29 June 2019

The Nutrient Cycle

Executive Summary

The nutrient cycle describes how nutrients move from the physical environment into living organisms, and subsequently are recycled back to the physical environment. This movement of nutrients, essential for life, from the environment into plants and animals and back again, is a vital function of the ecology of any region. In any particular environment, the nutrient cycle must be balanced and stable if the organisms that live in that environment are to flourish and be maintained in a constant population (MARTIN 2010). Currently, large parts of humankind influence the nutrient cycle in such a way that we remove nutrients from the land and discharge them into aquatic environments. On the one hand, this leads to soil depletion on the land, and on the other hand, an overabundance of nutrients and pollution of water sources.

Nutrients — the fuel of life

Nutrients are chemical elements that all plants and animals require for growth. On the earth, there is a constant and natural cycle how these elements are incorporated when an organism grows, and degraded if an organism dies. The nutrients used in the largest amounts are the non-mineral elements, i.e. carbon (C), hydrogen (H) and oxygen (O). These elements are mainly taken up as carbon dioxide (CO2) from the air, and water (H2O) by the roots (JOENSSON et al. 2004). They make up 95-98% of the mass of all living beings (MAHENDRAPPA 2007). But they are, however, not sufficient for life to exist.

Further elements are important to fuel life on earth: Nitrogen (N) and Phosphorus (P), Potassium (K) as well as Calcium (Ca) and Magnesium (Mg) are highly important, in particular for plant growth and agriculture. These elements are often referred to as macro nutrients. Their uptake is about 100 times that of micro nutrients. Further nutrients, that plants take up in a much smaller amount and that are essentially consumed by humans, include Boron (Bh), Copper (Cu), Iron (Fe), Chloride (Cl), Manganese (Mn), Molybdenum (Mo) and Zinc (Zn) and others. These are called micro nutrients (JOENSSON et al. 2004).

Natural nutrient cycles

The basic nutrient cycle. Source: USDA NRCS & NSTA (2010)
The basic nutrient cycle. Source: USDA NRCS & NSTA (2010)

These nutrients – essentially chemical elements – are continuously in a circular movement, the nutrient cycle. The nutrient cycle is hence a general term that describes how nutrients move from the physical environment into living organisms, and are subsequently recycled back to the physical environment (MARTIN 2010). Nutrients in the soil are taken up by plants, which are consumed by humans or animals, and excreted again by them — or they are released back into the environment when organisms die (e.g. plants lose their leaves). Microorganisms in the soil break this matter down, and again make nutrients available in their mineral form, which makes it possible for plants to take them up again (see also nutrient requirements of plants).

Essentially, all nutrients that plants and also human beings require to survive are cycled in this way. In relation to water management and sanitation, it is mainly N, P and K that are of high priority. They are the most important nutrients to sustain plant growth and agriculture, and thus humanity.

How humans influence nutrient cycles

Nutrient removal by harvesting crops. Source: UNKNOWN (n.y.)
Nutrient removal by harvesting crops. Source: UNKNOWN (n.y.)

As described above, nutrients are continuously recycled in a natural ecosystem. In recent decades, population growth and resulting human activities such as large-scale farming have caused some significant changes in nutrient cycles.

With harvesting crops, nutrients are removed from the soil. For centuries, dung from animals has been used as a fertiliser to restore the nutrients back to the soil, and in many cultures – e.g. in Europe, or also in China, also human excreta have been recycled back to agricultural fields. Hence, nutrients went back into the soil at roughly the rate they had been withdrawn. However, with the introduction of water-borne sewage, this cycle was interrupted and replaced by a linear system that transports nutrients away from soils and into watercourses (see also water pollution).

Furthermore, agriculture also influences the nutrient cycle in another way: agriculture accelerates land erosion — because ploughing and tilling disturb and expose the soil — so more nutrients drains away with runoff (see also soil degradation). And flood control contributes to disrupting the natural nutrient cycle. Typically river floods would redistribute nutrient-rich sediments to lower lands where it is again available for ecosystems. Instead dams trap sediment or embankments confine it to the river until it washes out to sea. So too much nutrients from eroded soil and from human and animal waste ends up in lakes and oceans, where it spurs massive, uncontrolled blooms of algae. Once they die and fall to the bottom, their decay starves other organisms of oxygen, creating "dead zones" and contributing to the depletion of fisheries (VACCARI 2009).

Degraded Soils. Source: REKACEWICZ (2002)
Degraded Soils. Source: REKACEWICZ (2002)

Essentially, the human alterations to the nutrient cycle leads to an excess of nutrients in aquatic ecosystems and a serious lack of nutrients in agriculture. Worldwide, more and more soils are deplete of nutrients, with serious consequences to agricultural production and food security.

