DOI: 10.5061/dryad.0gb5mkm3p
title: Rewetting prolongs root growing season in minerotrophic peatlands and mitigates negative drought effects

description: Root phenology influences the timing of plant resource acquisition and
 carbon fluxes into the soil. This is particularly important in fen
 peatlands, in which peat is primarily formed by roots and rhizomes of
 vascular plants. However, most fens in Central Europe are drained for
 agriculture, leading to large carbon losses, and further threatened by
 increasing frequency and intensity of droughts. Rewetting fens aims to
 restore the original carbon sink, but how root phenology is affected by
 drainage and rewetting is largely unknown. We monitored root phenology
 with minirhizotrons in drained and rewetted fens (alder forest,
 percolation fen and coastal fen) as well as its soil temperature and water
 table depth during the 2018 drought. For each fen type, we studied a
 drained site and a site that was rewetted ~25 years ago, while all the
 sites studied had been drained for almost a century. Overall, the growing
 season was longer with rewetting, allowing roots to grow over a longer
 period in the year and have a higher root production than under drainage.
 With increasing depth, the growing season shifted to later in time but
 remained a similar length, and the relative importance of soil temperature
 for root length changes increased with soil depth. Synthesis and
 applications. Rewetting extended the growing season of roots, highlighting
 the importance of phenology in explaining root productivity in peatlands.
 A longer growing season allows a longer period of carbon sequestration in
 form of root biomass and promotes the peatlands’ carbon sink function,
 especially through longer growth in deep soil layers. Thus, management
 practices that focus on rewetting peatland ecosystems are necessary to
 maintain their function as carbon sinks, particularly under drought
 conditions, and are a top priority to reduce carbon emissions and address
 climate change.
description: We used minirhizotrons to
 monitor root phenology.
 Thereto,
 transparent tubes were
 installed at an angle of 45° in the soil to insert a root image scanner
 (CI-600 In-Situ Root Imager; CID Bio-science Inc., Camas, WA, USA),
 taking c. 350° scans (image size: 21.6 × 19.6 cm) of the tube-soil
 interface and thus roots at three depths (0–15, 15–30, and 30–45 cm). The
 aboveground part of the tubes was wrapped with mirror foil to reduce
 thermal differences, taped and covered with a cap to exclude light from
 the tubes. Installation of the tubes took place in mid-August 2017 and
 measurements began in April 2018. As our study sites are very dynamic,
 fast-growing systems, we consider this time sufficient for recovery from
 the disturbance caused by the installation. Three minirhizotrons were installed within a
 distance of 1 m within each of the five plots at each of the respective
 sites, but one tube was damaged during the study, resulting in a total of
 89 minirhizotrons (six sites × five plots × three tubes). In the rewetted
 coastal fen site (CW), images could only be taken down to a depth of 30 cm
 for 10 tubes, due to hard mineral soil limiting the depth of
 minirhizotrons. We measured biweekly until 15th October 2018 and then
 monthly until 5th December 2018, resulting in 16 image sampling
 events. We processed the
 sample images with our newly developed automated RootDetector, which
 outputs binary images in which white pixels represent detected root
 objects and black pixels represent background or non-root objects. We
 applied a topology-preserving thinning algorithm as implemented by the
 skeletonize function in scikit-image v. 0.17.1 to reduce the detected root
 objects to one pixel wide lines. The output was converted into root length in mm
 cm<sup>-2</sup>, given that root length is stronger related to
 root functionality (e.g., nutrient uptake) than mass. Soil
 temperature data was collected at 15-minute intervals at 5 cm and 15 cm
 depth by six loggers per site (HOBO, Onset Computer Corporation, Bourne,
 MA, USA). Groundwater table relative to soil surface was
 recorded at 15-minute intervals in a slotted PVC pipe using a CS456
 pressure transducer connected via an SDI-12 sensor to a CR1000 data logger
 (Campbell Scientific Ltd., Bremen, Germany) at the alder forest sites, by
 Dipper-PT loggers for the two percolation fen sites and the drained
 coastal fen, and by a Baro-Dipper and a Dipper-APT logger (SEBA
 Hydrometrie GmbH & Co. KG, Kaufbeuren, Germany) for the rewetted
 coastal fen site. Gaps in water table data recording for the rewetted
 alder forest site between 23rd August and 27th October 2018 resulted from
 water tables below - 70 cm, which exceeded the reach of the groundwater
 pipe at this site. We used R
 version 4.0.2 (R Core Team, 2020) for all statistical analyses and
 visualizations (R package <em>ggplot2</em>, version
 3.2.1).