The effects of water potential on some microbial populations and decrease kinetic of organic carbon in soil treated with cow manure under laboratory conditions

Abstract

The moisture content of the soil habitat is an important stress factor and effects on soil microbial population and activity. The objectives of this study were to test the effects of water potential on the decrease kinetic of soil organic carbon (OC), some microbial populations and their activities in a soil treated with cow manure. A semiarid soil was treated with raw cow manure (CM) at a rate of 20 g kg^-1. Three water potentials established for soil incubation were: saturation (SAT, 0 bars), field capacity (FC, -0.3 bars), and permanent wilting point (PWP, -15 bars). Fourth irrigation treatment was drying-rewetting cycle (D-W) between -0.3 to -15 bars. After 0, 10, 20, 40, 60 and 90 days of incubation, soils were sampled for analysis. Colony forming units of bacteria, actinomycetes and fungi and soil OC, basal respiration (BR) and substrate induced respiration (SIR) were measured. The effects of soil moisture, incubation time and their interactions on all of the studied properties and kinetic parameter for OC decrease were significant. The populations of bacteria and fungi were the highest in the soils incubated in DWC and PWP respectively. The population of bacteria decreased significantly with increasing time of incubation. The population of fungi was the lowest in soil incubated in SAT condition and decreased continuously. However fungal population in soil incubated in other moisture increased significantly with increasing time of incubation. The population of actinomycetes was the highest in soil incubated in SAT condition in the early stages of incubation but it was decreased significantly with increasing time in soils incubated in SAT, FC, and DWC. Soil BR and SIR were significantly higher in SAT condition and lower in PWP condition compared to those in other water potentials. However both BR and SIR decreased significantly during soil incubation. Soil OC was significantly higher in SAT and PWP conditions due to unsuitable soil aeration and moisture for OC mineralization. The power function equation was the best model for decrease kinetic of OC in soil incubated in SAT and PWP conditions. However the second order equation was the best model for decrease kinetic of OC in soil incubated in FC and DWC conditions.