Fertilizers cause problems with water quality when they runoff into rivers or percolate into groundwater. In fact, agriculture (including livestock agriculture) is the largest source of nonpoint water pollution in the US. (For information on water flows and water quality in the US, you can visit a web site maintained by the USGS - click water data.)
There are basically two types of water pollution, in terms of their sources, and each is responsible for approximately half of the water pollution in the US:
Point source pollution, which, as the name implies, is pollution that comes from a discrete source, such as where a pipe carrying factory wastes dumps into a river.
Nonpoint source pollution, again as the name implies, is pollution that comes from more diffuse sources, such as runoff from parking lots and roads, or from agricultural fields.
Nitrates (as in fertilizers containing NO3- ) are highly water soluble, and so move readily with surface runoff into rivers or with water percolating through the soil profile into the groundwater below. A 1998 assessment of nonpoint sources of N and P to waters in the US (conducted by the Ecological Society of America; see supplementary reading list) determined that only about 18% of the nitrogen that is applied to fields as fertilizers leaves the fields in the form of produce. This means that the remaining 82% is left behind as residue or in soils, where it either accumulates, erodes with soil (often to surface waters), leaches to groundwater, or volatilizes into the atmosphere (sometimes in forms that contribute to acidic deposition, formation of tropospheric ozone, or act as "greenhouse gases."). [Elsewhere, I have read estimates that say that 40 - 60% of nitrogen applied as fertlizers is actually used by plants, with the remainder left being in soil or otherwise lost from the system....And, more recently, given improvements in efficiency of fertilizer application in the US, it appears that most of the applied nitrogen IS removed with the agricultural outputs [Science Vitousek article 2009] -- not so for China, however!
Much of the concern about fertilizers and water quality relates to nitrates, which can cause health problems in humans (as well as other problems, described below). When ingested, nitrates are converted into nitrite in the intestine, which then combines with hemoglobin to form methemoglobin. Methemoglobin has a reduced oxygen-carrying capacity, and is particularly problematic in children, who are most readily affected by this "nitrite poisoning" or "blue baby syndrome." Elevated levels of nitrate are common in groundwater in agricultural areas; maybe some of you use well water in your homes, and are familiar with the need to have the water tested for nitrate if you live in an agricultural region. (We drink well water at my home, and have our water tested fairly regularly for NO3 and other contaminants.)
Levels of nitrate in water that aren't harmful to humans may be harmful to some species of amphibians. You may know that amphibians around the world are in decline -- much unexplained illness, death, reproductive failure, and deformity. Dr. Andy Blaustein here at Oregon State University is involved in studies of these declines, and points out that different factors are probably important to various degrees in various places. However, his lab group has reported that certain amphibians here in western Oregon are sensitive to nitrates and nitrites at concentrations that are considered very modest by human standards. It may not be coincidence that one of the species that has disappeared most alarmingly from much of its historical range [the Oregon spotted frog], which is lowlands that are now under intensive agriculture, is especially sensitive to nitrogen in the water.
Phosphates are also applied abundantly in fertilizer, and contaminate water. Unlike nitrate, however, phosphate is not water soluble, so moves only with soil movement , as it adheres to soil particles. It is the least plentiful of the "big three" nutrients (N, P, and K), and phosphate shortages are already realities in some portions of the world (e.g., parts of Africa). When it erodes on soils from agricultural fields, it is essentially nonrecoverable, washing into sediments in oceans. Recent (2001) attempts to estimate the global P budget conclude that P is accumulating in the world's soils (that is, inputs, largely from fertilizers, animal feeds, and animal wastes, are greater than removals in harvested crops and meat). The result of this imbalance between input and output is that the net P storage in soil and fresh water ecosystems of the world is estimated to be about 75% higher than during preindustrial times. A large portion of this P accumulation is in agricultural soils, as might be expected. A major problem associated with this increased P content of soils is that any factors that increase soil erosion will also increase runoff of P with soil to streams, rivers, lakes and coastal regions.
The runoff of nitrate and phosphate into lakes and streams fertilizes them, and causes accelerated eutrophication (eu = true or well; trophy = food) or enrichment of the waters.
