Search

Floodplain ecosystems play a significant role in the global carbon (C) cycle, particularly due to their C sink potential in hardwood floodplain forests. However, in these forests, interactions between a heterogeneous micro‐relief and anthropogenic landscape changes make estimating C loss through soil CO2 efflux difficult. To determine the drivers of soil CO2 efflux, we selected six hardwood floodplain forests at the lower middle Elbe River, which were distributed among different relief positions (low‐lying or high‐elevated) in the active and former flooding zone. We measured soil CO2 effluxes over a full year using the closed‐chamber method. Based on the response of soil CO2 efflux to soil moisture and temperature, annual efflux rates were determined, which were then related to soil properties, such as pH, texture, soil organic carbon (SOC) and nitrogen (N) content. Soil CO2 efflux ranged between 1006 (±99) and 2214 (±118) gC m−2 year−1. Maximum efflux occurred in a former floodplain forest that was disconnected from Elbe River water table fluctuations. SOC‐specific soil CO2 efflux (gC gSOC−1 year−1) was smallest in low‐lying forests of the active flooding zone and reflected by the appearance of redoximorphic mottling close to the soil surface. Fine texture (<6.3 μm), SOC and N were related positively and electric conductivity, C/N and pH negatively to total soil CO2 efflux. Soil pH and fine texture were the strongest univariate predictors for total soil CO2 efflux (both R2 = 0.59). Fine texture, pH and C/N ratio explained 66% of the variance in total soil CO2 efflux according to multiple linear regression. We conclude that, in hardwood floodplain forests, soil CO2 efflux is mainly controlled by fine texture and soil pH. Fine texture can be related to soil moisture and nutrient availability and may have a positive effect on the activity of microorganisms. Highlights: Soil CO2 efflux ranged between 1006 (±99) and 2214 (±118) gC m2 year−1, whereby the maximum was measured in a disconnected floodplain forestMaximum soil CO2 efflux was measured in a disconnected floodplain forestSOC‐specific soil CO2 efflux was smallest in forests where redoximorphic mottling occurred close to the surfaceFine texture (<6.3 μm) and pH were the strongest predictors for annual soil CO2 efflux [ABSTRACT FROM AUTHOR]

Climatic changes affect soil formation and biogeochemical processes on various spatial and temporal scales. This alters the interaction and feedback mechanism between long-term soil development and present biogeochemical cycles. To identify these mechanistic transformations and thus implications on soil function and ecosystem services, continuous and highly resolved observations are fundamental.In spring 2017, we established the Ebergoetzen measurement site in order to investigate biogeochemical processes and related soil formation under two distinct microclimatic conditions. The sites is located in a small valley on Triassic sandstone bedrock with active soil acidification and early-stage podsolization. The area is covered by a ~50-year-old beech forest (Fagus sylvatica) and separated into two topographical units: a south-facing slope and a north-facing slope. Both slopes were equipped with a state-of-the-art sensor network to monitor environmental parameters and biogeochemical fluxes. Soil moisture and temperature, as well as climatic parameters are continuously monitored. Each slope was equipped with suction plates to collect soil water (-150 hPa) at four different soil depths (15, 30, 50 and 70 cm). Soil solution is collected on a bi-weekly basis and analyzed for carbon and nutrient concentrations. Additional litter traps (0.5 m²) were installed and sampled regularly to estimate the seasonal element inputs from solid matter. Dissolved element and nutrient inputs are quantified in samples from 12 stem-flow and throughfall collectors. Twelve individual trees were equipped with sap-flow meters to continuously monitor water uptake by trees.At the north-facing slope, soil temperature and evapotranspiration were seasonally reduced compared to the south-facing slope, leading to higher water availability and leaching, thus lower soil pH values – and a potential deceleration of biochemical turnover. Based on these initial conditions, we plan to assess annual and interannual fluctuations and relate these changes to soil development processes, such as soil organic matter transformation and relocation within the profile, as well as changes in the plant-soil interactions and effects on the biogenic silicon cycle. An SQL-database was designed to automatically collect and structure all data and make it available for possible collaborations and large-scale comparisons. [ABSTRACT FROM AUTHOR]

Soil CO2 efflux is a major pathway in the global C cycle and comprises root respiration as well as microbial respiration. Soil respiration is therefore tightly linked to root activity and C release from roots as root exudates are rapidly decomposed by soil microorganisms. As the rhizosphere is characterized by high C turnover rates and high spatial variability of processes, a method is needed to account for this spatial and temporal variability of CO2 production in the rhizosphere. Planar optodes were previously applied to measure the distribution of pH and O2 in the rhizosphere. So far, CO2 measurements were only a few times successfully conducted in saturated soils, although the importance of soil CO2 respiration is usually higher under aerobic conditions. We therefore tested whether planar optodes can be applied to measure CO2 in the rhizosphere of unsaturated soils. Maize (zea mays) plants were grown in 5 rhizoboxes filled with sandy soil. After four weeks, planar optodes sensitive to CO2 (PreSens GmbH, SF-CD2R - range: 0 – 1 % pCO2) were attached to the rhizosphere surface. We selected 2-5 positions with visibly growing roots per box and attached 20-40 cm² optodes at each position. The CO2 concentration was measured at three different volumetric soil water contents (VWC), a) 21%, b) 28% and c) after saturation at 38%. The change of CO2 signal was monitored over a period of 15 hours after saturation of samples to test the optode equilibration time. The obtained images were calibrated and analyzed using ImageJ v. 1.51 (NIH).Gradients of CO2 were clearly visible around root tips. Around mature root parts, the increase of CO2 was only half as strong as for young root parts, likely due to high root respiration in the growing root parts and due to microbial decomposition of root exudates. The equilibration time for the rhizosphere was 10 hours, while in bulk soil equilibrium was not reached after 15 hours. This suggests that optodes should be attached to samples and equilibrated for at least 15 hours prior to measurements. Soil moisture had a strong effect on the measured CO2 concentration. For 21% VWC the average CO2 concentration was 0.23 µmol CO2 l-1 while it was 0.38 µmol CO2 l-1 for 28% VWC and increased to 1.47 µmol CO2 l-1 after saturating the samples to 38% VWC. For 28% and 38% VWCs, CO2 gradients were clearly visible around the roots, while no gradients were detectable at VWC of 21%. This is likely due to the increased diffusion of CO2 into the optodes under higher VWC. Statistical comparisons between VWC levels were highly sensitive, indicating that small changes in soil WC will greatly affect the measured CO2 concentration and thus may be a strong confounding variable for treatment comparisons in unsaturated soils. However, provided that the soil moisture is kept constant in all samples, optode measurements of CO2 can be used in moist soil samples to quantify relative differences in CO2 concentration between treatments. [ABSTRACT FROM AUTHOR]