Soil organic matter dynamics under elevated CO2 : mechanisms of impact

Abstract

Soil carbon (C) is one of the most relevant reservoirs of this element on Earth and plays a critical role in climate regulation. The effects of rising atmospheric carbon dioxide (CO2) concentrations on soil C cycling and soil C stocks have been extensively investigated but the mechanisms by which the impact eCO2 may be altered are still not well understood. This is hindering our capacity to improve the predictions of the fate of C stocks and further impacts on climate change. In this thesis, I explored different mechanisms mediating the impact of eCO2 on SOM dynamics accounting for effects on both C inputs and outputs. This involved isotopic techniques, which allowed me to quantify two isotopically different C pools: a new C pool, derived from the recently fixed CO2 by plants and an old C pool, defined as the pre-existent C in the organic substrates or native soil. By separating these different C pools, I could assess gross C losses and gains, obtaining more accurate measurements of changes in the soil C reserves due to eCO2. The experiments included in this thesis were carried out in a Free Air CO2 enrichment (FACE) experiment located in a mature Eucalyptus woodland and in controlled conditions, using whole plant-soil systems with components extracted from this ecosystem. The mechanisms explored in this thesis include: the role of different belowground organic substrates altering the effects of eCO2 on their decomposition, the effects of root mediation and the influence of nutrient availability, particularly of phosphorus, mediating SOM decomposition under eCO2 conditions. Moreover, I also explored the effects of this climate change factor on soil microbial communities and their role in soil C cycling, including symbiotic fungi (mycorrhizae) found in plant's roots. The results presented in this thesis contribute to filling the gap in knowledge regarding the influence of eCO2 on soil C cycling in mature forests with P limiting conditions and the mechanisms influencing altered soil C dynamics in such ecosystems. Overall, it can be concluded that isolating the confounded effects of eCO2 on C inputs and outputs from soil C stocks with eCO2 provides accurate information of the impact of eCO2 on C processes than direct measurements of the stocks. Specifically, enhanced gross loss of C with eCO2 were confirmed, an observation that while often hypothesised is elusive. This effect was stronger for root litter, a fresh C substrate. Long-term consequences of such impacts are likely important for the ecosystem C balance and should be further investigated. Moreover, the enhancement in SOM decomposition under eCO2 conditions due to a rhizosphere priming effect was not observed for this P-limited woodland and thus, I provide evidence that this effect might be contingent on resource availability (nutrients ""particularly P -and water) that changes seasonally and with depth in the soil profile. Finally, I demonstrate that soil C cycling responses to eCO2 are contingent to P availability and that low soil P conditions do not necessarily lead to increased SOM decomposition, as low N conditions are reported to do. Therefore, generalisations of the impacts of nutrient availability on SOM decomposition under eCO2 conditions should be avoided. More information about the specific mechanisms determining changes in soil C dynamics with P limitation under eCO2 is needed to better predict changes in global soil C stocks.

Date of Award	2019
Original language	English