Study On The Regulation Of Straw Return To The Field On The Passivation Of Heavy Metal Cd In Soil

Straw return is a good way to recycle agricultural waste,and a large amount of organic matter can be imported into the soil.A number of studies have shown that the soil environment,straw source,foreign additives and other conditions will affect the decomposition and transformation process of straw organic matter in the soil,resulting in differences in soil organic matter components,affecting soil heavy metal activity and crop accumulation effect.In the actual cultivation process,in order to facilitate the planting of the next season of farmland,it is often necessary to promote the rapid decay of straw organic matter by adding humectants（cellulose decomposition bacteria,lignin decomposition bacteria,etc.）or changing environmental conditions,but this tends to increase soil Cd activity and improve the absorption and accumulation of heavy metal Cd by plants.Therefore,finding ways to strengthen the straw return process,improve soil fertility characteristics,weaken the bioavailability of soil heavy metal Cd,and reduce the risk of heavy metals entering the food chain through soil-crop systems is an urgent problem to be solved.In this paper,the effect of straw return was studied by regulating soil moisture conditions,C/N ratio（nitrogen fertilizer）,tillage method,and addition of Fe₂O₃or bentonite,and the appropriate straw return regulation methods were screened out by comparing the differences in straw decomposition process,soil nutrient change,soil organic matter species and content,soil heavy metal Cd activity and vegetable cd accumulation under different regulation methods.The variation characteristics of soil Cd morphology during straw return under this regulatory method were investigated,and the structural characteristics and molecular weight characteristics of soil organic matter were characterized by ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscopy,and the relationship between the organic matter formed by straw return and the activity change of heavy metal Cd was clarified by redundancy analysis.The main conclusions are as follows:（1）The addition of Fe₂O₃significantly reduced the decomposition rate of straw in the soil,and the addition of Fe₂O₃and bentonite had little effect on soil nitrogen and phosphorus nutrients,which increased the total soil carbon content,but significantly reduced the water-soluble organic matter content by 25.73%,and there was no significant difference in vegetable yield and nutritional quality compared with the control.The regulation of C/N and the change of water conditions increased the decomposition rate of soil organic carbon after straw return,increased the content of soil alkaline-hydrolyzed nitrogen,available phosphorus and available potassium,and the content of water-soluble organic matter,which promoted the growth of vegetables and improved vegetable yield.Under the control C/N mode,the fresh weight,plant height and root weight of vegetables increased by 29.7%,22.85%and 18.32%respectively,and the chlorophyll content in vegetables was significantly increased.（2）The addition of Fe₂O₃and bentonite in the process of straw return can reduce the available Cd content of the soil,of which the addition of Fe₂O₃has the largest decrease,up to 41.77%;The adjusted C/N ratio had no significant effect on the available Cd content in the soil.Changing the water conditions significantly increased the available Cd content of the soil.The Cd content of vegetable stem and leaf parts in the control group was 0.343 mg/kg,which was significantly higher than the limit value of leafy vegetables（0.2mg/kg）specified in the"National Standard for Food Safety Food Safety Cd Limit"（GB2762-2017）;Tillage,regulation of water conditions and addition of 0.3%bentonite did not have a significant effect on the Cd content of edible parts of vegetables.C/N was regulated to reduce the Cd content of edible parts of vegetables to 0.257 and 0.265 mg/kg,respectively,but the nitrate content of edible parts of vegetables was increased（it was secondary（moderate）pollution with nitrate content）,and the heavy metals and nitrate content did not meet food safety standards overall;The addition of Fe₂O₃and bentonite with an amount of 0.01%,0.05%and 0.1%can reduce the Cd content of stem and leaf parts to less than 0.2 mg/kg,and the decrease of Fe₂O₃at the same concentration is greater than that of bentonite,which can be reduced to 0.14mg/kg,and the nitrate content of vegetable stems and leaves can be significantly reduced to meet food safety requirements.Combining the effects on soil nutrients,the effects of soil Cd activity,and the uptake of Cd by vegetables and their growth status,the addition of Fe₂O₃was a more suitable method for regulating straw return.（3）The addition of different concentrations of Fe₂O₃to control the decomposition rate of straw return during the cultivation period of straw return to the field,the decomposition rate reached51.87%-71.30%on the 60th day,compared with the control,the addition of different concentrations of Fe₂O₃reduced the decomposition rate of straw by 17.41%-27.5%,and increased the total organic carbon of the soil;The soil unstable DOC content after straw return was reduced by 1.62%-17.45%,and the proportion of HA/HS was increased by 15.43%-20.67%.Promotes the formation of stable organic matter in the soil.The soil available Cd content showed a downward trend from day 5onwards,with the inflection point occurring earlier than that of the control group（day 15）,and since day 15,the soil available Cd content of each treatment group was lower than that of the control group,and the available Cd content of the treatment group was reduced by 15.35%-22.57%on day60 of culture.（4）According to the ultraviolet visible and three-dimensional fluorescence spectroscopy analysis of DOC and HA,Fe₂O₃regulatory treatment increased the content and molecular weight of doc aromatic components,promoted the increase of humus-like acid-like substances in DOC,improved the structural stability of soil DOC,increased the molecular weight of HA,increased the ha aromatic components,and also promoted the substitution of ha aromatic functional groups and the generation of quinone compounds,so that the fulvic acids in the HA fluorescent components were reduced and promoted the increase of stable humusoids.The redundancy analysis showed that the soil available Cd content was negatively correlated with the aromatic component content and molecular weight of DOC,the aromaticity,molecular weight and the substitution of the oxygen-containing functional groups of aromatic compounds,and the correlation analysis showed that the soil available Cd content and the structural characteristics of HA were more correlated.It shows that the increase in the aromatic structure of organic matter and the increase in molecular weight,as well as the increase in the substitution of oxygen-containing functional groups of aromatic compounds,reduce the content of available Cd,while Fe₂O₃strengthens the humusification process in the soil after the straw is returned to the field,promotes the formation of stable humus in HA,and thus reduces the activity of heavy metal Cd in the soil.