Rhizosphere priming in response to soil temperature and moisture

Background/Question/Methods

Root-soil interactions in the rhizosphere significantly affect soil organic matter (SOM) decomposition. This rhizosphere effect can range from 50% inhibition to 3-fold stimulation (median value of ~70% stimulation, based on mostly greenhouse experiments), depending on plant, soil, and environment interactions. There is large uncertainty as to how soil temperature and moisture affect rhizosphere priming. Here we report results from four experiments to explore the influence of soil temperature and moisture on rhizosphere priming of SOM and litter decomposition. In the first two experiments, we grew sunflower and soybean in a farm soil in a continuous ¹³C-labeling greenhouse. In the third experiment, we planted a C₃ grass in the “C₄” soil at a tall-grass prairie in Kansas. In the fourth experiment, we buried ¹³C-labled Avenaroot litter to two soil depths in field lysimeters dominated by annual grasses in California. Soil temperature and moisture were either manipulated (by warming with buried cables or infrared heaters, rainout shelters and controlled irrigation) or monitored as natural variations during the growing season. Rhizosphere priming was calculated as the difference in SOM or litter decomposition with and without the presence of live roots.   

Results/Conclusions

In the first experiment, soil warming (2.7-5 ^oC) significantly enhanced rhizosphere priming of SOM decomposition by up to threefold, and increased plant biomass to a lesser extent. In the second experiment, a severe drying-wetting treatment on sunflower significantly reduced plant biomass and rhizosphere priming, while a moderate drying-wetting treatment on soybean did not affect plant biomass or rhizosphere priming. In the third experiment, compared to the ambient condition, soil warming (2 ^oC at 15 cm depth) intensified rhizosphere priming, whereas delayed rainfall (lengthening the dry interval between rainfall events by 50%) had no or modest effect on rhizosphere priming, and the interaction between soil warming and delayed rainfall on rhizosphere priming was antagonistic. In the fourth, ongoing experiment, we will measure rhizosphere priming of root litter decomposition throughout a growing season, and relate the magnitude of rhizosphere priming to changes in soil temperature and moisture. In summary, there was no consistent effect of soil temperature and moisture on rhizosphere priming, and there is still more to learn about how temperature and moisture interact with other system attributes to affect rhizosphere priming.