| 英文摘要: | Multiple lines of existing evidence suggest that climate change enhances root exudation of organic compounds into soils. Recent experimental studies show that increased exudate inputs may cause a net loss of soil carbon. This stimulation of microbial carbon mineralization (‘priming’) is commonly rationalized by the assumption that exudates provide a readily bioavailable supply of energy for the decomposition of native soil carbon (co-metabolism). Here we show that an alternate mechanism can cause carbon loss of equal or greater magnitude. We find that a common root exudate, oxalic acid, promotes carbon loss by liberating organic compounds from protective associations with minerals. By enhancing microbial access to previously mineral-protected compounds, this indirect mechanism accelerated carbon loss more than simply increasing the supply of energetically more favourable substrates. Our results provide insights into the coupled biotic–abiotic mechanisms underlying the ‘priming’ phenomenon and challenge the assumption that mineral-associated carbon is protected from microbial cycling over millennial timescales.
Plants direct between 40–60% of photosynthetically fixed carbon (C) to roots and associated microorganisms via sloughed-off root cells, tissues, mucilage and a variety of exuded organic compounds1, 2. Elevated CO2 concentrations in the atmosphere are projected to increase the quantity3, 4 and alter the composition5, 6 of root exudates released into the soil. It seems less clear to what extent changing inputs will cause a net loss of native (or ‘old’) organic C (ref. 7). A better understanding of the mechanism underlying soil C loss is pivotal in predicting how the large soil C stocks may respond to global change. Exudate-induced soil C loss is commonly attributed to a ‘priming effect’—that is, a short-term increase in microbial mineralization of native soil C as a result of fresh carbon inputs to the soil8. Although the process of ‘priming’ has received great attention in ecosystem sciences in recent years8, 9, our knowledge of the underlying mechanism is limited. It is often supposed that bioavailable exudate compounds induce greater microbial activity and enzyme production because they serve as ‘co-metabolites’8, 10. Co-metabolism is defined as the mineralization of a non-growth substrate (for example, certain forms of native soil organic C) during growth of a microorganism on a bioavailable carbon and energy source (for example, exudate compounds)11. This mechanism is often invoked to increase the physiological potential of decomposers for the mineralization of native soil C (refs 8, 10) (Fig. 1a). However, as noted by Kuzyakov and co-workers12, direct experimental evidence in support of this mechanism is scarce because most studies have aimed at identifying ‘priming effects’ rather than the underlying mechanism.
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![Proposed mechanisms for the exudate-induced acceleration of the microbial mineralization of native carbon (/`priming effects[rsquor]) in the rhizosphere.](http://www.nature.com/nclimate/journal/v5/n6/images_article/nclimate2580-f1.jpg)
