MICROSOM · Microbial competition as a driving force of primed soil organic matter decomposition

Final Activity Report Summary - MICROSOM (Microbial competition as a driving force of primed soil organic matter decomposition)

During the project realisation of the estimations of ‘priming effects’ (PE), that is accelerated mineralisation of soil organic matter during long-termed microbial decomposition of 14C-glucose, 14?-cellulose, plant residues and sewage sludge were carried out. The growth strategies of soil microorganisms were related to apparent and real PE. Apparent PE corresponded to one to three days after the addition of available substrates, i.e. to short-termed response, and was performed by microorganisms with r-strategy. K-strategists were responsible for real PE, observed during 30 to 54 days after soil amendment. Two opposite relationships between the amount of added substrate and PE were found out. If the amount of added carbon was lower than 15 % of the microbial biomass, the amount of added substrate was the limiting factor for PE. If the added substrate amount exceeded 50 % of the microbial biomass, then the latter was the limiting factor for PE. Based on 14C-labelled substrates along with natural differences in the abundance of 13C between C4 and C3 plants, the mobilisation of old recalcitrant carbon by glucose addition was pinpointed. It was found out that the effect of unfavourable soil-ecological factors, such as low temperature, shortage of available organic carbon and contamination of soil with sewage sludge, resulted in the domination of microorganisms with K-strategy and the formation of an oligotrophic microbial community. Our work to estimate the active part of soil microorganisms in situ revealed that: 1. microbial growth rates after the input of root exudates were 1.5 times higher than in soil with plant residues, while substrate affinity to root exudates was twice that of the residues. Thus, for the first time under soil conditions, it was experimentally confirmed that the fast-growing r-strategists benefited from the available substrates, while during the mineralisation of the less easily available substrates slow-growing K-strategists dominated. 2. r-strategists were responsible for the production of extra carbon dioxide (CO2) after the input of small amounts of available substrate to the soil. Hence, the acceleration of microbial metabolisms, rather than of native soil organic matter, provided extra CO2. 3. a significant increase in microbial growth rates was observed under elevated CO2, independent of soil type, plant species or nitrogen fertilisation. Plants growing under elevated CO2 caused a shift in the ecological strategy of the soil microbial community towards a higher contribution from fast-growing r-selected species. Such fast-growing microorganisms could rapidly mineralise the increased root exudation caused by elevated CO2, preventing the activation of those slow-growing microorganisms that were responsible for the mineralisation of recalcitrant soil organic carbon. This microbial auto-regulation mechanism, which counterbalanced the effect of elevated CO2, confirmed our hypothesis. 4. microbial growth rates increased by between 20 and 60 % under elevated CO2 levels. However, only a tiny amount of active microorganisms caused this increase, illustrating its impact. Remarkably, the generation time of these active microorganisms which was estimated in soil in our experiments was 100 to 1 000 times shorter than that of the entire microbial community. Thus, although the amount of active microbial biomass was tiny, they were very powerful. Through our research MICROSOM discovered that competition between microorganisms with different growth strategies enabled the development of a unique self-regulation mechanism. This prevented the exhaustion of soil organic matter under elevated concentration of CO2 in the atmosphere, illustrating the immediate relevance of our research and reinforcing our determination to further advance it beyond the project completion.

Data: CORDIS, © European Union