Nutrient limitation in soils regulate the effects of elevated CO2 on soil N cycling at BIFoR-FACE (UK) and Euc-FACE (Australia)

Increasing atmospheric CO₂ concentrations due to human activities, is projected to enhance photosynthesis and carbon storage of forest ecosystem. However, it is unclear how nutrient limitation will constrain the projected CO₂ fertilization effect. Therefore, it is essential to evaluate how nutrient limitation will affect the response of forests to rising CO₂ concentration and how it will feedback on nutrient availabilities and more especially nitrogen (N) which can become limiting with time.

The purpose of this research is to evaluate the response of N cycling processes to elevated CO₂ enrichment in a N-limited northern deciduous temperate forest in the UK and a phosphorus (P)-limited Eucalyptus dominated forest in Australia. The research was conducted in two Free Air Carbon Dioxide Enrichment (FACE) facilities: BIFOR FACE located near Birmingham, UK and EucFACE in New South Wales, Australia. Furthermore, EucFACE (P-limited forest) received partial phosphorus fertilization to study its effect on N cycling.

We employed a ¹⁵N pool dilution method to assess gross protein depolymerization and gross mineralization at both study sites, along with the measurement of nitrous oxide emissions, extracellular soil enzyme activities and nutrient pools.

Results from the N-limited forest (BIFOR FACE) indicate that elevated CO₂ increased belowground carbon allocation, resulting in higher root biomass, dissolved organic carbon, microbial biomass and soil respiration. The additional carbon belowground stimulated net mineralization (+ 30%) (p<0.05) over a year of monthly measurement. Gross mineralization and ammonium immobilization were only enhanced in summer (+ 47%), whilst gross nitrification was overall downregulated (- 47%) and N₂O production was un-affected by elevated CO₂. This suggests that root exudates selectively influence microbial communities promoting SOM decomposition to enhance ammonium availability (+15%) (p<0.05) for trees. In the phosphorus-limited forest, carbon pools, nitrogen depolymerization and mineralization were unaffected by elevated CO₂. But elevated CO₂ increased soil nitrate pool (+ 37%) (p<0.05) and decreased soil moisture (-13%) indicating a potential reduction in denitrification activity.

Taken together, results from this research outline how nutrient limitation drives the response of a forest ecosystem to elevated CO₂. Although at EucFACE, P limitation was alleviated in the initial phase of the experiment (Hasegawa et al., 2016), it quickly truncated plant growth and carbon feedback in soils. While, at BIFOR FACE, N limitation has been alleviated via enhanced soil N mineralization and N supply to sustain plant growth enhancement for seven years. However, how long the N supply will be maintained in the face of declining nitrogen deposition in future climates remains uncertain. Results from this research aim at improving our understanding of forest response to future climate by unveiling the role of nutrient limitation in future C uptake.

Hasegawa, S., Macdonald, C.A., Power, S.A., 2016. Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus-limited Eucalyptus woodland. Glob. Change Biol. 22, 1628–1643. https://doi.org/10.1111/gcb.13147