J. F. Soussana

Temporal and among-site variability of inherent water use efficiency at the ecosystem level

Half-hourly measurements of the net exchanges of carbon dioxide and water vapor between terrestrial ecosystems and the atmosphere provide estimates of gross primary production (GPP) and evapotranspiration (ET) at the ecosystem level and on daily to annual timescales. The ratio of these quantities represents ecosystem water use efficiency. Its multiplication with mean daylight vapor pressure deficit (VPD) leads to a quantity which we call “inherent water use efficiency” (IWUE*). The dependence of IWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment. IWUE* is analyzed for 43 sites across a range of plant functional types and climatic conditions. IWUE* increases during short-term moderate drought conditions. Mean annual IWUE* varied by a factor of 3 among all sites. This is partly explained by soil moisture at field capacity, particularly in deciduous broad-leaved forests. Canopy light interception sets the upper limits to canopy photosynthesis, and explains half the variance in annual IWUE* among herbaceous ecosystems and evergreen needle-leaved forests. Knowledge of IWUE* offers valuable improvement to the representation of carbon and water coupling in ecosystem process models

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO2 is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO2 concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO2 stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO2 in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO2 would be to increase N-import to the grassland ecosystem through symbiotic N2 fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N2 fixation in legumes will be enhanced under elevated CO2, ii) this enhancement of N2 fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO2 under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO2, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO2. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N2 fixation usually promotes shoot growth (above-ground C storage) in elevated CO2. However, the consequences of this larger N input under elevated CO2 on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO2 on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.

Productivity, Respiration, and Light-Response Parameters of World Grassland and Agroecosystems Derived From Flux-Tower Measurements

Abstract Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO2 exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO2 fluxes in a sagebrush-steppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167–183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grünwald, K. Havránková, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J. M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11:1424–1439). Maximum values of the quantum yield (α  =  75 mmol · mol−1), photosynthetic capacity (Amax  =  3.4 mg CO2 · m−2 · s−1), gross photosynthesis (Pg,max  =  116 g CO2 · m−2 · d−1), and ecological light-use efficiency (εecol  =  59 mmol · mol−1) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO2. Maximum values of gross primary production (8 600 g CO2 · m−2 · yr−1), total ecosystem respiration (7 900 g CO2 · m−2 · yr−1), and net CO2 exchange (2 400 g CO2 · m−2 · yr−1) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO2, with mean net uptake of 700 g CO2 · m−2 · yr−1 for intensive grasslands and 933 g CO2 · m−2 · d−1 for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO2, this does not imply that they are necessarily increasing their carbon stock.

Long-term effects of CO2 enrichment and temperature increase on a temperate grass sward

Perennial ryegrass swards were grown in large containers on a soil, at two N fertilizer supplies, and were exposed over two years in highly ventilated plastic tunnels to elevated (700 μL L-1 [CO2]) or ambient atmospheric CO2 concentration at outdoor temperature and to a 3°C increase in air temperature in elevated CO2. These swards were either fully irrigated (kept at field capacity) in each climatic condition (W+), or received the same amount of water in the three climate treatments (W-). In the latter case, the irrigation was adjusted to obtain a soil water deficit during summer and drainage in winter. Using a lysimeter approach, the evapotranspiration, the soil water balance, the productivity (dry-matter yield) and the water use efficiency of the grass swards were measured. During both years, elevated CO2 increased the annual above-ground drymatter yield of the W- swards, by 19% at N- and by 14% at N+. Elevated CO2 modified yield to a variable extent during the growing season: a small, and sometime not significant effect (+6%, on average) was obtained in spring and in autumn, while the summer growth response was stronger (+48%, on average). In elevated CO2, the temperature increase effect on the annual above-ground dry-matter yield was not significant, due to a gain in dry-matter yield in spring and in autumn which was compensated for by a lower summer productivity. Elevated CO2 slightly reduced the evapotranspiration during the growing season and increased drainage by 9% during winter. A supplemental 3°C in elevated CO2 reduced the drainage by 29–34%, whereas the evapotranspiration was increased by 8 and 63% during the growing season and in winter, respectively. During the growing season, the soil moisture content at W- and at the high N supply declined gradually in the control climate, down to 20–30% of the water holding capacity at the last cut (September) before rewatering. This decline was partly alleviated under elevated CO2 in 1993, but not in 1994, and was enhanced at +3°C in elevated CO2. The water use efficiency of the grass sward increased in elevated CO2, on average, by 17 to 30% with no significant interaction with N supply or with the soil water deficit. The temperature increase effect on the annual mean of the water use efficiency was not significant. Highly significant multiple regression models show that elevated CO2 effect on the dry-matter yield increased with air temperatures above 14.5°C and was promoted by a larger soil moisture in elevated compared to ambient CO2. The rate of change in relative dry-matter yield at +3°C in elevated CO2 became negative for air temperatures above 18.5°C and was reduced by a lower soil moisture at the increased air temperature. Therefore, the altered climatic conditions acted both directly on the productivity and on the water use of the grass swards and, indirectly, through changes in the soil moisture content.

