21. Global Biogeochemical Cycles

21.2.1 Global Carbon Cycle

The biological carbon cycle is characterised by very high rates of CO₂ uptake and release. Besides being released as a result of respiration of plants and heterotrophic organisms, CO₂ is also lost from burning vegetation and the burning of fossil fuels. The net gain from gross photosynthesis and respiration by plants (autotrophic respiration) is the production of organic material by plant cover; this is known as net primary production(NPP; Chap. 12). Thus, NPP quantifies the amount of plant material produced that may be available for other organisms, unless it is not harvested for human use, used for bioenergy or burned by vegetation fires. Owing to the methods used to determine annual NPP (sampling, drying, weighing; modelling; deduction from remote sensing proxies), one should be aware that published NPP does not account for root exudates, the export of carbohydrates to mycorrhizae, rhizobia or the rhizosphere, interannual leaf and root turnover, or herbivory. Despite these shortcomings, NPP has remained an important quantity in global carbon cycle studies for estimating yield potentials or assessing impacts of pollution.

The global map of NPP (Fig. 21.4) shows higher biomass production rates in the tropics than in the lower latitudes. This is mainly due to the period of time for production (growing season length), which is either limited by water surplus or shortage or by temperatures at higher latitudes (Schulze 1982). But even at low latitudes, NPP is often limited by sunlight due to frequent cloud cover in tropical regions (Nemani et al. 2003). These global patterns of NPP are strikingly similar to those of plant biodiversity (Sect. 20.​3, Chap. 20, Fig. 20.​11).

Owing to their increased demand for energy and land, humans have strongly interfered with the natural carbon cycle (Fig. 21.5). Since industrialisation in the nineteenth century, large quantities of CO₂ have been and are still being released into the atmosphere, mainly due to emissions from fossil fuel burning, cement production and release of CO₂ by changes in natural vegetation during LULUC. These anthropogenic fluxes are larger than the natural budget of gross photosynthesis and respiration of the biosphere. The human signal thus significantly perturbs the natural carbon cycle, causing a massive increase in atmospheric CO₂ concentrations, which drive global warming. The year 2016 was the first year in which atmospheric CO₂ concentrations at Mauna Loa (Hawaii) stayed above 400 ppm year round (www.​esrl.​noaa.​gov/​gmd/​ccgg/​trends/​). Within the past two decades, a large amount of research has been carried out to understand the natural carbon source and sink capacities of terrestrial ecosystems and the oceans.

At the global scale, the carbon cycle has large reservoirs of C in the ocean and in soils (Fig. 21.5). Carbon in atmospheric CO ₂ is about 2% of the amount of C in the oceans. However, it compares well with the amount of C bound in the biomass of plants, but is only half of the C stored in soils.

The mean residence time (MRT) (defined as C pool over flux) of a CO₂ molecule in the atmosphere is about 3–7 years. However, because atmospheric CO₂ concentrations are determined not only by assimilation but also by respiration (Fig. 21.5), the overall residence time of CO₂ cannot be given by just one value but rather needs to be given for a certain process. For the period between 2000 and 2014, CO₂ in the atmosphere increased by 1.2–2.5% per year, but by around 3% per year in 2015 and 2016 (www.​esrl.​noaa.​gov/​). During this time, anthropogenic CO₂ emissions continuously increased by about 3.5% per year between 2000 and 2005 and by 1.8% per year between 2006 and 2015 (Le Quéré et al. 2016). Overall, the terrestrial biosphere has been a carbon sink at highly varying magnitudes over time. This terrestrial carbon sink accounted for about 31% of the anthropogenic CO₂ emissions between 2006 and 2015 (Le Quéré et al. 2016). The reasons for this C sink include CO₂ fertilisation effects on plant growth, increasing N deposition and a lengthening of the growing season, particularly in boreal and temperate regions.

The carbon cycle depicted in Fig. 21.5 does not consider other C-containing trace gases, such as methane (Chap. 16), which can have an even greater effect on climate than CO₂ (Fig. 21.6). Methane emissions, produced in wetlands around the globe, such as Siberia, are of a magnitude similar to that of methane emissions originating from waste management and agriculture, that is, from rice cultivation and ruminants (www.​globalcarbonproj​ect.​org). However, most parts of the terrestrial biosphere also act as (small) sinks for methane, that is, their uptake of methane by soil microorganisms exceeds emissions (Fig. 21.6).

