Allocation responses to nitrogen addition depend on photosynthetic demand and nitrogen acquisition strategy

Background/Question/Methods Anthropogenic activity has increased nitrogen availability to terrestrial ecosystems, increasing plant nitrogen uptake. However, the allocation of this nitrogen to different organs and processes is not well understood. Some models predict that the increased nitrogen uptake will be allocated to photosynthetic enzymes as a way to increase photosynthetic capacity. However, photosynthetic optimization theory suggests that photosynthetic demand, not nitrogen availability, drives photosynthetic capacity. An extension of this theory predicts that increased nitrogen will be allocated to the construction of new leaves and stems as opposed to roots. Notably, neither hypothesis considers the role of nitrogen acquisition strategy, which may impact the relative effect of soil nitrogen availability on plant uptake and allocation. We tested these competing hypotheses in species with different nitrogen acquisition mechanisms, a nitrogen-fixing plant (soybean) and a non-nitrogen-fixing plant (cotton), under different levels of soil nitrogen availability and a gradient of photosynthetic demand implemented using shade cloths. We measured leaf, stem, and root biomass as well as leaf area. We also analyzed biomass tissue for carbon and nitrogen content. Nitrogen allocation was calculated as the percent of nitrogen in total biomass found in each organ.

Results/Conclusions The total biomass nitrogen increased with nitrogen availability in both species, with the greatest increases seen in the higher light availability treatments. All plants grown in 80% shade, regardless of nitrogen availability, had low amounts of nitrogen in their tissues as expected with the photosynthetic optimization theory. However, overall photosynthetic demand caused no change in nitrogen allocation to the leaves in either species. With increasing nitrogen availability, plants allocated increasingly more nitrogen to leaves at the expense of stems, an effect that coincided with an increase in leaf biomass allocation. Nitrogen availability caused larger increases in allocation of nitrogen to leaves in soybean compared to cotton. Allocation of nitrogen to the stems decreased with light as that nitrogen was re-allocated to the roots. Nitrogen availability increased stem allocation of nitrogen in both species while decreased the same amount of nitrogen in cotton roots by ~10 %. The soybean roots were unaffected by nitrogen fertilization. These results support the idea that nitrogen demand limits whole plant photosynthesis through a limitation of leaf quantity, rather than leaf quality.