Nutrient co-limitation of an annual grassland ecosystem response to elevated CO2

Background/Question/Methods

As anthropogenic carbon dioxide (CO₂) emissions rise, compensatory carbon uptake by terrestrial ecosystems will depend on the degree of net primary production (NPP) limitation by other resources. Some elevated CO₂ experiments have found that the response of NPP was progressively limited by nitrogen (N) over time. Little attention has focused on other potentially limiting nutrients, such as phosphorus (P). Here, we evaluated the potential for N and P to limit NPP within the Jasper Ridge Global Change Experiment, which manipulates CO₂, temperature, N deposition, and rainfall in a California annual grassland.  Limitation of NPP by N & P was assessed indirectly in the JRGCE, through analysis of ecosystem N & P pools, nutrient mineralization via litter decomposition, tissue nutrient concentrations, N:P ratios and phosphatase enzyme activity. An additional mesocosm experiment factorially enhanced N, P and CO₂ to directly test for NPP limitation by these factors.

Results/Conclusions

In the JRGCE, we infer that N deposition (all years) and elevated CO₂ (some years) pushed this ecosystem from N limitation towards P limitation, resulting in co-limitation by these two nutrients. Several lines of evidence support this hypothesis. The NPP response to elevated CO₂ was N limited, evidenced by the large increase in NPP when elevated CO₂ and N deposition occurred together, as compared to elevated CO₂ alone.  The N pool in biomass also decreased under elevated CO₂ alone, and increased with elevated CO₂ and N deposition combined.  In some years, soil P availability, plant biomass P pools and concentrations declined under elevated CO₂ as compared to ambient conditions. We also found lower rates of N and P mineralization from litter under elevated CO₂ in some years. Declining N & P concentrations in aboveground biomass over time (constant ratio N:P), suggested increasing co-limitation of NPP by N and P under elevated CO_2. The mesocosm experiment supported this interpretation; NPP was strongly co-limited by N & P, especially under elevated CO₂.  In contrast, soil phosphatase activity in the JRGCE was either unchanged or declined under elevated CO₂. This response may have been constrained by N limitation, allocation patterns, or competition with soil microbes. We argue that the “shadow of P limitation” may be more common than previously thought, and we present conceptual models contrasting N versus P limitation, progressive limitation versus interannual variation in other control factors, and the potential importance of multiple nutrient limitation for future estimates of terrestrial carbon uptake.