NH₃ and N₂O emission and nitrogen loss rate from biogas liquid produced by pig slurry after topdressing on vegetable fields

Application of the byproducts from anaerobic digestion of animal manures is a promising solution in recycling animal wastes as resources. By the end of 2008, about 0.13 billion tons of liquid and solid digested residues have been produced in China. The predominating portion of which is in liquid state. The produced biogas liquid after anaerobic digestion is characterized by the high concentrations of dissolved organic carbon (DOC), nitrogen (N), potassium (K) and phosphorus (P). These digested slurries are very often applied on the top of vegetable fields not only as valuable nutrient resources, but also increases soil fertility and plant nutrition. As the high availability of the nutrients in biogas slurry, N loss would happen through ammonia (NH3) volatilization and nitrous oxide (N2O) emission. To understand the loss of N after topdressing of biogas liquid from anaerobic digested pig slurry (DPS), a field experiment was carried out in a vegetable greenhouse in winter and early summer. The NH3 and N2O emissions were assessed and soil dissolved mineral and organic nitrogen contents were determined, the nitrogen loss and N efficiency of biogas liquid were calculated and the key drivers of the dynamics in soil NH3 and N2O emissions were discussed. The results showed that NH3 emission was observed immediately following every topdressing time of DPS and tended to increase in the following 48 h. The N2O emission was burst after the first topdressing time, and slightly increased in the subsequent topdressing periods and the emission rate tended to be steady. The average NH3 and N2O emission rates from DPS treatments were significantly higher than those from chemical fertilizer. Overall 16.4%-23.2% of the N contained in the biogas liquid lost through NH3 and N2O emissions in winter, and 24.7%-27.5% lost in summer. The temperature, water and pH conditions in soil should bring enough attention with regard to the fertilizer with high concentrations of NH+4-N, NO-3-N and DON.