Lawns are a major part of urban ecosystems. Turf grasses represent one of the major crops in the United States. The area of lawns combined would be about equal to the size of the entire state of Wisconsin.
Lawns have several environmental benefits, including carbon sequestration, reducing urban heat, and reducing. However, if they are poorly managed by lawn owners, this can negatively affect the environment.
Like all other plants, turfgrass requires water, light, and nutrients to grow adequately. The most limiting nutrient for plant growth is nitrogen. While plants can obtain some of that nitrogen from the soil, it is often necessary to supply fertilizers to meet the nitrogen requirements for plant growth.
Nitrogen is very mobile in the soil, and there is a risk that nitrogen can be leached, and thus negatively affecting groundwater quality. Fortunately, there are ways to mitigate nitrate leaching when fertilizing lawns. Lawn owners, and people who manage lawns, can select the right nitrogen source, apply it an adequate rate and select the right time to apply fertilizer.
Nitrogen source. There are many types of fertilizers on the market, so it can be confusing to choose the right one. One of the most important things is to pick a fertilizer containing slow-release nitrogen. This can also be referred to as controlled-release or water-insoluble nitrogen. These fertilizers will release nitrogen over a long period (a few weeks to several months) that will more or less follow nitrogen uptake by the turf. Natural fertilizers also deliver nitrogen over a longer period. This type of fertilizer needs to go through a microbial decomposition phase for nitrogen and other nutrients to be released.
Using slow-release fertilizers is important. Plants will only absorb the nitrogen they need at the time, and excess nitrogen can be lost to the environment. The nature of the fertilizer (i.e., natural vs. synthetic) actually has little impact on nitrate leaching losses. Improperly using natural fertilizers can also result in negative environmental impacts.
Nitrogen rate. The amount of nitrogen required to maintain a healthy lawn will vary according to the length of the growing season. This depends on the local climate, but also on the soil type in your yard. In contrast with other nutrients such as phosphorus and potassium, nitrogen requirements cannot be determined from a soil test. The easiest way to determine nitrogen requirements for a lawn is to consult local extension specialists in turfgrass science.
Providing too much nitrogen is likely to result in higher losses through leaching (and other processes). It also increases the risk of fertilizer burn. Providing too little nitrogen can lead to the thinning of the turfgrass stand, which promotes weed invasion.
Previous research shows that unfertilized turf is less efficient in reducing overall runoff, which can increase phosphorus losses compared to an adequately fertilized, and thus thicker, turf stand. Another good practice is to leave grass clippings on the soil when mowing. The clippings will decompose and release nutrients. This can account for as much as 50% of the required nitrogen, so it is an efficient way to reduce fertilizer use and saves money.
Application timing. Another method to reduce nitrate leaching is to split the required amount of nitrogen into several applications over the growing season. Similar to the use of slow-release fertilizer, this will ensure a small and consistent supply of nitrogen for the turfgrass to use. Fertilizer applications should be made when the grass is actively growing (spring and fall) because this in when the plant needs the most nitrogen. Conversely, fertilizer applications should be avoided when turf is dormant (drought periods, winter, etc.) because nitrogen uptake is slow during these periods. Applying nitrogen should also be avoided before heavy rainfalls to reduce losses through runoff and leaching (so check the weather forecast!)
In addition to these good practices, there are other simple gestures that can help protect water quality when managing a lawn. For example, keeping the grass longer, by mowing it to a height of 3 inches (8 cm) reduces runoff. It also results in a more developed root system, which is more efficient in absorbing water and nutrients. When applying fertilizer, it is also important to avoid spreading it on hard surfaces like driveways, and pavement. Clean up any accidental spills: collecting the fertilizer with a broom and spreading it back on the lawn is the safest way to do so.
Answered Guillaume Grégoire, Laval University, Canada. Dr. Grégoire recently published a paper in the Journal of Environmental Quality about nitrate losses from residential lawns.
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