These biogeochemical cycles are important to the environment because this is how each respective chemical moves through the environment. Disrupting these cycles will impact organisms across the planet in multiple ways, as we rely on these cycles for our survival.
(To review each cycle, see these links: phosphorous cycle, nitrogen cycle, and the carbon cycle.)
Every living organism is made up of carbon, nitrogen, and phosphates. Nitrogen and carbon are found in amino acids which make up proteins. Phosphates make up DNA and ATP. Thus, the availability of these elements is of great importance to the existence of living things.
Human activities, such as the burning of fossil fuels, change the distribution of carbon throughout the cycle. The increased amount of carbon dioxide in the atmosphere is causing the planet to warm. The carbon cycle and the amount of carbon found in the atmosphere, the earth, and the oceans has acted as a sort of control for the longterm stability of temperatures across the planet.
You can read more about the carbon cycle and climate change on this page from the American Museum of Natural History.
The carbon cycle and its reservoirs of carbon:
Another example of the importance of these biogeochemical cycles is the disruption of the nitrogen cycle by humans, particularly the use of fertilizers. Some have argued that the nitrogen cycle has actually been altered by humans more than any other cycle. You can read about this in detail here.
The increase in nitrates in our waterways from fertilizers changes the chemistry of ecosystems, resulting in algae blooms that deplete dissolved oxygen and cause dead zones.
How too much nitrogen affects oceans, lakes, and other aquatic ecosystems:
See these related answers on Socratic on the importance of the carbon cycle, how the phosphorus cycle affects humans, and why is the nitrogen cycle important to living things.
The available data thus shows that the unbalanced human-induced inputs of carbon, nitrogen and phosphorus into the biosphere are altering environmental N:P ratios and that these altered environmental N:P ratios are affecting the metabolism and growth rates, and therefore the life histories and competitiveness of various microbes, plants and animals33. The observed metabolic shifts associated with organisms N:P ratio change21 provide support for the hypothesis that exceedances of the optimal N:P ratios can reduce growth rates34. Several studies have verified this hypothesis in diverse unicellular organisms, zooplankton and fish in aquatic ecosystems; there is evidence that this hypothesis can also be extended to a large number of terrestrial plants and animals, albeit with a few exceptions35 (Fig. 4).
The observed increasingly imbalanced inputs of N and P require substantial upregulation of the homoeostatic and flexibility mechanisms of the different organisms and communities and their corresponding energy costs, and in some cases it is likely that they will exceed the homoeostatic and flexibility limits, that is, the limits in the imbalance of the inputs of N and P that these homoeostatic and flexibility mechanisms can cope with36. Organisms and communities present a certain degree of homoeostasis, a certain capacity to maintain roughly constant stoichiometries by adapting their functioning and by acquiring more of the limiting nutrients. However, the homoeostatic capacity and the ranges of stoichiometric flexibility of organisms and communities are limited, and differ among species. The species that are stoichiometrically more homoeostatic than others tend to have higher biomass; and ecosystems dominated by more homoeostatic species tend to have higher productivity and stability37. Less homoeostatic species tend to have greater nitrogen and phosphorus concentrations and lower N:P ratio38. This suggests that fast shift rates of N:P ratio can be more detrimental for less homoeostatic species, with consequences for community composition and carbon cycling.
