Contribution of Liquid Manure and Inorganic Fertilizers to the Production of N2 O in Dairy Forage Production Systems.

Intensive dairy production generates large amounts of manure which is typically used as fertilizer for the production of forage, together with inorganic N fertilizers. Dairy forage production systems typically receive high annual inputs of N, having a great potential for large N₂O emissions. The present study was carried out in three dairy silage corn production fields of the Central Valley of California where liquid manure containing N mainly as ammonium was applied in irrigation water together with inorganic N fertilizers. The objectives of the study were to determine (i) the proportion of the N inputs emitted to the atmosphere as N₂O, and (ii) the main sources (liquid manure vs. inorganic N fertilizer) of the N₂O. Total N inputs and N₂O emissions were assessed over two corn cropping seasons at each farm, together with soil inorganic N (NH₄⁺-N, NO₃^--N), moisture and temperature. In order to determine the proportion of liquid manure and inorganic N in the emitted N₂O, the field data were complemented with 48-hour laboratory incubations of the three soils amended with liquid manure and ¹⁵N labeled or unlabeled inorganic fertilizers. Further, both the N inputs and field N₂O samples were analyzed for their stable isotope signature (δ¹⁵N-N₂O). Results showed that between 0.63 and 1.26% of the applied N was released as N₂O. Multiple regression analysis suggested that emissions were controlled by soil moisture and inorganic N availability. The incubations with ¹⁵N-labeled inorganic N indicated that mixtures of liquid manure and inorganic N released a larger portion of the inorganic N than the same amount of each N amendment alone. The δ¹⁵N-signature of the N₂O collected in the field was best explained by the finding that the δ¹⁵N values of N₂O produced under laboratory conditions depended mainly on N availability, soil moisture, and N₂O production rate. Results of this study suggest that a tighter management of N fertilization and irrigation with small and incremental instead of large inorganic N applications produce lower N₂O emissions.