Fertilization of the biosphere, feedback mechanisms

An independent estimate of the continental biospheric sink is obtained from local inventories of forest and ecosystem carbon stocks. The global balance shows systematic variations over two decades. The gross forest sink is around 2.4 GtC/year, with significant contributions at tropical and temperate latitudes. This flow is partially offset by an inverse flow of around 1.3 GtC/year linked to human modifications of tropical ecosystems (balance of tropical deforestation and reforestation). All in all, the net forest sink is therefore of the order of 1.1 GtC/year for the decades 1990-2000. Precise studies of forests are being carried out to assess the variability of the carbon sink. For example, the systematic measurement of tree size in the Amazon seems to indicate a reduction in the carbon sink due to increased tree mortality, the origin of which is still unclear.

Satellites make it possible to quantify vegetation from space by analyzing reflected solar radiation. The relative measurement of reflectance in the visible red (absorbed by plants) versus near infrared (scattered by plants) leads to the calculation of the NDVI index(Normalized Difference Vegetation Index), which can be linked to the primary production of terrestrial vegetation. As the relationship is not ideal, other indices have been developed using remote sensing (FPAR : Fractional Photosyntetically Active Radiation, LAI : Leaf Area Index, etc.).

More recently, it has been discovered that sensors on board satellites dedicated to measuring _pCO2 (GOSAT and OCO-2), can also measure a fluorescence signal from active chlorophyll. Monitoring the carbon sink of terrestrial vegetation should therefore make significant progress in the near future.

Based on spatial data available since the 1980s, it is now possible to map the evolution of net primary production on a global scale. Compilation of these maps reveals an increase of around 6 % for the 1980s and 1990s, as well as a large inter-annual variability (± 2 GtC/year) correlated with that of the rate of increase in atmospheric _pCO2. Subdivision of the signal into latitudinal bands shows that variability in plant primary production is concentrated in the tropical zone.

Transient decreases in the biospheric sink correspond to the El Nino phases of the Southern Oscillation (ENSO), originating in a drop in precipitation in the western Pacific over Australia and the islands of Southeast Asia. Conversely, the negative phases of the ENSO oscillation (La Nina) correspond to a shift in precipitation towards the west of the Pacific and to increases in primary productivity in the same continental zones, leading to falls in the rate of increase in atmospheric _pCO2.

This _pCO2-ENSO correlation is not perfect, however, particularly for the period following the volcanic eruption of Mount Pinatubo in 1991. This violent eruption injected sulfur oxide into the stratosphere, rapidly transforming it into fine sulfate particles. Specialized satellites measured the global dispersion of these volcanic aerosols, which led to transient cooling at ground level for several years, particularly during the summer season.

Radiation balances for the tropical zone show that direct irradiance at ground level decreased after the Pinatubo eruption, but that diffuse irradiance increased significantly at the same time. Several studies suggest that this increase in diffuse solar irradiance favored photosynthesis and stimulated the sink of the tropical terrestrial biosphere. This mechanism would explain the decreases in the rate of increase in atmospheric _pCO2 that followed violent eruptions with the injection of volcanic aerosols into the stratosphere (Pinatubo in 1991, El Chichon in 1982 and Agung in 1963). However, the overlap with ENSO variability makes it difficult to distinguish the impact of volcanic eruptions on the Earth's biosphere.

To understand and predict the response of vegetation to environmental variations, it is useful to return to the biochemical and physiological processes that control plant growth. Laboratory cultures can be used to quantify the effects of light, water,_CO2 and temperature on photosynthesis and respiration in different plant species. Plants can also adapt to external conditions, for example by increasing leaf stomata density as _pCO2 decreases.

Small-scale greenhouse cultures are not fully satisfactory and representative for studying plant response. For this reason, several laboratories have developed open-atmosphere, but controlled, systems to study plant response under realistic conditions. These FACE (Free-Air_CO2 Experiment) experiments have been carried out at numerous sites in the USA and a few other countries.

The synthesis of FACE data shows that carbon uptake is sensitive to atmospheric _pCO2, but that the fertilization effect is saturated at high _pCO2. At high pCO2 levels of around 550 ppm, photosynthesis and carbon assimilation rates increase by 20-40 % for a variety of plant types, from trees to herbaceous plants, both wild and cultivated. For the same plant species, _pCO2 has an impact on several plant characteristics: total mass, height, diameter, number of branches or leaves. Experiments confirm the importance of photosynthesis type, with C3 plants being more sensitive to _pCO2 than C4 plants, which have adapted by developing a pre-concentration of_CO2 in their mesophyll cells.