Phosphorus (P) loss from cropland is a concern in the Lake Champlain Basin and many other watersheds both in the U.S. and globally. Crops need adequate P for optimal performance and yield, however excess P lost to runoff can contribute to eutrophication of freshwater. Unlike nitrogen (N), P reacts readily with soil constituents (i.e, Al, Ca, Fe, clays) and is generally tightly bound. As such, P loss from crop fields is often associated with soil erosion and surface water runoff. With the exception of soils that have received large amounts of manure or fertilizer applications, only a very small fraction of total soil P is available for plant growth.
Where soil P levels are elevated due to historical P applications, P is more soluble and more easily lost to runoff water. While this is advantageous for crop growth, if this runoff water enters ditches or surface waters it may contribute to water quality deterioration. Practices that keep more soil and P on fields are thus mutually beneficial for agricultural production and water quality.
While wastewater treatment plants, urban/residential areas, and forests all contribute P to Lake Champlain, agriculture is considered to contribute 38% of the total P load. Recent policy changes in Vermont have increased the focus on controlling P losses from the agricultural sector. As part of this effort, the idea of regulating tile drains as a “point source” of P has been considered.
The objective of tile drains is to lower the seasonally high water table in poorly drained fields to create more favorable growing conditions. While there are many agronomic benefits of tile drains (higher yield potential, lower compaction potential, warmer soils), there are also soil conservation and water quality benefits. In fact, USDA previously promoted tile drainage to reduce erosion losses and facilitate other conservation practices such as no-till.
Tile drains have been under increased scrutiny recently due to the potential for N and P transport in tile flow. Due to their much different behavior chemically, N is more vulnerable to leaching and movement in tile drain flow. While some degree of P leaching to tile drains can occur, it greatly depends on several variables including field hydrology (i.e., amount of runoff leaving in surface runoff or tile drainage) soil texture, soil P levels, manure management, and tile depth. Two years of monitoring tile drain flow and surface runoff at one of our research sites has shown that over 90% of total and soluble reactive P loss was due to surface runoff, despite the majority of runoff being from tile drains.
Other studies have shown that P losses from tile drains can account for a larger fraction of P loss. For example, some studies in the Midwest have shown that over 50% of total P loss can occur via tile drain flow. These fields tend to be large, flat fields with little slope and therefore generate lower amounts of surface runoff. Our soils and landscapes in the Northeast are more topographically diverse with variable drainage, and tend to generate substantial amounts of surface water runoff, particularly during the non-growing season.
Few field studies have been conducted in NY or VT with the specific objective of comparing P export in tile drainage and surface runoff from the same field or similar fields. More research is needed to quantify P and N loss differences between tile-drained and undrained fields to better understand how hydrology and management impact losses, and to evaluate different best management practices that can mitigate losses.
Research has shown that tile drains can decrease erosion/total P loss. In addition, we know that in most cases P loss from crop fields is dominated by surface water runoff. Given this and the fact that there’s a clear lack of field-based research systematically addressing P loss from both tile-drained and undrained fields, regulating tiles would not be based on the best available science. Our fields in the Northeast are different than those in the Midwest or Canada, and more studies are warranted to better understand potential water quality tradeoffs associated with tiledrained fields. We know that nutrient management practices based on field studies are the best way to minimize nutrient losses to runoff water, whether surface runoff or tile drainage.