Agriculture and the Chesapeake Bay

Introduction

The Chesapeake Bay is one of Maryland's most important natural resources. The Bay is a major contributor to Maryland's economy through both the jobs provided by the tourism and fishing industries and the tax revenues that result from their activities. The vitality of both tourism and fishing directly depends on the Bay's water quality. Poor water quality impacts tourism from fears of water borne diseases, and it directly impacts the fishing industry through waterway closings and fish mortality.

Source of the Pollution

There are many contributing sources of nutrients that flow into the Chesapeake Bay. These include sewage treatment facilities, septic systems and agricultural wastes. Although agriculture is improving methods of food production to reduce nutrient losses, nutrient losses from agriculture are increasing to meet the demands for food of a human population that is increasing in size (Kohn, 2004).

This pollution is in the form of nitrogen and phosphorus present in manure and chemical fertilizers (Williams, 1995; Letson and Gollehon, 1996; Kohn et al, 1997). As these nutrients leach and runoff into the ground and nearby waterways, they are transported to the Bay from its watershed. Figure 1 shows source loading of nitrogen and Figure 2 shows source loading of phosphorus from major rivers within the Bay's watershed. The Susquehanna river is the largest source of nitrogen and phosphorus pollution in the Bay (Thomann et al, 1994). This is attributed in part to the large farming operations in Southern Pennsylvania. See Figure 3 for locations of the rivers within the watershed.

The Source of Excess Nutrients in the Bay

Nitrogen

Nitrogen contamination of both surface water and ground water is an environmental concern. Sources of nitrogen contamination include atmospheric nitrogen, fertilizer used in commercial and suburban settings, agricultural waste, both animal and crop, and human sewage. Surface runoff contributes nitrogen to contamination of water in streams, lakes and estuaries. Nitrogen that is volatilized into the air as ammonia returns to surface water in the rain. In addition, nitrogen leaches into ground water and resurfaces in water feeding streams and lakes. Nitrate (NO3-) leaches to ground water because it is a negatively charged ion in a porous medium (soil) that has an inherent negative charge. This negatively charged soil excludes nitrate, forcing it into the liquid phase (ground water) where it leaches, contaminating ground water and wells. Even if nitrogen is applied as urea or ammonia, it is rapidly converted to nitrate in soil. Nitrogen may also volatilize into the atmosphere as ammonia (NH₃). The ammonia is deposited into the Bay by precipitation. Nitrogen can also be denitrified to atmospheric gas (N₂). Denitrification is the conversion of nitrates (NO3-), to nitrites (NO₂^-), to nitrous oxide (NO₂), and finally to nitrogen gas (N₂) which is lost to the atmosphere. This nitrogen gas is not a pollutant, however, by products of denitrification (e.g. nitrous oxides) affect the ozone layer.

Phosphorus

It is well known that excess application of nitrogen, either as manure or chemical fertilizer, can result in loss of nitrogen to surface water. In contrast, P was thought to be fixed in soil in a relatively stable from, and the conventional wisdom was that excess P would accumulate in soils and only run off if there was erosion. Management of P on farms was a matter of preventing erosion using favorable tillage and cropping strategies. Recently, however, it has been discovered that with excessive application of P to soils over a period of several years, the soils become saturated with P and runoff can occur even when erosion is controlled.

Increased soil test P from excessive application of fertilizer, manure, and crop residues has been linked to greater soluble P in runoff. No-till practices, although they reduce soil erosion, may actually make runoff of soluble P worse, because P is kept near the soil surface. Still controversial is the soil test P level at which soluble P becomes a problem and the interaction of soil type with P solubility.

Impacts on the Bay

Once transported to the Bay, these nutrients spur the growth of algae and negatively impact fish stocks. Large algae blooms reduce light penetration and, thus, kill the submerged aquatic plants which protect many small aquatic animals and form the basis of the Bay's food chain. Also, as these blooms die and decay, oxygen levels at the bottom of the Bay can drop, further affecting the small aquatic species. The larger fish that prey on these smaller species are also impacted.

Solutions

Although agricultural production has been recognized as one of the major contributors of nutrients that reduce water quality in the Bay, local agriculture should be preserved. If regional farmers go out of business, further urbanization may increase the rates of nutrient losses.

Policies that would drive agriculture from Maryland would transfer the problems to other areas of the nation. This may concentrate nutrient losses in these areas which merely relocates the problem rather than solves it.

The only realistic solution is to improve farm management practices to increase the efficiency of nutrient utilization on farms. In fact, this strategy has been working. Modern agricultural practices continue to reduce the environmental impact of food production. At the same time, the world population continues to grow, increasing the demand for food. As a result, although nutrient losses to water resources continue to decrease as a fraction of the food produced, these losses have increased in recent years as food production has increased.

Increasing the efficiency of nutrient utilization on farms is the only option to decrease the nutrient losses to the environment.

References

Kohn, R. A. 2004. Use of animal nutrition to manage nitrogen emissions from animal agriculture. Proceedings of the Mid-Atlantic Nutrition Conference, Mar. 24-25, Timonium, MD, pp.25-30. PDF

Kohn, R. A., Z. Dou, J. D. Ferguson, and R. C. Boston. 1997. A sensitivity analysis of nitrogen losses from dairy farms. Journal of Environmental Management. 50: 417-428. PDF

Letson, D. and N. Gollehon. 1996. Confined animal production and the manure problem. Choices. Third Quarter: 18-24.

Thomann, R. V., J. R. Collier, A. Butt, E. Casman, and L. C. Linker. 1994. Response of the Chesapeake Bay Water Quality Model to Loading Scenarios. Annapolis: U.S. Environmental Protection Agency for the Chesapeake Bay Program. p. I-2,II-9, II-15.

Williams, P. 1995. Animal production and European pollution problems. Animal Feed Science and Technology. 53: 135-144.

Back to Manure Net