Modifications to the fumigation procedure to measure microbial biomass C in wet soils under pasture: Influence on estimates of seasonal fluctuations in the soil biomass

Abstract

Modifications to the chloroform-fumigation procedure to determine three indices of microbial biomass (viz. flushes of CO 2-C and mineral-N on incubation and C extractable in 0.5 M K 2SO 4) in wet, winter samples that smeared and compacted during sieving were investigated with four soils from under pasture. Rapid, partial air-drying of the soil before fumigation appeared satisfactory for subsequent incubation measurements, with CO 2-C flush values then usually being appreciably higher than in undried samples. With spring samples that gave discrete soil particles on sieving, this mild drying treatment had no effect on CO 2-C flush values, and only a small, inconsistent effect on mineral-N flush; extractable-C flush values were, however, lowered significantly. The possible suitability of increased shaking speed for determining extractable-C flush in wet, compacted samples was briefly examined. All of these biomass indices varied seasonally, whether compacted samples were partially dried or undried, with fluctuations greatest for CO 2-C flush. Fluctuations were generally not directly related to field moisture content when the partially-dried samples were used. The ratios of CO 2-C flush: mineral-N flush (both calculated with 0–10 days incubation periods for fumigated and unfumigated soil) were usually low in spring and partially-dried winter samples; ratios of only 0.85–1.20 were found in three of the soils, which suggests that the CO 2-C flush was still being under-estimated. An alternative calculation procedure (using fumigated soil alone, with CO 2-C produced during 10–20 days being subtracted from that produced during 0–10 days) gave CO 2-C: mineral-N flush ratios that were closer to those reported elsewhere, averaging 4.15 ± 0.46 and 4.76 ± 0.32 in spring and partially-dried winter samples respectively.