Soil regulates nutrients: Nature's recycling system

When nutrient cycles become more open

Natural ecosystems are highly efficient at recycling nutrients, but human activities can significantly alter these cycles. Compared with many human-managed systems, natural ecosystems tend to retain and recycle a larger proportion of their nutrients internally. This contrast is sometimes described as the difference between relatively closed and more open nutrient cycles (Mariotte et al., 2018).

In many human-managed systems, nutrients are moved away from where they originate. Agriculture provides a clear example. When crops are harvested, nutrients stored in plants leave the field and travel through food systems to towns and cities. After consumption, many of these nutrients enter wastewater or other waste streams instead of returning to the agricultural soils where they were originally taken up.

To sustain crop production, farmers often replenish soil nutrients by applying fertilisers. Fertilisers play an important role in food production, but they also represent an external input into the nutrient cycle. When nutrients are applied in excessive amounts or at times when crops cannot absorb them efficiently, they may be lost from the agricultural system. For example, nitrate is highly soluble and can be transported with water through the soil and eventually reach groundwater or surface waters, where it can contribute to environmental problems such as excessive algal growth, leading to eutrophication and reduced water quality.

You have already seen how this process affects water quality in the previous lesson on soil and water regulation. Here, we examine the same process from a different perspective: when nutrients leave the soil, they are not only a potential pollutant but also a valuable resource that is no longer available for future plant growth.

Human settlements create an additional challenge for nutrient cycling. Food and other products transport nutrients from rural areas to towns and cities, where they often become concentrated in waste streams. As a result, some places experience nutrient depletion, while others face nutrient surpluses and pollution (Trimmer & Guest, 2018).