Habitat filtering and niche differentiation jointly explain species relative abundance within grassland communities along fertility and disturbance gradients

Deterministic niche-based processes have been proposed to explain species relative abundance within communities but lead to different predictions: habitat filtering (HF) predicts dominant species to exhibit similar traits while niche differentiation (ND) requires that species have dissimilar traits to coexist. Using a multiple trait-based approach, we evaluated the relative roles of HF and ND in determining species abundances in productive grasslands. Four dimensions of the functional niche of 12 co-occurring grass species were identified using 28 plant functional traits. Using this description of the species niche, we investigated patterns of functional similarity and dissimilarity and linked them to abundance in randomly assembled six-species communities subjected to fertilization/disturbance treatments. Our results suggest that HF and ND jointly determined species abundance by acting on contrasting niche dimensions. The effect of HF decreased relative to ND with increasing disturbance and decreasing fertilization. Dominant species exhibited similar traits in communities whereas dissimilarity favored the coexistence of rare species with dominants by decreasing inter-specific competition. This stabilizing effect on diversity was suggested by a negative relationship between species over-yielding and relative abundance. We discuss the importance of considering independent dimensions of functional niche to better understand species abundance and coexistence within communities.

Elevated [CO2], temperature increase and N supply effects on the accumulation of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO2] (700 μl l−1 [CO2]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha−1 year−1) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO2 concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO2] reduced it. At the last sampling date, elevated [CO2] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO22] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha−1, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha−1. There was no significant effect of a 3 °C temperature increase under elevated [CO2] on C accumulation in the OM fractions above 50 μm.

Methane emission measurements in a cattle grazed pasture: a comparison of four methods

Abstract. Methane (CH 4 ) is considered to be the second main contributor to the global greenhouse gas effect, with major CH 4 emissions originating from livestock. Accurate measurements from ruminating herds are required to improve emission coefficients used in national emission inventories, and to evaluate mitigation strategies. Previous measurements of enteric methane emissions from domestic animals have been carried out in artificial conditions such as laboratory chambers, or by fitting individual animals with capillary tubes and using SF 6 as a tracer. Here we evaluated the reliability of eddy covariance technique (EC), already used for CO 2 fluxes, for continuous CH 4 measurements over a grazed field plot. Analyzer accuracy and reliability of eddy covariance technique were tested against field scale measurements with the SF 6 tracer technique, Gaussian plume model and emission factors (i.e. IPCC). Results indicate a better agreement between EC and SF 6 method when grazing heifers were parked close to the EC setup. However, a systematic underestimation of EC data appeared and even more when the distance between the source (ruminating heifers) and EC setup (mast) was increased. A two-dimensional footprint density function allowed to correct for the dilution effect on measured CH 4 and led to a good agreement with results based on the SF 6 technique (on average 231 and 252 g CH 4 ha −1 over the grazing experiment, respectively). Estimations of the CH 4 budgets for the whole grazing season were in line with estimates (i.e. emission factor coefficients) based on feed intake and animal live weight as well as SF 6 technique. IPCC method Tier 2, however, led to an overestimation of CH 4 fluxes on our site.