Interactions Between Phosphorus And Zinc In A Loessial Soil-Alfalfa System On The Loess Plateau

Phosphorus?P?is one of the essential nutrients for plants and an important material guarantee for agricultural production.Zinc?Zn?is an essential micronutrient for plants,it plays a vital role in plant growth and is also an essential trace element for humans.The Loess Plateau is one of the areas with the most serious soil erosion and the most fragile ecological environment in the world.The soil is barren and agricultural productivity is low.Due to the specific soil properties of the calcareous soil on the Loess Plateau,such as high p H,high Ca CO₃ and low organic matter,soil Zn availability becomes very low.In order to improve the utilization efficiency of phosphorus and zinc fertilizers by crops grown in calcareous soils on the Loess Plateau and improve the phosphorus and zinc nutrition,alfalfa,which is sensitive to phosphorus fertilizers,was selected as the research object.We carried out greenhouse pot experiments,P fertilizer?in the form of monopotassium phosphate?KH₂PO₄??and Zn fertilizer?in the form of zinc oxide?Zn O??were mixed into the soil at different rates.There were three P levels,i.e.0,20,and 80 mg P kg^–1 dry soil,two Zn levels,i.e.0 and 200 mg Zn kg^–1 dry soil.Then,we explored the interactions between soil P level and Zn level by measuring the dry mass of alfalfa,P and Zn concentrations in alfalfa shoots and roots,soil alkaline phosphatase activity,amounts of rhizosphere carboxylates,rhizosphere and bulk soil p H,etc.We also explored the responses of alfalfa growth and P-Zn uptake to homogeneously and heterogeneously applied P and Zn fertilizers into soil.The following results were obtained:?1?There are interactions between soil P level and Zn level on shoot dry mass,root dry mass,root mass ratio,shoot P content,root P content,shoot Zn content and bulk soil p H.?2?When P and Zn fertilizers were homogeneously added into the soil in the pot at different rates,alfalfa obtained highest total dry mass when soil was supplied with 80 mg P kg^-1 soil and 200 mg Zn kg^-1 soil.In the same Zn application treatment,shoot P concentration and root P concentration increased by 5.0–40.5%and 7.1–25.7%with the increase of soil P application rate,respectively.In the same P application treatment,shoot Zn concentration and root Zn concentration increased by 191.5–385.7%and 553.5–784.1%with the increase of soil Zn application rate,respectively.P-utilization efficiency of alfalfa decreased by 5.0–25.9%with increasing soil P concentration and Zn-utilization efficiency of alfalfa decreased by 83.2%with increasing soil Zn concentration.P and Zn deficiency would increase soil alkaline phosphatase activity.P deficiency would promote the secretion of rhizosphere carboxylates,especially the secretion of tartrate.Rhizosphere p H was lower than bulk soil p H.?3?When the distribution of P and Zn in the soil was heterogeneous,root P content of the P-and Zn-rich areas was higher than that in the P-and Zn-deficient areas,the amountof rhizosphere tartrate in P-rich areas was significantly lower than in P-deficient areas.Therefore,the application of P and Zn fertilizers in the soil can significantly promote the growth of alfalfa and the accumulation of P and Zn,but excessive application of P and Zn fertilizer will reduce the P-and Zn-utilization efficiency of alfalfa.The results of this study can provide a theoretical basis for rational fertilization in the calcareous soil on the Loess Plateau and increase the utilization efficiency of P and Zn fertilizersby plants,and have an important guiding significance for further exploring the mechanism of P-Zn interactions in soil-plant systems.