Nitrogen deposition determines soil microbial functional responses to chronic elevated CO2 in a dry grassland

Background/Question/Methods

Ever-increasing atmospheric carbon dioxide (CO₂) and nitrogen (N) deposition are intertwining factors associated with global changes. Extensive efforts have been devoted to understand the effects of eCO₂ and N on plant and soil processes, however, the effects on belowground microbiome yet remain less well understood.  Microbes drive important nutrient cycling and other soil biogeochemical processes, thus to understand whether and how N deposition influences the effects of elevated CO₂ on soil microbial community and processes is of great importance for incorporating the microbial contributions to the whole ecosystem responses to climate change.

We investigated soil microbial community in a grassland ecosystem subjected to ambient CO₂ (aCO₂, 368 ppm) or elevated CO₂ (eCO₂, 560 ppm) under ambient nitrogen deposition (aN, 2 g/m²) or elevated nitrogen deposition (eN, 4 g/m²) for more than a decade. This study was carried out at the BioCON (CO₂, N, biodiversity) experiment site located at the Cedar Creek Ecosystem Science Reserve, MN, USA. We chose 48 plots manipulating CO₂, N treatments to investigate the CO₂ and N interactions, which were carried out by: 1) DNA extraction, purification, quantification and GeoChip hybridization; 2) Raw data progressing and the downstream statistical analysis; 3) Network reconstruction to visualize the interactions in microbial communities; 4) CO₂ and N interaction modeling on microbial community.

Results/Conclusions

Under the aN condition, a majority of microbial C, N cycling genes, as measured by GeoChip 4.0, were increased in relative abundance by eCO₂ or remained unchanged. Consistently, net N mineralization, ammonification and nitrification processes rate were increased by eCO₂. Under the eN condition, a majority of the microbial C, N cycling genes were decreased in relative abundance by eCO₂ or remained unchanged, owing to the reason that the plant allocated more C from soil to plant biomass. Consistently, plant C content was increased while soil C content and net N mineralization process rate were decreased by eCO₂. Most positive node links and highest link density were observed in eCaN samples of constructed C, N cycling networks because of the priming effect. eCO₂ effect on microbial functional gene abundance was antagonistic with eN effect, revealing the potential balance of ecosystem functioning with continously increasing CO₂ concentration and N deposition.

Collectively, these results demonstrated that eCO₂ effects on grassland microorganisms were contingent on N conditions, N effect was antagonistic with CO₂ effect on microbial community.