Plants, microbes and soil-redox in salt marshes: Intricate interactions and responses to global warming

Abstract

As part of blue carbon ecosystems, salt marshes exert outsized leverage on global carbon cycling. Their soils act as highly effective carbon sinks, with substantial rates of carbon sequestration and long-term storage capacity. However, the low redox potentials that underpin this sequestration also create favourable conditions for microbial methanogenesis. Bidirectional interactions between both plant-soil and microbe-soil regulate the processes that determine soil redox conditions in salt marshes. However, the understanding of these regulative processes remains limited. Further, we lack fundamental insight into how these plant-soil and microbe-soil redox interactions will respond to global warming and in turn mediate warming effects on carbon cycling. To address these research gaps, I investigated these interactions under both controlled conditions and in situ warming, leveraging the MERIT (Marsh Ecosystem Response to Increased Temperatures) whole-ecosystem warming experiment. This thesis is structured into six chapters: an introduction to the study context (Chapter 1), four studies that address the research gaps (Chapters 2-5), and a unifying synthesis that discusses the results from previous chapters (Chapter 6). The first study (Chapter 2), assesses plant-soil redox interactions combining mesocosm and field study with high resolution oxygen profiling using planar optodes. Results highlight that, roots can act both as net reducers and net oxidizers in wetland soils, and that the direction of this plant effect is inversely correlated with background redox conditions. I find that plant effects on soil reduction are net reducing, due to the comparably well aerated soils of the study system, a minerogenic Wadden Sea salt marsh. The second study (Chapter 3), examines early-stage (1-2 years) warming effects on decomposition processes, using the Tea-Bag Index approach. Results show that, increased temperature accelerated decomposition rates. However, warming effects on litter stabilization were restricted to higher elevated zones and soil layers. This suggests that the reducing soil conditions suppress the response of belowground litter stabilization processes to warming. The third study (Chapter 4), shows results from mid-stage (5 years) warming effects on microbial functioning (i.e., exo-enzymatic activities) and putatively active microbial community structure (i.e., 16S sequencing on total RNA). Results indicate that, exo-enzymatic activities tend to decrease with warming. Additionally, microbial community structure remains largely stable under warming, however shifts towards phyla with capacities to degrade complex carbon compounds occur in the higher elevated zones. These findings suggest, that warming can induce drought stress in higher elevated zones, causing subtle shifts with implications for carbon cycling. The fourth study (Chapter 5), addresses warming effects on soil redox conditions and links them to methane fluxes and activity of methanogenic and methanotrophic potential processes. Results reveal that, soils become more reducing with warming. In the pioneer zone, this effect is accompanied by increasing methane fluxes and a higher ratio of methanogenic to methanotrophic potential processes. Findings from this study challenge the common hypothesis that hydrology, especially in the lower elevated marsh zones, outweighs warming effects in salt marshes. Overall, this work emphasizes that interactions between plants and soil and between microbes and soil influence soil redox conditions, and that soil redox conditions are of central importance for the global carbon cycle. It shows that hydrological constraints frequently outweigh direct warming effects, but that sustained warming ultimately shifts microbial functioning and redox dynamics toward enhanced methanogenesis and methane release. This thesis demonstrates that salt marsh vulnerability to climate change is best understood through the lens of bidirectional plant-soil and microbe-soil redox interactions.