Process-based characterization of the soil carbon balance in hardwood floodplain forests of the lower middle Elbe River

Abstract

I assessed key drivers for processes in the soil carbon (C) balance of hardwood floodplain forests at the lower middle Elbe River. Floodplains play a significant role in the global C cycle, particularly due to their soil organic carbon (SOC) storage potential in hardwood floodplain forests. In floodplains, C input occurs from deposition of dead plant debris and fluviatile sediments. C losses are mainly driven by flood-induced soil erosion and SOC mineralization. These processes vary by relief position and vegetation type. However, anthropogenic landscape modifications have affected the natural flooding regime and the vegetation composition. The interaction between natural conditions and anthropogenic modifications complicate the understanding of processes in the C balance of hardwood floodplain forests. To determine the driving mechanisms for these processes, I selected 50 floodplain study sites along the lower middle Elbe River and categorized them into hydrologic situation (low and high relief position; active and former flooding zone) and vegetation type (forest and grassland; old and young forest). The aim of my dissertation is to understand the processes controlling the soil C balance of hardwood floodplain forests. To characterize the processes controlling the soil C balance of hardwood floodplain forests, I related SOC stocks, SOC stability, and soil CO2 efflux (through autotrophic and heterotrophic soil respiration) to vegetation (e.g., forest age, basal area) and soil characteristics, particularly pedological traits (e.g., hydromorphic features, soil texture, pH, C/N ratio). SOC stocks were determined up to a depth of 1 m and compared to topsoil SOC stocks. SOC density fractions, SOC mineralizability and microbial biomass in top- and subsoils were analyzed to identify drivers for SOC stability. Soil CO2 effluxes were measured over a full year using the closed-chamber method. Based on the response of soil CO2 efflux to soil moisture and temperature, annual rates were determined. Additionally, the applicability of a low cost CO2 sensor (K33SOIL) for in situ soil measurements was tested to improve the spatial and temporal resolution of soil CO2 flux studies. SOC stocks were unaffected by vegetation type (grassland and forest) but greatest in low relief position and in the active flooding zone. SOC stocks ranged between 99–149 t ha-1 and were thereby similar to other temperate hardwood floodplain forest but also larger than terrestrial forests. SOC stocks in low lying forests of the active flooding zone were 50% greater compared to high elevated forests and to hardwood floodplain forests of the former flooding zone. Fine soil texture (< 6.3 µm) was the most important univariate predictor for SOC stocks, followed by flooding duration. A multiple linear regression showed that fine texture, pH, C/N ratio and forest age explain 86% of variance in SOC stocks. Consequently, fine texture was the most important driver for SOC stabilization to organomineral complexes, explaining 43–64% of variance in mineralizable C and the heavy density fraction (HF) of SOC. The HF was the most important SOC pool (contributing > 64% in top- and subsoil) and further confirmed the importance of fine texture for SOC storage. Thus, SOC stocks and SOC stabilization were strongly controlled by proxies for floodplain relief and sedimentation processes, such as flooding duration and fine soil texture. Soil CO2 efflux ranged between 1006–2209 gC m 2 y 1 (corresponding to 10–22 t ha 1 y 1) and was also closely related to fine texture and soil pH (R2 = 0.75), confirming a close relationship between the C balance and relief-affected features. Soil CO2 efflux was decreased at high pH (i.e., close to neutral). This result fits with the finding that SOC stocks were greatest at high pH. The largest total soil CO2 efflux occurred on sites, with highest fine texture content. This seems to contradict with the finding, that fine texture is the main driver for SOC storage. However, this effect was related to an indirect positive effect of fine texture on soil moisture and SOC content, which are important for microbial mineralization and root vitality. Total soil CO2 efflux occurred thereby partly in amounts comparable to tropical forests. Relative soil CO2 efflux (in gCO2-C gSOC-1 y-1) revealed that the smallest efflux occurred in low lying sites, where largest SOC stocks and fine texture contents occur. These sites where also represented by greatest pH and hydromorphic features appearing close to the soil surface. These results again suggest that SOC is protected by oxygen scarcity and stabilization to fine soil particles in low lying forests. To cover further CO2 flux measurements with high spatial and temporal resolution, I approved the applicability of a low cost CO2 sensor module in precision and accuracy in floodplain soils. To sum up, the process-based soil C balance in hardwood floodplain forests of the lower middle Elbe River was mainly controlled by properties representing the hydrologic situation, such as fine soil texture, soil pH and C/N ratio, the depth of hydromorphic features, and flooding duration. These findings indicated that oxygen scarcity, allochthonous C input, and SOC stabilization through the accumulation of fine soil particles are the main processes contributing to SOC preservation and mitigation of C loss through soil CO2 efflux in hardwood floodplain forests. Thus, hardwood floodplain forests act as more efficient C sinks once located in the active flooding zone where flooding and sedimentation processes occur. My dissertation thereby provides important information that should be considered during floodplain management. Furthermore, my studies underline the importance of hardwood floodplain forests as a considerable SOC reservoir but also highlight the relevance of the hydrologic situation for future climate change scenarios.