Effects of Plant Diversity on Root Decomposition and Root Turnover in Grasslands

Root processes – decomposition, production, and mortality – are intrinsic parts of ecosystem carbon cycling and yet have been understudied in the context of biodiversity-ecosystem functioning relationships. In a long-term grassland diversity experiment (the Jena Experiment), I investigated (1) whether and how plant diversity affected root decomposition and (2) how plant diversity enhanced standing root biomass and influenced root turnover. Plant diversity may affect root decomposition via three non-mutually exclusive pathways: (1) root litter quality, (2) soil biota, and/or (3) soil abiotic conditions. In Chapter 3, via three decomposition experiments, I demonstrate that plant species richness negatively affects root decomposition via the root litter quality pathway and the soil environment pathway (including soil biotic and abiotic conditions). The presence of grasses negatively affects root decomposition while the presence of legumes positively affects root decomposition. In Chapter 4, I further explored the pathways driving the relationship between plant diversity and root decomposition using structural equation modeling. The final structural equation model suggests that root chemistry is a universal pathway for explaining the effects of plant diversity on root decomposition and that Oribatida are also involved in root decomposition. Most importantly, I directly show that different measures of plant diversity influence root decomposition via different pathways. In Chapter 5, I shift my focus to root production and mortality which collectively determine standing root biomass. I monitored in situ root dynamics biweekly for one growing season using minirhizotrons. I show that plant species richness consistently enhances standing root length throughout the observational period. However, plant species richness did not affect rates of root length production, mortality, or turnover. Only root lifespan increased with increasing plant species richness. The lack of significant diversity effect on root length-based measures may be due to the age of the studied communities. In mature grasslands, plant species richness may only have a minor effect on root turnover and one growing season may be too short to detect such a small effect. The results of this dissertation collectively provide new insights into the relationship between plant species richness and soil carbon stock in mature grasslands. Based on the new paradigm of soil carbon formation, plant species richness may enhance soil carbon stock through a greater input of partially decomposed root residuals (reduced root decomposition) and a higher input of root exudates (increased standing root biomass).:ACKNOWLEDGEMENTS .......................................................................................III 1 GENERAL INTRODUCTION ............................................................................... 1 1.1 Biodiversity-Ecosystem Functioning Research in the Context of Global Biodiversity Loss ……........................................................................................... 1 1.2 Biodiversity-Ecosystem Functioning Relationships .............................................................................................................................. 2 1.3 Effects of Plant Diversity on Decomposition ................................................... 6 1.4 Effects of Plant Diversity on Root Turnover .................................................. 10 1.5 Scope and Contents of this Dissertation ...................................................... 14 2 GENERAL METHODS ......................................................................................17 2.1 The Jena Experiment ................................................................................... 17 2.2 Root Decomposition Experiments ................................................................ 18 2.3 Applying Structural Equation Modeling to Understand the Diversity-Decomposition Relationships ............................................................................. 22 2.4 In-situ Monitoring of Root Turnover Using Minirhizotrons ............................. 25 3 PLANT SPECIES RICHNESS NEGATIVELY AFFECTS ROOT DECOMPOSITION IN GRASSLANDS ................................................................................................27 4 ROOT CHEMISTRY AND SOIL FAUNA, BUT NOT SOIL ABIOTIC CONDITIONS EXPLAIN THE EFFECTS OF PLANT DIVERSITY ON ROOT DECOMPOSITION..44 5 THE EFFECTS OF PLANT SPECIES RICHNESS ON ROOT TURNOVER IN EXPERIMENTAL GRASSLANDS...........................................................................74 6 GENERAL DISCUSSION..................................................................................105 6.1 Summary of Contents in this Dissertation.................................................... 105 6.2 Advancing Our Understanding of Root Decomposition in BEF Research.... 106 6.3 Time Matters for the Diversity–Root Turnover Relationship......................... 112 6.4 Outlook........................................................................................................ 117 REFERENCES...................................................................................................120 SUMMARY .........................................................................................................137 ZUSAMMENFASSUNG .......................................................................................142 LIST OF PUBLICATIONS....................................................................................148 AUTHOR CONTRIBUTION STATEMENTS..........................................................150 DECLARATION OF INDEPENDENT WORK........................................................159 CURRICULUM VITAE .........................................................................................16