Effects of phosphorous application on arsenic toxicity to and uptake by rice seedlings in As-contaminated paddy soils

Abstract

Arsenate (AsV) has been proven to share the same absorption pathway from soil to rice roots as phosphate (Pi). Therefore, soil application of phosphorous was assumed able to reduce arsenate uptake by rice. However, solution cultivations, pot tests and field experiments have shown differing results regarding the efficacy of phosphorous applications. Moreover, the effects of Pi on Arsenic (As) uptake by rice in As-enriched soils may depend on multiple factors, such as soil properties, As levels in soils or the application rate of Pi. In this study, we aimed to determine the effects of a wide range of application rates of Pi on As release and phytotoxicity in soils. Two geogenically As-enriched soils, two background soils, and two As-spiked soils were tested. The rice seedlings were cultivated in the soils treated with Ca(H2PO4)2 for 40days. The results show that Pi application into soils did not inhibit As uptake and accumulation in rice seedlings for all soils. For the soils with high retention capacity of Pi, the Pi treatments did not result in any obvious change in As concentration of the soil solution while the Pi concentration in the soil solution increased but did not reach a level that can compete with As for rice uptake. For the soils with low retention capacity of Pi, the application of Pi largely increased both the As and Pi concentrations in the soil solutions, which could be attributed to competitive adsorption by solids. This resulted in a small increase in the Pi/As molar ratio of the soil solution and an absence of competitive uptake by rice. Conversely, the As toxicity was aggravated when As concentration in soil solution was increased with high Pi application. We conclude that for paddy soils with high levels of As content, Pi application at a much higher level than plant requirements may be needed, which is not appropriate due to environmental concerns. We suggest future testing be aimed at determining whether foliar applications of P would induce enough competitive uptake between P and As to sufficiently reduce As uptake by rice seedlings grown in As-contaminated soils.