OBJECTIVES

1. to determine the short- and long-term effects of different agricultural management systems on surface soil nitrogen pools and transformations,
   
2. to assess potential sources of groundwater pollution by nitrates,
   
3. to quantify the uptake and recycling of nitrogen by crop plants under the different management systems,
   
4. to investigate the influence of the management systems on soil microbial biomass and activity,
   
5. to evaluate the efficiency with which applied nitrogen was used in the Ohio MSEA management systems.

NEED

     Improved nitrogen management in agricultural systems should lead to increased efficiency in the use of fertilizers and reduced risks of nitrate contamination of groundwater. To develop improved methods of nitrogen management, it is essential to understand the ways in which farming systems influence the cycling of nitrogen in plants and soil.

SUMMARY OF RESULTS, CONCLUSIONS, AND BENEFITS

1. Soil inorganic N concentrations in the corn phases of the rotations exhibit short-term responses reflecting differences in N application rates. Seasonal patterns in microbial biomass-N suggested a slight increase following anhydrous ammonia application.
   
2. In the spring of 1992, levels of soil nitrate were slightly higher in the chisel-plow systems that were planted to corn in 1991 than in the ridge-till system planted to corn in 1991, but levels of labile organic N were lower.
   
3. One year after establishment of the experimental plots, surface soil total N content did not change significantly, and was not affected by the management system.
   
4. In the first-year corn, initial differences between the management systems, in N uptake by corn plants, disappeared by the time of harvest; there were no differences in total N uptake, yield, or N export in grain.
   
5. In the first year corn systems, net losses of applied N from the surface soil/plant system were lower in the ridge-till management system, suggesting more efficient use of applied N in this system.

     Conclusions based on data from the inital years of the project are only tentative; the longer-term effects of the management systems on crop growth and soil processes are not known and are the subject of continuing research. In addition, 1991 was an extraordinarily dry year, and 1992 extraodinarily wet, at the Ohio MSEA site.

APPROACH

     Three management systems were established in 1991 at the Ohio MSEA site in Pike County, Ohio. They were: (A) continuous corn, (B) a corn/soybean rotation, and (C) a corn/soybean/wheat-vetch rotation. Systems A and B were chisel-plowed and disked following corn, system C was ridge-tilled. Nitrogen inputs in systems A and B were primarily in the form of anhydrous ammonia during the corn phase. System C received liquid manure, in addition to reduced amounts of anhydrous ammonia, in its corn phase. Each phase of each rotation was established on 0.4 ha plots in a complete randomized-block design, with three replicate blocks.

     The surface 15 cm of soil was sampled periodically for determination of ammonium-N and nitrate N concentrations, potentially mineralizable nitrogen (PMN), dissolved organic N (DON), microbial activity and biomass-N, and total N and C. Soil nitrogen mineralization rates were estimated using an in situ soil core incubation technique. Above-ground plant samples were taken periodically for biomass N and C determinations.

DISCUSSION OF RESULTS

     Changes in soil inorganic and organic N concentrations in the first year corn phases of the chisel-plow and ridge-till management systems are summarized in Figure 1. Large increases in soil NH₄-N concentrations following anhydrous ammonia application (June 1, 1991) were evident in both systems. Soil NO₃-N concentrations also increased following anhydrous ammonia application. Seasonal patterns of soil microbial biomass-N suggested a slight increase following anhydrous ammonia application. At the end of the first year, there were significant differences between the management systems in the concentrations of residual soil nitrate-N; the chisel-plow corn systems had slightly more NO₃-N than the ridge-till corn system. Conversely, the amount of N bound in labile organic forms was significantly greater in the ridge-till corn systems than in the chisel-plow corn systems. The total N concentrations in the soils of both the chisel-plow and ridge-till management systems were similar, even though the chisel-plow systems received over twice the amount of N fertilizer than the ridge-till system.

     In June and July, 1991, corn plants in the chisel-plow systems accumulated N (above-ground) more rapidly than in the ridge-till system, but by August, differences between the systems were no longer significant. Differences in total biomass and N content, and grain harvest between the systems, were not significant. The net use of applied N was greater in the ridge-till corn system in 1991, even though this system received less than half the amount of applied N than the chisel-plow corn systems. It is likely that the excess N applied in the chisel-plow systems was lost from the surface soil/plant zone, either through volatilization, denitrification, or leaching below the 15 cm soil depth. In the first year of establishment, the ridge-till corn system appeared to utilize applied N more effectively, and had a reduced potential for N leaching, than the chisel-plow corn systems which received greater amounts of inorganic N fertilizers.