The availability and imbalance of nitrogen and phosphorus influence plant productivity, fitness, and allocation to roots and arbuscular mycorrhizal fungi

Background/Question/Methods

The availability of limiting resources varies across habitats, because of both natural gradients and anthropogenic inputs. This variation in resource availability affects plant productivity, allocation strategies, fitness, and interactions with other organisms. For example, plant uptake of phosphorus frequently depends on association with arbuscular mycorrhizal fungi: phosphorus limited plants should allocate heavily to AMF. However, plant phosphorus limitation depends on both the absolute abundance of phosphorus and its abundance relative to other limiting resources such as nitrogen. This imbalance of nitrogen and phosphorus (extreme N:P ratios) could lead to even stronger limitation and even higher allocation to AMF, especially if both nutrients are scarce. In contrast, nitrogen limited plants might have high allocation to roots but low allocation to AMF because uptake of nitrogen probably occurs directly via plant roots. To test these ideas, we conducted a fertilizer addition experiment in an old-field in southwest Michigan. We fertilized with 19 different combinations of urea and triple superphosphate fertilizers to manipulate the abundance of N and P as well as their relative abundance (imbalance; N:P ratio).  

Results/Conclusions

We found that the abundance of nitrogen was by far the most important factor affecting productivity, fitness, and allocation of both big bluestem (Andropogon gerardii) and smooth brome (Bromus inermis). Nitrogen increased aboveground biomass, roots, and seeds, but decreased proportional allocation to seeds and soil hyphae of AMF. Species differed in the degree to which they changed their allocation strategies in response to nitrogen. In contrast, phosphorus availability did not directly affect species’ allocation to leaves, roots, or soil hyphae of AMF, but it did affect allocation to seeds. The imbalance of nitrogen and phosphorus (N:P ratio) was important in determining allocation to roots: both species reduced root allocation when P was extremely scarce relative to N. Interestingly, N:P ratio did not affect allocation to soil hyphae of AMF. Thus, both the abundance and imbalance of nitrogen and phosphorus affected plant allocation strategies. Plant productivity, allocation, fitness, and interactions with other organisms responded to different aspects of resource availability. This result indicates that increased nitrogen availability, for example via anthropogenic nitrogen deposition, might affect communities differently, depending on whether phosphorus is scarce and whether N:P ratios are in balance.