Global change effects on microbial-mediated soil organic carbon cycling processes

Soil microorganisms, including bacteria, fungi, and archaea, are the main actors of global C cycling. Global change factors, such as nitrogen deposition, warming, and elevated CO2 (eCO2), can influence microbial communities and, consequently, carbon pools in the soil. Understanding the microbial community's response to global change factors is crucial for predicting and managing soil carbon cycling. Such understanding is important for efforts to maximize soil carbon storage in order to reduce the earth's surface carbon emissions and associated climate warming. In this PhD thesis, my primary focus lies on 1) investigating the response of microbial carbon use efficiency and the resulting necromass to anthropogenic nitrogen enrichment; 2) exploring the influence of global change factors, including climate warming and elevated CO2, on microbial necromass in soil and its consequential impact on the formation of associated soil organic matter; 3) unraveling the global patterns and drivers of microbial CUE in soil. I generally used a global meta-analysis approach to test hypotheses about microbial attributes’ responses to global changes. In Chapter 3, I assessed the impact of anthropogenic nitrogen (N) enrichment on microbial Carbon Use Efficiency (CUE; the ratio of the C amount used for growth to the C taken up) within the ecosystem. The results revealed a notable enhancement in microbial growth rate and CUE, particularly when applying the 18O-labelled water approach. This increase was expected to result in more microbial biomass and microbial-derived C formation. Indeed, N addition increased microbial necromass, while N effects on bacterial necromass were stronger than those on fungal necromass (Chapter 2). In Chapter 4, I investigated the impacts of warming and eCO2 on microbial necromass. The findings revealed that experimental warming led to a specific increase in bacterial necromass, suggesting that bacteria possess a competitive edge, driven by faster growth rates and more effective competition for readily available resources, particularly in response to short-term warming. In contrast, my meta-analysis indicated an absence of a discernible effect of eCO2 on microbial necromass. This lack of response is attributed to the difference in the response between microbial biomass and necromass. Even when microbial biomass undergoes significant changes over a short period, these alterations may not immediately manifest in necromass. Consequently, the evaluation of microbial necromass and its associated carbon formation in response to eCO2, coupled with climate change, necessitates a more protracted assessment, emphasizing the importance of ongoing or new experimental studies for a comprehensive understanding. In Chapter 5, I delved into the underlying regulatory factors and patterns governing soil microbial CUE on a broad scale. The findings revealed a noteworthy increase in CUE with latitude. This elevation in CUE at higher latitudes correlated with amplified microbial biomass and necromass synthesis, contributing to a more stable soil organic carbon pool. This observation aligns with the understanding that high-latitude regions harbor substantial soil microbial carbon and soil carbon stocks.