Denitrification in soil treated with beef‐feedlot manure

Abstract

Abstract Effects of yearly applications of beef‐feedlot manure on denitrification in an irrigated silty clay loam soil (pH 7.0) were studied in the field. Mass balance calculations could not account for large quantities of N. Plots that had received 85 cumulative t/ha (metric) of dry manure (1.0% N) had 2830 kg/ha of unaccountable N loss after two years. Plots that had received the highest manure treatment (1043 cumulative t/ha) had the least unaccountable N loss (700 kg/ha). Relative to control plots, one meter soil profile distributions showed increased total N and organic C at depths of 50 cm in plots that had received 522 and 687 t/ha/yr of manure, respectively, after two years. Indirect evidence in manured soil of reducing conditions necessary for denitrification was found in increased soil solution concentrations of Fe and Mn, and lowered O2 and increased CH4 concentrations in the soil atmosphere. Soil and soil solution samples taken in the summer were found to contain NO3‐N. Profile distributions of relative gas concentrations in the soil atmosphere were successfully obtained using an access tube sampling technique. N2O, an end product of denitrification, was detected at depths of 50 to 100 cm under two plots that had received 306 and 687 t/ha/yr of manure. Increased CO2 also was recorded throughout the surface meter of soil in those two plots.