Effects Of Milk Vetch Returning To Field On Cd Absorption And Translocation Of Rice

Milk vetch,as a legume green manure crop,can fix nitrogen and be easily decomposed,and its return to the field has an impact on the soil environment of the paddy field and the growth of rice.In order to study the Cd absorption and transport effects of double-cropping rice in lightly Cd-polluted rice fields,the preliminary exploration of the Cd-reducing effect and physiological mechanism of the double-cropping rice in lightly Cd-polluted rice fields was aimed at Safe and high-quality agricultural production in areas with high Cd pollution provides a new approach and theoretical reference for reducing Cd.In this paper,a field experiment for five consecutive years（2016-2020）was used to study no fertilization（CK）,single application of milk vetch（GM）,single application of chemical fertilizer（F）,and application of milk vetch combined with chemical fertilizer（F+GM）A total of 4treatments,Milk vetch is used as a winter planting green manure and returned to the field.The main findings are as follows:（1）The Cd content of double-cropping rice is reduced by application of milk vetch.Early rice CK has the highest Cd content in various parts（roots,stems and leaves,grains）of the plant.F+GM has the lowest Cd content in roots,stems and leaves of early rice.The Cd content of root of F+GM is 30.5%～78.5%lower than that of F,and that of F+GM stem and leaf is 25.2%～57.1%lower than F.The Cd content of GM’s early rice root and grain is lower than that of CK and F.The Cd content of GM’s root is 37.0%-80.2%lower than that of CK,and the Cd content of GM’s grain is57.1%-87.4%lower than that of CK.The Cd content of GM’s root is lower than F reduced by 2.5%～49.4%,the Cd content of GM grain is 33.1%～33.6%lower than that of F.In addition,the Cd content of CK and F early rice grain at the mature stage was higher than 0.2 mg·kg^-1（exceeding the national safety standard）,and GM and F+GM were lower than 0.2 mg·kg^-1（not exceeding the national safety standard）.The Cd content of the late rice of F+GM and GM is lower than that of F.The Cd content of root of F+GM is 13.4%～31.3%lower than that of F,and the Cd content of stem and leaf of F+GM is 8.4%～35.3%lower than that of F.The Cd content of F+GM grain is26.7%～50.0%lower than that of F.The Cd content of GM’s root is 26.7%～70.9%lower than F,the Cd content of GM stem and leaf is 1.4%～56.9%lower than F,and the Cd content of GM grain is 28.6%～58.5%lower than F.（2）The active of Cd is reduced milk vetch as manure in paddy soil.The return of milk vetch has no significant effect on the total Cd content of the paddy soil,and reduces the effective Cd content of the paddy soil.The soil available Cd content of early rice GM and F+GM was significantly lower than CK,and the soil available Cd content of late rice GM and F+GM was lower than CK and F.The effective Cd content of the paddy soil decreases,and the rice’s absorption of soil Cd decreases accordingly.（3）The Cd transfer from the stems and leaves to the grains were restricted by milk vetch incorporated as manure in double-cropping rice,and reduces the grain Cd distribution ratio.The stem-leaf-grain transport coefficient of F+GM and GM decreased,and the stem-leaf-grain transport coefficient of GM was lower than CK by8.3%～52.2%,and F+GM was lower than F by 4.5%～27.8%;F+GM stem-leaf-grain transport coefficient ratio CK is reduced by 20.0%～35.1%,which is 33.3%～45.7%lower than F.In addition,F+GM had the lowest Cd accumulation and distribution ratio in the roots of early rice at maturity,followed by GM;GM had the lowest Cd accumulation and distribution ratio in grains,followed by F+GM;and GM had the highest distribution ratio in stems and leaves,followed by GM.F+GM.Late rice F+GM has the lowest distribution ratio of grains,followed by GM;F+GM has the highest distribution ratio of stems and leaves.Astragalus vulgaris returned to the field reduced the Cd transport coefficient between the stems and leaves of rice and the grains,weakened the transport capacity of Cd from the stems to the grains,and allowed Cd to accumulate in the stems and leaves,resulting in a higher distribution ratio in the stems and leaves,and the distribution in the grains.The ratio is low.（4）Pressing milk vetch enhances the photosynthetic capacity of double-cropping rice in lightly Cd-polluted rice fields,prolongs the photosynthetic function period of rice,and delays the senescence of rice leaves.Compared with CK,both F+GM and GM had a more positive impact on the photosynthetic characteristics of double-cropping rice,especially early rice.The SPAD and NDVI of early rice GM and F+GM were higher than CK;F+GM had the highest net photosynthetic rate and stomatal conductance,and the lowest intercellular CO₂ concentration.In addition,to the middle and late stages of rice growth,the photosynthetic index of F decreased,and the decline of photosynthetic indexes of F+GM and GM was smaller than that of F,resulting in higher photosynthetic indexes of F+GM and GM in the middle and late stages of growth.（5）Plowing milk vetch in lightly Cd-polluted rice fields effectively promotes stable and increased production of double-cropping rice.Compared with CK,GM increases the yield of double-cropping rice.Compared with CK and F,F+GM has increased production rates of 46.9%～124.4%and 4.8%～13.4%for five consecutive years,and the average annual output increase rates are 84.0%and 8.7%,respectively.Optimizing rice plant traits and yield components are the main reasons for the increase in double-cropping rice yield in lightly Cd-polluted paddy fields.In addition,the pressure of milk vetch improves the stability of double-cropping rice output,which is specifically manifested in reducing the inter-annual yield variation coefficient of double-cropping rice and increasing the yield sustainability index.（6）In lightly Cd-polluted rice fields,the Cd content of double-cropping rice grains at maturity stage is significantly positive with the Cd content of roots,stems and leaves,stem-leaf-grain Cd transport coefficient,and grain Cd distribution ratio relationship.The Cd content of early rice grains was significantly positively correlated with the soil available Cd content.Cd content in early rice grains is significantly negatively correlated with SPAD,NDVI and stomatal conductance.