Progress in the mechanism for the promotion of nitrogen loss reduction during the acidification storage of animal slurry

Abstract: Acidification storage of animal slurry is one of the key technologies to effectively regulate the relationship between microorganisms, environment, and nitrogen for the high reduction of nitrogen loss and efficiency in animal slurry. This review aims to systematically investigate the nitrogen migration and transformation during the acidified storage of animal slurry in recent years. Three acidifiers were used, including the strong acid, hydrolyzed acidic salt, and easily decomposed organic matter. Three types of acidizing storage were then selected, namely long-term, medium, and long-term acidizing in the storage pool and short-term acidizing in the application. Finally, the economic cost and application effectiveness were comparatively evaluated in this case. More importantly, a systematic analysis was performed on the reduction of nitrogen losses and enhancement of fertilizer effectiveness, according to the technical route in the acidification storage. Furthermore, future research directions were addressed to identify the shortcomings of existing technologies for the acidified storage of animal slurry. Specifically, the nitrogen migration and transformation in the acidification storage process of animal slurry were divided into organic nitrogen mineralization, ammonium nitrogen fixation, inhibition of inorganic nitrogen conversion, and nitrification. Two reasons were attributed to reducing the nitrogen loss in the acidification storage of animal slurry. The addition of acidifiers was one way to change the dynamic balance of NH4+ and NH3 by lowering the pH of animal slurry, in order to promote the formation of NH4+ while reducing the emission of dissolved NH3 in animal slurry. The microbial activity decreased with the decreasing pH and mineralization of organic matter in animal slurry, thus reducing the production and emission of NH3. The acidification storage of animal slurry reduced the NH3 emissions by 15%-77%. The degradation rate of nitrogenous organic compounds decreased with the addition of acidifiers, leading to the lower production of ammonium nitrogen. And the conversion rate of ammonium nitrogen to nitrate nitrogen was also reduced, thus further reducing the conversion of nitrate nitrogen to N2O. To sum up, the nitrogen losses were reduced to alter the microbial action and chemical equilibrium state. In addition, the long-term acidizing process presented more stable acidizing and a wider application range during the application, compared with the medium and long-term acidizing in the storage pool and the short-term acidizing. The acidification storage of animal slurry can be expected to stabilize and retain NH4+ in the animal slurry. Therefore, acidification and returning to the field can improve the content of effective nutrients (such as N and P) in the soil for the high efficiency of soil fertility. However, the low efficiency of manure fertility and the secondary pollution can also be found in the unreasonable acidification storage and application in practice. It is a high demand for the quantitative analysis of nitrogen migration and transformation pathways in the process of fecal water storage, the complex acidifiers, and the post-acidification animal slurry in agricultural fields. The assessment and response to environmental risks can be launched for the research and development of acidification storage equipment in the future.