Cold Storage and Pretreatment Incubation Effects on Soil Microbial Properties

A long-standing dilemma for soil microbial assays is how best to store soil samples between sampling and analysis. We studied the effects of sample handling and storage on methods used to determine soil microbial biomass, structure, and function. For this study, one forest soil (Gilpin), and two agricultural soils (Granby and Hoytville) were selected with five commonly used sample pretreatments: (i) fresh soil; (ii) air drying for 14 d followed by rewetting (65% water-holding capacity) and incubation (25°C) for 14 d (D/R); (iii) 28 d at 4°C; (iv) 28 d at -20°C; and (v) 28 d at -80°C. Immediately after pretreatments, soils were analyzed for fatty acid methyl esters (FAMEs), total DNA (tDNA), seven enzyme activities, microbial biomass C, and respiration. Drying and rewetting significantly reduced microbial biomass, respiration, most enzyme activities, tDNA, and total FAME concentrations compared with fresh soil in all three soils. The percentage of fungal FAME markers and two enzyme assays were unaffected by 4°C storage in all soils, and microbial biomass C was unchanged in Hoytville and Gilpin soil at -20 and -80°C. Total DNA was unchanged in the Granby soil at -80°C, and in the Hoytville soil at both -20 and -80°C compared with fresh soil. Total FAME was reduced by all storage treatments in all three soils. We concluded that storage should be avoided whenever possible, particularly for extraction of FAME and total DNA, but that 4 or -20°C is the best storage method for FAME analysis, and -80°C is preferable for DNA analysis. Microbial biomass C and enzyme activities were least affected when stored at 4 or -20°C. The D/R treatment was the least desirable soil preparation method for microbial analyses, and we recommend that this pretreatment be avoided.

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