Short-term wavelike dynamics of bacterial populations in response to nutrient input from fresh plant residues

The objectives of the research were to investigate short-term dynamics of bacterial populations in soil after a disturbance in the form of fresh organic matter incorporation and to investigate how these dynamics are linked to those of some environmental parameters. To reach these objectives, soil bacterial populations, mineral nitrogen, pH, and redox potential (ROP) were monitored daily for 1 month after incorporation of clover-grass (CG) plant material in microcosm experiments. Colony-forming units (CFUs) and direct microscopic counts of FDA-stained and FTTC-stained bacteria increased immediately after incorporation of the plant material, dropped within 2 days, and fluctuated thereafter. Harmonics analysis demonstrated that there were significant wavelike fluctuations with three or four significant peaks within 1 month after incorporation of clover-grass material. Peaks in CFUs were 1-2 days ahead of those in direct counts. Ammonium (NH4) concentrations increased from the start of the experiments until nitrification commenced. Nitrate (NO3) concentrations dropped immediately after plant incorporation, and then rose monotonically until the end of the experiments. There were no wavelike fluctuations in NH4 and NO3 concentrations, so that bacterial fluctuations could not be attributed to alternating mineral N shortages and sufficiencies. pH levels rose and declined with NH4 levels. ROP dropped shortly before NH4 concentrations rose, and increased before NH4 concentrations decreased; there were no regular fluctuations in ROP, so that temporary oxygen shortages may not have been responsible for the observed fluctuations in bacterial populations. Thus, for the first time, regular wavelike dynamics were demonstrated for bacterial populations after perturbation by addition of fresh organic matter to soil, and several potential reasons for the death phase of the fluctuations could be excluded from further consideration.