Pulsing your way out of trouble: Soil temporal carbon dynamics and competitive interactions in plants

Background/Question/Methods

Root carbon (C) exudation initiates a chain of reactions involving soil microbial community and microbial predators, leading to nutrient enrichment (i.e. 'microbial loop'). However, C exudation was also shown to stimulate nutrient immobilization, thereby reducing plant growth. Both mechanisms depend on C exudation, but generate two opposite soil nutrient dynamics. We suggest that this apparent contradiction between 'microbial loop' and 'immobilization' hypotheses can be reconciled when considering temporal C dynamics. We address two possible modes of C exudation: (1) Continuous; (2) Pulsed. Each C dynamic should influence the soil microbial community differently, resulting in opposing nutrient availabilities. We carried out two experiments: the first manipulated the initiation time of temporal C dynamics and the second manipulated both temporal C dynamics and soil nutrient availability. In both experiments, a fast-growing wheat plant was planted together with a slow-growing sage plant in the same pot, but the focus was on the performance of the wheat plant. We hypothesized that wheat plants should perform better under C pulses (leading to nutrient enrichment) than under continuous C supply (leading to nutrient limitation). However, this pattern should be stronger when C dynamic begins early during the growing season of the wheat plants.

Results/Conclusions

Temporal C dynamics restricted the performance of both plant species. Moreover, this pattern was stronger when C supply began early during the wheat growing season. Contrasting only the pulsed and continuous C supply regimes, exemplified that wheat plants performed better under C pulses. Under rich soil conditions, the addition of both C and nitrogen (N) had the most prominent effect on soil characteristics, bacterial abundance and plant performance. Specifically, when N was added, continuous C supply led to a higher abundance of soil bacteria, while causing wheat plants to exhibit a growth pattern typical to nutrient stress conditions. When taking into consideration all measured variables and looking for differences emerging from temporal C dynamics, it appears that C pulses differed both from the control and from the continuous C supply. This separation based on temporal C dynamics supports our assertion that C pulses lead to below-ground chain reactions in the microbial community cascading up to affect plant performance. Deciphering the complex temporal dynamics and interactions which control the availability of growth-limiting nutrients to plants is expected to generate a broader picture of the ecological links between above- and below-ground interactions, which are currently at the cutting edge of soil science.