Background/Question/Methods Anthropogenic N deposition currently represents a relatively small, but continuous, addition of N into ecosystems. However, because N is frequently limiting to plant growth, particularly in terrestrial systems, this small flux can have substantial effects on ecosystem productivity and function. Unfortunately, many N addition experiments are not set up to ask questions about the impact of N deposition, adding rates of N that greatly exceed ambient deposition rates. This underscores the need to understand how N supply rate changes carbon pools and flux rates. We quantified the response of grassland plant biomass and ecosystem carbon flux in an experiment at Cedar Creek Ecosystem Science Reserve in Minnesota, USA in which N was added annually as slow-release urea at three rates (1 g, 5 g, and 10 g m-2 year-1) since 2007. These rates represent approximately double, five times, and ten times ambient N deposition. We hypothesized that net sequestration of C would increase with N supply rate due to increasing biomass and decreasing soil respiration. We measured soil efflux (SR) biweekly, and net ecosystem exchange (NEE; NEE = ER - GPP) monthly, from April through October 2020, and collected below and aboveground biomass in early August. Results/Conclusions We found a unimodal response in aboveground biomass, with the greatest increase in biomass in the lowest N addition treatment (p = 0.036). Belowground biomass decreased linearly, with the lowest belowground allocation in the highest N addition treatment (p = 0.039). GPP, ER, and SR flux rates were consistently higher in the lowest N treatment compared to Control (p = 0.03, 0.04, and 0.001, respectively). There was no difference in the net rates CO2 uptake (NEE; p = 0.18) in any treatment, despite the increases in GPP, ER, and SR. These results show that a doubling of current N deposition rates in this area (currently ~0.9g m-2 year-1) would lead to substantially increased soil respiration rates and aboveground biomass, but negligible changes to total ecosystem CO2 uptake. Many past studies have found that N additions equal to or in excess of 10 g N m-2 year-1 repress soil respiration, but our work indicates that low rates of N addition can stimulate CO2 efflux from the soil. These findings provide new insights on the impacts of low rates of N addition on grasslands and suggests that continued N deposition may change rates of plant growth and carbon fluxes.
