Early response of biological nitrogen fixation in a mature oak dominated forest to elevated atmospheric CO2 fumigation at BIFoR-FACE

Elevated atmospheric carbon dioxide concentrations are stimulating photosynthesis and carbon sequestration. However, the extent of photosynthetic stimulation in forests under future climates is highly uncertain given that nutrient limitation in soils may constrain the CO₂ fertilization effect. The Birmingham Institute of Forest Research (BIFoR), University of Birmingham established the only global mature temperate deciduous forests Free Air Carbon Dioxide Enrichment (FACE) experiment to study the response of forests to future climates. Fumigation of the forest with ~550 ppm CO₂ started in 2017 and will continue until at least 2026. Soil nutrients cycling including nitrogen transformation in response to elevated atmospheric CO₂ (eCO₂) fumigation is currently investigated to determine the role of nutrient availability in carbon capture by forests. In this paper, we show preliminary results of the response of asymbiotic biological nitrogen fixation (BNF) in soils and epiphytic bryophytes at BIFoR-FACE following a year of eCO₂ fumigation. It is hypothesized that the demand for available nitrogen by trees will increase under eCO₂ and that competition of roots and soil microbes for available nitrogen will enhance asymbiotic BNF to at least meet microbial metabolic nitrogen demands in the long run. Surface soils (0-5 cm) and epiphytic feather moss (Hypnum cupressiforme) growing on oak tree stems in the FACE site were  collected during the second year of eCO₂ fumigation for the quantification of BNF activity using the ¹⁵N₂ assimilation methods (Saiz et al. 2019). Samples were incubated in 50 mL serum bottles under in situ conditions, followed by the analysis of soil and tissue samples for ¹⁵N signature on an Isotope Ratio Mass Spectrometer for the quantification of BNF activity.

The BNF activity under eCO₂ were 369% higher than in soils under ambient atmospheric CO₂. BNF rates associated with feather mosses (Hypnum cupressiforme) did not differ between the eCO₂ and control plots; however, rates under eCO₂ on average were 60% lower than in the control plots. Unlike soils, the moisture of feather mosses correlated significantly (R² = 51%) with BNF activity. Among nutrients in soil with implications for BNF activity, the concentrations of Mg, K, Co and Ni were significantly lower in soils under eCO₂ than in the control plots, while in feather moss tissues no differences were observed.  Our preliminary results show that eCO₂ fumigation primed asymbiotic BNF activity in soils. An enhancement of BNF together with the observation of a relatively low nutrient content under eCO₂ points to important changes in nitrogen cycling processes in the early years of CO₂ fumigation. Further detailed studies are underway to fully disentangle controls on nitrogen availability to trees under future climates.

 

Reference

Saiz, E, Sgouridis, F, Drifjhout, F & Ullah, S. 2019. Biological nitrogen fixation in peatlands: comparison between acetylene reduction assay and ¹⁵N₂ assimilation methods. Soil Biol. Biochem:131:157-165