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This study aimed to explore the roles of selenite (Se) on nitrogen conservation and greenhouse gases (GHGs) mitigation during the composting process. Six levels of Se(IV) dosages (i.e. 0, 2, 4, 6, 8 and 10 mg/kg) were examined for 80-day composting of goat manure and wheat straw mixtures, where the different blending proportions were marked as T1 (Control), T2, T3, T4, T5 and T6, respectively. The results showed that adding Se(IV) was beneficial for reducing NH 3 by 3.50-42.41% by buffering pH and promoting nitrification. For N 2 O, it showed different responses to different Se(IV) dosages, and it was increased by 29.62-71.29% in T2-T4 but reduced by 30.45-69.54% in T5-T6. Methane (CH 4 ), another main component of GHGs, was increased by 1.35-107.42% by adding 2-10 mg/kg Se(IV). To further evaluate the effect of Se(IV) on GHGs, global warming potential value was calculated, which was 103.32-499.80 and minimum value was in T5. Furthermore, the physicochemical indexes, especially temperature and OM, had vital effects on microbial community. Overall, the results obtained from this study demonstrated that the application of Se (IV) in composting was reasonable to generate Se-rich organic fertilizer, and the 8 mg/kg was suggested from perspectives of nitrogen conservation and GHGs reduction., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

To explore the effect of microelement selenium on greenhouse gas emission, nitrogen loss and related functional genes during the composting. Selenite and selenate were respectively mixed with goat manure and wheat straw and then composted the mixture without selenium regarded as control. The results indicated adding selenite prolonged the thermophilic phase and improved the organic matter degradation, while the selenate presented the opposite results. Selenite and selenate influenced ammonium transformation while prompting the formation of nitrate. Compared to the control, adding selenite and selenate both decreased NH 3 emissions (by 26.7%-53.1%) and increased the total nitrogen content of compost. The addition of selenium increased mcrA in the early phase of composting, thereby promoting CH 4 emission (by 3.5-18.4%). Meanwhile, adding selenate significantly reduced nirK abundance and consequently reduced N 2 O emission. Moreover, selenate added treatment presented the highest compost maturity (88.77%) and the lowest global warm potential (117.46 g/kg CO 2 -eq.) among all treatments., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

[Display omitted] • Adding selenate enhanced the removal of total ARGs during composting. • Bacterial hosts were the primary determinant of ARGs abundance. • Selenium could affect MGEs, oxidative stress response and type IV secretion system. • Selenium-resistant genes affect the microbial abundance and altered ARGs. • Selenate was recommended for the production of selenium-rich organic fertilizers. Co-composting of selenium and livestock manure is an effective way to produce selenium-rich organic fertilizer. However, antibiotic resistance genes (ARGs), as emerging contaminants in compost, respond to selenium remains unknown. In this study, the effects of selenite and selenate on ARGs during goat manure composting were investigated. Compared to the control, the abundance of ARGs decreased by 8.39% with selenate addition, whereas increasing by 9.75% with selenite addition. ARGs degradation in selenate-added treatment was driven by a succession of potential hosts, primarily including Proteobacteria. Meanwhile, mobile genetic elements, oxidative stress response and type IV secretion system genes were down-regulated following selenate exposure, inhibiting the transmission of ARGs through horizontal gene transfer. Both selenite and selenate reduced the aminoglycoside and beta-lactam resistance genes while enriching the multidrug resistance genes in compost. In conclusion, selenate improved the removal of ARGs and was recommended for producing selenium-rich organic fertilizer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Selenate added treatment had more SOL-Se and EXC-Se than selenite added treatment. • Selenium promoted the compost humification, especially selenite. • Selenite and selenate enhanced the different key bacteria in humus transformation. • Selenium increased the genes encoding carbohydrate-active enzymes. This study investigated the variation of selenium fractions and their effects on humification during composting. Selenite and selenate were added to a mixture of goat manure and wheat straw for composting. The results demonstrated that the bioavailable Se in the selenite added treatment (9.3–13.8%) was lower than in the selenate added treatment (18.1–47.3%). Meanwhile, the HA/FA of selenite and selenate added treatments were higher than in control, indicating that the selenium addition (especially selenite) promoted the humification of composting. Importantly, selenite enriched the abundance of Tepidimicrobium and Virgibacillus which were responsible to improve humification performance. Selenate increased the abundance of Thermobifida and Cellvibrio which facilitated the composting humification. The genes encoding CAZymes involved in the degradation of organic materials were also analyzed, and selenium could contribute to the synthesis of humus. KEGG pathway analysis revealed that the selenite addition promoted amino acids and carbohydrate metabolism compared to the control. [ABSTRACT FROM AUTHOR]