Speciation Transformation And Coupling Mechanism Of Phosphorus And Iron In Red Soil Under Flooding-Drying Condition

Phosphorus in soil is easily fixed,and its bioavailability is often the main limiting factor for crop growth in many farmland soils.Paddy rice-upland crop rotation is the main farming system in the Yangtze River Basin and the southern part of China.The strong changes of hydrothermal conditions and redox potential affect the speciation,migration and bioavailability of soil phosphorus.Great progress has been made in the study of iron form change and phosphorus availability under the paddy rice-upland crop rotation system.However,the soil chemical processes affecting the transformation of iron and phosphorus forms in the paddy rice-upland crop rotation system are less involved,and the corresponding microscopic mechanism is still not clear.In this study,we selected the red soil in Qiyang,Hunan Province as the object,and carried out soil culture in the laboratory to simulate the paddy rice-upland crop rotation conditions,in order to explore the mechanism of phosphorus and iron transformation in soil under different flooding-drying conditions.The Hedley phosphorus fractionation and the modified Zhang-Jackson inorganic phosphorus fractionation method,and the thin film gradient diffusion technique?DGT?were jointly applied to elucidate the chemical speciation transformation of phosphorus,The traditional solution extraction method was used to explore the speciation of soil iron?Fe?II?,the change of content of amorphous iron and free iron?,These results could reveal the main controlling factors and mechanism of phosphorus transformation and bioavailability in soil and water rotation system,and clarify the role of iron in phosphorus transformation,activation and fixation and their coupling relationship.This study could provide a basis for understanding the soil chemical process of phosphorus cycling under the water and drought rotation,rational application of phosphate fertilizer and efficient management of nutrients.The main conclusions obtained are as follows:?1?Effects of anaerobic methods and soil properties on the transformation of soil phosphorus and iron.Under flooding condition,the pH of red soil increases and tends to be neutral,the the Eh decreases.For the high phosphorus soil,the effect of treatment to the pH and Eh is in the following order:the ascorbic acid submerged group>natural flooding group>nitrogen flooding group?H-AA>H-CK>H-N₂?.The availability of phosphorus in the flooding process is mainly affected by the transformation of iron-aluminum oxide combined with phosphorus form.The results of Hedley phosphorus classification indicate that the main forms of phosphorus in high-phosphorus and low-phosphorus soils are iron-aluminum-bound phosphorus?NaOH-Pi?,and the content of each phosphorus component is in the order:NaOH-Pi>NaHCO₃-Pi>HCl-Pi>H₂O-Pi.During the equilibrium period of soil flooding,the availability of phosphorus temporarily increases,and in the subsequent flooding,the availability of phosphorus gradually decreases.Zhang-Jackson's inorganic phosphorus classification results show that the inorganic phosphorus forms in high-phosphorus soil are mainly iron phosphorus and aluminum phosphorus,followed by occluded phosphorus?O-P?and calcium-phosphorus?Ca-P?.During the in the equilibrium period of flooding,the amount of aluminum phosphorus and calcium phosphorus temporarily increases,and the amount of aluminum phosphorus and calcium phosphorus are converted to iron phosphorus and occluded phosphorus during the flooding process.During the process of drying,the amount of aluminum phosphorus decreases,and that of O-P phosphorus increases.Ascorbic acid can promote the conversion of soil inorganic phosphorus to a stable state.The results of thin film gradient diffusion technique?DGT?show that with the increase of flooding time,the effective phosphorus concentration extracted by DGT increases,and the increase of pH during soil flooding may be an important reason resulting in the increase of soil phosphorus supply capacity.Speciation analysis of iron shows that Fe?II?iron and amorphous iron increase significantly in the soil after flooding,and the content of crystalline iron decreases.During flooding the activation degree of soil iron increases,and decreases during drying.?2?Effects of the addition of organic matter on the transformation of soil phosphorus and iron forms.The effect of humic acid on pH and Eh of flooded soil is larger than that of biochar,and the activation effect of humic acid to soil iron is stronger than that of biochar.Hedley classification results show that the addition of organic matter significantly increases the availability of soil phosphorus during the equilibrium period,but during flooding process the availability of soil phosphorus gradually reduces,and the activation effect of humic acid to soil phosphorus is stronger than that of biochar.The film gradient diffusion technique?DGT?shows that the addition of organic matter increases the bioavailability of soil phosphorus with the increase of flooding time.?3?Effects of rice growth period on the transformation of soil phosphorus and iron.The results of Hedley phosphorus classification show that NaHCO₃-Pi and NaOH-Pi increase significantly in the early stage of rice growth,while HCl-Pi and H₂O-Pi decrease significantly.In the initial stage,maybe O-P phosphorus transforms to NaHCO₃-Pi and NaOH-Pi.During the growth process,the amount of NaHCO₃-Pi decreases,while the amount of NaOH-Pi and HCl-Pi increases significantly,and the availability of phosphorus decreases.Speciation analysis of iron shows that the activation of iron in rice growth period increases,the content of amorphous iron increase,and the content of crystalline iron decreases;the content of amorphous iron decreases and the content of crystalline iron increases during the mature period.As bioavailability of soil phosphorus reduces,the oxygen secretion of rice roots may be an important mechanism to ensure normal phosphorus supply for rice.