华中地区水旱轮作酸性土磷、铁形态转化及机理

Speciation transformation and mechanism of phosphorus and iron in acid soils in paddy-upland rotation in central China

为探讨华中地区水旱轮作体系酸性土壤磷与铁的形态转化以及磷生物有效性变化,本研究于2018年和2019年水稻收获后采集该区域典型酸性红壤(低磷铁比)和黄棕壤(高磷铁比),采用张守敬-杰克逊无机磷分级方法和梯度扩散薄膜(DGT)技术,对干湿交替培养样品磷形态、生物有效性磷(DGT-P)与铁(DGT-Fe)、Fe(Ⅱ)及无定形氧化铁(Fe)等进行定量分析。结果表明:淹水过程中黄棕壤和红壤pH逐渐升高趋于中性;二者氧化还原电势(Eh)在淹水期间降低,土壤环境呈明显的还原条件。淹水过程黄棕壤和红壤中Fe与Fe均增加,干燥后均明显降低。黄棕壤与红壤磷形态以铁结合态磷(Fe-P)与闭蓄态磷(Oc-P)为主,干湿交替过程中黄棕壤Oc-P向Fe-P转化,红壤Fe-P向Oc-P转化。在水-土界面以下120 mm土壤垂直剖面内,黄棕壤DGT-P和DGT-Fe释放水平先增加后减少;红壤DGT-Fe增加,DGT-P减少,黄棕壤P和Fe的同步释放相关性更显著。研究表明,干湿交替条件下酸性土壤磷形态主要是Fe-P与Oc-P之间转化,低P/Fe红壤以磷闭蓄化为主,高P/Fe黄棕壤Fe-P增加,以铁吸附磷为主;淹水导致DGT-P含量增加,生物有效性增加;干燥过程磷闭蓄化加深,但磷有效性并无明显降低,这与土壤磷水平有关。

This study aimed to investigate the transformation of iron and phosphorus and phosphorus availability in two typical acid soils at paddy-upland rotation system in central China.Yellow Brown soil(High P/Fe)and Red soil(Low P/Fe)were collected in 2018 and 2019after harvest.Phosphorus species,bioavailable phosphorus,and the iron Feand Fewere quantified during alternating wetting and drying via sequential extraction procedure and Diffusive Gradients in Thin-films technology.The results showed that the pH of Yellow Brown soil and Red soil increased during incubation,respectively,and tended to be neutral.During the flooding period,the Eh decreased,and the soil environment presented obvious reduction conditions.Both Feand Feincreased,and later decreased significantly after drainage.The phosphorus species mainly included iron bound phosphorus(Fe-P)and occluded phosphorus(Oc-P)in Yellow Brown soil,and Red soil mainly contained these two kinds.Both bioavailable phosphorus(DGT-P)and bioavailable iron(DGT-Fe)were released in the soil profile of 0 mm to 120 mm during incubation.The correlation was more significant in Yellow Brown soil.This study suggests that the soil redox conditions change the iron and phosphorus forms as the most important environmental factors.Fe-P and Oc-P are transformed after the soil underwent alternate wetting and drying.Red soil has low P/Fe ratio,and phosphorus is obviously occluded by iron.Yellow Brown soil has high P/Fe ratio,resulting in increase in Fe-P,and iron absorbed phosphorus as the main process.The increase in phosphorus bioavailability depends on the phosphorus level.