BtM, a Low-cost Open-source Datalogger to Estimate the Water Content of Nonvascular Cryptogams

Nonvascular cryptogams are crucial for several ecosystem functions. For example, they are responsible for almost 50%of the biologically fixed nitrogen. We have designed a datalogger to address questions in the field of cryptogamic ecology.

The methods are measuring in situ the period these organisms remain hydrated, relating bryophyte activity with environmental conditions. The main advantage of this technique is that it is a low cost, open-source, and easy-to-assemble datalogger, which can be built without technical knowledge. The method can be applied to a wide range of model ecosystems and organisms, from biocrust in drylands to peat bogs in boreal regions.

Prepare a soldering iron and a spool of wire. Wait for the soldering iron to heat up, and moisten the cleaning sponge. Cut the pin header strips to the desired length for the temperature and humidity sensor, microcontroller, and RTC clock and microSD breakout modules.

To solder the pin header strips into the sockets, pre-heat the desired join with the tip of the soldering iron. Then, apply a small amount of material from the solder wire, enough to fill up the junction. Finally, remove the soldering iron, and wait for the junction to cool down.

Using the same technique, begin to assemble the components to the printed circuit board. First, solder the resistors. Then, solder the sockets for the operational amplifiers, the SHT7x sensor, and the RTC clock and microSD breakout modules.

Next, solder the two transistors. The board also needs to be soldered now, using pin headers. Finally, solder the connectors to the board.

Now, solder the SHT7x humidity-temperature sensor into a pin header or extension cable to reinforce the leads. Prepare a multimeter in the continuity testing or conductivity testing mode. Use the multimeter to verify that there are no short circuits between any of the pins or connections.

Double-check the positive and negative terminals of the power supply. Also, verify that each solder joint creates a stable connection between the component pins and the copper tracks of the circuit. To connect the battery terminals and cable clips to the board, use any cutting tool to strip approximately four millimeters of each wire end, exposing the conductive core.

Then, introduce each cable into the appropriate terminal, and tighten the screw with the screwdriver. Ensure and double-check the correct polarity of the cables, especially those of the power supply. Test the strength of the connection by pulling the cables slightly, verifying that everything is firmly connected.

To further reduce power consumption, remove the power LED of the microcontroller board by either desoldering or cutting off the LED diode from the board. Finally, mount the BtM board in a weather-proof enclosure to keep moisture away from the electronics. Mount the humidity-temperature sensor outside of the box, leaving it connected to the BtM board.

Route the eight pairs of crocodile clips needed for conductance measurements to the outside of the weatherproof enclosure. Last, clip each moss strand with the crocodile clips. To ensure that the specimens are dry, perform the calibration at noon, on a day with low air humidity and preceded by at least one, and preferably two, dry days.

Select a community of moss or lichen that is healthy and well-structured. Connect the datalogger to the moss or lichen by placing the crocodile clips at a central position of the communities in the cases of bryophytes, fruticose lichens, and foliose lichens. For fruticose lichens, attach the clips in the thallus.

For mosses, attach the clips directly on the stem of an individual. In the case of foliose lichens, place the clips on the border of the thallus. Keep a minimum distance of approximately five millimeters between electrodes.

Ensure that the clips are not easily detached before starting measurements. Start the measurements by turning on the datalogger, and leave the BtM board running for approximately three minutes to stabilize the recorded values. Perform a pre-calibration test to estimate the amount of water required in each watering event.

Connect the clips to the sample. Then, add water until the conductance reaches a value that does not increase with the addition of more water. This is the maximum conductance value of that sample and will be used to establish the watering steps of the calibration.

Wait until the conductance measures return to the initial values. Use a small spray to moisten the samples with a quantity of water equivalent to 1/10 of the predetermined amount of water required to achieve the maximum conductance in the sample. Wait until the moss or lichen fully absorbs the water and the conductance measurements are stable before watering again.

Repeat until the conductance reaches the maximum value and the moss or lichen is fully hydrated. Each calibration test should take around 15 minutes, depending on the interval between the waterings, which should be one to two minutes. After finishing the calibration, take the microSD card from the BtM board, and copy the data file to a computer.

Shown here, is a figure depicting the evolution of the conductance measurements during the calibration phase. Desiccation curves of Homalothecium aureum and Dicranum scoparium reveal variability among the samples of the same species. The intra and interspecies variability found were quite large and can be attributed to differences in biomass and morphology of each stem.

Due to its ease of assembly, this device overcomes the technical constraints of designing and building a datalogger, enabling the establishment of medium and longterm monitoring networks. This technique paves the way for researchers in biogeography and ecophysiology to explore when, how, and why known vascular cryptogams, such as lichens or mosses, are growing in different regions and throughout environmental gradients. Furthermore, this technique can help understand the drivers of success of a species or individuals within ecological communities or establish what determines the delivery of ecosystem services by these key organisms.