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Many terms often used to describe old-growth forests imply that these forests are less vigorous, less productive and less stable than younger forests. But research in the last two decades has yielded results that challenge the view of old-growth forests being in decline. Given the importance of forests in battling climate change and the fact that old-growth forests are shrinking at a rate of 0.5% per year, these new results have come not a moment too soon. This book is the first ever to focus on the ecosystem functioning of old-growth forests. It is an exhaustive compendium of information that contains original work conducted by the authors. In addition, it is truly global in scope as it studies boreal forests in Canada, temperate old-growth forests in Europe and the Americas, and global tropical forests. Written in part to affect future policy, this eminently readable book is as useful for the scientist and student as it is for the politician and politically-interested layman.

The production and export of terrestrial DOM is intimately related to processes in soils and the critical zone, and society relies on the sustained functioning of these systems (Küsel et al., 2016). Understanding mechanisms of SOM turnover and DOM production are thus crucial for proper management of ecosystems. Advancements in analytical technology – namely in soft ionization techniques coupled to ultrahigh resolution Fourier transform mass spectrometers – have revolutionized our view of ubiquitous and ultra-complex organic mixtures and have made the analysis routine in biogeochemistry (Minor et al., 2014; Smith et al., 2018; Zark and Dittmar, 2018). Knowledge about terrestrial DOM remains scarce, compared to advances in the fields of aquatic and marine sciences. Although terrestrial DOM is now seen as a valuable indicator and integrator of fast ecosystem responses (Roth et al., 2015, 2014), our understanding of molecular ecosystem imprints and potential new source or process markers is still limited.

This dissertation presents modern approaches for the identification of molecular markers and marker patterns as indicators for natural processes that transform dissolved organic matter (DOM) in the Critical Zone. The attached manuscripts describe the transformation of plant-derived DOM in the litter layer, soils and aquifers. The results of these investigations show that the interdisciplinary integration of untargeted ultrahigh-resolution mass spectrometry data on DOM composition with hydrological, isotopic and microbial information can substantially improve our understanding of the functioning of the Critical Zone.

Microorganisms have the ability to control the biophysical properties of their membrane lipids to thrive in natural environments. Different microbial species possess different membrane lipid compositions, even the composition of a single species is not always constant, but depends on the habitat conditions. Microorganisms adjust their membrane lipid composition by modifying the types of glycerol backbone (chain length, saturation, and branching) and head groups. Therefore, signatures of membrane lipids in the environmental samples could be used as biomarkers reflecting the physicochemical conditions and the community structures. With the increasing knowledge of lipid structures and their properties, the interpretation of lipid-based biomarker in environmental samples would be more accurate. By using lipidomics, this thesis aimed at (i) identifying new lipid isomers to help improve the precision of its derived proxy, (ii) expanding the application of lipid biomarkers to the terrestrial subsurface critical zone for tracing the signal of surface down, (iii) providing a full lipid profile in the soils, investigating their sources and potential possibility as novel chemotaxonomic markers for bacteria.

Acknowledgments We thank Steffen Ruehlow, Agnes Fastnacht, Karl Kuebler, Iris Kuhlmann, Heike Geilmann, and Petra Linke for technical support in establishing the experiment and with stable isotope analyses. We also thank Markus Lange, Daniel Read, and Hyun Gweon for helpful discussions. Funding AM has received funding from Max Planck Society and the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 655240. AM has also received a career orientation grant from the Jena School for Microbial Communication (JSMC) that funded the laboratory visits. DFG SFB Aquadiva funded part of this work. ; Peer reviewed ; Publisher PDF

A.A.M. has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska–Curie grant no. 655240 and all other UK-based authors were funded by the UK Natural Environment Research Council under a Soil Security Programme grant (NE/M017125/1). We also acknowledge Kathleen Eismann for her help in sample preparation for metaproteomic analysis; Heiko Moossen, Petra Linke and Steffen Ruehlow for assistance with stable carbon isotope analyses and all the land owners who provided sampling access. The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information file, and from the corresponding author on request. The mass spectrometry proteomics data generated during the current study are available in the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD010526. ; Peer reviewed ; Publisher PDF

In order to predict which ecosystem functions are most at risk from biodiversity loss, meta-analyses have generalised results from biodiversity experiments over different sites and ecosystem types. In contrast, comparing the strength of biodiversity effects across a large number of ecosystem processes measured in a single experiment permits more direct comparisons. Here, we present an analysis of 418 separate measures of 38 ecosystem processes. Overall, 45 % of processes were significantly affected by plant species richness, suggesting that, while diversity affects a large number of processes not all respond to biodiversity. We therefore compared the strength of plant diversity effects between different categories of ecosystem processes, grouping processes according to the year of measurement, their biogeochemical cycle, trophic level and compartment (above- or belowground) and according to whether they were measures of biodiversity or other ecosystem processes, biotic or abiotic and static or dynamic. Overall, and for several individual processes, we found that biodiversity effects became stronger over time. Measures of the carbon cycle were also affected more strongly by plant species richness than were the measures associated with the nitrogen cycle. Further, we found greater plant species richness effects on measures of biodiversity than on other processes. The differential effects of plant diversity on the various types of ecosystem processes indicate that future research and political effort should shift from a general debate about whether biodiversity loss impairs ecosystem functions to focussing on the specific functions of interest and ways to preserve them individually or in combination.