N Mineralization of Dairy Manure from Anaerobic Storage Ponds.

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.