The changes in community composition in response to altered N–P availability by nitrogen enrichment can result from multiple processes. These are some reported examples: First, the N2-fixing community loses its competitive advantage over other species and decreases in abundance39. Second, increased N:P availability ratio in soils and waters favours the slow-growing species with high optimal N:P ratios at the expense of the fast-growing species with lower optimal N:P ratio34. Third, by favouring species with slower growth rates, food sources with higher N:P ratios decrease the rates of energy transfer through food webs40, favouring shorter trophic webs with fewer predators41, thus potentially decreasing biodiversity. Fourth, in phosphorus-limited zones of the coastal waters, zooplankton (for example, copepods) adapt their feeding behaviour to counteract the resource limitation in selecting higher phosphorus-containing organisms (for example, ciliates) rather than nutrient-limited phytoplankton42. Fifth, in the open ocean, altered nutrient input ratios reduce food-web diversity, alter phytoplankton community composition and could increase toxic blooms of phytoplankton43. Looking at the past may reveal how changes in the N:P ratio influence marine biota. Diatom frustule size seems to have decreased through the Neogene, linked with a decrease in phosphorus availability44. Further back in time, large episodic accumulations of phosphorite in the early Paleozoic, Permian, Jurassic, early Cretaceous and Cenozoic periods45 indicate periods of well-suited sedimentation conditions combined with a high biological productivity, potentially driven by enhanced delivery of P by continental weathering. Most of these events were associated with widespread anoxic conditions, sea-level changes, tectonics and biodiversification44,45,46,47. For example, phytoplankton adapted to phosphorus-rich environments is thought to have diversified during the early Palaeozoic in open waters overlying deeper anoxic zones48. Enhanced phosphorus content compared with nitrogen, however, does not necessarily confer benefits to the marine biomass. An example is the possible decline in growth efficiency and animal biomass in benthic marine ecosystems in the Triassic, which led to a slow recovery after the Permian mass extinction that coincided with a period of supposed low-nitrogen but high-phosphorus delivery to the ocean49. Marine ecosystems in the geological past thus appear to have been highly sensitive to changes in the N:P ratio, although most events recorded a decrease in this ratio during transition periods ranging from 104 to 106 years. A strong increase in this ratio over a few decades, as is now occurring in response to human activities, seems unprecedented in Earth’s history.
The imbalanced human-induced inputs of N and P are also affecting the functioning of ecosystems. For many regions of the Northern Hemisphere, human nitrogen inputs are converting originally nitrogen-limited ecosystems into a state of nitrogen saturation, with nitrogen losses to aquatic ecosystems and with high rates of nitrogen volatilization50. The response of tropical ecosystems to nitrogen addition is more uncertain. Some models suggest that tropical forests will be unresponsive to nitrogen enrichment because it is already plentiful and phosphorus is the key limiting nutrient8,51. However, the diversity of conditions with nutrient limitation is very high in tropical forests52,53, and therefore uncertainty is large. Furthermore, increased nitrogen loadings can affect ecosystem phosphorus cycling, as observed in a wide variety of terrestrial ecosystems54, by favouring higher plant phosphorus uptake e.g. through enhanced activity of soil phosphatases55, root phosphatases56 or changing symbiotic fungi57. Nonetheless, in the long term, these mechanisms seem quantitatively insufficient to deliver enough phosphorus to alleviate phosphorus limitation58. On the other hand, eventual negative effects of deposited NH4+ on litter decomposition, as frequently observed in northern ecosystems31, could slow nutrient cycling and further reduce phosphorus availability and ecosystem productivity.
Few studies have focused on the role of changes in atmospheric deposition or riverine input of nutrients (including phosphorus) on marine biota and productivity. Krishnamurthy et al.59 have shown that changes in the deposition of atmospheric nitrogen and phosphorus should only have a weak effect on marine productivity and the marine carbon sink on a global scale, because these depositions are small relative to the input of these nutrients from deeper waters by ocean mixing and upwellings. However, the potential for compensatory effects of these changes in nutrient input with the expected decrease in the vertical supply of phosphorus from deep water due to marine stratification60 remains largely unknown. Because anthropogenic riverine supply and atmospheric deposition are concentrated near coasts, it is the coastal zone that is most affected by phosphorus changes on short timescales61.
The responses of communities and ecosystems to environmental change are likely to be more complicated than predicted from N and P alone or the ratios between them. There are many complexities in ecosystem functioning including multiple factors and nutrients limiting productivity: for example, nitrogen deposition also depletes ecosystems of calcium as soils are acidified, promoting calcium limitation (for example, in northern temperate forests), while there is evidence that N, P and K all limit different components of biomass production in lowland tropical rain forests53. However, analysing C:N:P interactions and the isolation of the N:P impacts in our analysis is already one important step ahead with respect to previous studies.
