Dana Sullivan, Wes Wood, Frank Owsley, Lee Norfleet, Brenda Wood, and Joey Shaw

It’s 6:00 a.m. on a clear, still morning in June. The day promises to be hot and windy. As the early morning light streams over the bermudagrass pasture, the sound of a motor can be heard. This motor is running a sprinkler system, which is spreading swine waste on bermudagrass pasture.

While this could be a scene from a farm in rural Alabama, it is not. This event occurred near Auburn where AAES researchers conducted monthly field-scale experiments from June until September for two years.

Swine waste application at a site near Auburn, Alabama.

Animal manure is a major component of agricultural byproducts, and on many farms it may be used as an economical source of nutrients. Farmers in many southeastern states apply swine waste to land as a method of disposal as well as a soil amendment. AAES researchers have determined that ammonia (NH3-N) volatilization from land-applied swine waste may account for a substantial amount of nitrogen (N) losses from pasture systems.

A substantial percentage of N in swine waste consists of ammonium (NH_4-N), leading to increased concentrations of ammonia within the soil. Atmospheric ammonia contributes to formation of complex sulfates, which cause smog and acid rain. Rates of ammonia volatilization are dependent on temperature, humidity, wind, pH, and concentration of ammonium. Specifically, volatilization rates following application of swine waste increase with temperature, wind-speed, pH, and ammonium content and decrease with humidity. Decreasing pH values are indicative of ongoing volatilization and as pH decreases, ammonia volatilization slows. Volatilization losses vary between swine waste applications, but tend to follow a diurnal pattern, peaking with maximum daily temperatures at midday.

Until recently, most research in this area has been conducted in laboratories or closed field experiments. Information obtained was difficult to apply to real situations because experimental designs were often in partially or totally enclosed systems and the rates at which ammonia moved within the system were significantly different from those in a natural environment. The objective of this study was to determine ammonia volatilization rates and measure total volatilization losses from swine effluent applied to established bermudagrass pastures using a field-scale technique.

Three circular 0.284 acre bermudagrass plots were established near Auburn, Alabama, on a Hiwassee sandy loam in 1998. Plots received three applications of swine effluent (liquid waste material) and ammonium-nitrate based on Auburn University Soil Testing Laboratory results. Swine effluent applications were determined by the total phosphorus (P) content with a target rate of 8.2 pounds of P per acre. Supplemental ammonium nitrate was applied immediately following application of swine waste to meet crop nitrogen (N) requirements of 100 pounds of N per each acre.

Applications began during the 1998 growing season and were repeated in 1999. Ammonia volatilization was measured on plots receiving swine waste using a passive field scale technique developed at Auburn University (see article in Highlights Vol. 46, No.4, “Inventiveness in the Air: AAES Researchers Develop Improved Method for Measuring Ammonia Volatilization”). A single rotating mast 10 feet in height was placed in the center of each plot, and a separate mast placed upwind of plots was used to determine background volatilization rates. Daily volatilization measurements began immediately at the start of each application series and continued for approximately two weeks.

Ammonia volatilization rates increased following each application of swine effluent to bermudagrass pasture with peak daily losses ranging from 3.4 to 4.7 pounds of ammonia per acre in 1998 and 3.0 to 11.2 pounds of ammonia per acre in 1999 (see figure). In both years ammonia volatilization peaked immediately following application with a rapid decline over approximately 10 days. Approximately 60% of total ammonia volatilization took place within four days of application, and by day 10 ammonia losses were nearly complete. During each of three application periods, total applied N losses ranged from 13 to 16% and 9 to 28% during the 1998 and 1999 growing seasons, respectively.

Typical pattern of ammonia volatilization following swine waste applicaitons.

Strategies to minimize ammonia volatilization include efficient N utilization based on crop requirements, and split fertilizer applications to mediate unnecessary N losses. Infiltration of swine effluent with percolating water also influences ammonia volatilization losses; thus, incorporation of swine waste into soil may reduce the potential for volatilization. Because volatilization losses are greatest during hot, dry, and windy conditions, timing applications to avoid prime volatilization conditions may restrict gaseous N losses.

Current air quality standards do not regulate N emissions from ammonia volatilization following swine waste application. However, European standards suggest a critical loading rate of only 9 to 13 pounds of N per acre of land per year from grasslands. Furthermore, ammonia has a short half-life of approximately seven hours and may redeposit quickly near the emission source as a gas, liquid, or salt. Ammonium redeposition leads to stressed crop conditions, potential phytotoxicity, and acidification of soils.

Ammonia volatilization rates increased after applications of swine effluent to bermudagrass pasture. As much as 28% of total N applied as swine effluent may be lost through volatilization, consequently affecting total N available to crops. Thus it is recommended that ammonia volatilization be limited with time management strategies in conjunction with application methods and nutrient management plans.