How do exogenous additives affect the direction of heavy metals and the preservation of nitrogen in pig manure compost?

Most of the heavy metals in pig manure originate from feed additives, such as copper and zinc. When these heavy metals are introduced into agricultural soil, they can significantly increase the heavy metal content in crops, posing a threat to both the environment and human health. While pig manure is rich in nitrogen, an essential nutrient for crop growth, a substantial amount of nitrogen is lost in gaseous form during the composting process, impacting the quality of the compost. Moreover, this process results in the emission of significant amounts of greenhouse gases. Therefore, it is crucial to comprehend and regulate the production of greenhouse gases from manure. The addition of heavy metal passivators can alter the behavior of heavy metals and the composting environment, thereby influencing the nitrogen release during composting. This review consolidates recent research on the impact of pig manure compost additives on heavy metal passivation and nitrogen retention, offering valuable insights and future research directions.

By adding exogenous additives to pig manure, the migration path of heavy metals can be altered, reducing the bioavailability of heavy metals and fixing their role. The main passivators for heavy metals are categorized as physical, chemical, and microbial. Currently, there is significant research focusing on physical passivating agents, while microbial additives have shown promising applications in heavy metal passivation, emerging as one of the most effective methods for reducing the bioavailability of heavy metals in local composting. In terms of compost nitrogen retention, similar to heavy metal passivation agents, additives are divided into physical, chemical, and microbial categories. Physical additives primarily work through physical adsorption, ion exchange, co-precipitation, and molecular interaction to decrease the release of NH₃ and N₂O during composting, while chemical additives slow down nitrogen-containing gas emissions through chemical reactions to minimize nitrogen loss. Microbial additives involve the addition of small amounts of microbial agents to alter carbon and nitrogen metabolism in compost, along with microbial activity, to decrease ammonia emissions.

Numerous studies have made significant advancements in heavy metal passivation and nitrogen retention. However, there are existing limitations, particularly in the limited practical application of research findings on a large scale. The cost of production for additives remains a crucial issue that needs to be addressed. It is essential to focus on the development of sustainable additives, starting with microbial additives, creating reusable options, and minimizing the reliance on physical and chemical additives to enable the recycling of pig manure. This approach is key to achieving efficient and sustainable progress in composting livestock and poultry manure.

This study has been published in Frontiers of Agricultural Science and Engineering in 2024, Volume 11, Issue 3. DOI: 10.15302/J-FASE-2023487