Effects of soil flooding on P transformations in soils of the Mesopotamia region, Argentina Cesar Eugenio Quintero 1 , Flavio Hernán Gutiérrez-Boem 2 *, María Romina Befani 1 , and Norma Graciela Boschetti 1 1 Facultad de Ciencias Agropecuarias, Universidad Nacional de Entre Ríos, CC 24, 3100 Paraná, Entre Ríos, Argentina 2 Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martín 4453, 1417 Buenos Aires, Argentina Accepted December 6, 2006 Summary In the Mesopotamia region (Argentina), rice is cropped on a wide range of soil types, and the response of rice to fertilizer application has been inconsistent even in soils with very low levels of available phosphorus. Phosphorus transformations in flooded soils depend on soil characteristics that may affect phosphorus availability. This study was conducted to deter- mine which soil characteristics were related to the changes in P fractions during soil flooding. Soils were chosen from ten sites within the Mesopotamia region that are included in five different soil orders: Oxisols, Ultisols, Alfisols, Mollisols, and Vertisols. Soil phosphorus (P) was fractionated by a modified Hedley method before and after a 45 d anaerobic-incubation period. Changes in the inorganic P extracted with resin depended on soil pH and were related to the exchangeable- Fe concentration of soils (extracted with EDTA). Inorganic P extracted with alkaline extractants (NaHCO 3 and NaOH) in- creased due to soil flooding. This increase was related to the organic-C (OC) percentage of soils (r 2 = 0.62, p < 0.01), and ranged from 13 to 55 mg kg –1 . Even though previous studies showed that P associated with poorly crystalline Fe played an important role in the P nutrition of flooded rice, in this study, there was no relationship between ammonium oxalate– extractable Fe and P changes in soils due to flooding. Our results suggest that in the Mesopotamia region, changes in P fractions due to soil flooding are related to soil OC, soil pH, and soluble and weakly adsorbed Fe. Key words: phosphorus fractions / rice / flooding / incubation / phosphorus transformation  2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/07/0408-500 500 DOI: 10.1002/jpln.200625015 J. Plant Nutr. Soil Sci. 2007, 170, 500–505 1 Introduction Phosphorus (P) availability increases when soils are flooded due to increased phosphate diffusion towards roots and the release of phosphate to the soil solution (Young and Ross, 2001). When soils are subjected to reducing conditions, iron (Fe) oxides are solubilized, and the associated P is released (both adsorbed and occluded forms). On the contrary, when a flooded soil is drained, P availability decreases (Seng et al., 1999; Scalenghe et al., 2002; Huguenin-Elie et al., 2003). Intermittent periods of flooded and drained conditions are common in rice-production systems. These alternating wet- dry conditions may affect P absorption by crops due to changes in P availability (Huguenin-Elie et al., 2003). Mesopotamia is the main rice-production region of Argentina, and it has a diverse range of soil types, from Oxisols and Ulti- sols in the N to Mollisols and Vertisols in the S (INTA, 1990). Rice is cropped on all these different soil orders. Soil-P avail- ability in these soils ranges from very low to moderate, and phosphate fertilizer is used at rates from 10 to 30 kg P ha –1 when P soil tests are <10 ppm (Bray 1 method). The re- sponse of rice yields to fertilizer application has been incon- sistent even in soils with very low levels of available P (De Battista and Arias, 2006). These results suggest that methods successfully used to evaluate phosphorus availabil- ity for other grain crops in the same region are not suitable for predicting P availability for rice. The release of phosphates from reduced Fe oxides in flooded soils may be one of the reasons for the inconsistent relationship between crop re- sponse to fertilization and the amount of phosphates extracted by a soil test. Some studies showed that Fe phosphate compounds were the main source of P for flooded rice crops (Shahandeh et al., 1994). Flooding increases solubilization of Fe oxides due to the reduction of ferric (Fe 3+ ) to ferrous (Fe 2+ ) ion. Scalenghe et al. (2002) observed that the amorphous oxides were pre- ferentially involved in this process as there was a selective solubilization of Fe from the oxalate-extractable forms. As a consequence of the Fe reduction, the P associated with these oxides is released to the soil solution. The amount of P released to the soil solution depends on soil characteristics involved in the reduction process (abundance of Fe oxides, their crystallinity, and the soil organic-matter (SOM) content), and the degree of soil saturation with P. Organic matter (OM) favors reduction as it is the main elec- tron donor (Scalenghe et al., 2002). Therefore, different soils release different amounts of P when undergoing the same flooding event. Predicting this P release based on soil proper- ties would improve the evaluation of P availability for rice crops. In a previous study, soils from the Mesopotamia region that had been cropped with rice for several years were examined with sequential P extractions (Hedley fractionation). Soils that had been cropped with rice for longer periods had reduced pools of labile organic P and residual P, while every pool of labile inorganic P increased. These results suggested that rice cropping produced a P transfer from less labile pools * Correspondence: Dr. F. H. Gutiérrez-Boem; e-mail: gutierre@agro.uba.ar