Eutrophication is a natural process that typically occurs as lakes age. However, human-caused, accelerated eutrophication (called "cultural eutrophication") occurs more rapidly, and causes problems in the affected water bodies, as described below. It is estimated that 50-70% of all nutrients reaching surface water (principally N and P) originate on agricultural land as fertilizers or animal waste. (In the US, farm animals produce about 130 times as much waste as the country's people do! As of 2006, hogs in North Carolina alone produced as much feces and urine daily as do the combined human populations of New York + Los Angeles + Chicago + Houston!) One clear example of agriculturally-related inputs is the Lake Erie basin, where farms (crop and livestock) are estimated to contribute as much nitrogen to the lake as would the sewage of 20 million people, twice the population of the Lake Erie basin!
Urban and industrial runoff also contribute to eutrophication. You have probably heard of the bans on (or reductions in allowed amounts of) phosphates from detergents? Those bans arose because of concerns about cultural eutrophication. Sewage discharges also contribute to eutrophication. These are largely point sources though, and have been easier to control than nonpoint, diffuse sources such as agricultural runoff.
In general, excess N is particularly a problem in coastal marine regions, where N is often more limiting than P, whereas excess P is more threatening to freshwater systems.
Rich nutrient input stimulates growth of algae which change the lake or stream as their populations increase. This is particularly the case when they undergo population explosions, referred to as "blooms." Basically, the fertilizers make the lake more productive, as they stimulate algal primary productivity.
However, from a multiple use perspective, such stimulation has undesirable consequences:
(1) Penetration of light into the water is diminished. This occurs because the algae forms mats as a result of being produced faster than they are consumed. Diminished light penetration decreases the productivity of plants living in the deeper waters (and hence their production of oxygen).
(2) The water becomes depleted in oxygen. When the abundant algae die and decompose, much oxygen is consumed by those decomposers. Oxygen in the water is also lowered by the lack of primary production in the darkened, deeper waters.
(3) Lowered oxygen results in the death of fish that need high levels of dissolved oxygen ("DO"), such as trout, salmon and other desirable sport fish. The community composition of the water body changes, with fish that can tolerate low DO, such as carp predominating. As you can imagine, changes in fish communities have ramifications for the rest of the aquatic ecosystem as well, acting at least in part through changes in food webs.
(4) Further, some of the algal species that "bloom" produce toxins that render the water unpalatable.
Essentially, the entire aquatic ecosystem changes with eutrophication.
Many aquatic systems have been cleaned up dramatically as result of new and improved sewage treatment plants, the improved control over point sources of pollutants, and regulation of pollutants such as phosphate in detergents. (Much of the improvement came about as a result of passage and implementation of the US Clean Water Act in 1972.) A dramatic example in the US is the recovery of Lake Erie.
The Clean Water Act has, however, been unable to address adequately the control of nonpoint sources of pollutants, such as agricultural inputs. Further, it affects only the waters of the US. Hence, eutrophication and other water quality problems attributable to such sources continue to threaten aquatic systems.
For example, the Mississippi River carries so much nitrogen (and phosphorus) that each summer a "dead zone" the size of New Jersey forms in the Gulf of Mexico, where the river empties. The zone is usually most apparent between approximately May and September, when runoff from snowmelt and spring rains brings an accumulation of nutrients to the river and thus to the Gulf. The warmer, lighter river water spreads out over the heavier salt water, and, since the river water is so enriched with nitrogen and other nutrients, it feeds massive blooms of algae near the surface. When the algae die, sink to the bottom and are decomposed, the decomposers use tremendous quantities of oxygen, depleting its concentration in the water. This zone is devoid of bottom-dwelling fish and crustaceans, such as shrimp, which can't compete with the decomposers for oxygen. The low oxygen region isn't confined just to the depths, but in some years, extends to within several meters of the surface. The anoxic (zone with essentially zero dissolved oxygen) and hypoxic (< 2 ppm dissolved O2) zones dissipate later in the autumn, when winds, currents, and temperature conditions foster mixing of the water (the vertical temperature-induced stratification breaks up, allowing mixing of oxygen-rich surface waters with deeper, oxygen-poor waters). There are serious concerns that Louisiana's valuable coastal fisheries are imperiled-- as is true for the Chesapeake Bay and other regions where such dead zones occur.
