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.