Estimation of Effective Cation Exchange Capacity and Exchangeable Iron in Paddy Fields After Soil Flooding

In flooded soils, the concentrations of exchangeable Mn²⁺ and, especially, Fe²⁺ can be high and must be considered when determining the cation exchange capacity (CEC) of the soil under flooded conditions. However, these reduced forms of Mn and Fe are oxidized and precipitated during the extraction process used in traditional CEC methods. This procedure underestimates the exchangeable portion of these cations and, consequently, the CEC value of the flooded soil. We introduce a pH-gradient-based model to predict ECEC and exchangeable Fe²⁺ in flooded soils, circumventing oxidation artifacts inherent in conventional methods. The objective of this study is to propose an alternative to estimate the exchangeable Fe²⁺ and the effective CEC (ECEC) of flooded soils. To achieve this goal, 21 surface samples (0–20 cm) of soil from rice fields were collected and distributed in the cultivation regions of southern Brazil. The soils were flooded for 50 days. The soil solution was collected on the first day and after 50 days of flooding and pH, Na, K, Ca, Mg, Fe and Mn were determined. In these samples, exchangeable cations (K, Na, Ca, Mg, Mn, Al and H + Al) were determined to calculate ECEC and CEC at pH 7 of unflooded soil and after 50 days of flooding. There was a wide range of variation in the exchangeable cation contents among the soil samples. The K contents ranged from 0.12 to 0.54 cmol_c kg⁻¹, the Na contents from 0.00 to 1.18 cmol_c kg⁻¹, the Ca contents from 0.48 to 37.31 cmol_c kg⁻¹, the Mg contents from 0.10 to 15.53 cmol_c kg⁻¹, the Mn contents from 0.01 to 0.36 cmol_c kg⁻¹, the Al contents from 0.10 to 1.74 cmol_c kg⁻¹ and the H + Al contents from 2.01 to 8.42 cmol_c kg⁻¹. The results were used to develop models to predict ECEC and exchangeable Fe content after 50 days of flooding. Estimating the ECEC after flooding using the pH gradient before and after flooding yielded values closer to CEC pH 7.0, correcting for the possible underestimation of the ECEC during flooding. The amount of exchangeable Fe estimated was higher than the exchangeable Fe determined, correcting the possible underestimation of these quantities determined during flooding. It is concluded that the estimations of ECEC after flooding through the equation <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>E</mi><mi>C</mi><mi>E</mi><mi>C</mi></mrow><mrow><mi>a</mi><mi>f</mi><mi>t</mi><mi>e</mi><mi>r</mi></mrow></msub><mo>=</mo><mi>E</mi><mi>C</mi><mi>E</mi><mi>C</mi><mo>+</mo><mfenced separators="|"><mrow><msub><mrow><mi>p</mi><mi>H</mi></mrow><mrow><mi>s</mi><mi>o</mi><mi>l</mi><mo>.</mo><mi>a</mi><mi>f</mi><mi>t</mi><mi>e</mi><mi>r</mi></mrow></msub><mo>−</mo><mtext> </mtext><msub><mrow><mi>p</mi><mi>H</mi></mrow><mrow><mi>s</mi><mi>o</mi><mi>l</mi><mo>.</mo><mi>b</mi><mi>e</mi><mi>f</mi><mi>o</mi><mi>r</mi><mi>e</mi></mrow></msub></mrow></mfenced><mtext> </mtext><mo>×</mo><mtext> </mtext><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><mo>(</mo><msub><mrow><mi>C</mi><mi>E</mi><mi>C</mi></mrow><mrow><mi>p</mi><mi>H</mi><mn>7</mn></mrow></msub><mo>−</mo><mtext> </mtext><mi>E</mi><mi>C</mi><mi>E</mi><mi>C</mi><mo>)</mo></mrow><mrow><mo>(</mo><mn>7</mn><mo>−</mo><mtext> </mtext><msub><mrow><mi>p</mi><mi>H</mi></mrow><mrow><mi>s</mi><mi>o</mi><mi>l</mi><mo>.</mo><mi>b</mi><mi>e</mi><mi>f</mi><mi>o</mi><mi>r</mi><mi>e</mi></mrow></msub><mo>)</mo></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula>, where <i>pH_sol.before</i> is pre-flooding soil pH, <i>pH_sol.after</i> is after flooding pH, <i>ECEC_after</i> is effective CEC after flooding and the exchangeable Fe²⁺ after flooding through the equation <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>e</mi><mi>x</mi><mi>c</mi><mo>.</mo><mi>a</mi><mi>f</mi><mi>t</mi><mi>e</mi><mi>r</mi><mo>.</mo><mi>e</mi><mi>s</mi><mi>t</mi><mi>i</mi><mi>m</mi><mi>a</mi><mi>t</mi><mi>e</mi><mi>d</mi></mrow></msub><mo>=</mo><msub><mrow><mi>E</mi><mi>C</mi><mi>E</mi><mi>C</mi></mrow><mrow><mi>a</mi><mi>f</mi><mi>t</mi><mi>e</mi><mi>r</mi></mrow></msub><mo>−</mo><mtext> </mtext><mfenced separators="|"><mrow><mi>C</mi><mi>a</mi><mo>+</mo><mi>M</mi><mi>g</mi><mo>+</mo><mi>K</mi><mo>+</mo><mi>N</mi><mi>a</mi><mo>+</mo><mi>M</mi><mi>n</mi></mrow></mfenced></mrow></semantics></math></inline-formula> where <i>Fe_{exc.after.estimated}</i> is estimated exchangeable Fe²⁺ after flooding corrected the problem of underestimating the values of these variables by analytical methods, demonstrating its viability for use in flood-prone soils.