Fall leaves: Putting nutrients in their place
Courtesy photo
The vibrant colors of fall leaves are a beauty to behold every year in our region. The stunning display marks the end of summer and transition into winter. The leaves of fall provide more than just aesthetics, however. They also bring carbon, nitrogen, phosphorus, and potassium, along with other nutrients that can be used to feed gardens, and adding them to your compost pile is a great way to harness this nutrition. On the other hand, if leaves are left to make their way into local waterways, such as Coeur d’Alene Lake, this same nutrition can pose a threat to the health of our aquatic ecosystems.
Phosphorus and nitrogen are essential nutrients for growth. When leaves and pine needles shed in cool fall weather, they decompose and release nutrients. When it rains, fallen leaves may wash into lakes and rivers through storm drains and direct overland runoff. Excess nutrients that reach aquatic ecosystems can feed future algae and aquatic plant growth. Subsequent decomposition of this excess organic material consumes dissolved oxygen in the water column, posing a threat to organisms that depend on it for life. In Coeur d’Alene Lake, depletion of dissolved oxygen can also promote geochemical reactions that release mining-related heavy metal contaminates, such as lead, arsenic, zinc, and cadmium from lakebed sediments.
Nutrient inputs to waterbodies are a concern throughout the year but especially in autumn when leaves fall and wash into our lakes and rivers. A 2016 United States Geological Survey study in Madison, Wisconsin found that stormwater nutrient levels peak during fall months when large amounts of organic detritus (decaying organic material) littered streets. This nutrient load spike can be managed, however. The study found leaves contributed 56% of annual total phosphorus load in urban stormwater compared to 16% when streets were cleared of leaves prior to rain events.
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