21.2.2 Global Water Cycle

The water cycle (Fig. 21.7) (Schlesinger and Bernhardt 2013; Chap. 10) is characterised by the large water reservoir of salt water in the oceans, accounting for about 96.5% of the Earth’s water pools. Only 2.5% of water on Earth is freshwater, of which about 70% is locked up in glaciers on Antarctica and Greenland (Gleick and Palaniappan 2010). Thus, only a very small fraction, in fact less than 1%, of water on Earth can potentially be used by terrestrial ecosystems and for human life. Nevertheless, the terrestrial biosphere has a strong impact on the Earth’s water cycle, being responsible for large water vapour fluxes, that is, evapotranspiration (ET, also called latent heat flux λE) (Chaps. 10 and 16). Ecosystem ET returns about 60% of total terrestrial precipitation back to the atmosphere. This large flux is highly variable spatially and temporally, depending on incident solar radiation, environmental conditions and ecosystem type (Chap. 16). Furthermore, a H₂O molecule has a MRT (calculated as reservoir/flux) in soil of only about 300 days; the MRT in the atmosphere is much smaller (MRT: 9 days). In contrast, the MRT for water in the ocean is about 3200 years. The MTR in continental groundwater is 300 years.

The net transport of water vapour from the ocean to land is smaller than the fluxes above the oceans or landmasses. Precipitation evaporates in coastal regions, and this evaporated water is then precipitated over the interior of the landmasses again. A “wave” of rain, with evaporation and condensation events, “rolls” across the continents. In the case of the Eurosiberian region, it has been calculated that the same water molecule undergoes five to seven evaporation and condensation cycles before it reaches the Pacific Ocean. During these processes, losses by surface run-off and seepage occur, so that precipitation of about 1000 mm in the montane regions of Central Europe is reduced to 300 mm in Central Siberia and 150 mm in Eastern Siberia (Schulze et al. 2002). As a consequence of these precipitation patterns, vegetation changes accordingly: in the very dry (and cold) areas in Eastern Siberia, larch replaces pine and spruce, mainly owing to drought-associated fires.

With the large amounts of water vapour and precipitation that are converted globally, it may appear unlikely that humans can alter this cycle, particularly since they predominantly use surface freshwater, that is, the excess of the hydrological balance (Chap. 16). However, some effects of human activities are visible on regional and global scales (WBGU 1999), particularly:

• Changes in river discharge due to changes in LU and water diversion for urban settlements. These start with LC changes from forest to agriculture and continue with intensive mechanical management practices, fertilisation, irrigation and use of pesticides, potentially leading to soil degradation and erosion on agricultural land, and end with the redistribution of water via pipelines for drinking water and wastewater of cities. Water withdrawals from groundwater have already lowered regional groundwater levels, affecting plant water uptake and leading to drastic changes in species composition, but also facilitating saltwater intrusion into groundwater aquifers in coastal regions.

• Changes in run-off caused by dam building and irrigation (partially from fossil groundwater storage) and city water supplies. Well-known examples of such effects are the drastically reduced flow of the Colorado River in Arizona, the drying out of the Aral Sea and changes in water levels of the Nile River as caused by the Aswan Dam in Egypt.

• Changes in the distribution of precipitation due to the release of aerosols from combustion to the atmosphere (Fig. 21.8). Aerosols create additional condensation nuclei while the water vapour remains constant. Since there are more condensation nuclei, droplet size decreases, which delays precipitation. As a result, the vapour remains in the atmosphere as a haze, which does not develop into rain (Toon 2000). This explains the statistically established periodicity in weekly precipitation (Cerveny and Balling 1998) in the Eastern USA, where aerosol concentration increases rapidly at the beginning of the week and reaches a maximum on Wednesday/Thursday. This delays rainfall from the atmosphere. Precipitation in the first half of the week is low, increases only in the second half of the week, and reaches its statistical maximum on Saturday. Statistically, the least amount of precipitation falls on Monday. With ongoing climate change, it is also predicted that precipitation patterns across the globe will shift, causing floods in some parts and droughts in other parts (IPCC 2012). The current and predicted changes in precipitation patterns might have tremendous implications for terrestrial ecosystems and the distribution of species.

Life on Earth is not possible without water. Plant life in particular is always exposed to a trade-off between starvation and withering. Once stomata open to assimilate CO₂ during leaf photosynthesis, water is lost simultaneously owing to transpiration. Thus, global water and carbon cycles are tightly connected by plants. The ratio between photosynthesis (carbon gain) and transpiration (water loss) is called water use efficiency (Chap. 10), an ecosystem function that plays a central role in connecting global water and carbon cycles. Recently it was shown that the fertilisation effect of increased atmospheric CO₂ during the last 20 years increased the water use efficiency of Northern Hemisphere boreal forests (Keenan et al. 2013), but it has emerged that the increase in photosynthesis is in fact caused by nitrogen deposition. An increase in water use efficiency in forests has also been observed during spring droughts in Switzerland, but not in grasslands (Wolf et al. 2013). Such contrasting ecosystem responses reflect different adaptive strategies between vegetation types, which are in turn important to biosphere–atmosphere feedbacks in the climate system (Chaps. 10 and 16). Currently, one third of global terrestrial evapotranspiration is lost from cropland and grazing land (Oki and Kanae 2006). One of the highest priorities in agricultural research is breeding for high water use efficiency since 70% of current world’s water use is for irrigation (Condon et al. 2004).

