Most of us are familiar with the threats of declining biodiversity, deforestation, and global heating. However, nutrient cycles remain less well understood by the general public and by environmentalists.
Nitrogen and phosphorus are the two primary biological nutrients that circulate through Earth’s ecosystems. Every living organism on Earth requires both elements to form proteins and vital organic compounds. Both are required for our genetic DNA. Cells require nitrogen and phosphorus to make proteins, enzymes, and other organic compounds essential for life.
We typically add nitrogen and phosphorus to our gardens and farms in animal manure and synthetic fertilizer. However, human activity has so thoroughly disrupted Earth’s natural nutrient cycles that we have degraded soils and created aquatic dead zones.
Human influences
The rapid decimation of large terrestrial mammals was the first step in humanity’s disruption of nutrient cycles. Early farming communities and entire civilizations – the Maya and Mesopotamians, for example – collapsed after depleting their soils. Farmers learned about manure, compost, biochar, and crop rotation to help stabilize soils, but the rapid growth of humanity eventually depleted soils throughout the world.
During the nineteenth century, European nations mined potassium nitrate (KNO3) and imported bird and bat guano from Pacific islands to enrich their exhausted soils. As reserves of these nitrogen sources depleted, scientists sought ways to convert atmospheric nitrogen into ammonia. In Germany, Fritz Haber succeeded and by 1913, BASF chemical company was producing 20 tonnes of ammonia per day.
Industrial fertilizer has allowed the modern growth of human population. However, as we so often learn in ecology, there exist unintended consequences.
The use of fertilizer introduces new sources of nitrogen and phosphorus to the ecosystem, and concentrates these nutrients within certain watersheds. Typically, we think of a “nutrient” as a good thing. Nutrients make things grow. However, ecology is never that simple.
By pulling nitrogen and phosphate out of the environment and concentrating these elements in our agricultural and residential septic run-off, we have overloaded certain watersheds. The annual loading is now about 8.5 million tonnes of phosphorus and 54 million tonnes of nitrogen per year. Typically, if the local plant community cannot take up the added nutrient load, the nutrients move through groundwater, ditches, and streams into lakes and oceans.
Throughout the world, lakes and marine shorelines become “over-productive” (eutrophic) where a few plant or bacteria species feast on the nutrients and choke out other lifeforms. Eutrophication can create dead zones and putrid lakes. Fish and amphibians can die out, leaving festering swamps. Swamps, of course, are part of nature too, but human activity has vastly accelerated this process and altered critical habitats.
Algae blooms virtually killed Lake Erie, between Canada and the US, Lough Neagh in the UK, Lake Taihu in Jiangsu China, Green and Fern Ridge lakes in the northwest US. We’re seeing this repeated around the world: thousands of dead or swampy former lakes. When algae blooms die off, they deplete oxygen, killing other organisms. Anoxia in the Baltic Sea, for example, has been caused by excessive nutrients. Similar ocean anoxic events are linked to present and past mass extinctions of marine life.
The disruption of Earth’s nutrient cycles remains as urgent as global heating and biodiversity loss. To come up with solutions, we must first understand the natural nutrient cycles of a healthy ecosystem.
The Nitrogen Cycle
Nitrogen gas, N2, comprises about 78 % of our atmosphere, but is not readily available for organic use. Certain bacteria in Earth’s soils can capture nitrogen and convert it to ammonia, NH3, which they use for their own growth and reproduction, leaving some surplus for plants to absorb.
These nitrogen-fixing bacteria also live in the roots of certain plants, such a beans, peas, clover, and alfalfa. In a typical symbiosis, these plants provide the bacteria a home and carbohydrates. In return, the bacteria convert nitrogen to usable ammonia. Any extra ammonia remains in the soil for other plants.
This leads to the common practice of crop rotation in gardens and farms. After a particular food crop has depleted the soil of nitrogen, the grower may plant a legume crop to restore nitrogen. Farmers in the Indus, Yangtze, Huang Ho, and Mesopotamian river valleys had figured this out by 5,000 years ago, long before anyone understood organic chemistry.
Herbivores get their nitrogen by eating plants and play a significant role in distributing nutrients throughout the ecosystem. According to a study by Christopher Doughty and colleagues, in the past, marine mammals, seabirds, fish, and terrestrial animals, “likely formed an interlinked system, recycling nutrients from the ocean depths to the continental interiors,” moving nutrients from concentrated hotspots into biomes where other plants and animals could use them. However, the role of animals has been greatly diminished through biodiversity loss. Doughty estimates that due to anthropogenic extinctions and attrition, the capacity of fish, birds, and mammals to distribute nutrients has decreased by 94% across land and ocean.
Nitrogen concentration from fertilizers may help sequester a some carbon in terrestrial ecosystems, the one possible positive impact. However, a study by Peter Vitousek and others, “Human alteration of the global nitrogen cycle,” showed that human disruption of the nitrogen cycle has:
Doubled the rate of nitrogen input into terrestrial ecosystems
Increased the greenhouse gas N2O globally, contributing to photochemical smog
Depleted calcium and potassium in soils, undermining long‐term soil fertility
Contributed to acidification of soils, streams, and lakes
Increased the eutrophication of lakes, rivers, estuaries, and coastal oceans
Diminished biological diversity
Reduced coastal marine fisheries
Taking action
As with virtually all ecological challenges, the scale of human enterprise remains a primary driver. Soils can naturally replenish nutrients after a modest harvest of crops, but not after an endlessly increasing harvest. Watershed ecosystems can process a certain increase in nutrient flow but not an endlessly increasing flow. On a national and regional level, we have to ask: what are nature’s limits?
In local gardening, small farming, and in industrial farming, we need to avoid nitrogen and phosphorous fertilizers, and use all fertilizer and manure sparingly. Both gardeners and farmers must consider how much nutrient load their crops can actually absorb, and apply no more than this.
Farmers, gardeners, residents, and industry need to manage the uptake from their nutrient flow. This can be achieved with swales, dry wells, rainwater catchment, aerobic treatment, and bioremediation. All residential and industrial septic and sewage systems also need be maintained, cleaned, and inspected, to ensure optimum operation.
Ranchers, pet owners, and small farms have to manage manure run-off. Manure should be removed from fields, isolated from precipitation, and composted prior to use as a soil additive.
Clearing, paving, road building, logging, and construction all reduce natural plant uptake and increase nutrient flow into waterways. Road ditching and culverts should attempt to restore natural groundwater flow, not collect water in ditches.
Biological techniques use bacteria, fungi, and plants to remove or metabolize nutrients and pollutants. Bioremediation occurs naturally in healthy ecosystems and can be enhanced by design. Disturbed shorelines should be replanted with native species, especially those with high nutrient uptake, such as cattails (typha species). Certain useful mushroom species, such as Garden Giant (Stropharia rugosoannulata) can absorb nutrients and metabolize pollutants.
We can reverse the trend of increasing marine dead zones and eutrophic lakes. To achieve this, however, we must accept the evidence that nature’s bounty comes with limits.