Study On The Effect Of Exogenous Selenium On Aerobic Composting Of Goat Manure And Its Fraction Variations

Selenium（Se）is an important trace element for humans,who obtain Se from the soil-plant-human system mainly through the food chain.Soil is the starting point of selenium biofortification,however,about 51%of Chinese soils are deficient in selenium and nearly 100million people have insufficient selenium intake.Application of inorganic selenium fertilizer（Na₂SeO₃ or Na₂SeO₄）to increase soil selenium content is a common measure for selenium supplementation in China.However,only 5%of the inorganic selenium fertilizer can be used by plants,and most of which are leached by rainwater,thus leading to excessive accumulation of selenium in soil and groundwater,causing damage to the environment and humans.Therefore,it is necessary to find alternatives to inorganic fertilizers in order to improve the efficiency of Se fertilizer application and reduce its potential risk to the ecological environment.Aerobic composting is an effective,practical and economical technology for the production of selenium-enriched organic fertilizer.From the perspective of bioavailability and safety,organic fertilizers enriched with selenium are superior to inorganic selenium fertilizers.However,there is still not well-explored about the effect of selenium addition on composting,and its fraction variations during composting.In this study,goat manure and straw were used as raw materials for composting with the same concentration of different types of inorganic selenium（Na₂SeO₃ and Na₂SeO₄）,and the mixed raw materials without selenium were marked as control for aerobic composting for 80days.The effects of Na₂SeO₃ and Na₂SeO₄ on physicochemical properties,greenhouse gas emissions,nitrogen retention,resistance genes and humification during aerobic composting were investigated,and the morphological changes of selenium and its biological effectiveness in selenium-rich organic fertilizers were studied by continuous leaching technique,which combined with macrogenomics revealed the potential mechanism,and the main conclusions obtained were as follows:（1）Compared with control,adding Na₂SeO₃ prolonged the thermophilic phase and improved the organic matter degradation,while the Na₂SeO₄ presented the opposite results.Adding Na₂SeO₃ and Na₂SeO₄ prompted the formation of NO₃^–-N,decreased NH₃ emissions（26.7–53.1%）,increased the total nitrogen content of compost.Adding Na₂SeO₃ and Na₂SeO₄increased mcr A gene,thereby promoting CH₄ emission（3.5–18.4%）.In addition,adding Na₂SeO₄ significantly reduced nir K abundance and consequently reduced N₂O emission.Among all treatments,Na₂SeO₄ treatment presented the highest compost maturity（88.8%）and the lowest global warm potential（117.5 g/kg CO₂-eq.）.（2）During composting,Na₂SeO₃ and Na₂SeO₄ treatments showed higher fulvic acid/humic acid than the control,indicating that the addition of Se（especially Na₂SeO₃）promoted humification of composting.Microbial analysis showed that Na₂SeO₃ increased the abundance of Tepidimicrobium and Virgibacillus,while Na₂SeO₄ promoted the enrichment of Thermobifida and Cellvibrio,thus enhancing compost humification.The increase in genes encoding carbohydrate-active enzymes further indicated that Na₂SeO₃ and Na₂SeO₄contributed to humus synthesis.In addition,adding Na₂SeO₃ promoted amino acid and carbohydrate metabolism.The bioavailable Se content of Na₂SeO₃ treatment（9.3–13.8%）was always lower than that of Na₂SeO₄ treatment（18.1–47.3%）,probably due to the higher humification in Na₂SeO₃ treatment.（3）Compared with control,adding Na₂SeO₄ reduced the abundance of Antibiotic resistance genes（ARGs）during composting,while the opposite result was observed in the Na₂SeO₃ treatment.Adding Na₂SeO₄ mainly reduced the abundance of Proteobacteria,which inhibited the spread of ARGs through vertical gene transfer,resulting in a decrease in the number of ARGs.Meanwhile,Na₂SeO₄ reduced the abundance of mobile gene elements（plasmids and class 1 integrons）,oxidative stress response,and type IV secretion system genes,which inhibited the spread of ARGs through horizontal gene transfer pathways.Although,Na₂SeO₃ and Na₂SeO₄ had different effects on ARGs,both Na₂SeO₃ and Na₂SeO₄ reduced aminoglycoside and beta-lactam resistance genes and enriched multidrug resistance genes.Overall,adding Na₂SeO₄ reduced the composting GWP,improved the compost humification process and the removal of ARGs,which is recommended for the production of selenium-rich organic fertilizers,but the enrichment of multidrug resistance genes caused by the addition of Na₂SeO₄ in the composting process should be further considered.