Differential plant responses to pulsed nitrogen additions

Background/Question/Methods

Temporal variability is a common, and commonly overlooked, component of nutrient availability and cycling. For example, over the span of a few days, fires in longleaf pine forests can deliver 10x more nitrogen to the soil than plants can put into biomass in a single year, yet the ability for plants to access this short-lived pulse has not been explored. For nutrient pulses to influence community composition, plants must differ in their ability to access the pulse, which itself must occur frequently or be very large. To test for plant-level differences in access to ephemeral nitrogen, we supplied plants with a pulse of ¹⁵N-labeled ammonium nitrate (¹⁵NH₄¹⁵NO₃), and examined assimilation over time. We compared (a) how quickly plants began uptake of the pulse and (b) the amount of nitrogen assimilated into plant tissue. We compared root-level traits (root length, root area, root diameter, the number of root tips) to (a) and (b) in four plant species, Pinus palustris, Lyonia lucida, Vaccinium formosum, and Ilex glabra. All are woody species are found inhabiting longleaf pine forests in North Carolina, but differ substantially in leaf carbon:nitrogen. 

Results/Conclusions

We expected that plants with fine, branching roots would take up more nitrogen, and would begin uptake faster than those with large roots. P. palustris produced nearly 9x the length of roots than L. lucida, and had an average root diameter that was less than half that of L. lucida. However, L. lucida produced many shorter roots than P. palustris, as evidenced by their similar root tip densities (P. palustris: 4.9 tips cm^-2; L. lucida: 3.5 tips cm^-2). We anticipate that these differences will correspond to differences in the uptake of a nitrogen pulse. Due to its overall greater root system size, we expect that P. palustris will take up more nitrogen than L. lucida. However, since L. lucida produced relatively more root tips than P. palustris, we further expect that L. lucida will reach its maximum uptake rate faster than P. palustris.

It remains unclear how plant-level differences scale up to community-level dynamics. Current competition models are based almost exclusively on the ability of plants to access spatially heterogeneous nutrients. For systems that experience substantial temporal nutrient variability, like longleaf pine forests, neglecting this aspect of nutrient availability may limit our ability to understand community assembly and persistence mechanisms.