Global change effects on soil C and soil C cycling

The concentration of carbon dioxide (COx) in the atmosphere represents the balance between COx uptake and COx release by the terrestrial and oceanic biosphere. In the absence of anthropogenic greenhouse gas emissions, COx uptake and release are approximately in balance, and atmospheric COx concentrations remain relatively constant over time. However, fossil fuel burning and land-use change have increased greenhouse gas emissions considerably, with an increase in global mean temperatures as a consequence. In addition, the changes in global temperatures are inducing alterations in precipitation patterns, and fossil fuel emissions are increasing the amount of reactive nitrogen deposition. These changes will, alone and in combination, affect the COx exchange of terrestrial ecosystems with the atmosphere through changes in plant productivity, soil nutrient and water balances, and carbon storage in biomass and soils. In this thesis, I have addressed this topic by evaluating the effects of four major global changes (elevated COx, rising temperatures, increasing N deposition, and changing water availability) on soil carbon (C) storage and soil C cycling in terrestrial ecosystems. Using a database with results from global change manipulation experiments across a range of different ecosystems, and a soil sampling campaign along an altitudinal gradient in tropical forest, I assessed whether global changes affected C storage in soils and tried to uncover the mechanisms behind the effects of global change on soil C content and soil C cycling. First, I have tested whether elevated COx concentrations can increase soil C content through increased plant productivity, and found that elevated COx accelerates soil C cycling, but does not increase soil C content. Effects of nitrogen (N) availability were important in the COx response of soil C processes, as we found that soil C content did increase in elevated COx when sufficient amounts of N fertilizer were added. In addition, I have shown that initial soil N content affected responses of fine roots, microbial decomposition and soil C content to elevated COx. Secondly, I have tested the hypothesis that N fertilization consistently decreases decomposition processes, and found a strong negative effect of N fertilization on microbial biomass and activity. I have revealed possible mechanisms behind these effects and indicated that N fertilization can thus lead to increases in soil C content. Thirdly, I analyzed a dataset of warming experiments to test whether warming consistently stimulates microbial decomposition rates. I did not find a consistent increase in microbial respiration or soil respiration, due to large variation between experiments. Here, I concluded that interactions and feedbacks related to soil water and nutrient status, and ecosystem-specific responses were causing the large range of observed effects. Futher, in an analysis of combined elevated COx and warming experiments, I have shown that elevated COx and warming can alleviate (part) of each other's limitations, resulting in synergistic interactions between different global change drivers. But more often, effects of combined global changes are unpredictable, suggesting the need for global models to validate their models structure against results from studies combining multiple global changes, rather than against effects of individual global changes. Lastly, due to spatial variability and differences in sensitivity of different terrestrial ecosystems, I have indicated the importance of a high quality documentation of site variables to be able to explain the observed effects of global change drivers. In this thesis, I have increased the understanding of mechanisms and patterns behind effects of N fertilization and elevated COx concentrations, and have synthesized and clarified some effects and responses patterns in warming experiments. In addition, I have indicated gaps in our current understanding of soil C cycling processes due to lack of data, or low data quality. To increase the understanding of mechanisms behind global change effects on soil processes, I call for a more integrated approach where effects on soil C inputs, C pools, and C losses are simultaneously addressed, with a stronger attention for documentation of, and effects on, soil nutrient and water balances