Establishing
and maintaining vegetation on disturbed soils can be challenging.
Post-construction soils are typically compacted and have been stripped of the
soil horizons that contain organic matter and nutrients, thereby creating a soil
(usually the subsoil) that is low in fertility, infiltration, water holding
capacity, and biology. The standard practice for reestablishing vegetation,
particularly for erosion control and slope stabilization, is to apply seed with
commercial fertilizer, whether through broadcasting, drilling, or hydroseeding.
Research is showing that runoff from sites where application of commercial
fertilizers, used in vegetation establishment for construction site erosion
control, may be doing more harm than good. In fact, fertilizer application for
vegetation establishment near storm water conveyance systems or surface water
bodies may be the leading pollutant source in a given watershed.
As part of
the 1972 Clean Water Act (Section 303(d)), the USEPA has frequently listed
streams for Total Maximum Daily Load (TMDL) designation for specific pollutants.
Since 1995, nutrients have been one of the most frequently cited TMDL water
impairing pollutants with 5,625 reported cases impairing 3,511 listed water
bodies across the US (USEPA 2007). With 35% of our surface waters listed as
severely impaired, and 75% of us living within 10 miles of one of these impaired
bodies, we can certainly do more to make our nation’s water bodies more suitable
for swimming, fishing, and drinking (USEPA 2007).
Recent
published research from land grant universities is consistently showing nutrient
loading in runoff can be significantly reduced by using compost erosion control
blankets instead of conventional practices such as hydroseeding and broadcasting
of seed and fertilizer. Research conducted at the University of Georgia showed
that hydromulch released 2.5 times more total nitrogen (N), 8 times more
nitrate-N, 8 times more total phosphorus (P), and 9 times more soluble P in
runoff relative to compost blankets used for erosion control vegetation
establishment. In a follow up study, conducted jointly by Auburn University and
the University of Georgia, straw mulch with seed and fertilizer, released 13
times more total N and 33 times more soluble P in runoff relative compost
blankets used for slope stabilization.
Similarly,
research conducted by Texas A&M University showed compost erosion control
blankets relative to seed + fertilizer reduced total nitrogen by 88%,
nitrate-nitrogen by 45%, total phosphorus by 87%, and soluble phosphorus by 87%
(Mukhtar et al, 2004). Iowa State University reported that compost erosion
control blankets used for slope stabilization on highway embankments reduced
total nitrogen, total phosphorus, and soluble phosphorus by 99% relative to seed
and topsoil applications (Persyn et al, 2004).
Why do
compost erosion control blankets release so much less nitrogen and phosphorus in
storm runoff? There are two reasons, the form of the nutrients (or species) and
the capacity to reduce runoff volume. Compost erosion control blankets supply
nitrogen and phosphorus in organic
form. Organic nutrients are slow release, which helps to sustain plant growth,
and are also less mobile in runoff than inorganic (or mineral) nutrient forms,
such as ammonium-nitrogen, nitrate-nitrogen, and orthophosphate. Commercial
fertilizers, whether broadcast or used in hydroseed mixtures, typically use
inorganic nutrient forms. Additionally, compost erosion control blankets absorb
much more rainfall than conventional seeding and erosion control management
practices, thereby reducing runoff volumes. Published research shows that
hydromulch increases runoff volume by 137%, and straw mulch by 200%, relative to
compost erosion control blankets when used for slope stabilization applications.
Nutrient loading (or any pollutant loading) is directly proportionate to the
volume of runoff generated from a site or watershed surface.
Finally,
while the form of these nutrients (organic vs inorganic) can affect its mobility
or transport potential during a runoff event, once these nutrients enter a
receiving water the form can be even more critical. Inorganic nutrients are
generally soluble in water and thereby immediately available for plant uptake
(organic and particulate bound nutrients typically are not), which is why they
elicit a rapid growth response on land. Unfortunately, the same is true once
these nutrients enter a water body. Because these soluble nutrients are
immediately bio-available to aquatic
plants they can trigger a rapid growth response once they enter a waterway. This
leads to algae blooms, ultimately eutrophication and fish kills, and if left
unchecked, the collapse of the local aquatic ecosystem. While both nitrogen and
phosphorus can contribute to aquatic eutrophication, phosphorus is often of
greatest concern, as it is typically the limiting nutrient in aquatic
ecosystems. Just as nitrogen is the limiting nutrient in land based ecosystems
(which is why we apply more N than P for vegetation establishment), contribution
of the limiting nutrient to any ecosystem (terrestrial or aquatic) elicits the
greatest growth response. The surface water concentration of total P and soluble
P in which eutrophication is triggered is only 0.1 and 0.03 mg/L, respectively.
Typical storm runoff concentration for phosphorus is 0.4 mg/L. Consequently,
water treatment facilities are regulated so they do not discharge greater than 5
mg/L of total P into our surface waters (Brady and Weil, 1996).
The water
concentration of nitrate-nitrogen that can be toxic to humans if ingested is
only 10 mg/L.
The total
annual loss of nutrients due to soil erosion in the US is estimated to be over
42 million tons costing society over $27 billion per year (Brady and Weil,
1996). Choosing best management practices that not only reduce erosion and
sedimentation but also nutrient loading to receiving waters should be of concern
to all of us considering water quality issues, the potential effect on our
aquatic ecosystems, and the price we all pay for mitigation, loss of natural
capital, and reduction in ecosystem services. Prevention is always cheaper than
treatment.