3.1. Monitoring
The study period was from August 20th to November 3rd 2021. Climate data (air temperature, precipitation amount, wind speed and direction, relative humidity, air pressure, global radiation) were available from the rooftop of IGB ~300 m away. Additionally on site, precipitation (tipping bucket raingauge, 0.2 mm/tip, precision ±3% of total rainfall; AeroCone® Rain Collector, Davis Instruments, Hayward, USA) was recorded with a CR800 Datalogger (Campbell Scientific, Inc. Logan, USA) logging every 15 min. Temperature was recorded (every 5 min) with BetaTherm 100K6A1IA Thermistors T107 (Campbell Scientific, Inc. Logan, USA; tolerance ±0.2°C (over 0°–50°C)), with a CR300 Datalogger (Campbell Scientific, Inc. Logan, USA). Precipitation and temperature data were verified against available data from the German Weather Service (DWD) of the “Berlin-Marzahn” station (Station ID: 420), ~12 km north of the study site.
Precipitation for stable water isotope analysis was collected using a HDPE deposition sampler (100 cm2 opening; Umwelt-Geräte-Technik GmbH, Müncheberg, Germany). Overall, 32 daily and 15 bulk (interval ~weekly) samples with precipitation >1 mm (to limit evaporation effects) were collected between July and November 2021. Further, daily precipitation samples were collected ~350 m away from the study site with an autosampler (ISCO 3700, Teledyne Isco, Lincoln, USA) at a 24 hours interval. All autosampler bottles were filled with a paraffin oil layer > 0.5 cm in thickness (after IAEA/GNIP, 2014) to avoid evaporative effects. Additionally, groundwater samples were taken weekly with a submersible pump (COMET-Pumpen Systemtechnik GmbH & Co. KG, Pfaffschwende, Germany) from a well on IGB grounds ~300 m away from the site.
For isotope analysis of the liquid water samples at the IGB laboratory, samples were filtered (0.2 um cellulose acetate) and decanted into 1.5 ml glass vials (LLG LABWARE). They were analysed by cavity ring-down spectroscopy (CRDS) with a L2130-i Isotopic Water Analyser (PICARRO, INC., Santa Clara, CA) using four standards for a linear correction function and which were referenced against three primary standards of the International Atomic Energy Agency (IAEA) for calibration (VSMOW2 (Vienna Standard Mean Ocean Water 2), GRESP (Greenland Summit Precipitation) and SLAP2 (Standard Light Antarctic Precipitation 2)). Liquid samples were injected six times and the first three injections discarded. To screen for interference from organics, the ChemCorrect software (Picarro, Inc.) was applied and contaminated samples discarded. After quality-checking and averaging multiple analyses for each sample, the results were expressed in δ-notation with Vienna Standard Mean Ocean Water (VSMOW). Analytical precision was 0.05 ‰ standard deviation (SD) for δ18O and 0.14 ‰ SD for δD.
Stable isotopes of atmospheric water vapour (δv) were measured in-situ at the tree-dominated and grassland sites, respectively at 0.15 m, 2 m and 10 m height to capture the effects of vegetation heterogeneity and potential turbulence within an urban surface boundary layer. To monitor the elevation profile above the grassland, a 10 m flag mast with ~ 100 cm long perpendicular poles at required sample points was set up (Fig. 2). At the tree site, we measured directly at the trunk within the canopy of the maple tree. The measurement campaign started on 20.08.2021 above the grassland and on 03.09.2021 in the tree canopy.
We performed in-situ real-time sequential measurements of water vapour via CRDS (Picarro L2130-i, Picarro Inc., Santa Clara, CA, USA) placed in a box between the sampling sites. Air inlets and CRDS were connected with polytetrafluoroethylene (PTFE) tubing (1.6 mm x 3.2 mm). We used PET bottles covered with aluminum foil to prevent the inlets from rain and sun exposure. Each tube inlet (Fig. 2) was sampled for 20 min in resolution of seconds. Then sampling was switched automatically to the next one; resulting in a 2-hourly resolution for each inlet. We only used the data when a measurement showed stable values (i.e. ranges of 2 ‰ for 2H and 0.3 ‰ for 18O). The first 5 min of data after switching inlets were always discarded to avoid memory effects. Prior the vapour entering the CRDS unit, a preceding sub-micron particulate filter was connected to prevent liquid water from entering by creating a low dew point by lowering the air pressure. The sample flow rate was at 0.04 L min−1. Water vapour concentrations were always above 6000 ppm (this is where the concentration dependent deviation becomes low and thus measurement precision is not compromised).
