The Priming Effect: How Plant Root Exudates Make Soil Carbon More Susceptible to Microbial Degradation
Rates of decompositional processes performed by soil microbes are influenced by a variety of factors including temperature, water availability, and the presence of minerals. As plant materials are broken down by microbes, released organic carbon compounds can bind to soil minerals, becoming much less accessible to further decomposition. These bound pools of organic carbon can be stored in soils for years, decades, or centuries depending on local site conditions. However, microbiologists have long observed a phenomenon known as “the priming effect,” in which the addition of small amounts of unbound organic carbon results in microbial degradation of older pools of mineral-bound soil carbon. Elevated atmospheric CO2 levels recently have been shown to cause plant roots to increase their secretion of small carbon molecules (“exudates”), which has significantly increased the importance of understanding how the priming effect works. In a recent study, a team of scientists co-led by Lawrence Livermore National Laboratory and Oregon State University used a combination of microbial community analysis and high-resolution mass spectrometry (NanoSIMS) to examine the mechanistic basis of the priming effect in soil microcosms. When a variety of different carbon compounds associated with root exudates were added to the soils via an artificial root system, they were shown to directly disrupt associations between older carbon and soil minerals. Liberated carbon was rapidly consumed by soil microbes, and the team was able to follow correlated shifts in microbial community composition and elevated CO2 production. Different types of exudate compounds had varying degrees of ability to strip stored carbon from minerals, a particularly significant observation since elevated atmospheric CO2 shifts both the amounts and types of exudates that plants produce. These results represent a new breakthrough in understanding the molecular-scale mechanisms underlying the priming effect and could significantly advance our ability to predict impacts of climate change on carbon cycling in terrestrial ecosystems.
Keiluweit, M., J. J. Bougoure, P. S. Nico, J. Pett-Ridge, P. K. Weber, and M. Kleber. 2015. “Mineral Protection of Soil Carbon Counteracted by Root Exudates,” Nature Climate Change, DOI: 10.1038/NCLIMATE2580.