Carbon efficiency for nutrient acquisition (CENA) by plants: role of nutrient availability and microbial symbionts

BackgroundIn a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.ScopeWe broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.ResultsWe explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.ConclusionsThe updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.