Available Soil Water and its Effect on Microbial Activity in the Vadose Zone

Estimation of microbial activity in soils is a complex and often difficult process. In this presentation, we describe several new methods developed to estimate microbial activity in unsaturated intact soil cores. These methods, together with a previous characterization of the pore volume distribution of the soil, were then used to demonstrate a shift in the measured microbial activity when the soil water available to the microbes was considered rather than the total gravimetric water content. Prior microbial activity studies have added substrate solution drop-wise to the soil and distributed it throughout the soil by mixing. Although this method distributes the substrate, it alters the in situ pore volume distribution, increases initial microbial activity, and when constant amounts of substrate are added to soils of varying moisture content, varies the initial substrate concentration among the samples. Under ideal circumstances, these limitations could be overcome by measuring microbial activity in unsaturated intact soil cores that initially have a homogeneous distribution of substrate at a constant concentration. In this study, intact soil cores were first saturated at 5°C with a 14C-labeled glucose solution and then drained to the desired moisture content in 8 hours or less in a geocentrifuge at 4°C. Intact aliquots of the cores were then incubated, sacrificing samples at 30, 60, and 90 days. Respired CO2 was quantitatively recovered and measured via liquid scintillation. The volume of soil water available to microbes ranging in size from 0.2 to 1 microns was estimated from the pore volume distribution (accounting for compaction in the geocentrifuge), and correlated with the measured microbial activity at each water content. Results indicate that for the soil tested, there is a significant amount of soil water that is inaccessible to the microbes. Thus, it is not adequate to measure microbial activity for a certain soil type, correlate it with gravimetric moisture content, and then assume that the microbial activity / moisture relationship can be used to estimate relative amounts of microbial activity at various water contents in a similar soil type. Rather, the microbial activity should be correlated with available water content, since the pore volume distribution and percentage of inaccessible water will vary among soils.