Ammonium carbonates are a group of fertilizer materials that include ammonium bicarbonate, ammonium carbonate hydrate, and ammonium carbamate. They can be synthesized from diverse nutrient-bearing liquid waste streams but are unstable in a moist environment. While extensively utilized several decades ago, their use gradually decreased in favor of large-scale, facility-synthesized urea fertilizers. The emergence of sustainable agriculture, however, necessitates the recovery and reuse of nutrients using conventional feedstocks, such as natural gas and air-derived nitrogen, and nutrient-containing biogenic waste streams. To this extent, anaerobic digestion liquid presents a convenient source of solid nitrogen and carbon to produce solid fertilizers, since no significant chemical transformations are needed as nitrogen is already present as an ammonium ion. This review describes detailed examples of such feedstocks and the methods required to concentrate and crystallize solid ammonium carbonates. The technologies currently proposed or utilized to stabilize ammonium carbonate materials in the environment are described in detail. Finally, the agricultural efficiency of these materials as nitrogen and carbon source is also described.

New scalable methods are needed to provide sustainable solutions for nutrient recovery in the form of solid fertilizer materials and their environmental stabilization from anaerobic digestion liquid byproducts. In this work, two nutrient metal containing double salts, Mg(NH₄)₂(CO₃)₂·4H₂O and Zn(NH₃)CO₃, were synthesized using naturally abundant Mg- and Zn-carbonate minerals and ammonium (bi)carbonate salts. This constituted a conceptually new synthetic approach, different from the previous work in which the aqueous solution was utilized. We also showed that the materials could be easily scaled to 20 g quantities sufficient for soil testing. The crystalline structure of the resulting materials was confirmed using powder XRD and thermal analysis showed properties distinctly different from those of parent ammonium (bi)carbonates. Accordingly, a reduction in NH₃ volatilization in soil was measured with up to 20% more NH₄⁺ recovered after the soil experiments at 80% water holding capacity. Further, inhibition of the agriculture-beneficial bacteria Bacillus subtillis in a nutrient medium was dramatically reduced when compared to the ammonium bicarbonate alone, suggesting decreased negative effects on soil biota. Finally, Mg(NH₄)₂(CO₃)₂·4H₂O and Zn(NH₃)CO₃ matched the kinetic nitrogen need of lettuce plants better than the ammonium carbonate control while also keeping it in a form that will be available in the future. The utility of magnesium and zinc double salts in agriculture is paramount if environmentally benign and nutrient-efficient fertilizers from liquid digestate waste are to be enabled.