The lack of nutrients in agriculture is often made up by applying artificial fertilisers. However there are several problems related to this strategy:

  • Artificial fertilisers usually contain only the most important macro nutrients nitrogen, phosphorus and potassium and lack other essential trace elements. This eventually leads to a soil deplete in micro nutrients.
  • Phosphorus is a finite resource. "China, Morocco and the United States alone currently produce almost two thirds of global phosphate. Estimates on the remaining amount of phosphorus vary, as do projections about how long it will take to deplete the irreplaceable resource entirely. Figures for easily exploitable reserves range from 60-130 years (TIESSEN 1995; STEEN 1998), but all sources agree that continued phosphorus production will decline in quality and increase in cost. It imperative that we begin recycling phosphorus and returning it to the soil to decrease the need for mined phosphorus as artificial fertiliser. Within a half century, the severity of this crisis will result in increasing food prices, food shortages and geopolitical rifts."
  • Nitrogen, on the other hand, is available abundantly. 78% of the air is composed of nitrogen. However, the process of producing nitrogen is highly energy consuming and relies mostly on fossil fuels – which, on the other hand, are finite.
  • The element potassium is available abundantly, and reserves are predicted to last for a long time. However, most of the potassium is contained in small proportions in a large number of mineral formations which makes extraction laborious.

Essentially, the problem with the human alterations to natural nutrient cycle is the one that we are extracting nutrients from the soil, and discharging them essentially in aquatic environment – this leads to a heavy imbalance with severe consequences.

Consequences

Sites with dead zones, February 2008. As visible on the picture, the problem is crucial In particular in the so called "developed countries" in North America and Europe - where sewer-based wastewater management is common. Source: AHLENIUS (2008)
Sites with dead zones, February 2008. As visible on the picture, the problem is crucial In particular in the so called "developed countries" in North America and Europe - where sewer-based wastewater management is common. Source: AHLENIUS (2008)

These human-induced alterations in the nutrient cycles lead to an imbalance in the availability of nutrients, whose consequences, in particular with regard to water, are grave:

  • Depletion of soils: The accumulation of nutrients in the seas means that they are depleted elsewhere, i.e., to a large extent from soils. This leads to the fact that many of the fruit and vegetables that animals and humans are consuming, essentially contain less nutrients, minerals, vitamins etc. that they have some decades ago. Applying more artificial fertilisers is not the solution: it is energy and cost intensive; it can lead to salinisation, and, because artificial fertilisers which are mainly based on the three main components Nitrogen, Phosphorus and Potassium, are essentially incomplete. Furthermore, soils that do not contain adequate amounts of organic material (e.g. from compost, dead plant material etc.), are not able to adsorb much of the nutrients added, and also have a low water holding capacity.
  • Depletion of nutrient sources: While nitrogen can be gained from the air, it is an energy intensive process that is mainly based on the use of fossil fuels. Other components of artificial fertiliser, are based on fossil resources, such as phosphorus. The amount that is easily mined, is limited (see above). If these easily mineable resources are deplete, this means that phosphorus prices will increase drastically.
  • Affordability & food security: "Fertilisers are bound to world market prices which are already substantially high for many farmers from developing countries. An increase in price, as is to be expected in the case of phosphorus, will make them unavailable for many farmers. This may make agricultural products, especially in developing countries, more expensive and thus lead to a decreased food security." (CONRADIN 2007).
  • Eutrophication of waterways and dead zones: "Fertiliser runoff and wastewater discharge contribute to eutrophication, uncontrolled blooms of algae in rivers, lakes and oceans, feeding on nitrogen and phosphorus from fertilisers. When they die, their decomposition depletes the water of oxygen and slowly chokes aquatic life, producing "dead zones." The largest dead zone in American waters, topping 20,000 square kilometres in July 2008, is off the Mississippi delta. More than 400 dead zones now exist worldwide, covering a combined area of more than 245,000 square kilometres." (CASTELVECCHI 2009) (see also water pollution).

Outlook

At the moment, humans influence natural nutrient cycles in an unsustainable way, and in a one-way direction. Too many nutrients end up in the sea, and are lacking on the land, leading to the above mentioned consequences. A new approach in nutrient management is needed, essentially incorporating a new way to look at what we commonly call wastewater too: this is, not to consider wastewater as a waste, but as a resource, full of nutrients that can be recycled and reused (see reuse and recharge section).

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