Reconciling turnover models of roots and soil organic carbon with radiocarbon measurements

authored by

Bernhard Ahrens

supervised by

Georg Guggenberger

Abstract

Terrestrial ecosystems and soils are major actors in the Earth’s carbon cycle, and tightly linked to the evolution of atmospheric CO2 concentrations and climate change. Soils alone store several times more carbon than the atmosphere, and carbon cycling in soils could hence have substantial impact on atmospheric CO2 concentrations. To understand the timescales of carbon cycling in terrestrial ecosystems, radiocarbon measurements are an important tool. Yet, results from radiocarbon measurements have often conflicted with results other measurement techniques: In the study of root turnover, radiocarbon has yielded turnover times that are much longer compared to those attained by other methods, such as sequential coring or minirhizotrons. In the study of soil organic carbon turnover, radiocarbon has pointed to pools that cycle on centennial to millennial timescales. Empirical evidence, however, has suggested that individual compounds turn over more rapidly. This dissertation's overarching goal is to reconcile turnover models of roots and soil organic carbon with radiocarbon data by incorporating new process understanding into these models. The first part of the dissertation reconciles radiocarbon contents of fine roots with observations of root lifetimes from minirhizotrons. Previously root turnover had mainly been estimated by a one-pool model. This kind of model assumes an equal likelihood for root death throughout the lifetime of a root. Minirhizotron observations, however, have pointed to higher likelihoods of root turnover at the beginning of a root’s lifetime. In this thesis, a framework was developed that allows using minirhizotron and radiocarbon data in conjunction to estimate mean fine-root residence times. Survival functions from the field of survival analysis were used to estimate mean fine-root residence times from lifetime data of individual roots. Convoluting fine-root survival functions with the atmospheric radiocarbon bomb curve allowed performing a joint estimation of mean fine-root residence times from radiocarbon and minirhizotron data. The second part of the dissertation develops a new soil organic carbon profile model that incorporates mechanistic descriptions of microbial and organo-mineral interactions. The aim is to reconcile apparent millennial radiocarbon ages of soil organic carbon in the subsoil with other observations by considering the contribution of microbial decomposition limitation and organo-mineral interactions. A version of the model parametrized with site-specific sorption capacities was contrasted with a more generic parametrization of sorption capacity. With this generic formulation of sorption capacity based on clay and silt content, between-site differences of radiocarbon depth gradients could be represented. After calibration to profiles of soil organic carbon and radiocarbon, model experiments were used to study the importance of individual processes and their interaction for explaining radiocarbon depth gradients. A special focus was put on how different levels of sorption capacity interact with microbial substrate limitation. This approach allowed us to reconcile apparent millennial radiocarbon ages with mechanisms of microbial decomposition and sorption capacity instead of chemical recalcitrance. The mechanistic framework developed in this thesis can be used to better understand soil organic matter turnover, the belowground parts of the global carbon cycle, and eventually its response to global warming.

Organisation(s)

Faculty of Natural Sciences

Type

Doctoral thesis

No. of pages

232

Publication date

2021

Publication status

Published

Sustainable Development Goals

SDG 13 - Climate Action, SDG 15 - Life on Land

Electronic version(s)

https://doi.org/10.15488/11347 (Access: Open)