ESTCP Project #1011, Rhizosphere
heavier than would be used normally for establishing the grass. Extra seed is to account for
losses from poor germination and seedling die-off due to petroleum contamination and poor
growth conditions, such as drought. The goal is to get a good plant cover on the soil and
thorough root growth and penetration in the soil.
6.4.3 Fertilizing: Rate
In many cases, there has been a tendency to add fertilizer, primarily nitrogen, to yield a final
carbon:nitrogen ratio that was considered optimum--or similar to the carbon:nitrogen ratio of
bacterial cells. This seems to make sense, but petroleum is mostly carbon, and petroleum-
contaminated soils can have an exceedingly high carbon level. As petroleum is metabolized,
carbon is eventually lost from the soil as evolved carbon dioxide (CO2), but nitrogen remains in
the system, cycling among various pools in the soil. Because nitrogen is added as a fertilizer salt,
adding sufficient nitrogen to yield an optimum carbon:nitrogen ratio can cause osmotic stress to
both microbes and plants. This can result in poor or no seed germination or poor plant growth
simply due to the salt-effect of the fertilizer. It is similar to spilling fertilizer on your lawn or
adding salt to soil. The osmotic effect is very detrimental.
Two approaches are useful. One is to add fertilizer as you would for seed establishment using the
general guidance for establishing a lawn or garden. The other is to add as much nitrogen as can
be added without stunting plants. The maximal level for nitrogen additions without inhibiting
microbial activity is approximately 2000 mg N / kg soil water. Note that this approach is based
on soil water content rather than soil. The challenge to this approach is that soil water content
varies as soil wets and dries. A reasonable way to address nutrient additions is to add nutrients
based on soil water concentrations of 2000 mg nitrogen / kg soil water, and use soil water
content that is equivalent to a soil water matric potential of -33 KPa. We used this approach at
our three demonstration locations.
The problems seem to come when you add too much nitrogen to what is already present in the
soil. For many sites, there will be little available nitrogen in the soil and nitrogen applications
can be made assuming that there is effectively no residual nitrogen. However, if earlier fertilizer
applications have been made, they should be considered. At Galena, the soil had been fertilized
earlier and some residual fertilizer remained. Our fertilizer additions inhibited seed germination
until microbial processes lowered the nitrogen in the soil.
6.4.4 Fertilizing: Type of Fertilizer
There are proprietary fertilizers on the market, specifically aimed at bioremediation and
phytoremediation. Data supporting the benefits of these products are quite scarce and often not
critically defensible. For example, CRREL reviewed the marketing literature for a product
marketed as a "petroleum remediation enhancer" that showed graphs of concentrations
decreasing with time. However, the petroleum was jet fuel, the soil was sand, it was tilled every
day, it was hot and windy, and there were no control treatments for comparison. Most of the
petroleum almost certainly simply volatilized. Users of products need to know the test conditions
in addition to the marketing data and presentations.