A tale of nitrogen cycling in two Free Air CO2 Enrichment (FACE) experiments: BIFoR-FACE and EucFACE

Rumeau, Manon ORCID: 0000-0001-7731-6247 (2025). A tale of nitrogen cycling in two Free Air CO2 Enrichment (FACE) experiments: BIFoR-FACE and EucFACE. University of Birmingham. Ph.D.

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Abstract

The constraint of tree growth under elevated atmospheric CO2 (eCO2) concentration by limiting soil nutrients, particularly nitrogen (N) in temperate ecosystems, can reduce projected carbon (C) storage from 25% to 75%. This uncertainty arises from the lack of experimental data on mature forests that experience stronger nutrient limitation than young forest ecosystems which have been studied to date. Therefore, it is essential to evaluate how eCO2 will affect nutrient transformations and availability in mature forest soils to assess if tree N demands can be met under eCO2. The purpose of this research was to evaluate the response of soil N cycling processes to eCO2 in two contrasting mature forests; in a N-limited deciduous temperate forest dominated by 180-year-old oak (Quercus robur L.) trees in the UK, and in a phosphorus (P)-limited Eucalyptus (Eucalyptus L.) dominated forest in Australia. This study seeks to identify the mechanisms by which forest ecosystems may modulate N availability under eCO2, understand the interactions between C, N and P cycling and provide experimental data to predict nutrient constraints to the CO2 fertilization effect under future CO2 enriched atmospheres.
The research was conducted in two Free Air Carbon Dioxide Enrichment (FACE) facilities: Birmingham Institute of Forest Research (BIFoR) FACE located near Birmingham, UK, and EucFACE in New South Wales, Australia. Furthermore, EucFACE was fertilized with P to momentarily alleviate/reduce P limitation. We employed a 15N labelling approach to measure N transformations (i.e. gross mineralization, depolymerization, N fixation, nitrification and denitrification). When possible, methods were adapted for in-situ measurements to preserve roots and soil structure whilst introducing newly adopted 15N labelling techniques.
At BIFoR-FACE, we found a ~30% increase in net and gross in-situ N mineralization, delivering an extra 26 kg N ha-1 y-1, meeting the higher N uptake from trees estimated at 10 kg N ha-1 y-1. Organic and inorganic N-mining via primed microbial activity were especially enhanced in the rhizosphere suggesting a stronger rhizosphere priming effect (RPE) under eCO2. Furthermore, the tenfold difference in N mineralization rates between rhizosphere and bulk soils suggests that by expanding the rhizosphere relative volume, trees under eCO2 may be able to meet higher N demand. Conversely, gross nitrification was downregulated at the macro-scale due to higher fine root biomass reducing nitrification hot-spots, and, at the micro-scale of the rhizosphere suggesting a N conservation strategy by trees. Moreover, an additional experiment simulating changes in root exudate composition highlighted that N-mining enhancement under eCO2 was driven by the increase in exudate C:N ratio rather than the amount of C added, challenging the original theory focusing RPE mainly on C input. Therefore, at BIFoR-FACE, a faster and tighter N cycle facilitated by plant-soil interactions and N conservation strategies at the ecosystem scale supported C biomass gains under eCO2 after six years of enrichment. However, with declining N deposition and a high soil C:N ratio, BIFOR FACE sits at a tipping point where microbes could start outcompeting trees for N resources with a likelihood of N limitation in the long run.
In contrast, nutrient cycling at EucFACE, in a highly P-limited forest, showed a very different response. No sign of a higher C allocation belowground was detected as root biomass was lower under eCO2 in the 0-10 cm layer. N-mining remained largely unaffected under eCO2 despite the observed accumulation of soil nitrate. C, N and P mining were coupled together and slowed down with P fertilization. But this decrease was partly offset under eCO2 suggesting that P fertilization initiated a faster C, N, P cycling under eCO2.
Taken together, results from this research indicate that the response of mature forests to eCO2 is not consistent but rather depends on which nutrient is limiting forest growth and likely the degree of nutrient limitation. These diverging responses highlight that predicting global C storage of mature forests requires either the multiplication of such experimental facilities to cover the diversity of forests worldwide; or a deeper understanding of the mechanisms behind forest response to eCO2; or both.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Ullah, Sami UNSPECIFIED UNSPECIFIED MacKenzie, Angus UNSPECIFIED UNSPECIFIED Reay, Michaela UNSPECIFIED UNSPECIFIED Sgouridis, Fotis UNSPECIFIED UNSPECIFIED Carrillo, Yolima UNSPECIFIED UNSPECIFIED
Licence:	All rights reserved
College/Faculty:	Colleges > College of Life & Environmental Sciences
School or Department:	School of Geography, Earth and Environmental Sciences
Funders:	Natural Environment Research Council
Subjects:	G Geography. Anthropology. Recreation > G Geography (General) S Agriculture > SD Forestry
URI:	http://etheses.bham.ac.uk/id/eprint/15988

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