The optimization of phosphorus (P) fertilization in rice paddies requires an accurate estimate of soil P availability. Several accepted methods are available for assessing P availability in upland crops, however, they are not necessarily adequate for the determination of available P pools in redoximorphic soils due to the strong link between P mobility and Fe redox cycling [1][2]. The effects of rice plants on shaping the chemical gradients and microbiological activity in the rhizosphere adds complexity to P cycling in these soils [3]. The aim of this study was to identify the most suitable method to estimate P availability for plant uptake in rice paddy soils across a wide range of differing P contents, and to evaluate the role of plant in shaping the rhizospheric P mobility. To this aim, twelve paddy soils, differing for total P concentration and representative of paddy rice soils in the Po region for texture, pH and organic matter content, were analysed for available P with several methods aimed at quantifying different potentially available soil P pools, including soluble and exchangeable P (0.01 M CaCl2, 0.5 M Na2CO3, anion exchanging resins), as well as P retained on Fe and Mn oxyhydroxides that could be potentially released under anoxic conditions (EDTA, citrate-ascorbate, oxalate). Rice plants were cultivated in these soils for 60 days under submerged conditions. The concentrations of dissolved Fe(II), molybdate reactive phosphorus (MRP), and dissolved organic carbon (DOC) in soil porewaters were analysed weekly in pots with and without plant. Above and belowground plant biomass, P concentration in tissues (Pplant), and the expression of root P transporters were evaluated at the end of the growth period. The strong positive correlation between Pplant and resins-P and CaCl2-P across all soils indicated that these methods best represented P availability for rice grown under reducing conditions. Specifically, CaCl2-P estimates P dissolution in soil porewater, while resins-P, that measures the soil re-supply capacity after P depletion in the solution, was the best correlation method with Pplant. Conversely, despite the strong relation between the release of P and Fe(II) in the porewaters under anoxic conditions, the chemical methods based on P release after the solubilization of different Fe pools were barely correlated with Pplant. The poor estimation offered by EDTA, citrate-ascorbate and oxalate could be explained by the temporal shift between maximum porewater P concentrations induced by the reductive dissolution of Fe minerals, achieved at 14 days after flooding, and P uptake by plants. Indeed, during germination rice plants utilize P reserves in the seeds and start to depend on soil available P after around 15 days from germination and greatly increase with root development [4], when P concentrations in solution begin to decline probably due to re-adsorption onto different mineral phases and/or precipitation with Fe(II) ions [5]. In addition, the correlation between P availability estimated by the chemical methods and Pplant varied according to the soil P levels: in low P soil the correlation pattern was different compared to high P soils, suggesting the key role of plant strategies to overcome P limitation. The higher investment in root growth and the increased expression of high affinity P root transporters (OsPT1 and OsPT8) in low P soils could justify the poor correlation between Pplant with CaCl2-P observed in the soils with the lowest P contents. The poor correlation between Pplant and the methods estimating P release from Fe minerals, again weaker in P-deficient soils, could be explained by the lower value of Fe(II) in solution observe in planted than in unplanted pots, because of the oxidizing effect of root radial oxygen losses in the rhizosphere compared with the anaerobic bulk soil., and clearly Our results indicated anion exchanging resins as the best estimation of P availability under reducing conditions underline the importance of rhizosphere processes, particularly under P-limited conditions.

Soil-plant interactions governing phosphorus availability to rice

Martinengo Sara
First
;
Michela Schiavon;Veronica Santoro;Daniel Said-Pullicino;Luisella Celi;Maria Martin
Last
2022-01-01

Abstract

The optimization of phosphorus (P) fertilization in rice paddies requires an accurate estimate of soil P availability. Several accepted methods are available for assessing P availability in upland crops, however, they are not necessarily adequate for the determination of available P pools in redoximorphic soils due to the strong link between P mobility and Fe redox cycling [1][2]. The effects of rice plants on shaping the chemical gradients and microbiological activity in the rhizosphere adds complexity to P cycling in these soils [3]. The aim of this study was to identify the most suitable method to estimate P availability for plant uptake in rice paddy soils across a wide range of differing P contents, and to evaluate the role of plant in shaping the rhizospheric P mobility. To this aim, twelve paddy soils, differing for total P concentration and representative of paddy rice soils in the Po region for texture, pH and organic matter content, were analysed for available P with several methods aimed at quantifying different potentially available soil P pools, including soluble and exchangeable P (0.01 M CaCl2, 0.5 M Na2CO3, anion exchanging resins), as well as P retained on Fe and Mn oxyhydroxides that could be potentially released under anoxic conditions (EDTA, citrate-ascorbate, oxalate). Rice plants were cultivated in these soils for 60 days under submerged conditions. The concentrations of dissolved Fe(II), molybdate reactive phosphorus (MRP), and dissolved organic carbon (DOC) in soil porewaters were analysed weekly in pots with and without plant. Above and belowground plant biomass, P concentration in tissues (Pplant), and the expression of root P transporters were evaluated at the end of the growth period. The strong positive correlation between Pplant and resins-P and CaCl2-P across all soils indicated that these methods best represented P availability for rice grown under reducing conditions. Specifically, CaCl2-P estimates P dissolution in soil porewater, while resins-P, that measures the soil re-supply capacity after P depletion in the solution, was the best correlation method with Pplant. Conversely, despite the strong relation between the release of P and Fe(II) in the porewaters under anoxic conditions, the chemical methods based on P release after the solubilization of different Fe pools were barely correlated with Pplant. The poor estimation offered by EDTA, citrate-ascorbate and oxalate could be explained by the temporal shift between maximum porewater P concentrations induced by the reductive dissolution of Fe minerals, achieved at 14 days after flooding, and P uptake by plants. Indeed, during germination rice plants utilize P reserves in the seeds and start to depend on soil available P after around 15 days from germination and greatly increase with root development [4], when P concentrations in solution begin to decline probably due to re-adsorption onto different mineral phases and/or precipitation with Fe(II) ions [5]. In addition, the correlation between P availability estimated by the chemical methods and Pplant varied according to the soil P levels: in low P soil the correlation pattern was different compared to high P soils, suggesting the key role of plant strategies to overcome P limitation. The higher investment in root growth and the increased expression of high affinity P root transporters (OsPT1 and OsPT8) in low P soils could justify the poor correlation between Pplant with CaCl2-P observed in the soils with the lowest P contents. The poor correlation between Pplant and the methods estimating P release from Fe minerals, again weaker in P-deficient soils, could be explained by the lower value of Fe(II) in solution observe in planted than in unplanted pots, because of the oxidizing effect of root radial oxygen losses in the rhizosphere compared with the anaerobic bulk soil., and clearly Our results indicated anion exchanging resins as the best estimation of P availability under reducing conditions underline the importance of rhizosphere processes, particularly under P-limited conditions.
2022
XL SICA Congress - Conciliating Sustainability, Resilience and Food Quality: New challenges for a 2030 agriculture
Pisa
5-7 settembre 2022
CONCILIATING SUSTAINABILITY, RESILIENCE, AND FOOD QUALITY New challenges for a 2030 agriculture
1
192
rice, phosphorus, soil availability
Martinengo Sara, Michela Schiavon, Veronica Santoro, Daniel Said-Pullicino, Luisella Celi, Maria Martin
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/1880344
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