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Studies Find “Corn Belt” Agriculture Increases Gulf of Mexico Dead Zone

(Beyond Pesticides, February 1, 2008) Two studies of nutrient levels in the Gulf of Mexico, one by the United States Geological Survey and the other by Yale and Louisiana State University researchers, have recently been published. Both reveal that nutrient levels in the Mississippi watershed have risen significantly, and that the growing “dead zone” in the Gulf can largely be attributed to changes in agricultural practices in the last half-century.

The first report, entitled “Differences in Phosphorus and Nitrogen Delivery to The Gulf of Mexico from the Mississippi River Basin,” modeled nutrient delivery to the gulf in order to determine the watershed management practices required to reduce nitrogen discharge by 30 to 45 percent, and the size of the dead zone in half, by 2015. 13 states in the midwest contributed the overwhelming majority of inputs to the watershed. The largest contributor to nitrogen pollution of the watershed came from corn and soybean production, which also contributed to a quarter of the total phosphorus runoff into the Gulf.

The researchers concluded that future management strategies must “include recognition of important differences in the agricultural sources of N[itrogen] and P[hosphorus] . . . and better control of both N and P in close proximity to large rivers.” Recent efforts to reduce agriculture’s impact on waterways have included conservation and reduced tillage, which the report says, “are generally effective in increasing water infiltration and removing particulates from runoff, but have little effect on nitrate leaching, with more mixed results on dissolved forms of phosphorus.” In fact, “the N:P ratios in the waters delivered to streams from corn/soybean cultivation are, on average (0.164/0.023), seven times higher than the N:P ratios for the nutrient inputs to these lands.”

The second study, “Anthropogenically enhanced fluxes of water and carbon from the Mississippi River,” measured the rise in inorganic carbon inputs to the Gulf of Mexico over the last 50 years, using a 100-year data set. Carbon dioxide makes water more acidic, making it harder for coral reefs to grow, compounding the effects of the dead zone. In the report, researchers “show that the increase in bicarbonate and water fluxes is caused mainly by an increase in discharge from agricultural watersheds and has not been balanced by a rise in precipitation, which is also relevant to nutrient and pesticide fluxes to the Gulf of Mexico . . . Furthermore, land use change and management were arguably more important than changes in climate and plant CO2 fertilization to increases in riverine water and carbon export from this large region.”

“We’re learning more and more about the far-reaching effects of American agriculture on rivers and lakes,” said graduate student and researcher Whitney Broussard of the data. “If we want to clean the water, we have to steward the land with right agriculture.”

Conventional agriculture relies heavily on nutrient-intensive, quick-release fertilizers, which contribute a large proportion of total nutrient pollution of the Gulf watershed. Organic farming can reduce such inputs and instead sequester existing nutrients more successfully. To learn more about organic agriculture, visit Beyond Pesticides’ program page.

Sources: Des Moines Register (x2), Environment News Service, Louisiana State University


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