Influence of soil matric potential on 14C exudation from fungal propagules

The influence of soil matric potential (ψm) on 14C exudation from labeled fungal propagules incubated in two soils over 5 days was determined, using tensiometers set at 0, −50, −100, and −200 mbar ψm (1 bar = 100 kPa). In general, 14C exudation to soil (as a percent of initial label) tended to be greatest at 0 mbar and decreasing ψm. In a sandy loam soil, the range of 14C exudation from conidia of Bipolaris victoriae or sclerotia of Macrophomina phaseolina was 18.4–20.9 or 4.4–5.5% at 0 mbar. At −200 mbar, it was 5.5–6.0 or 1.2–1.7%, respectively. In a clay loam soil at 0 mbar, conidia of B. victoriae or B. sorokiniana lost 17.6–19.9 or 7.3–9.2%, respectively, of total 14C label as exudate. At −200 mbar 14.6 or 5.9–7.2% was lost, respectively. Sclerotia of M. phaseolina lost 5.1–5.7% 14C-labeled exudate to the clay loam soil at 0 mbar and 4.3–4.6% at −200 mbar. 14C exudation at ψm less than saturation was about 2–3 times greater in the clay loam soil than in the sandy loam soil. Calculated ratios of evolved 14CO2/total 14C-labeled exudate were lowest at 0 mbar and increased as ψm decreased, suggesting that oxygen availability and soil water content as regulated by ψm were related to soil respiratory utilization of exudate.