"Dead zones" have been reported from more than 400 areas around the world (Science 15 Aug. 08), with more than 245,000 sq km of ocean being affected. The number of "dead zones" has increased nearly exponentially since the 1960's -- approximately doubling every decade [UNEP 2003; BiioScience July '05].
As you may have heard, during several recent summers, beginning in ~ 2002, a zone of hypoxic waters (waters with low dissolved oxygen concentrations) has been reported off the Oregon and Washington Coasts. These events have adverse consequences for fisheries such as crabs, but the cause hasn't been clear, and river deposition of excess fertiity seemed unlikely as a cause. It appears that what happens is this: This region of the coast experiences frequent upwelling of nutrient-rich and oxygen-poor water from the deep, which fertilizes phytoplankton (floating algae) and causes their populations to grow rapidly ("bloom"). Typically, however, upwelling is interrupted every couple of weeks by a day or two in which the winds that drive upwelling slow, upwelling stop, and the organic matter that resulted from the nutrient enrichment can get carried out off the continental shelf to the deep ocean. These interruptions are known as "relaxation events." In several recent years, however, there have been fewer relaxation events, so the decomposing organic material accumulates, decomposers consume oxygen, and you get the picture -- hypoxia results. In the summer of 2006, the hypoxia was particularly bad, and monitoring revealed that there was disappearance of fish and mass die-off of many bottom-dwelling marine orgnisms in near-shore rocky environments. The problem recedes in autumn, when winds shift direction and promote ocean currents that flush the hypoxic waters off of the continental shelf.
A logical follow up question, of course, is "what has caused the change in winds that drive upwelling?" No one knows, with certainty at present. It is increasingly thought that the changing winds that have decreased the frequency of relaxation events may be the result of larger scale climate changes, in which case, may be essentially permament.
Studies are ongoing to determine the sources of the excess nitrogen in the case of the Gulf of Mexico dead zone (and others, world wide). Sources of phosphorus are also being examined -- although phosphorus is generally less limiting to plant production in marine environments than is nitrogen, so less attention is paid to phosphorus in these cases. As nitrogen inputs cause nitrogen to be less-limiting, however, phosphorus can become limiting even in marine environments, such that enrichment with phosphorus can also be problematic [Science 2011, pg 505]. If we can learn sources for nitrogen or other nutrients, these problems can be controlled (with concerted efforts!), but you can imagine that it is difficult to do an accurate source apportionment, given the size of the Mississippi River watershed!
There is consensus, however, that much (if not most) of the nutrients (in the case of the Gulf of Mexico and most other similarly-affected regions) come from agricultural lands (including feedlots). Amazing as it may seem, the Mississippi River drains more than 55% of US agricultural lands! Other sources for nitrogen compounds include insufficiently-treated human wastes and the transportation and industrial sectors. A study by the US Army Corps on Engineers concluded that about 70 % of the nitrogen in the Mississippi River at its mouth comes from a six-state area in the cornbelt upstream (Cons. in Practice '02). The value of the excess N in the Mississippi River is estimated at ~ $750 (this being the amount that the equivalent amount of fertilizer would cost if purchased.) Oviously, it will pay farmers to keep the N in their fields rather than let it runoff or leach into the rivers.
The problem can be reduced by decreasing soil erosion (which carries fertilizer with it!) and increasing efficiency of fertilizer application. Various approaches include enhance vegetation buffers along the streams and rivers that feed this area; the plants should slow erosion and also take up some of the excess nutrients themselves. More efficient and well-timed application of fertilizers will also be important as a part of the control strategy. Wetland restoration and enhancement programs are also likely to be important, as wetlands are an important site of denitrification (the process by which bacteria reduce nitrates (NO3) to molecular N2 or N2O which are then volatilized into the atmosphere.) The Federal government might decide to provide funding to farmers to work on all of these efforts (maybe has, by now). See the article by Mitsch et al. (listed on the supplementary reading list for this part of the course) for more information on this issue.
All of these fertilizer-related problems can be minimized by more careful and efficient application of fertilizer and by soil conservation strategies (which decrease erosion and the associated transport of fertilizers) as we will discuss later when we explore prospects for alternative agriculture.
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Page maintained by Patricia Muir at Oregon State University. Last updated Oct. 29, 2012.