Ammonia volatilization from Vietnamese acid sulfate paddy soil following application of digested slurry from biogas digester

In some areas of Vietnam, digested slurry from biogas digesters is discharged into canals without wastewater treatment. In order to reduce environmental pollution, utilization of digested slurry as a fertilizer in paddy fields is one promising measure. NH3 in the digested slurry is easily volatilized after application. Therefore, NH3 volatilization is a key factor for fertilizer design. In this study, we conducted laboratory experiments to examine the characteristics and mechanism of NH3 volatilization from Vietnamese paddy soil (acid sulfate soil) following application of digested slurry with the overall goal of evaluating digested slurry as a fertilizer. Incubation experiments under flooded conditions were conducted with Vietnamese acid sulfate soil and Japanese soil. Six fertilizer treatments were carried out, namely Vietnamese soil with no fertilizer, Vietnamese soil with urea, Vietnamese soil with digested slurry, Vietnamese soil with half the amount of digested slurry, Japanese soil with no fertilizer, and Japanese soil with digested slurry. pH, NHx–N, and SO42− in the floodwater and NH3 volatilization fluxes were measured for 1 week after application of digested slurry or urea. The results indicated that the amount of NH3 volatilization in Vietnamese soil is small compared to Japanese soil because the pH of floodwater from Vietnamese soil remains low (3.7–7.6) due to oxidation of pyrite, and this low pH inhibits NH3 volatilization. Therefore, we conclude that NHX–N in digested slurry can be efficiently utilized as a fertilizer in Vietnamese paddy fields with acid sulfate soils. Moreover, the results also indicate that relatively more NH3 is volatilized even in Vietnamese soil by the slowdown in pyrite oxidation and the buffering capacity of digested slurry when much digested slurry is applied. Therefore, NH3 volatilization can be reduced using methods such as a split application to maintain low floodwater pH.