Rhizosphere Regulation Of Efficient Utilization Of Phosphorus And Selenium In Alfalfa

Phosphorus(P)plays a vital role in plant growth.Due to the poor solubility and slow diffusion of P salts and high P sorption by particles,P availability in soils is often very low.Phosphorus deficiency in soil limits the production of many crops.Selenium(Se)is an essential element for animals and humans.Selenium deficiency can cause many health disorders.A proper amount of Se has a beneficial effect on plant growth and crops yield.Supplying Se to soils can increase plant Se contents,further increase Se content in the food chain,and is an important way for humans and animals to take a daily supplement of Se.There is a complex interaction between P and Se in soil and plants,and a proper application of P and Se can improve the yield and quality of crops.Alfalfa(Medicago sativa)is a perennial forage legume,it is sensitive to changes in soil P supply,and it is also able to accumulate Se.The rhizosphere environment determines the fractions and bioavailability of P and Se,and is essential to improve the P-and Se-utilization efficiency in crops.The main of this study was intended to investigate the interaction between P and Se on the growth and accumulation of P and Se grown in alfalfa in a loessial soil.We focused on the rhizosphere regulation of alfalfa roots on P and Se uptake,accumulation and translocation,aiming to investigate the rhizosphere regulation mechanism of the efficient utilization of P and Se under the arbuscular mycorrhizal fungi(AMF)-P-Se interaction.The main results are as follows:(1)The inhibitory concentration thresholds of selenate and selenite on the alfalfa growth were different,and the toxicity of selenate was greater than that of selenite.There was no significant effect on the germination rate of alfalfa seeds in different application between selenate and selenite.In all treatments,the seeds could germinate after seven days,but with the application rate of Se increasing,there was a significant inhibition on the growth of seedlings.Compared with the Se0 treatment,when the application rates of selenate and selenite were 2 mg kg-1 and 4 mg kg-1 respectively,the growth indexes of seedlings decreased significantly.The results of the germination experiment showed that the suitable application range of selenate and selenite were 0–2 mg kg-1 and 0–4 mg kg-1 for planting alfalfa in the loessial soil.(2)Combined application of phosphate and selenite had a concentration effect on P and Se uptake by plants.Moderate application of phosphate combined with selenite application had a positive effect on alfalfa growth and accumulation of P and Se,while excessive application of phosphate and selenite had an inhibitory effect.Optimal P-and Se-application rates for the loessial soil in this study were 5 mg kg-1and 1–2 mg kg-1,respectively,for the maximum P and Se accumulation and Puptake efficiency in alfalfa.As the P-application rate increased,exchangeable Se(EXSe)and organic-matter-bound Se(OR-Se)concentration decreased significantly by 12–23% and 23–47% respectively,in contrast,soluble Se(SOL-Se)and iron oxidebound Se(OR-Se)concentration increased significantly by 23–81% and 8–40% respectively.In the lower P treatment,P had a greater adsorption capacity than Se in soil,causing soluble Se concentration to increase,thereby increasing Se uptake by plants.Soil p H decreased by 0.23–0.75 unit with excessive P application.The decline of p H caused available Se to be transformed into stable Se form(Fe-Se),thereby inhibiting Se uptake by alfalfa.Supplying Se promoted stable P to be transformed into available P,thereby improving P-uptake efficiency.As the Se-application rate increased,the concentration of Ca8-P and Fe-P decreased significantly by 7–26% and 10–50% respectively,while Ca2-P concentration increased by 7.4–41%.The interaction of P and Se affected soil inorganic P-fraction and Se-fraction.The proper application ratio of phosphate and selenite played a vital role in promoting the uptake and utilization of P and Se by alfalfa.(3)The secretion of rhizosphere carboxylates and the change of alkaline phosphatase activity were important factors affecting the uptake of P and Se by alfalfa.The results showed that colonization by AMF and supplying P promoted plant growth,and had a positive effect on P and Se accumulation by alfalfa.In the P0+AMF treatment,the amounts of rhizosphere carboxylates and rhizosphere soil alkaline phosphatase activity were significantly higher than in other treatments.The concentration of EX-Se in rhizosphere soil was significantly positively correlated with the amounts of rhizosphere carboxylates.The release of carboxylate promoted the transformation of stable Se into available Se.Colonization by AMF and low-P environment promoted alfalfa roots to release carboxylates,especially citrate,and increased the rhizosphere soil alkaline phosphatase activity.It was a key process to improve the bioavailability of soil P and Se.(4)Localized P supply increased P bioavailability by promoting the release of carboxylates,especially tartrate,by alfalfa rootse,and weakening the competition between roots and microbes.The results of split-root experiment showed that localized P supply stimulated root proliferation and induced rhizosphere acidification.Compared with the homogeneous soil,when nutrients were distributed in patches in soil,the competition between microbes and roots was weaker,thus further promoting the nutrients uptake by roots.In the split-root system,the shoot to root-A(compartments without P and Se,or only with P)translocation factor of Se was 0.60–0.95.Compared with the homogeneous P and Se treatment,the translocation capacity of Se in the heterogeneous P and Se treatment was stronger.Localized Se supply was more beneficial to P uptake by alfalfa.The high translocation capacity of Se in alfalfa improved P-uptake efficiency.