The mass balance approach used here shows that limited P and N availability are likely to jointly reduce future carbon storage by terrestrial ecosystems during this century. A high sensitivity to model assumptions was found in this study, indicating that the future mobilization of phosphorus stocks, in particular soil phosphorus and interactions with microbial processes is critical for sustaining changes in future carbon stocks. Unfortunately, current process understanding is lagging behind model needs8,58. Knowledge about the potential for ecosystems to tap into the labile-phosphorus pool and for nitrogen deposition to affect the processes by which plants may try to take up phosphorus from the labile pool appears crucial to determine the future carbon sequestration potential of terrestrial ecosystems and specially of tropical ecosystems.
For the extra-tropical regions (Fig. 5), the calculated change in phosphorus supply was insufficient to support the projected changes in carbon stocks. This was mainly due to increases in P bound in soil carbon, which tends to have a lower C:P ratio than vegetation. This result is somewhat surprising given the paradigm that non-tropical systems are considered less phosphorus-limited than tropical systems62 but consistent with more recent work suggesting equal nutrient limitation across biomes63 and near future shifts from N to P limitations at high latitudes although nutrient limitations in the tropics decline8.
The magnitude of the marine sink of anthropogenic CO2 is more certain than the terrestrial sink64 and can be assessed using several methods, now all converging towards a figure of 2.4±0.5 Pg C per year (ref. 65). The marine sink is mostly due to physical and chemical mechanisms. It is mainly driven by increasing levels of atmospheric CO2, which tend to increase diffusion in regions that exhibit natural CO2 uptake (for example, North Atlantic) and to decrease outgassing in regions that experience natural CO2 outgassing (for example, equatorial Pacific). Thus, contrary to what happens on land, no additional phosphorus input is required to absorb most of the anthropogenic carbon fixed in the oceans. However, any increase or reduction of phosphorus input to the oceans from atmospheric deposition or from rivers potentially affects biological carbon fixation in the oceans and indirectly perturbs air-sea CO2 fluxes – but these effects are still largely unknown. In the oceans, with low living-biomass relative to its annual turnover, the carbon and nutrient cycles are tightly coupled. With rapid recycling of carbon and nutrients, the ocean carbon–climate feedback seems weak at the century timescale, but a remaining question is whether the changes in the C:N:P ratios in both the particulate and dissolved organic matter pools may provide a mechanism for biological processes to change the amount of carbon sequestered by the ocean66. A recent study67 suggests that coastal oceans are an anthropogenic sink of CO2 of 0.2 Pg C per year and that it is mainly due to phosphorus and nitrogen increasing accompanied by usually also increasing N:P ratios, biological activity and burial of carbon in organic shelf-sediments, but not in open ocean.
At crop level, our results would mean that mineable phosphorus reserves would be depleted by over 100% by 2100 given the lower and upper estimates for these reserves from Fig. 2. We observed that future phosphorus deficits for cereal crops (including wheat, maize and rice) may be especially large in Africa and Russia where the potential yields are not realized (Fig. 6), which is in agreement with previous work68 and which has large implications for the sustainability of global agriculture, and its geopolitical consequences.
In conclusion, a key and complex impact of the N:P ratio can be anticipated in a carbon- and nitrogen-enriched current and future world. The many lines of evidence reported here indicate that the changes in phosphorus and N:P ratios are, and will become more so, vital through their controlling role on organism and ecosystem functioning and structure, the carbon cycle, climate and agriculture. Nevertheless, we still have little knowledge of where, how and to what degree the imbalance in phosphorus and nitrogen additions to ecosystems will affect the structure and diversity of microbial, plant and animal communities, and their functioning, including for example organic matter decomposition, N2 fixation, N2O and NOx emissions or CO2 uptake, and the magnitude of the feedback of this altered Earth-system structure and functioning on nutrient cycling and climate change. However, our estimations indicate that the change in nitrogen and phosphorus supply is likely insufficient to support the projected changes in carbon stocks even for the extra-tropical regions, which is surprising because non-tropical systems are considered less phosphorus-limited than tropical systems.