The excessive nitrogen and phosphorus discharged into the water environment will cause water eutrophication and thus disrupt the water ecosystem and even exert biological toxicities. In this study, the absorption removal of nitrogen and phosphorus from the anaerobic tank in an anaerobic–anoxic/nitrifying system using four different kinds of biowaste-reclaimed biochars were investigated and compared. The effects of temperature and pH on nutrient adsorption removal were further investigated. The four kinds of biochar were successfully prepared and well characterized using a scanning electron microscope, fourier transform infrared spectroscopy, X-ray diffraction and Brunner−Emmet−Teller methods. Generally, there was no significant change in chemical oxygen demand (COD) and NH₄⁺-N removal efficiencies when treated by the different biochars, while the activated sludge biochar (ASB) displayed the highest total phosphorus (TP) removal efficiency. The initial TP concentrations (<40 mg/L) displayed no remarkable effects on the TP adsorption removal, while the increase of temperature generally enhanced TP and NH₄⁺-N adsorptions on the ASB. Besides, the increase of pH significantly promoted NH₄⁺-N removal but depressed TP removal. Moreover, the adsorption process of TP by the ASB complies with the secondary kinetic model, suggesting the chemical precipitation and physical electrostatic interaction mechanisms of TP adsorption removal. However, the adsorption of NH₄⁺-N conformed to the inner-particle diffusion model, indicating that the NH₄⁺-N adsorption was mainly involved with pore diffusions in the particles.

Continuous cropping can often deteriorate soil quality and reduce crop yield. Soil properties and microbial communities usually play a vital role in maintaining rhizosphere micro-ecosystem sustainability, which is yet to be addressed in continuous peony monoculture systems. Herein, variations in soil physiochemical properties were extensively investigated following 1, 4, and 10 years of continuous tree peony cropping, as well as microbial community diversity, composition, and predicted functions. The soil pH and contents of available Mg, Mn, Zn, and B significantly declined after 10 years of continuous monoculture, while the contents of soil organic carbon, nitrate, and available P, K, Fe, and Cu notably increased by more than 100%, implying an imbalance of soil nutrients resulting from long-term continuous cropping. High-throughput sequencing results indicated that the microbial community structure and composition were remarkably altered after either 4 or 10 years of continuous cropping, interfering with diverse microbial metabolic pathways and phenotype functions. In addition, the relative abundances of some beneficial bacteria dramatically increased, especially for Acidobacteriota and Bacillus members. Microbial selections or adaptations in response to soil nutrient changes were expected to remediate negative impacts of continuous cropping on soil quality. Findings in this study provide insights into the establishment of proper management strategies for sustaining soil quality to resist potential obstacles after long-term continuous peony cropping.

Heterotrophic nitrification–aerobic denitrification (HNAD) bacteria enable nitrogen removal under aerobic conditions and have attracted increasing attention in wastewater treatment. Although numerous studies have enriched HNAD strains and explored their nitrogen removal performance, the environmental responses and metabolic mechanisms underlying nitrogen transformation remain insufficiently evaluated. In this review, the ecological distribution and physiological characteristics of HNAD bacteria are summarized, followed by a systematic elaboration of the effects of nitrogen sources, carbon sources, C/N ratio, and environmental factors (e.g., temperature, pH, dissolved oxygen and salinity) on nitrogen removal and transformation performance. In addition, nitrogen depletion from the aqueous phase is likely to reflect both conversion to gaseous nitrogen products and assimilatory uptake into biomass, which may account for a non-negligible proportion of the apparent nitrogen removal in some strains. Accordingly, the nitrogen transformation mechanisms, encompassing nitrite/nitrate-mediated pathways, proposed hydroxylamine-associated pathways, and nitrogen assimilation, are critically examined, with attention to the strength and limitations of the available evidence. Furthermore, the regulatory roles of intracellular redox balance and electron allocation in mediating the interplay among nitrification, denitrification, and cellular respiration are addressed. Finally, the key challenges and future prospects of HNAD research and applications are highlighted. This review provides mechanistic insights into the metabolic versatility of HNAD bacteria and supports the development and evaluation of nitrogen removal strategies in advanced biological nitrogen removal systems.