Phosphorus: Transport to and availability in surface waters

Erosion and runoff are the main factors affecting the transport of P to surface waters. Because P is attached to soil materials, erosion largely determines the particulate P (PP) movement in the landscape.

Sources of PP in streams include eroding surface soil, stream banks, channel beds and plant material, such as leaves.

How to minimize phosphorus movement

To minimize P movement, it’s important to control erosion, especially in fertile agricultural landscapes.

In landscapes with permanent vegetative cover, such as forests or pastures, stream bank erosion is the primary source of sediment. This sediment has similar characteristics to the subsoil of the parent material, which is often low in P content.

How erosion adds phosphorus to surface waters

Eroded soil materials add P to surface waters in a very complex way. First, part of the sediment eroded from the landscape deposits at the toe slope, in depressional areas, along field boundaries or in grassy riparian zones. Sediment yield from the field to the water is only a portion of what eroded from the slope.

Then, as sediment moves, it prefers transporting the smaller particles (i.e., fine fractions) of material, while the larger particles tend to settle out. The eroded particulate matter usually has more P concentration and reactivity than the source soil. The enrichment of P may increase as much as six-fold as the movement of fine particles relative to coarser particles increases.

This is why Minnesota’s fine-textured soils (e.g., clay loam, silty clay loam, silty clay and clay soils) have a high potential of supplying P to surface water bodies if erosion occurs and the soil particles aren’t kept on the soil landscape.

Water runoff either across the soil surface or via subsurface flow can contain significant concentrations of dissolved P (DP).

How runoff removes P

As rainfall or snow melt moves across the soil surface, the water interacts with a thin layer of soil. During this process, P is extracted from the soil and plant material and dissolved in the runoff water.

Spring snowmelt can also cause standing vegetation (e.g., Conservation Reserve Program acres, alfalfa, native prairies) to lose DP. This is because the breakdown of plant cells by freezing and thawing releases the P contained in the tissue.

P removed from plant residue by rainfall or runoff may account for differences among watersheds and seasonal fluctuations in P movement.

Subsurface flow

Subsurface flow has low DP concentrations because the P-deficient subsoils sorb (absorb and adsorb) much of the soluble P contained in the water percolating through the soil profile.

Exceptions may occur in organic, permeable coarse and waterlogged soils with low ability to retain P. When surface waters become more and more overly enriched with phosphorus, it’s mostly due to fertilizer and other surface inputs, not subsurface flow.

As erosion increases, the PP fraction of total P increases, while the DP fraction decreases significantly.

You can reduce phosphorus losses from erosion and runoff by increasing residue cover on the soil surface through conservation tillage.

Tillage system

Results from a flat, tiled, clay loam site in Michigan show significantly lower losses of sediment, total P and soluble P for chisel tillage compared to moldboard plow tillage (Table 1).

However, sediment reductions were much greater than the reductions in total P or soluble P. Water flow, a combination of surface runoff and tile drainage, did not greatly differ between the two tillage systems. In other research, DP concentration in the runoff from no-till practices has often exceeded conventional practices.

Filter strips

Strategically placed and properly designed filter strips have effectively reduced erosion and P movement, especially PP. The strips must be level to prevent surface runoff water from channeling, and sufficiently wide to effectively filter out runoff contaminants.

However, these strips have had little consistent effect on reducing DP concentrations. Other measures to reduce the potential for P movement by erosion and runoff include terracing, contour tillage, cover crops, tile drainage and impoundments or small reservoirs. Most of these practices are more effective at reducing PP than DP movement in runoff.

Surface intakes

Installing surface intakes in depressional areas within the landscape serve as direct conduits from the land surface through the subsurface tile system to drainage ditches, streams and lakes.

It’s critical to prevent erosion or any lateral movement of nutrients or organic materials within the area drained by the surface intake. Because this isn’t always possible, there’s a rather high potential for P loss into drainage systems with surface tile intakes.

Decades of fertilization at rates exceeding crop removal rates have resulted in widespread increases in soil test P (STP), often above levels required for crop production.

In south-central Minnesota, for example, about 65 percent of the soil samples tested low or medium in extractable P in 1956. In 1991-93, 69 percent of the soil samples tested by the University of Minnesota Soil Testing Laboratory had soil test values greater than 21 ppm (very high).

Once STP levels become excessive, applying more P increases the potential for P movement and won’t provide any potential agronomic benefits.

Causes of high STP levels

While STP levels in the top 6 to 8 inches of the soil profile have increased over time due to fertilization, the adoption of conservation tillage systems has also influenced the level of STP in runoff from the soil surface (top inch).

Increased levels of STP at the surface of the soils – known as phosphorus stratification – have also occurred with reduced tillage. This happens when broadcast fertilizer and manure aren’t incorporated and when plant growth recycles subsoil nutrients to the soil surface. As a rule, the less tillage, the more phosphorus appears near the soil surface.

