ElevatedCO₂promotes long‐term nitrogen accumulation only in combination with nitrogen addition

Abstract

Biogeochemical models that incorporate nitrogen (N) limitation indicate that N availability will control the magnitude of ecosystem carbon uptake in response to rising CO2 . Some models, however, suggest that elevated CO2 may promote ecosystem N accumulation, a feedback that in the long term could circumvent N limitation of the CO2 response while mitigating N pollution. We tested this prediction using a nine-year CO2 xN experiment in a tidal marsh. Although the effects of CO2 are similar between uplands and wetlands in many respects, this experiment offers a greater likelihood of detecting CO2 effects on N retention on a decadal timescale because tidal marshes have a relatively open N cycle and can accrue soil organic matter rapidly. To determine how elevated CO2 affects N dynamics, we assessed the three primary fates of N in a tidal marsh: (1) retention in plants and soil, (2) denitrification to the atmosphere, and (3) tidal export. We assessed changes in N pools and tracked the fate of a (15) N tracer added to each plot in 2006 to quantify the fraction of added N retained in vegetation and soil, and to estimate lateral N movement. Elevated CO2 alone did not increase plant N mass, soil N mass, or (15) N label retention. Unexpectedly, CO2 and N interacted such that the combined N+CO2 treatment increased ecosystem N accumulation despite the stimulation in N losses indicated by reduced (15) N label retention. These findings suggest that in N-limited ecosystems, elevated CO2 is unlikely to increase long-term N accumulation and circumvent progressive N limitation without additional N inputs, which may relieve plant-microbe competition and allow for increased plant N uptake.

Reducing nitrogen inputs mitigates Spartina invasion in the Yangtze estuary

“…This suggests that soil N availability can be replenished by the microbial mineralisation of N from plant litter and soil organic matter in terrestrial ecosystems and does not decrease immediately after reducing N inputs, as in the coastal wetlands we studied (Figure S1). Similar N retention has been found in coastal wetlands (Pastore et al, 2016); however, N mineralisation is largely limited by anaerobic conditions (Chen et al, 2018), which slow the release of reactive N from the N pools retained in plant biomass and soil. Moreover, the remaining reactive N in the soil of coastal wetlands might be reduced by anaerobic denitrification (Canfield et al, 2010).…”

“…Denitrification can remove more than 80% of N from coastal wetlands, including the coastal marshes at the same study site in the Yangtze estuary (Li et al., 2020), and the N removal rate is significantly higher than that (56%) in terrestrial ecosystems (Scheer et al., 2020), making coastal wetlands a crucial buffer zone for relieving nutrient pollution. Furthermore, tidal flushing is another important factor contributing to significant N losses in these coastal wetlands (Pastore et al., 2016). Collectively, the high rate of N loss caused by denitrification and tidal flushing makes reducing N inputs a promising approach for restoring coastal wetlands from plant invasion.…”

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

“…The fate of N fertilizer differed among sites. At FC edge and interior, potential fates of NH 4 + included uptake by S. alterniflora , microbial nitrification, or export due to flushing (Pastore et al, 2016), The high H 2 S at TBC likely inhibited S. alterniflora N uptake and microbial nitrification (Joye & Hollibaugh, 1995); thus, the NH 4 + added by fertilization accumulated in pore water to a much greater extent at TBC than at the other sites with lower H 2 S.…”

Global change accelerates carbon assimilation by a wetland ecosystem engineer

“…Aluminum skirts at the base of each chamber (extending to 30 cm depth), in combination with low lateral movement of water at this site and the low mobility of NH 4 , ensured that minimal fertilizer was removed by tidal flooding [46]. Air temperatures measured in the middle and upper Phragmites canopy (175 and 230 cm, respectively) in one OTC from July-October were 2.0±2.8°C (24 h mean±SD) warmer than outside the chamber but above the native plant canopy (at 175 cm).…”