 30",, of growtli rales, liut the ratio ol respiration to growtli increases under suhoptimal conditions. 'I'he intercept of plots of respiration rate vs. growth rate \aried from < OOl to 0-4 d ', and the slopes of thest' relationships \aried from < 0-1 to IO. .Minimum maintenance metaholic rates of about ()•() 1 d ' ba\e been estimated for some microalgae by extrapolation ot the initial slope of the light cur\e for grow th to a h\'pothetical negative growth rate in darkness. The dimensionless total energy cost of synthesis ami maintenance for heterotr