To allow for later conversion of δv measurements into liquid water isotope values, temperature probes were installed at all heights near the tube inlets at both sites with BetaTherm 100K6A1IA Thermistors T107 (Campbell Scientific, Inc. Logan, USA; tolerance ±0.2°C (over 0°–50°C)), with a CR300 Datalogger (Campbell Scientific, Inc. Logan, USA) logging mean values every 5 min from secondly-resoluted data. To avoid tube condensation, heating cables (ILLw.CT/Qx, Quintex GmbH, Lauda-Königshofen, Germany) were installed and wrapped with the tube in insulation material. The cables were controlled via an automatic multi socket (Gembird 235 EG-PMS2, Gembird Software Ltd., Almere, The Netherlands) to prevent overheating in summer. To minimise condensation effects, the measurements were checked daily. The PTFE-tubes were flushed weekly or if required for 10 minutes per probe to remove any water . Data was discarded when condensation inside the system was identified in the respective tube.
By combining δv and temperature data from each inlet we derived the values for all heights of temperature dependent equilibrium fractionation from vapour to liquid with the correction formulated by Majoube :
\(\alpha=exp\frac{a\left(\frac{10^{6}}{T_{k}^{2}}\right)+b\ \left(\frac{10^{3}}{T_{k}}\right)+c}{1000}\)(1)
where α is the isotopic fractionation factor,Tk is the temperature (in K), and a ,b , and c are empirical parameters that vary depending on the isotopologue. All values of isotopic compositions are given in liquid phase and relative to Vienna Standard Mean Ocean Water (VSMOW).
To investigate the local evaporative effects, the line-conditioned excess (short lc-excess) (see ) was calculated. The lc-excess describes the deviation of the sample from the local meteoric water line (LMWL):
lc-excess = δ2H − a · δ18O −b (2)
where a is the slope and b the intercept of the weighted isotopic composition of the local precipitation. The LMWL was calculated by amount-weighted least square regression from daily precipitation isotopes measured at IGB from July until November 2021.
In order to assure stable values to offset variability in the field, stability of the CRDS was tested in the lab before installing the setup outside. During the sampling campaign, we calibrated once a week (cf. calibration periods ) with two standards. Stored in sealed glass containers, the standards were connected to the CRDS for two-point calibrations (liquid values: light: 2H -109.91 ‰/18O -17.86 ‰; medium: 2H -56.7 ‰/18O -7.68 ‰). We used measured water vapour concentrations and added linear regressions of temperature dependency slopes to correct for isotopic offsets and vapour concentration dependency (resembling the approach by Schmidt et al. ).
We also monitored sap velocities and stem circumference of the maple tree. Two sap flow sensors (SFM-4, Umwelt-Geräte-Technik GmbH, Müncheberg, Germany; ±0.1 cm/hr heat velocity precision) were installed at breast height (1.3 m) at the north and south side of the tree stem. The sap flow sensors work according to the heat ratio method by Marshall . Daily reference crop evapotranspiration (ET0) was estimated using the FAO Penman–Monteith method with “R”-Package “Evapotranspiration” . To investigate dynamics during the growing season, both daily mean sap velocity [cm h-1] and ET0 were then normalized (to sapvelocitynorm and ETnorm, respectively) by feature scaling. One dendrometer (DR Radius Dendrometer, Ecomatik, Dachau, Ger170; accuracy max. ± 4.5% of the measured value (stable offset)) was also installed to measure stem diameter dynamics at high temporal resolution. Sap velocity and stem increments were logged as 15 min intervals using a CR300 Datalogger (Campbell Scientific, Inc. Logan, USA). Throughfall amount was sampled manually at a height of 30 cm above ground using four rain gauges (Rain gauge kit, S. Brannan & Sons, Cleator Moor, UK) which were installed 1 m and 3 m, respectively, north and south of the tree’s stem.
Volumetric soil water content and soil temperature were measured at both sites (Fig. 2) by soil moisture temperature probes (SMT-100, Umwelt-Geräte-Technik GmbH, Müncheberg, Germany) in the upper soil at 6 cm depth. Recording took place with a CR800 Datalogger (Campbell Scientific, Inc. Logan, USA) with a 15 min frequency and a precision of ±3 % for volumetric soil water content and ±0.2 C for soil temperature. Groundwater level in one well was monitored with an automatic datalogger (groundwater level probe) at an interval of 15 min (see location in Fig. 1C).