Nutrient Loss from Agricultural Land Use
Phosphorus (P) and nitrogen (N) are the two major nutrients from agricultural land with the potential to degrade water quality. In the Cayuga Lake watershed, the various forms of fertilizers include:
Surface water runoff from agricultural lands to which nutrients have been applied may transport the following pollutants:
Ground-water infiltration from agricultural lands to which nutrients have been applied may transport soluble nutrients and chemicals, such as nitrogen, phosphorus, metals, and many other major and minor nutrients, and salts.
The nutrient of greatest concern in the Cayuga Lake watershed is phosphorus, the limiting nutrient for primary production in Cayuga Lake. The load and concentration of phosphorus ultimately determines the abundance, diversity, and types of plants and animals found in the lake ecosystem. Manure and fertilizers increase the level of available phosphorus in the soil to promote plant growth, but many soils now contain higher phosphorus levels than plants need (Killorn, 1980; Novais and Kamprath, 1978). Phosphorus can be found in the soil in dissolved, colloidal, or particulate forms.
A recent Cornell University study developed a nutrient mass-balance for several New York dairy farms. The study documented the excess of nutrients brought onto the farm as compared with nutrients leaving in products sold (Klausner 1995). Data from this report are summarized in Table 1.
Table 1. Nitrogen and Phosphorus mass balances on several New York dairy farms.
|Number of cows||45||85||120||45||85||120|
|Remaining on farm||64%||64%||76%||81%||69%||81%|
Source: Klausner, S. 1995. Nutrient management: Crop Production and Water Quality. 95CUWFP1, Cornell University, Ithaca NY.
It is evident from the mass balance investigation that excess nutrients are generated on dairy farms, representing a potential for loss of N and P to water resources.
Runoff and erosion can carry some of the applied phosphorus to nearby water bodies. Dissolved inorganic phosphorus (soluble reactive phosphorus) is probably the only form directly available to algae. Particulate and organic phosphorus delivered to waterbodies may be released when the sediments are exposed to a chemically reduced (anaerobic) environment. Under current conditions, the dissolved oxygen concentrations in Cayuga Lake remain high at all depths and all seasons, so release of phosphorus adsorbed to sediments is low. However, particulate and organic phosphorus compounds are available to support rooted aquatic plant growth (macrophytes) that are present at nuisance levels in the lakes shallow northern and southern basins.
Excessive nitrogen may also cause or contribute to a number of other water quality problems. Dissolved ammonia at elevated concentrations may be toxic to fish, especially trout or early life stages of many species. Nitrates in drinking water are potentially dangerous, especially to infants. The U.S. Environmental Protection Agency has set a limit of 10 mg/L nitrate-nitrogen in water used for human consumption (USEPA, 1989).
Nitrogen is naturally present in soils but must be added to increase crop production. Nitrogen is added to the soil primarily by applying commercial fertilizers and manure, but also by growing legumes (biological nitrogen fixation) and incorporating crop residues. Not all nitrogen that is present in or on the soil is available for plant use at any one time. Organic nitrogen normally constitutes the majority of the soil nitrogen. It is slowly converted (2 to 3 percent per year) to the more readily plant-available inorganic ammonium or nitrate.
The chemical form of nitrogen affects its impact on water quality. The most biologically important inorganic forms of nitrogen are ammonium (NH4-N), nitrate (NO3-N), and nitrite (NO2-N). Organic nitrogen occurs as particulate matter, in living organisms, and as detritus. It occurs in dissolved form in compounds such as amino acids, amines, purines, and urea.
Nitrate-nitrogen is highly mobile and can move readily below the crop root zone, especially in sandy soils. It can also be transported with surface runoff, but not usually in large quantities. Ammonium, on the other hand, becomes adsorbed to the soil and is lost primarily with eroding sediment. Even if nitrogen is not in a readily available form as it leaves the field, it can be converted to an available form either during transport or after delivery to waterbodies.
Monitoring data from the watershed indicates a significant range in nitrate concentration in streams draining the Cayuga watershed. Land use appears to be important; streams draining agricultural subwatersheds have higher concentrations of nitrate than streams in more forested areas. An interesting annual pattern is evident in the stream data. Higher concentrations are present in the winter, when there is little nutrient uptake by the vegetation.
Methods to Control Nutrient Loss
According to EPA (1996) the best management practices to control nutrient loss begin with a detailed site-specific plan relating the amount of nutrients applied to field conditions and needs of the crops. "Develop, implement, and periodically update a nutrient management plan to: (1) apply nutrients at rates necessary to achieve realistic crop yields, (2) improve the timing of nutrient application, and (3) use agronomic crop production technology to increase nutrient use efficiency. When the source of the nutrients is other than commercial fertilizer, determine the nutrient value and the rate of availability of the nutrients. Determine and credit the nitrogen contribution of any legume crop. Soil and plant tissue testing should be used routinely."
This commonly recommended management measure would reduce the potential for nutrient loss to both groundwater and surface waters. Another benefit is economic: most farms would reduce the amount of nutrients applied on a given site. Many producers in the Cayuga Lake watershed are already using systems that satisfy or partly satisfy the intent of this management measure, the only action that may be necessary is to determine the effectiveness of the existing practices and add additional practices. In the Cayuga Lake watershed, these voluntary measures can be implemented on individual farms as part of the whole farm planning process referenced above.
Specific data and information that may be important in refining a nutrient management plan include (EPA 1993):
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