Contributors to increased phosphorus loss in drainage water

Increased phosphorus loss due to greater phosphorus availability in the soil and rapid water transport.

Sewage pipe leading to water source.
Drainage water flowing out of a drainage outlet. Photo by Ehsan Ghane, MSU Extension.

Excess phosphorus (P) can cause harmful algal blooms and eutrophication, as seen in recent years in Western Lake Erie. Much of this excess phosphorus comes in dissolved reactive phosphorus (DRP). Jarvie et al. 2017 noted that 65% of the increased DRP loads in Western Lake Erie arose from the increased DRP delivery. Although there are many possible factors, Jarvie et al. 2017 noted two have been identified as key contributors to increased DRP load in the Western Lake Erie Basin. These two factors are increased water transport efficiency and higher phosphorus availability in the soil.

The first factor that contributes to increased DRP load to Lake Erie is the increase in soil P, which means elevated soil test P. The buildup of P near the soil surface is known as P stratification. Baker et al. (2017) found that the mean soil-test P (STP) in the top 1 inch of soil was 55% higher than that of samples used for fertilizer recommendations leading to excessive fertilizer application and increasing phosphorus buildup near the soil surface. For fields with high amounts of P stratification, Baker et al. (2017) recommends an occasional soil inversion with mixing. To combat the elevated soil test P, soil test and use variable-rate fertilizer application.

Also, Macrae et al. 2021 recommends coupling no-till in fine-textured soil with subsurface placement or band. Subsurface placement removes P from the path of surface runoff and moves P away from preferential flow paths. If subsurface placement is unfeasible, incorporate the fertilizer after surface broadcast.

The second factor that contributes to increased DRP load to Lake Erie is the rapid water transport because more land is subsurface drained over time and drainage systems designed with narrower drain spacings. Subsurface drainage is vital to crop production as it removes excess water from the field, creating a suitable environment for crop growth and improving trafficability. This exit pathway for excess water also allows phosphorus to bypass the natural filter of the soil and be transported more quickly to surface water. To combat the rapid water transport, use drainage conservation practices, such as controlled drainage and saturated buffers to reduce drainage discharge. These practices allow the management of water that can help minimize P loss.

One conservation practice cannot be the answer to surface water quality issues, but the adoption of a suite of best management practices (in-field and edge-of-field) could help improve surface water quality and crop yields. Examples of edge-of-field practices are controlled drainage and saturated buffers that can reduce nutrient transport in drainage discharge.

Michigan State University is researching controlled drainage and saturated buffers to quantify the effectiveness of these edge-of-field drainage conservation practices. This research can then be used to recommend best management practices, specifically related to decreasing DRP delivery through drainage discharge.

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