Protozoan predation and the turnover of soil organic carbon and nitrogen in the presence of plants

Abstract

The impact of protozoan grazing on the dynamics and mineralization of 14C- and 15N-labelled soil organic material was investigated in a microcosm experiment. Sterilized soil was planted with wheat and either inoculated with bacteria alone or with bacteria and protozoa or with bacteria and a 1:10 diluted protozoan inoculum. 14C−CO2 formation was continuously monitored. It served as an indicator of microbial activity and the respiration of soil organic C. The activity of protozoa increased the turnover of 14C-labelled substrates compared to soil without protozoa. The accumulated 14C−CO2 evolved from the soils with protozoa was 36% and 53% higher for a 1:10 and for a 1:1 protozoan inoculum, respectively. Protozoa reduced the number of bacteria by a factor of 2. In the presence of protozoa, N uptake by plants increased by 9% and 17% for a 1:10 and a 1:1 protozoan inoculum, respectively. Both plant dry matter production and shoot: root ratios were higher in the presence of protozoa. The constant ratio of 15N: 14+15N in the plants for all treatments indicated that in the presence of protozoa more soil organic matter was mineralized. Bacteria and protozoa responded very rapidly to the addition of water to the microcosms. The rewetting response in terms of the 14C−CO2 respiration rate was significantly higher for 1 day in the absence and for 2 days in the presence of protozoa after the microcosms had been watered. It was concluded that protozoa improved the mineralization of N from soil organic matter by stimulating the turnover of bacterial biomass. Pulsed events like the addition of water seem to have a significant impact on the dynamics of food-chain reactions in soil in terms of C and N mineralization.