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Title: Rhizosphere priming: a nutrient perspective

Author
item DIJKSTRA, FEIKE - University Of Sydney
item CARRILLO, YOLIMA - University Of Sydney
item PENDALL, ELISE - Western Sydney University
item MORGAN, JACK - Retired ARS Employee

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2013
Publication Date: 7/29/2013
Citation: Dijkstra, F.A., Carrillo, Y., Pendall, E., Morgan, J. 2013. Rhizosphere priming: a nutrient perspective. Frontiers in Microbiology. 4(216):1-8.

Interpretive Summary: With atmospheric carbon dioxide (CO2) concentrations continuing to rise primarily from fossil fuel burning, there is a critical need to understand the long-term consequences of these higher CO2 concentrations on Earth. Perhaps the most important consequence of rising atmospheric CO2 levels is a warming of the planet, but it has many other effects due to its uptake by plants in photosynthesis. Higher ambient CO2 often causes correspondingly higher levels of plant photosynthesis, which in turn have been shown to effect soil nutrient cycling via the transfer of recently-assimilated photosynthetic metabolites from the plant into the soil, in effect, feeding the soil microorganisms. The artificial enrichment of CO2 over a native semi-arid grassland was found to stimulate the rate at which N was released from soil organic matter via soil micro-organisms in one experiment, but not in another similarly designed experiment also conducted in a semi-arid grassland. This released N is an important nutrient for plants, and its enhanced release may be an important adaptive mechanism for plants to grow faster and sequester more carbon in the soil as CO2 levels increase. However, our results which differed in the two grasslands suggest that this response is most likely to occur in soils with inherently low N status, and not in soils limited by another plant nutrient, phosphorus. These results suggest that the indirect effects of CO2 on ecosystems, including their capability to sequester more carbon and keep it from returning to the atmosphere, will depend in part on the inherent soil nutrient status.

Technical Abstract: Rhizosphere priming is the change in decomposition of soil organic matter (SOM) caused by root activity. Rhizosphere priming plays a crucial role in soil carbon (C) dynamics and their response to global climate change. Rhizosphere priming may be affected by soil nutrient availability, but rhizosphere priming itself can also affect nutrient supply to plants. These interactive effects may be of particular relevance in understanding the sustained increase in plant growth and nutrient supply in response to a rise in atmospheric CO2 concentration. We examined how these interactions were affected by elevated CO2 in two similar semiarid grassland field studies. We found that an increase in rhizosphere priming enhanced the release of nitrogen (N) through decomposition of a larger fraction of SOM in one study, but not in the other. We postulate that rhizosphere priming may enhance N supply to plants in systems that are N limited, but that rhizosphere priming may not occur in systems that are phosphorus (P) limited. Under P limitation, rhizodeposition may be used for mobilization of P, rather than for decomposition of SOM. Therefore, with increasing atmospheric CO2 concentrations, rhizosphere priming may play a larger role in affecting C sequestration in N poor than in P poor soils.