Interaction between gas exchange rates, physical and microbiological properties in soils recently subjected to agriculture

The impact of conventional tillage (CT) or no-till (NT) management on soil microbial respiration as well as microbial abundance was studied in soils from the El Salado basin river (Buenos Aires, Argentina) recently subjected to agriculture under a corn-pasture rotation since 1996. Both management systems were monitored for several soil (micro)biological, physical and chemical properties during the second (1997) to fourth (1999) years from the beginning of the experiment. O₂ and CO₂ composition of the soil atmosphere and the rate at which soil consumes O₂ (qO₂) or produces CO₂ (qCO₂), under conditions that approximate the soil environment in the field, were quantitated following an experimental method and a mathematical model developed by ourselves Soil Sci. 166 (2001) 68] to interpret the data. qO₂ and qCO₂ expressed in terms of kg O₂ or CO₂-C per ha per day or per kg C of microbial biomass (microbial respiration), increased from the lowest values measured at 10–30% water-filled pore space (WFPS) up to 60% WFPS, decreasing thereafter. Low respiratory quotients, RQ (qCO₂/qO₂<1.0), were detected, with gas exchanges being slightly higher in NT than in CT. Correspondingly, higher bacterial and fungal biomass were measured in NT than in CT. Apparently, bacteria were more sensitive to high WFPS than fungi. When aerobic bacteria or fungi counts were compared at low or high WFPS, they differed significantly only in the upper soil profile whereas microaerophilic bacteria and fungi were significatively different in both depths tested (D1=5–10 cm; D2=15–20 cm). The results are discussed in terms of microbial metabolism behavior and abundance as a function of management and soil air/water balance in soils recently subjected to agriculture.