Effects Of Exogenous Selenium Application On Selenium Accumulation And Transformation In Mushrooms And Reuse Of Selenium-containing Spent Mushroom Substrate In Selenium Biofortification

Both the species of mushroom and exogenous selenium（Se）determine the absorption and transformation of Se by mushrooms.The bioaccessibility of Se in Se-enriched mushroom is affected by processing methods,and ultimately determines the impact of Se-enriched mushroom on human health.However,both the systematic comparison of Se biofortification in mushrooms among different exogenous Se treatments,and the effect of processing methods on Se bioaccessibility in mushroom have not been reported yet.In addition,the spent mushroom substrate produced by Se-enriched mushroom cultivation（Se-SMS）is rich in organic Se.Whether the Se-SMS can be used as organic Se fertilizer for Se biofortification in crops,and the effects and mechanisms of different pretreatment methods on Se bioavailability in Se-SMS amended soil are not clear.Consequently,the effects of exogenous inorganic and organic Se on the accumulation and transformation of Se in three common mushrooms（Pleurotus ostreatus,Lentinus edodes and Pleurotus eryngii）were compared systematically.And in order to provide scientific guidance for the processing of Se-enriched mushroom,the effects of two common processing methods（boiling and frying treatments）on Se content and bioaccessibility in P.eryngii fruit bodies were explored.The incubation experiment and pot experiment were combined,the sequential extraction procedure,DOM-Se isolation method and 3D-EEM combined with parallel factor analysis were used to explore the release pattern of Se from different Se-containing SMS in different types of soils.Moreover,combined with the absorption and transportation of Se by crops and the transformation of soil Se fractions and DOM-Se,the effects and mechanisms of Se-containing SMS amendment on soil Se bioavailability were revealed.The purpose of this study is to provide scientific basis for Se biofortification in mushrooms and the reuse of Se-containing SMS as organic Se fertilizer.Main results are as follows:1.The Se contents in fruit bodies of P.ostreatus,L.edodes and P.eryngii were significantly increased by the application of exogenous inorganic and organic Se（p<0.05）,and all the three kinds of mushroom had the highest utilization efficiency for Na₂Se O₃compared with Na₂Se O₄ and Se yeast.Although the bioaccessibility of Se in Se-enriched mushroom was high（70-90%）,there is no potential health risk with normal consumption of Se-enriched mushroom in high Se treatment（5 mg/kg）.That is because exogenous Se was transformed and ultimately accumulated in mushroom fruit bodies mainly in organic forms（50-90%）（especially Se Met）.Among three kinds of exogenous Se,Na₂Se O₃ is an ideal Se fertilier for Se biofortification in mushrooms.2.Both boiling and frying treatments reduced the Se concentration in P.eryngii fruit bodies to different degrees（6.6-46.0%）,though the effect of frying treatment was more significant（17.1-46.0%）.Frying treatment also reduced the bioaccessibility of Se in P.eryngii fruit bodies（4.4-32.0%）.However,boiling treatment has a relatively limited effect on the Se concentration in P.eryngii fruit bodies and increased the Se bioaccessibility（8.2-10.6%）in it,and is thus the recommended processing method for Se-enriched mushrooms.3.The fraction transformation of Se in SMS-amended soils was influenced by the types and pretreatment methods of SMS and soil types.Control-SMS（CK-SMS）amendment reduced the Se bioavailability in Lou soil and Red Soil,but it had no significant effect on the transformation of bioavailable Se to stable forms in soils.Natural stacking Se-SMS amendment increased the Se bioavailability in these two soils,but the corresponding composting treatment had the opposite effect on Se bioavailability and accelerated the transformation of soil Se to stable forms.Both the CK-SMS and Se-SMS amendment significantly increased the proportion of organic bound Se（OM-Se）in Lou soil（1.0-38.2%）,but had the opposite effect on the proportion of OM-Se in Red Soil（decreased by 1.0-13.5%）.Regardless of whether the SMS is Se-enriched or not,the proportion of humic acid-bound Se（HA-Se）in the two soils increased after the amendment of SMS（10.6-38.6%）,and the proportion of HA-Se in composting treatments were higher than that in the corresponding natural stacking treatments.Soil p H and DOC were the main contributors to the distribution and transformation of Se fractions in SMS amended soil.4.The effect of exogenous Se-SMS amendment on soil Se bioavailability varied with the species of exogenous Se and the pretreatment methods of SMS.Natural stacking selenate-SMS（Se（VI）-SMS）amendment significantly increased the Se bioavailability in Lou soil and Red Soil（about 18%）,while the corresponding selenite-SMS（Se（IV）-SMS）treatment had no significant effect on it.However,the composting exogenous Se-SMS（both Se（IV）-SMS and Se（VI）-SMS）treatments significantly reduced the Se bioavailability in soil（6.0-13.3%）and accelerated the transformation of soil bioavailable Se to stable forms.The amendment of two kinds of exogenous Se-SMS increased the proportion of OM-Se（11.2-22.5%）and HA-Se（19.1-40.1%）in Lou soil,and the promoting effect of composting treatments were higher than that of natural stacking treatments.The natural stacking exogenous Se-SMS amendment reduced the proportion of OM-Se in Red Soil（1.3-13.7%）,while the corresponding composting treatment had the opposite effect on it and increased the proportion of HA-Se（16.8-27.0%）.The speciation of Se in SMS was the key factor to determine the bioavailability of Se in exogenous Se-SMS amended soil,and it was also affected by soil p H and EC.5.Selenium concentration in green onion was significantly increased by the application of Se-containing SMS,thus it can be used as organic Se fertilizer for Se biofortification in crops.Among three kinds of Se-containing SMS,the composting Se-SMS treatment significantly promoted the Se accumulation in the edible parts of green onion.In all SMS treatment,the proportion of SOL-Se,OM-Se and HA-Se in soil after green onion harvest decreased compared with that before planting,but the proportion of exchangeable Se（EXC-Se）increased.The transformation of OM-Se（including HA-Se and hydrophobic organic matter bound Se（HON-Se））into EXC-Se was observed in all SMS amended soils,indicating the mineralization of OM-Se contributed to the improvement of soil Se bioavailability.6.Both the immobilization of soil Se caused by the amendment of CK-SMS and the promotion of Se absorption in cherry radish by the amendment of Se-containing SMS were persistent.The natural stacking Se（IV）-SMS is the most suitable reutilization way of Se-containing SMS for Se biofortification in cherry radish.After the harvest of the first and second season of radish,the transformation of OM-Se（including HA-Se and HON-Se）into SOL-Se and EXC-Se was observed in all SMS-amended soils,indicating OM-Se is the potential source of bioavailable Se in soil.Moreover,EXC-Se,OM-Se and HA-Se were the main Se sources in radish grown in soil amended with CK-SMS,while SOL-Se,HA-Se and HON-Se were the main contributors of radish Se in Se-containing SMS treatments.In conclusion,compared with Na₂Se O₄ and Se yeast,Na₂Se O₃ is an ideal Se fertilier for the cultivation of Se-enriched mushrooms.Boiling treatment is the recommended processing method for Se-enriched mushrooms.It is feasible to reuse Se-containing SMS as organic Se fertilizer for Se biofortification in crops,but the effects of Se-containing SMS amendment on the fraction transformation and bioavailability of Se in soil is influenced by soil types,the source and speciation of Se in SMS,and the pretreatment methods of SMS.Therefore,these factors should be considered comprehensively when Se-containing SMS is used for crops Se biofortification.