Comparison of physiological techniques for estimating the response of the soil microbial biomass to soil moisture

The influence of soil moisture status on the soil microbial biomass was determined using three physiological methods, i.e. fumigation-incubation (using either k c = 0.41 or a different k c value determined for each treatment), substrate-induced respiration (SIR) method (with samples either remoistened or non-remoistened immediately prior to determination), and the oxygen-uptake method. These techniques were applied to soil samples that had been incubated at 15, 25, 35, 45 or 55% moisture content for 6 h or 3, 10 or 30 days. The fumigation-incubation technique (k c = 0.41) predicted that microbial biomass was maximal at intermediate soil moisture contents. Fumigation-incubation (varying k c) and SIR (non-remoistened samples) predicted that microbial biomass remained relatively constant across the entire soil moisture gradient. SIR (non-remoistened samples) and the O2 uptake curve method predicted that microbial biomass increased with increasing soil moisture content. However, relationships between pairs of methods for the same samples were extremely weak, with r 2 values never exceeding 0.50. The possible reasons for poor agreement between these methods are discussed.