Applying livestock manure to land also elevates STP levels. This is a significant concern wherever livestock density is concentrated and the manure volume exceeds the land area available to cost-effectively apply manure.

P in runoff

Accumulating P near the soil surface increases the concentration and loss of P in runoff. Highly significant linear relationships similar to that shown in Figure 2 are frequently seen between the amount of P in the surface soil (STP) and DP concentration in surface runoff.

Though various methods accurately assess plant availability of STP, the relationship between soil P and the potential enrichment of bioavailable P (BAP) in runoff water is less clear.

Recently developed resin accumulators and iron-oxide impregnated strips have been shown to estimate BAP in both acid and alkaline soils. Further testing of these methods is necessary before they can be widely used as an environmental test for soil P bioavailability.

Loss of P in runoff is influenced by:

• The rate, method, and the time of P application.

• Source of P used.

• Amount and duration of rainfall.

• Vegetative cover.

Fertilizer method

Increased loss of P in runoff is frequently reported with increasing application rates of fertilizer P, dairy manure, swine manure and poultry litter.

The DP concentration in runoff from areas receiving broadcast fertilizer P is, on average, as much as 100 times greater than from areas where comparable rates were applied 2 inches below the soil surface. It’s been reported that incorporating dairy manure resulted in a five-fold reduction in the total P (TP) loss in runoff compared to areas receiving broadcast applications.

Timing

Timing between application of P and the first runoff event is also important, especially in situations involving manure. Losses of P from surface-applied poultry and swine manure have been reduced by up to 90 percent due to P sorption by the soil when rainfall did not occur for three days after application.

The majority of annual P loss in runoff is generally caused by one or two intense storms. The percentage of applied P lost will generally be greater if producers make broadcast P applications during periods of the year when intense storms or runoff is most likely, compared to those who apply P when runoff probabilities are less.

Phosphorus runoff is greatest during the planting season, a time of intense rain and minimum crop cover. Under some conditions, applying manure to frozen, sloping soils in the winter also greatly increases the potential for loss during spring runoff events.

Within the Minnesota River Basin, this combination of transport and P source/management factors has led to higher TP loadings in the river from March to June than all other months.

Tillage system

Conservation tillage systems, especially no-till, can reduce sediment (particulate) forms of P losses on nearly level, somewhat poorly-drained soil. However, their effect on soluble P losses varies depending on how the P fertilizer is applied.

If you switch from full-width tillage systems like chisel-field cultivator to conservation tillage systems, it must be accompanied by practices that place P fertilizers and manure below the soil surface or BAP in runoff may actually increase.

Cropland characteristics

All agricultural cropland can contribute nutrients to surface waters. However, some sites are more likely to contribute significant amounts than others. Highly erodible cropland enriched with fertilizer and manure nutrient inputs is more likely to degrade surface water quality due to greater runoff and soil losses.

Other highly probable sites for P contribution to surface waters are nearby feedlots and barnyards in near surface waters. Sites with the highest probability for P loss merit specific management practices to minimize how much they contribute to surface water.

Phosphorus losses from agriculture can be a significant source of P entering lakes and rivers. However, P budget and management information indicates that P losses in runoff represent a small portion (1 to 2 percent) of annual P inputs to agricultural land.

Many watersheds contain acreage that does not appreciably contribute to runoff of P, plus most of the annual P loss occurs during a few high-runoff events.

This suggests that management strategies to remediate water quality problems associated with losing P from the landscape will be most effective on high-risk, sensitive or source areas within a watershed, rather than implementing general strategies over a broad area.

General practices

Here are general management practices to apply to high-risk areas:

• Minimize erosion and runoff.

• Avoid P additions to very high testing soils where a crop response is unlikely.

• Incorporate or inject P inputs, such as fertilizer and manure, below the soil surface.

When runoff and erosion moves phosphorus from the landscape, it occurs as particulate P (PP) and dissolved P (DP). Generally, PP represents the majority (75 to 90 percent) of P transported from cultivated land via runoff and erosion.

PP is primarily associated with sediment and organic matter, and contains both organic and inorganic P. Although not immediately available, PP represents a major reservoir of P to aquatic vegetation and algae.

Some researchers have estimated that 20 to 40 percent of sediment inorganic P is potentially available. Additionally, PP can be transported as part of the suspended sediment load, potentially affecting downstream aquatic systems.

Dissolved P is considered most available for plant uptake and can have an immediate impact on aquatic vegetation and algal growth. Total P is the total amount of PP and DP contained in the water system.

Several studies have shown that when P inputs into lakes reduced, there was little decrease in algal growth. This suggests that other forms of P would be a better index for P bioavailability.

The bioavailable form of P (BAP) includes all of the DP and the portion of the PP that comes into solution, which ranges from 10 to 90 percent of the PP. It’s estimated that BAP represents 98 percent of the P that algae can use. So, BAP measurements, although more difficult to determine, can more accurately estimate long-term P availability to aquatic plants and algae.