Environmental Issues Caused by Fertilisers (AQA A-Level Biology): Revision Notes
Environmental Issues Caused by Fertilisers
In natural ecosystems, minerals like nitrate ions are returned to the soil when plants decompose. However, in agricultural systems, crops are harvested and removed, meaning these essential nutrients are not returned naturally. To maintain soil fertility and crop yields, farmers add nitrogen-containing fertilisers to replace these lost nutrients.
The shift from natural ecosystems to agricultural systems fundamentally changes nutrient cycling, creating a dependency on external fertiliser inputs to maintain productivity.
While fertilisers have provided significant benefits by increasing crop productivity and making food more affordable, their use has led to several environmental problems that require careful consideration.
Effects of nitrogen-containing fertilisers
Nitrogen is essential for biological molecules such as proteins and is needed for plant growth. Increased nitrogen availability boosts photosynthesis rates and improves crop productivity. However, the widespread use of nitrogen-containing fertilisers has created several detrimental environmental effects:
While fertilisers are essential for modern agriculture, their environmental impacts create a complex balance between food security and ecological protection.
Reduced species diversity
Nitrogen-rich soils favour the growth of fast-growing species like grasses and nettles. These competitive species outcompete other plants, leading to reduced biodiversity. Traditional species-rich habitats, such as wildflower meadows, can only survive when soil nitrogen concentrations remain low enough to prevent these aggressive species from dominating.
This explains why many conservation areas require careful management to prevent nitrogen enrichment from destroying their unique plant communities.
Water pollution through leaching and eutrophication
Excess fertiliser nutrients can be washed away from agricultural land, leading to two interconnected environmental problems: leaching and eutrophication.
Leaching process and consequences
Leaching is the process by which nutrients are removed from soil by water. When rainwater falls on fertilised land, it dissolves soluble nutrients such as nitrate ions and carries them through the soil. These dissolved nutrients move beyond the reach of plant roots and eventually reach watercourses including streams, rivers, and lakes.
This process has serious implications for human health and environmental quality:
Health and Environmental Risks:
- Drinking water contamination: High nitrate concentrations in drinking water can prevent efficient oxygen transport in babies and may be linked to stomach cancer in humans
- Environmental damage: The leached nutrients contribute to eutrophication in aquatic ecosystems
Eutrophication process
Eutrophication is the process by which nutrient concentrations increase in bodies of water. This natural process normally occurs slowly, but human activities, particularly fertiliser run-off, dramatically accelerate it. The eutrophication sequence follows these key stages:
Initial stages (1-3)
In most freshwater bodies, nitrate ions are naturally present in very low concentrations, making nitrogen a limiting factor for plant and algal growth. When nitrate concentrations increase due to leaching from fertilised land, nitrogen is no longer limiting. This allows both aquatic plant and algal populations to grow rapidly, with algae typically growing at the water surface where light is most abundant. This surface growth creates an algal bloom - a dense layer of algae.
Light limitation and plant death (4-6)
The dense algal bloom at the surface absorbs available light and prevents it from penetrating to deeper water levels. Light then becomes the limiting factor for plants and algae growing at lower depths, causing them to die. The lack of living plants and algae at depth means there is no longer a limiting factor for saprobiotic bacteria populations, which use the dead organic matter as a food source.
Oxygen depletion (7-9)
Saprobiotic bacteria require oxygen for respiration during decomposition. As their populations increase to break down the dead plant material, they create an increased demand for dissolved oxygen. This reduces oxygen concentration in the water, releasing more nitrates from the decaying organisms. Oxygen becomes the limiting factor for aerobic organisms such as fish, which ultimately die as oxygen is depleted.
Anaerobic conditions (10-11)
Without aerobic organisms to compete for resources, anaerobic organisms experience population growth. These anaerobic bacteria continue decomposing dead material but also release toxic waste products such as hydrogen sulphide, making the water unsuitable for most life forms and creating foul-smelling, polluted water bodies.
The progression from aerobic to anaerobic conditions represents a fundamental shift in ecosystem function, often leading to irreversible changes in aquatic environments.
Contributing factors
While artificial fertilisers represent the main cause of accelerated eutrophication, other factors can contribute to this process including:
- Organic manures from agriculture
- Animal slurry and human sewage
- Natural nutrient leaching from old grassland
- Soil disturbance from ploughing
Real-world implications
The environmental impact of fertiliser use can be observed in monitoring studies of affected water bodies. When fertilisers are applied to fields near lakes or rivers, changes in water quality typically become apparent within days to weeks. These changes include:
- Increased algal density and reduced water clarity
- Decreased dissolved oxygen levels
- Changes in aquatic species composition
- Long-term ecosystem degradation
The rapid timeframe of these changes demonstrates how quickly fertiliser pollution can impact aquatic ecosystems, making prevention far more effective than remediation.
Understanding these processes is essential for developing sustainable agricultural practices that balance food production needs with environmental protection.
Key Points to Remember:
- Leaching removes nutrients from soil and transports them to water bodies, causing pollution and eutrophication
- Eutrophication follows a predictable sequence where nitrogen limitation is replaced by light limitation, then oxygen limitation
- Algal blooms prevent light reaching deeper water, killing plants and creating conditions for bacterial decomposition
- Oxygen depletion kills aerobic organisms like fish and leads to anaerobic conditions with toxic waste production
- Species diversity decreases in both terrestrial and aquatic environments due to fertiliser effects