Nitrogen fixation accelerates phosphorus cycling rates under elevated carbon dioxide

Background/Question/Methods

The ability of N fixers to effectively colonize new terrestrial landscapes has major implications for carbon dioxide uptake, storage, and climate change. As part of the Jasper Ridge Global Change Experiment, we have initiated an experiment to explore how nutrients, particularly nitrogen (N) and phosphorus (P), interact with elevated carbon dioxide in controlling rates of N fixation and the persistence of symbiotic N fixers in ecosystems. Our experimental design consisted of full factorial carbon dioxide, N, and P fertilizations of intact soil mesocosms that we seeded with various members of Jasper Ridge grassland communities. Functionally, these included: an N-fixing forb, a non-N fixing forb, naturalized annual grasses, native perennial and late season annual grasses. We seeded the mesocosms as mono-cultures and as mixed communities to examine for within and between species effects. The top-soil was homogenized and pre-labeled with ¹⁵N prior to the experiment, thus providing an adequately distinct soil ¹⁵N end-member for estimating N fixation rates at ecosystem levels. Across treatments, we measured root phosphatase activities, nutrient and C contents of vegetation, and ¹⁵N isotope abundance.

Results/Conclusions

Here we report that N-fixing plants employ an N-rich strategy of P acquisition that enables them to successfully compete for soil P against other plants at ambient and elevated carbon dioxide concentrations.  Across all treatments (carbon dioxide, N, P), the N fixer releases substantial quantities of N-rich phosphatases (organic-P mineralizing enzymes) into the soil, whereas N appears to limit this P acquisition strategy in non-fixers. The N fixer’s investment in phosphatase increases significantly and linearly with biomass N:P ratios, is only slightly affected by elevated carbon dioxide concentrations, and can explain their very high P contents. N fixation rates were highest with added P, though N fixation contributed substantially to the N-fixer's N economy across all treatments. Using a simulation model of C, N, and P cycles to explore these interactions further, we find that the ability of N fixers to invest N into P mineraliztion can accelerate the P cycle and enrich P availability at the ecosystem level. Consequently, this interaction – N fixation, phosphatase production, P enrichment – could enable P-limited terrestrial environments take up and store additional carbon dioxide in the future. We will discuss the implications of our results with respect to global change and our basic understanding of C, N, P and N fixation interactions in ecosystems.