June 27, 2009
In recent years, there has been great uncertainty regarding the cause of the hypoxic zone (low oxygen) in the northern Gulf of Mexico. This has often been the result of a lack of data to support many of the prevailing theories regarding the size, duration and source of the problem. This paper looks at the available information and draws thefollowing conclusions.
First, the hypoxic zone is seasonal. While localized effects can be severe, vast “dead zones” with widespread negative effects on the fishing industry may be overstated. On the contrary, it is possible that the water flow from the Mississippi-Atchafalaya River Basin (MARB) delivers the basic nutrients required for the very existence of the northern Gulf fishing industry.
Second, fishing data since 1985 shows no negative impact nor any clear relationship between the fish catch, the flow of water through the MARB or the size of the seasonal hypoxic zone.
Third, there is also no clear evidence of a relationship between nitrogen and the size of the seasonal hypoxic zone. In recent years, as corn production has become more efficient and yields have increased, the nitrogen removed from corn fields in the grain may equal or exceed the amount of nitrogen applied in the fertilizer.
While many conclude that corn ethanol is the real reason for the large Gulf Dead Zones, a closer look shows that this is just not true.
There are several sources of nitrogen that contribute to algae growth in the Gulf.
1) Natural sources such as fixation, soil, etc.
2) Agricultural sources such as fertilizer application
3) Industrial sources such as waste water treatment
4) Municipal sources such as sewage, golf courses, and run‐off from lawns, etc.
There has been considerable finger‐pointing at agriculture as the source of N and, in particular, at corn because the total N application is relatively high.
We explored this further to determine the net N balance in relation to corn: our hypothesis was that since corn yield has increased considerably over the years then the nitrogen removed in the grain will have increased, thereby, resulting in a large increase in nitrogen use efficiency in corn.
It should be noted that between 1970 and 1980 the N removed was just over 50% of
the applied N. However, as yields corn increased without a corresponding increase in applied N, the ratio gradually improved until, for 2007, the N amount removed in the grain is about equal to the N amount applied.
Therefore, under present day cultural practices, the net balance for N applied and N removed in corn is such that there is no excess N available due to fertilizer use. The conclusion then is that any change in N entering the Gulf via the MARB, over time, is probably not related to the use of fertilizer N for corn.
Other possible sources.
The amount of N flowing through the MARB that originates from sewage has likely increased by a considerable amount. While difficult to calculate the exact number, we can assume that N output per person is relatively constant, while the population within the Mississippi watershed increased by 22% between 1970 and 2000.
Another source that is linked to population and the expansion of homes is that from the N applied to lawns.
The estimated area for lawns, which includes golf courses and other commercial grass areas, in 2005, was ~64K sq miles = 41 MM acres across the U.S. We estimate that 60% of the area falls within the Mississippi watershed, which would be 24.6 MM acres of lawns.
The typical recommendation for lawns works out to be 130 lb N/acre/season.
Therefore, the amount of N applied to lawns within the Mississippi watershed is 3.2 billion lbs, or 1.6 MM tons N per year.
Since most lawns are cut and mulched there is relatively little removal of N, unlike the grain in corn. Consequently, a major portion of the N applied to lawns may be available for leaching. While the total amount of N applied to lawns is approx 25% of the total N applied to corn, the net N available for leaching per acre is almost infinitely higher for lawns than from corn.
Complete study with charts