21.2.3 Global Nitrogen Cycle

The largest amount of nitrogen (Fig. 21.9) (Schlesinger and Bernhardt 2013) is stored in gaseous form in the atmosphere (4 × 10¹⁵ t, with a MRT of 10⁷ years). Storage in other compartments is negligible by comparison, even though flux rates are high. Biological N ₂ fixation (Chap. 11) is the starting point of biological N processes on continents and in oceans within the global nitrogen cycle, with higher rates in oceans than on land. Also, denitrification is higher in oceans than on land. The N cycle is particularly affected by industrial N ₂ fixation (Haber-Bosch process), resulting in NH₃ production, and by NO_x production during fossil fuel burning. The present rate of industrial N₂ fixation exceeds biological N₂ fixation. Also, total nitrogen fixation (biological + industrial) of 453 × 10⁶ t N exceeds the global rate of denitrification (409 × 10⁶ t N), apparently leading to a net N accumulation in terrestrial and aquatic ecosystems. Internal gross fluxes of organically and inorganically bound N in ecosystems exceed the net fluxes by about a factor of ten. Figure 21.9 does not depict the fate of reactive nitrogen and the processes of chemical transformation of reduced and oxidised nitrogen in the atmosphere and in the biosphere. Part of the fixed nitrogen is emitted again into the atmosphere and returns to the biosphere via nitrogen deposition. These processes are shown in Schlesinger and Bernhardt (2013).

Figure 21.9 shows the flux of N₂ by denitrification but does not specifically consider the flux of N₂O generated by anoxic incomplete denitrification (Fig. 21.10). The N ₂O flux is important in view of its slow turnover (lifetime of 114 years) and due to its global warming potential over a 100-year period, which is about 298 times larger than that of CO₂. N₂O fluxes are highest in the tropics and in regions with intensive agriculture such as Europe, India and China, mainly due to mineral fertilisation. However, very large N₂O fluxes can also be found following the restoration of permanent grassland, when fertiliser is added after ploughing, harrowing and resowing. N₂O fluxes were on average 2.9 g N m⁻² year⁻¹ (Merbold et al. 2014), representing 51% of the CO_2eq fluxes from the site (i.e. equivalents of CO₂, meaning CO₂, CH₄ and N₂O). Fluxes were driven by N inputs, environmental factors such as soil water content and temperature, and by (a lack of) plant productivity.

An additional important N-containing trace gas is NO_x, which contributes to the formation of ozone in the atmosphere. NO _x formation is clearly associated with high industrial activities in the USA, Europe and East Asia and caused by car traffic. In the Southern Hemisphere, only South Africa contributes to NO_x production. NO_x acts initially as fertiliser for terrestrial vegetation, up to a point where negative impacts prevail. During the 1980s and 1990s, high N deposition in Europe and North America caused significant stress on forests, particularly for those on acidic, nutrient-poor soils, resulting in imbalanced nutrition, impairing mycorrhizal and, root growth, up to tree death (Chap. 11). Chronic N deposition also strongly affects plant species composition in many ecosystems worldwide, for example by favouring grass encroachment in forest understorey or enabling tree invasions into bogs.

21.2.4 Global Sulphur Cycle

The global sulphur cycle is important because of its effects on global aerosol production, which in turn affects the water cycle. Also, SO₂ emissions from coal burning significantly changed the cation balance in soils of the Northern Hemisphere, and soils in Europe have not recovered despite large-scale liming. S deposition in combination with N deposition was responsible for forest decline in Europe and North America in the 1980s (Chap. 11). Present SO₂ emissions have decreased to pre-industrial levels over Europe but remain very high over East Asia (Piao 2009; Li et al. 2016).

The S cycle (Fig. 21.11) is characterised by high exchange rates across oceans caused by the release of dimethyl sulphide (DMS) from marine algae that contain chlorophyll a and c (dinoflagellates, green algae, diatoms and red algae). The MRT of DMS is about 1 day as it is oxidised to sulphate, which returns to the ocean by rain. The flux of sulphate is lower on land, where anthropogenic sources of S predominate, contributing to the formation of aerosols (ammonium-sulphate). During scrubbing of smoke from power plants sulphur is bound to Ca, forming gypsum, to be used in construction.