The chloroform fumigation efficiency in water-saturated soils increases by mixing sand and decreasing packing thickness

• CHCl3 fumigation efficiency is low in water-saturated soils.
• Mixing sand into water-saturated soils increases the fumigation efficiency.
• Decreasing soil packing thickness increases the fumigation efficiency.
• Placing the same size of samples into the same-sized containers is required.

The chloroform fumigation efficiency for measuring microbial biomass in water-saturated soils is low due to limited diffusion of chloroform. To suggest modifications for the improvement of the fumigation efficiency, the effects of sand mixing and soil packing thickness on the fumigation efficiency in water-saturated soils were investigated through four experiments. Two soils with contrasting organic carbon content were used to examine if the modifications are applicable regardless of microbial biomass level. The soils were waterlogged to mimic rice paddy soils, and sand was mixed to decrease water content by either adding an increasing amount of sand to a decreasing amount of soil (Exp-1) or an increasing amount of sand to a constant amount of soil (Exp-2). Soil packing thickness was decreased by packing a decreasing amount of soil into same-sized containers (Exp-3) or by placing the same amount of soil into progressively wider containers (Exp-4). Overall, the effects of sand mixing and packing thickness on the fumigation efficiency did not differ with soil microbial biomass level. In both soils which were originally saturated with water, increasing the amount of sand mixing decreased (P < 0.001) soil water content and increased (P < 0.001) the concentration of extracted organic carbon by fumigation (EOCf) by up to 303% in Exp-1 but decreased (P < 0.001) EOCf by up to 80% in Exp-2, likely due to the concomitant increase in soil packing thickness. Decreasing soil packing thickness increased (P < 0.001) EOCf by up to 128% (Exp-3) and 298% (Exp-4). It was also found that the amount of soil packed affected the fumigation efficiency (Exp-2 and 3). We suggest using the same amount of sample and same-sized beakers for fumigation is more practical to minimize the confounding effect of soil amount and packing thickness on the fumigation efficiency of water saturated soils and thus to help the comparison of microbial biomass between soils.