Regulatory Effect Of Humus Active Fraction On Soil Cd Effectiveness And Critical Threshold For Reducing Cd Accumulation In Rice Seeds

Soil cadmium（Cd）contamination in paddy fields poses a serious threat to rice seed safety and population health.The effects of the main active components of HAs,huminic acid（HA）and fulvic acid（FA）,on the biological effectiveness of Cd in soil differed significantly,with the former passivating soil Cd and the latter possibly activating soil Cd.The overall effect of the organic material was the result of the combined effect of its active components.In this study,the effects of HAs main active components HA and FA and their different combinations of ratios（HA/FA）on Cd sorption properties and leaching characteristics as well as soil p H,effective sulfur content,DOM content and humus composition in soils were investigated by field plot tests with typical purple rice soils in southwest China,combined with FT-IR and chemical extraction and other analytical techniques.Then,we analyzed the relationship between the accumulated Cd uptake by rice and the changes of Cd form and effective state Cd in purple rice soils under different HA/FA treatments.The critical threshold of HA/FA for blocking soil Cd activity and bioeffectiveness were determined.It provides a scientific basis for the targeted utilization of organic materials containing humic acids（HAs）.The main findings are as follows:（1）The addition of HA,FA and HA/FA all promoted the adsorption of Cd on soil.With the decrease of HA/FA,the amount of Cd adsorbed by the soil gradually decreased,the adsorption intensity gradually decreased and the adsorption rate gradually increased.The more HA had a stronger promotion effect on soil Cd sorption capacity,while the promotion effect of FA on soil Cd sorption decreased with the increase of its addition.Compared with the CK treatment,the maximum soil sorption of Cd was increased by9.23%and 44.62%in the 1%HA and 10%HA treatments,respectively,58.46%and27.69%in the 1%FA and 10%FA treatments,respectively.The maximum sorption of Cd by soil was increased by 36.85%to 74.62%in different HA/FA treatments.（2）Under the same conditions,the leaching rate of Cd in purple soil increased with the addition of HA,FA and HAs compared to the pure water control（CK）.The promotion effect of FA treatment on the leaching capacity of Cd was greater than that of HA treatment,and the effect on the leaching capacity of soil Cd could reflect the change of Cd activity in soil,and FA promoted the Cd activity in soil more compared with HA treatment.The Cd solubilization rate was positively correlated with both HA and FA additions by linear fitting,with the equations:y₁=0.07×HA+0.125（R²=0.807）and y₂=0.206×FA+0.124（R²=0.916）,respectively.When HA/FA was 8/2,the leaching rate of soil Cd was the smallest and increased by 84.62%compared with CK.When HA/FA was 1/9,the leaching rate of soil Cd was the largest and increased by 161.54%compared with CK.When HA/FA≥5/5,the leaching rate of Cd gradually increased with the decrease of HA/FA.（3）The different HA/FA and the three organic fertilizer treatments for testing increased the humus carbon content of the soil,increased the hydrophobic fraction of humus in the soil,and improved the relative molecular mass and aromaticity of humus in the soil.The effects of different HA/FA treatments on the slope of the curve Sr（slope in the wavelength range of 275～295 nm and 350～400 nm）of UV-Vis spectra of humic acids extracted from soils differed significantly,and Sr could qualitatively reflect the molecular weight and photochemical potential of organic matter,and Sr was inversely proportional to the relative molecular mass in soils.In this study,compared with the CK treatment,Sr increased by 1.54%to 6.15%in the HA/FA≥4/6 treatment and decreased by 1.54%to 7.69%in the HA/FA≤2/8 treatment.Thus,the relative molecular masses of huminic acid in soils treated with HA/FA≥4/6 were greater than those treated with HA/FA≤2/8,indicating that the critical threshold of HA/FA for regulating the relative molecular mass size of humus fractions in soils was between 4/6 and 2/8,which is consistent with the critical threshold for regulating changes in the effective state Cd content in soils.（4）HA can increase p H and decrease effective state Cd content in soil,while FA can decrease p H,increase effective sulfur content and increase effective state Cd content in soil.The effects of different HA/FA on soil p H,effective sulfur content and effective state Cd content are the result of the combined effect of both.According to Pearson correlation analysis,HA addition showed a significant positive correlation with soil p H（p<0.05）,while FA addition showed a significant negative correlation with soil p H（p<0.05）,and FA addition also showed a significant positive correlation with soil effective sulfur content at tillering stage of rice（p<0.05）.The effective state Cd content in soil was significantly negatively correlated with HA addition（r=-0.853,p<0.05）and positively correlated with FA addition（r=0.758,p<0.05）.（5）The effect of HAs on the effective state Cd and Cd accumulation in rice seeds was governed by the ratio of their active component content.The treatment of HA/FA≥4/6 played a blunting effect on soil Cd,Cd effectiveness was significantly reduced,and the content of Cd in rice seeds was reduced,compared with the control treatment CK,seed Cd was reduced by 15.2%～33.3%.While HA/FA≤2/8 treatment played an activating effect on soil Cd,Cd effectiveness increased significantly,and the Cd content of rice seeds increased by 24.2%～42.4%compared with the control treatment CK.Therefore,to ensure the quality and safety of rice,organic materials containing HAs with a ratio of HA/FA≥4/6 should be selected.