Microbial mechanisms underlying rhizosphere priming of litter decomposition

Background/Question/Methods

It is well documented that plant roots can substantially alter rates of litter decomposition in soil. However, the mechanisms underlying “priming” are poorly understood. Identifying the mechanisms by which roots alter the microbial mediation of decomposition is essential to predicting the impacts of changing environments on soil decomposition processes. We examined effects of Avena fatua roots on ¹³C-labeled root litter decomposition in a California grassland soil over two simulated growing-seasons, with focus on microbial community function and composition. The abundance, composition and functional potential of soil microbial communities were analyzed by qPCR, Illumina MiSeq sequencing of 16S and ITS amplicons and GeoChip 4. 

Results/Conclusions

The presence of the annual grass roots consistently suppressed rates of labeled litter decomposition. Presence of plants significantly altered the abundance, composition and functional potential of microbial communities. Significantly higher signal intensities of genes capable of degrading low molecular weight organic compounds (e.g., glucose, formate and malate) were observed in  microbial communities from planted soils, while microorganisms in unplanted soils had higher relative abundances of genes involved in degradation of some macromolecules (e.g., hemicellulose and lignin). Additionally, microbial communities from planted soils showed molecular characteristics of water stress (higher signal intensities of proV and proW, genes involved in osmotic stress response). The results from functional gene analyses suggest two possible mechanisms for the reduced rates of litter decomposition: 1) microbial preferential utilization of simple substrates from plant root exudates and 2) reduction in microbial activity due to soil drying from evapotranspiration. We propose a conceptual model of the mechanisms by which plants modulate microbial mediation of decomposition.