Thursday, 9 October 2008: 8:45 AM
George R. Brown Convention Center, 371F
Large confinement-style dairies, as are common in the western US, generate dilute manure-containing wastewater (typically <2% solids), which is stored in anaerobic lagoons prior to land application. The rate and amount of organic N that mineralizes following land application of anaerobic lagoon water remains uncertain. Low total N and high ammonium content can lead to variable results in the typical laboratory aerobic soil incubations. Aerobic soil incubations were conducted at 22°C in sand-soil mixes with (1) unaltered dairy lagoon water samples added at rates required to reach the soil available water holding capacity, and (2) the solid (>0.3 µm) fraction of these samples, from which most of the dissolved C and N had been removed. For the solid fraction, the cumulative net mineralization after six weeks averaged 40% (range 34-45%, n=6) of the added organic N. Results for the unaltered lagoon waters (n=7) were highly variable, ranging from apparent net immobilization of added ammonium to 51% net mineralization. Other measurements indicated that biotic denitrification, ammonia volatilization, and microbial biomass uptake were not responsible for the differences in apparent net mineralization. Chemodenitrification (reaction of HNO2 and organic matter to form N gases) may have contributed to the variability. Soils treated with lagoon waters with high ammonium concentrations (>350 mg/L) had high transient nitrite concentrations. Soil nitrite concentrations measured at week 1 were negatively correlated (R2=0.83) with net organic N mineralized at wk 6. These results suggest that when undiluted lagoon waters with high ammonium concentration are applied to soils, N may be lost via chemodenitrification. When diluted lagoon waters or lagoon waters with low ammonia concentrations are applied to soil, 40% or more of the added organic N solids >0.3 µm can be expected to mineralize in six weeks.