(Posted by Tom Fisher.)For the last 400 years agriculture has been an important component of the Chesapeake Bay watershed. Native Americans cleared small patches of forest for squash and corn, and the flood of European colonists in the 1600s followed their example and added tobacco for local use and export to Europe. Wheat and other grains in the 1700s and 1800s led to widespread clearing of forests, but poor management practices resulted in soil erosion that left a clear signal in the sediments that is still visible in cores retrieved from the bay. The introduction of European soil conservation methods in the 1800s helped stabilize a denuded landscape, and abundant oysters and submerged grasses cleared the waters.
Agriculture entered a new phase in the 1900s. Urban areas were growing and expanding into former farmlands, creating demand for agricultural production, and mechanization improved the efficiency and yields of the agricultural remaining lands. Nationwide, agriculture was moving further south and west where longer growing seasons provided better conditions than the cooler, short growing season in the NE. Many formerly cleared lands that were too wet or otherwise marginal for agriculture went back into forest, leaving only the better arable lands for production. A second phase of intensification of the remaining agricultural lands was introduced by widespread application of fertilizers – the green revolution of the 1950s and 1960s. Crop yields doubled and then doubled again, as farmers increased the applications of fertilizers and suppressed weeds with herbicides. Use of green manures (clover or other N-fixing plants) and animal manure from widely dispersed, small-scale barns declined, keeping arable lands in near-continuous production. Small excesses of phosphorus accumulated in surface soils, and highly soluble nitrate leached to groundwater from nitrogen-rich soils in the winter. At the same time, the growing poultry industry on Delmarva created an enormous demand for feed grains (corn, wheat, soy), more than could be supplied by local growers. This drove local farms away from vegetables and fruit to grain production to supply the poultry growers, and grains were imported from the Mid-West to supply the shortfall that local farms could not produce. The concentrated animal operations of the poultry industry resulted in larger quantities of manure than could be readily absorbed locally, and manure was disposed of on local soils, rather than being spread widely. Phosphorus accumulated still further in surface soils, and nitrate in groundwater increased and began appearing in local streams.
The green revolution also touched urban areas. Individual homeowners began to desire thick green lawns with smiling wives and cavorting children, and lawn care companies became a growing industry for both homes, apartment buldings, public spaces, and golf courses. USDA recommendations for lawn fertilization, reconfigured as pounds per 100 square feet, are equivalent to those of corn, the most heavily fertilized crop on agricultural lands. Just as on agricultural lands, rain infiltrating through fertilized lawns leaches excess nitrogen to groundwater, and overland flows during heavy rains carries P to storm drains and streams. Currently, fertilizer sales in the Chesapeake region are 40-50% for non-agricultural use. Compounding the problem, increasing human populations resulted in larger waste streams from treatment plants.
The intensity of agricultural production and urban discharges were beginning to influence the bay. Nitrogen leached from agricultural and urban lands via groundwater, which over years and decades moved towards streams, and phosphorus in surface soils was eroded and leached during storms. The fertilizing elements that should have been growing plants on land began to grow algae in the waters of the Bay. First, the dissolved oxygen in bottom waters began to decline as the additional algal production sank to the bottom, fueling bacterial respiration. Then submerged grasses began to decline as algal slicks on their leaves robbed them of the light that they needed to grow and reproduce. Oysters, decimated by over-harvesting and diseases, were unable to remove significant quantities of the excess algal production. Currently, the bay is very green, and those who venture into the waters to swim are challenged to see their feet. Much of the Bay’s bottom waters below 10 meters (30 feet) are devoid of oxygen, oysters, and fish in summer. Submerged grasses make occasional appearances in dry years when nitrogen and phosphorus leaching from land is reduced because of low river discharge, only to be wiped out in a wetter year when greater quantities of nitrogen and phosphorus are transported. The intensity of agricultural production and urban use of fertilizers and sewage production has stimulated the high production of Chesapeake Bay, with the classic symptoms of eutrophication: bottom water oxygen deficits, loss of submerged grasses, and turbid, green waters.
What is the scientific evidence to support this narrative? Nitrate in groundwater has increased in parallel with fertilizer sales (jump to figure 1). Compared to streams draining forests, streams draining agricultural lands are now enriched in nitrogen in proportion to the amount of agriculture (jump to figure 2). During storm events, phosphorus concentrations increase dramatically due to leaching and erosion of phosphorus-rich soils (jump to figure 3). The intensity of modern agriculture and sewage production and fertilizer use in urban areas is clearly enriching streams draining into the Bay in nitrogen and phosphorus and creating low oxygen conditions and losses of bay grasses and water transparency. It is our own waste, either due to food production on farms or from disposal of our excreta, which creates the problems in the Chesapeake.
The trick now is to learn how to deal with our own waste as a society in equitable and cost-effective ways. We have clearly abused the use of plant-stimulating fertilizers in both urban and agricultural areas, and we have resisted using our own wastes for alternative products such as fertilizer or energy production. Regulations on agricultural fertilizer use are needed, and their application on urban areas should be greatly curtailed or eliminated. We clearly need agriculture for food production, and regulations that potentially put small farmers out of business may lead to larger agribusinesses with less incentive to comply and more ability to resist regulations. Going forward, we need to consider the marginal economics of agriculture, and potentially be willing to pay more for food that is grown green, not literally green, but with less fertilizer and less loss of nitrogen and phosphorus to streams. There are many solutions available, but most will require changes, some small and others larger, in how we live on the land that drains to the Chesapeake Bay.
Figure 1 (above). Bottom panel: the history of fertilizer sales in three agricultural counties on Delmarva in the second half of the 20th century. The exponential increase in fertilizer use enriched soils in nitrogen and phosphorus and resulted in greatly increased crop yields. Top panel: the history of the amount of nitrate (NO3) in surface groundwater. Nitrate increased in parallel with fertilizer applications, and now groundwater nitrate frequently exceeds the maximum amount allowable in drinking water. As a result, surface groundwaters in most agricultural areas generally can not be used for drinking water.
Figure 2 (above). Current conditions for the amount of nitrate (NO3) in non-tidal streams on coastal plains in North America and Europe as a function of agricultural land use in the drainage basin. As agricultural land use expands to greater than 50% of total basin area, the amount of nitrate increases exponentially due to substitution of agriculture (a nitrate source) for wetlands and forest (where nitrate can be removed). In contrast, forested basins, represented as stars in the lower left corner of the graph, discharge very little nitrate despite atmospheric deposition of nitrogen. Plant uptake and soil interception remove most biologically available N in forests.
Figure 3 (above). Effects of a large rain event in late September 2004 on stream chemistry. Top panel: as stream stage (water depth and discharge) increased, seston (particulates in water) dramatically increased as a result of soil erosion and mobilization of stream sediments. Middle panel: both total phosphorus (TP) and dissolved phosphate (PO4) were mobilized to very high concentrations during the high flow period due to erosion and leaching of P-rich soils, and concentrations required many days to recover to previously lower levels. Lower panel: total nitrogen (TN) increased briefly during the storm due to mobilization of particulate nitrogen, but nitrate (NO3) decreased due to dilution of high-nitrate groundwater by lower concentrations in rainfall and overland flow during the event.