VARIABILITY IN LOSS OF NITROGEN FROM INORGANIC CHEMICAL FERTILIZERS AND COW MANURE APPLIED TO MIDWESTERN LOAM SOILS

Degradation of nitrogen in agricultural loam soils was investigated in a field study in northeast Iowa, USA. The objective of the study was to find suitable field methods to enhance the degradation of soil nitrate. Seven different methods were examined. The target parameters were types of fertilizers (i.e., manure versus inorganic chemicals), added carbon source, irrigation, and assimilation by plants. The predetermined land use methods were designed in 21 experimental plots. Soil samples were collected from the surface and from 0.3-cm depths. A weather station was set up in the field to gather climate data.

On the surface, the maximum average concentration of nitrate was much higher in the chemically fertilized plots (500 mg/km) than those fertilized organically (120 mg/kg). This was attributed to the excessive ammonia volatilization from the applied cow manure. Rapid degradation of nitrate was observed in the 0.3-cm-deep samples of the plots that had been intermittently irrigated. Subsequently, the soil nitrate level dropped from 155 mg/kg to 50 mg/kg in a matter of 3 weeks. This is because higher soil moisture content in the irrigated plots lowered the oxygen level that favored denitrification. Ethanol was added as an additional carbon source to some of the plots. Although there are some indication that ethanol restricted the release of nitrate into the manure-treated plots, the data are not conclusive. It was not clear whether ethanol restricted soil nitrification or nitrogen was lost from soil by volatilization. The highest degradation of soil-nitrate was observed in those plots that grew corn, received cow manure, and were not irrigated. The lowest level of nitrate was recovered from these plots a few weeks after the second fertilization (38 mg/kg). Soils in these plots were depleted of nitrogen by ammonia volatilization from manure, and through the uptake by corn plants. Simultaneously, nitrification of organic nitrogen to nitrate was restricted by the dry soil condition (no irrigation). Rain events helped nitrification on the surface, but promoted denitrification at depth. Higher temperature somewhat enhanced nitrification by stimulating the nitrifying bacteria in the soil.