biogeochimie.fr

Forest: lung of the Earth?

The photosynthesis is a biochemical reaction which synthetize organic matter from atmospheric CO2. This is performed in chlorophyllian organisms, and particularly in forest ecosystems. This reaction also releases O2 by oxidizing water molecules. Thus, it is commonly and wrongly believed that the forest is a lung for the planet. Besides the fact that a lung is exchanging O2 to CO2, this idea is dynamically wrong. As for animals, vegetation needs energy which is generated by biochemical reaction of respiration, producing CO2. A mature forest shows a biogeochemical equilibrium: inputs and outputs of carbon in this system are the same. A forest is, however, a sink of carbon of the order of 60 to 120 Mg of carbon per hectare depending the type of forest (temperate, tropical, or boreal). These values can reach 150 to 400 Mg of carbon per hectare if we consider the entire ecosystem, that is to say including soil. More than 1,000 Pg of carbon are stocked in such ecosystems, explaining why the deforestation is a disturbing action. During a biomass combustion, the organic carbon is transformed to atmospheric CO2. All extra carbon in the atmosphere due to biomass or fossil combustion is controlled by the oceans: CO2 is absorbed as dissolved carbonic acid, acidifying the ocean. Knowing the harmful impacts of this on natural ecosystems, several unanswered questions are raised about the consequences of long-time sequestration of carbon in the deep ocean.

The sea without water

There are two types of watersheds on Earth: those that reach the sea, called exorheic basins, and those that do not, called endorheic basins. While the former are relatively common, the latter are much less known even when they represent 18% of the Earth's surface. They are found on every continent: the largest are in Central Asia, North Africa, Arabian Peninsula, Australia, South Africa, Western America, and Andean Altiplano. The regional dry climate leads to closed systems: the evaporation losses are equal to the rainfall inputs. They often end by a salt lake, called terminal lake, resulting from the continuing accumulation of soluble salts. This is the case of the Caspian Sea, Dead Sea, or Lake Chad, for example. These systems are in a fragile equilibrium possibly influenced by human activities with irreversible consequences; water supply is a central geopolitical issue often source of conflict. The leading example is those of the Aral Sea, located between Kazakhstan in the north and Uzbekistan in the south. Historically fed by the Syr Darya and the Amu Darya, the terminal lake has lost 90% of its volume since 1960. In 1920, the USSR diverted water from these two rivers using transverse channels to irrigate cotton crops, an important source of income at the time. Although in precarious balance for a long time, it was 40 years later during the agricultural intensification that the most drastic losses appeared, with social and environmental consequences: scarcity of fishes for Kazakhs and Uzbeks fishermen, hypersaline waters toxic to lake species, dispersion of pesticide-contaminated dust. The various attempted rehabilitations have so far never been successful.

Plastic ocean

Our current activities are largely dependent on plastic material. We produce 280 millions of tons of plastic each year from 8% of our worldwide petrol production. This synthetic polymer, however, is little recycled, and consequently represent a large portion of our wastes. Continental production reaches littoral environments by the dense hydrographic network, adding to the coastal wastes from urban activities especially increased during the summer period. The physical properties of plastic promote its transport in every directions; compounds of low density may float (such as the polyethylene) while those with high density reach the oceanic floor (such as the polyvinyl chloride). The degradation rate is close to zero in such conditions without light and oxygen in the deep layers; these plastics constitute persistent pollutants. Plastic wastes may have different size – from microparticles to bottles or plastic bags – that can move passively with the water currents. The NASA has recently modeled the transportation of all the particulates through ocean gyres in the last 35 years, capable to develop the “garbage islands” where there is a concentration of plastic wastes. This pollution create various health issues for marine organisms. For example, animals at the end of the food chain, such as albatrosses and penguins, accumulate more pollutants than primary consumers. Nine tenth of seabird species ingest fortuitously fragments of plastics, either by confusing plastics for food, or by bioaccumulation. Mollusks that filter water (up to several hindered milliliters of water per hour) are also sensitive to aquatic pollutants. Plastic wastes were recently located in the arctic Barents Sea.

