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Soil Solution Sampling for Organic Acids in Rice Paddy Soils

International Rice Research Institute (IRRI), Crop, Soil, and Water Sciences Division, DAPO Box 7777, Metro Manila, Philippines

View larger version (26K): [in a new window]   Fig. 1. Illustration of plastic rectangular pots (51 by 28 by 36 cm) used for the experiments. Zero-tension displacement (ZTD) sampling port(1), suction sampling port using porous tubes surrounded by a glasswool–sand layer (PT-S) (2), and suction sampling port using porous tubes (PT) (3). Centrifugation (CFG) sampling used soil taken from depth 5 to 10 cm.

View larger version (19K): [in a new window]   Fig. 2. Comparison of soil solution sampling methods for paddy soil based on organic acid detection and recovery from soil (A) with added rice straw and (B) with added acetic acid standard. ZTD, zero-tension displacement; PT, porous tube; PT-S, porous tube placed in sand layer; and CFG, centrifugation. Formic acid values for graph A were multiplied by 100. Percentage recoveries of the known amount of added standard are shown above the columns in graph B. Bars = ±1 standard deviation. ANOVA at 0.05 level revealed significant differences between sampling methods for graph B (both millimolar and percentage recovery values) but not A. Different letters indicate significant differences by Tukey's HSD at = 0.05, and are the same for millimolar and percentage recovery data.

View larger version (40K): [in a new window]   Fig. 3. Factors evaluated as possible sources of variation between sampling methods based on organic acid recovery using organic acid standard mix at known concentration: (A) pH difference; (B) with and without suction during sampling; and (C) presence of soil and sand particulates. Bars = ±1 standard deviation. ANOVA revealed no significant differences at the 0.05 level between treatments within each organic acid.

View larger version (21K): [in a new window]   Fig. 4. HPLC chromatogram showing organic acid peaks at different retention times (min) using an ion exclusion column run at 40°C, 0.75 mL min–1 flow rate, 10-µL sample volume, and 210 nm detection: (A) organic acid standard mixture showing small unknown ion peak; and (B) soil solution sample showing a large ion peak which may overlap with oxalic acid.

View larger version (61K): [in a new window]   Fig. 5. Factors evaluated as possible sources of interference from soil solution components regardless of the sampling method used: (A) organic acid standard mixture with various levels of Fe, NaCl, and oxalic acid; (B) soil solution sample with and without cation exchange membrane resin (CEMR) strips; and C) organic acid standard mixture with and without CEMR strips. Formic acid values were multiplied by 10 in graph B. Bars = ±1 standard deviation. Different letters represent significant differences by Tukey's HSD at = 0.05 where treatment differences within each organic acid were significant by ANOVA at the 0.05 level.