Sources and Sinks of Greenhouse Gases from European Grasslands and Mitigation Options: The ‘GreenGrass’ Project

Adapting the management of grasslands may be used to enhance carbon sequestration into soil, but could also increase N2O and CH4 emissions. In support of the European post-Kyoto policy, the European \u27GreenGrass\u27 project (EC FP5, EVK2-CT2001-00105) has three main objectives: i) to reduce the large uncertainties concerning the estimates of CO2, N2O and CH4 fluxes to and from grassland plots under different climatic conditions and assess their global warming potential, ii) to measure net greenhouse gas (GHG) fluxes for different management which reflect potential mitigation options, iii) to construct a model of the controlling processes to quantify the net fluxes and to evaluate mitigation scenarios by up-scaling to a European level

Long-term steady state ¹³C labelling to investigate carbon turnover in plant soil systems

International audienceWe have set up a facility allowing steady state 13CO2 labeling of short stature vegetation (12 m2) for several years. 13C labelling is obtained by scrubbing the CO2 from outdoors air with a self-regenerating molecular sieve and by replacing it with 13C depleted (-34.7±0.03‰) fossil-fuel derived CO2 The facility, which comprises 16 replicate mesocosms, allows tracing the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied during 2 yrs to temperate grassland monoliths (0.5×0.5×0.4 m) sampled in a long term grazing experiment. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow (0.67–0.88 volume per minute) with modified air (43% scrubbed air and 57% cooled and humidified ambient air) at mean CO2 concentration of 425 µmol mol-1 and d13C of -21.5±0.27‰. Above and belowground CO2 fluxes were continuously monitored. The difference in d13C between the CO2 at the outlet and at the inlet of each canopy enclosure was not significant (-0.35±0.39‰). Due to mixing with outdoors air, the CO2 concentration at enclosure inlet followed a seasonal cycle, often found in urban areas, where d13C of CO2 is lower in winter than in summer. Mature C3 grass leaves were sampled monthly in each mesocosm, as well as leave from pot-grown control C4 (Paspalum dilatatum). The mean d13C of fully labelled C3 and C4 leaves reached -41.4±0.67 and -28.7±0.39‰ respectively. On average, the labelling reduced by 12.7‰ the d13C of C3 grass leaves. The isotope mass balance technique was used to calculate the fraction of "new" C in the soil organic matter (SOM) above 0.2 mm. A first order exponential decay model fitted to "old" C data showed that reducing aboveground disturbance by cutting increased from 22 to 31 months the mean residence time of belowground organic C (>0.2 mm) in the top soil

Long-term steady state ¹³C labelling to investigate soil carbon turnover in grasslands

We have set up a facility allowing steady state ¹³CO₂ labeling of short stature vegetation (12 m²) for several years. ¹³C labelling is obtained by scrubbing the CO₂ from outdoors air with a self-regenerating molecular sieve and by replacing it with ¹³C depleted (&minus;34.7&plusmn;0.03&permil;) fossil-fuel derived CO₂ The facility, which comprises 16 replicate mesocosms, allows to trace the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied to the study of soil organic carbon turnover in temperate grasslands. We tested the hypothesis that a low disturbance by grazing and cutting of the grassland increases the mean residence time of carbon in coarse (&gt;0.2 mm) soil organic fractions. <br><br> Grassland monoliths (0.5&times;0.5&times;0.4 m) were sampled from high and low disturbance treatments in a long-term (14 yrs) grazing experiment and were placed during two years in the mesocosms. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow with air at mean CO₂ concentration of 425 &micro;mol mol^&minus;1 and &delta;¹³C of &minus;21.5&plusmn;0.27&permil;. Fully labelled mature grass leaves reached a &delta;¹³C of &minus;40.8 (&plusmn;0.93) and &minus;42.2&permil; (&plusmn;0.60) in the low and high disturbance treatments, respectively, indicating a mean ¹³C labelling intensity of 12.7&permil; compared to unlabelled control grass leaves. After two years, the delta ¹³C value of total soil organic matter above 0.2 mm was reduced in average by 7.8&permil; in the labelled monoliths compared to controls. The isotope mass balance technique was used to calculate for the top (0&ndash;10 cm) soil the fraction of ¹³C labelled carbon in the soil organic matter above 0.2 mm (i.e. roots, rhizomes and particulate organic matter). A first order exponential decay model fitted to the unlabelled C in this fraction shows an increase in mean residence time from 22 to 31 months at low compared to high disturbance. A slower decay of roots, rhizomes and particulate organic matter above 0.2 mm is therefore likely to contribute to the observed increased in soil carbon sequestration in grassland monoliths exposed to low disturbance

Comparaison de 3 méthodes de mesure de la transpiration de jeunes arbres

Trois méthodes de mesure de la transpiration ont été comparées sur de jeunes noyers (Juglans regia L, cv Lara) cultivés en bacs, pendant une période de 115 j en 1990. La mesure de transpiration par pesée a servi de référence et a été comparée à la mesure du flux de sève dans le tronc par bilan thermique, ainsi qu'à la mesure de transpiration par échanges gazeux. De plus les mesures de flux de sève ont été multipliées sur le même arbre afin, d'une part, d'estimer la sensibilité de cette méthode et, d'autre part, d'évaluer le stockage d'eau entre ces 2 hauteurs. Un calage précis de la mesure du flux de sève a été obtenu dans le cas où les arbres ont subi une courte période de sécheresse intense, ce qui a conduit alors à un très bon accord avec la pesée, qu'il s'agisse des valeurs horaires ou journalières sur une période relativement longue (115 jours). La mesure de la transpiration par échanges gazeux est systématiquement sous-estimée. II semble que cette sous-estimation puisse être corrigée par un étalonnage précis de tous les capteurs en cause et par la mise en œuvre d'une correction de la pression dans le calcul des teneurs en vapeur d'eau de l'air. Enfin, les stockages ou déstockages d'eau dans l'arbre n'ont pu être quantifiés correctement malgré la très bonne sensibilité de la méthode de flux de sève (précision : 3 %).Comparison of 3 methods for measuring the transpiration rate of young trees. Three different methods of measuring the transpiration in young potted walnut trees (Juglans regia L, cv Lara) were compared over a 115-d period in 1990. Weighing of the pots was used as the reference for comparison with the sap flow rate given by a thermal method and the rate of transpiration by gas exchange determination. Several sap flow meters were installed at different heights on the same tree in order to estimate the storage of water between them. A good calibration of the sap flow measurement method was obtained in those trees which had been subjected to a short period of severe water stress, leading to an excellent agreement with hourly or daily values of transpiration by weighing. Gas exchange determination systematically underestimated transpiration. Nevertheless it seems possible to improve gas exchange determination of transpiration by: i) accurate calibration of all the sensors; and ii) introducing a pressure correction in the calculation of water vapour density in the air. Although the accuracy of the sap flow method was high (3%), it was not possible to assess within tree water movement correctly