Space debris

Manmade contamination is not limited to continents and oceans. Despite the lack of knowledge, the space is largely subject to pollution since the first launches of artificial satellites in 1957. To date, more than 4,600 launches have been made, and now there are about 2,600 satellites orbiting the Earth, corresponding to 5,000 tons of material. Only 800 of them are currently in service, ensuring various functions (Earth and space observation, telecommunication, geographical positioning…). They are located at different altitudes according to their function: orbit between several hundreds and tens of thousands kilometers for mobile satellites, and 35,786 km for the fixe ones (geosynchronous satellites) rotating in 23 hours, 56 minutes, and 4 seconds. Orbital explosions, instrument aging, and impacts of foreign body result in a considerable amount of debris that can reach high speeds (ten kilometers per second or more). NASA monitors 13,000 catalogued objects larger than 10 cm (decommissioned satellites and satellite fragments), and estimates to about 200,000 debris of 1–10 cm, and 35,000,000 debris of 0.1–1 cm; these numbers are constantly increasing. Space debris pose risk of collision with functional satellite, damaging it through abrasion and deterioration, a virtuous cycle that creates more space projectiles. There is also a risk of falling debris that are not completely consumed during reentry in the atmosphere. The Inter Agency Space Debris Coordination Committee (IADC) was established in 1993 to identify, investigate, and mitigate the impacts of space debris. Unfortunately, no solution financially feasible has yet been found.

The Oil Palm is awarded to…

The African oil palm (Elaeis guineensis, Araceae family) is native to West Africa and grown for its oil from the abundant fruits since the early 20th century. For some years now, the global production of palm oil exceeded the soybean oil production with more than 50 million tons in 2012, according to the Food and Agriculture Organization of the United Nations (FAO). This palm is cultivated on three continents (Africa, Asia, and America), but Indonesia and Malaysia alone constitute more than 85% of world production. Palm oil is found mostly in food (such as margarine or cooking oil), but also in many derivatives consumed daily from food (confectionery, crisps...), cosmetics (soap) or chemical industry. More recently, this lucrative market encouraged the agrofuel industry. However, the palm monoculture in tropical areas becomes increasingly widespread at the expense of natural environments (currently, nearly 10 million hectares). The acceleration of palm oil demand implies several consequences. The first one is directly related to the habitat loss through deforestation: many species are currently threatened, such as Sumatran and Bornean orangutans. The second consequence concerns the geochemical carbon balance: the removal of existing vegetation by fire (CO2 emissions with the consequent impacts on the climate), and the destruction of peatland. Indeed, peats represent 3% of the land surface, but they store one third of the world's soil carbon. The drainage performed on soils for the culture of palms promotes the release of CO2 by organic degradation. For example, 25% of Indonesian palm plantations are carried out on former peatlands. Finally, the health aspect related to the presence of unsaturated fatty in palm oil remains a subject of debate within the scientific community.

Once upon a time the biological invasion

The tumbleweed is a symbol of the Sergio Leone’s movies, rolling bush on the music of Ennio Morricone. This is the upper part of a plant formerly used for the manufacture of soaps: Salsola tragus (Amaranthaceae family) or Russian thistle. Originally from the arid steppes of Eurasia, this species was accidentally introduced during the 1970's in the United States in South Dakota during import flaxseed contaminated by Russian thistle seeds. During the fall, mature plants become dry and brittle; the upper part could be detached from its base and rolled in the wind. It carries several hundreds of thousands of seeds; sometimes it colonizes inhospitable environments by its low ecological requirements (for example, high thermal amplitude or salt tolerance). Despite its popularity, Salsola tragus is a typical illustration of the biological invasion: outside of its natural environment, the ecological balance is lost by creating new competitions and it becomes invasive. Today, the Russian thistle is found on almost all United States and causes many societal consequences: destruction of wheat field, obstacles on the road, or unsightly storage near fences. The human activities promote the inadvertently disturbance of whole territories by several invasive species.

The town honey and the country honey

As in the Fontaine’s fable written in 1668 – The town mouse and the country mouse – the country is synonymous with quality of life. However, in recent years, the rural environment does not seem very hospitable to honeybees (Apis mellifera), endangering the reproduction of flowering plants. According to a report written in 2008 by the French food safety authority (Afssa), the decrease in the number of these social Hymenoptera results to a combination of forty factors grouped into biological causes (predation, parasitism and viral/bacterial infection), chemical causes (more than 400 plant protection chemicals), environmental causes (biodiversity loss) or on beekeeping practices. Into the chemical factors, it is necessary to distinguish between the treatments applied directly to the bees (antibiotics or biocides to eliminate parasites, such as Varroa destructor) and the treatments applied to the most cultivated fields. In 2013, the European food safety authority (Efsa) assures that three systemic neurotoxic insecticides of the neonicotinoid class (imidacloprid, thiamethoxam and clothianidin) have an impact on the behavior of honeybees. Along with this chemical pressure and since the advent of intensive monoculture, rural landscapes have been changed: the area of natural habitats, such as hedgerows, was significantly reduced (over 50% in some European regions in a century). This directly impacts the biological diversity, including the honey plants essential for bees. Despite the pollution, the plant biodiversity in urban environments seem to be in favor of the development of these insects in the cities, encouraging several beekeepers to produce "honey from Paris".