Temperature Response of Soil Organic Matter Decomposition Rates: Construction and Applications of a Temperature Gradient Block

Our research examines how changing temperatures influence soil organic matter decomposition. And our aim is to define the temperature response and help improve carbon climate feedback predictions. Recent studies reveal broadly varying temperature responses for soil organic matter decomposition, but oversimplified matrix limit insight.

Improved methodology will allow collecting data for better mechanistic understanding. To begin, place the soil sample in a metal tin and mix it thoroughly until homogeneous. Distribute the mixed soil into 22 pre-labeled 60 milliliter glass vials.

Using weighing paper, add about three grams of soil into each 60 milliliter vial. With a pipette, adjust the soil in each vial to the desired water holding capacity based on the funnel method. After adjusting the water content to the target percentage water holding capacity, record the exact weight of each vial.

Cap each vial using cotton wool to allow ambient air exchange. Pre-incubate the vials in a temperature-controlled laboratory set to 20 degrees Celsius for 10 days. To begin preparation for loading, adjust the water holding capacity one hour before starting the incubation.

Check the heater control to ensure it has reached the desired temperature 10 minutes before placing samples in the block. Then turn on the ambient conductor located in the laboratory. Connect the flushing system and connect the gas analyzer to the computer.

To begin collecting blanks, connect the flushing line to the injection system of the gas analyzer and record the carbon dioxide concentration in parts per million directly from the flushing line. For a second blank measurement, place five 60 milliliter glass vials in an empty holder. Insert one flushing line into each vial.

A minute later, remove the flushing line and cap each vial. Measure the carbon dioxide inside the vials after four hours. Next, open the temperature gradient block and place the computer and flushing system on top of the block lid.

Place the sample boxes on a movable table. Prepare a spreadsheet to record the closing time of each vial and simultaneously visualize the time displayed on the gas analyzer. Start loading samples into the temperature gradient block in batches of eight.

Place uncapped samples inside the block, starting from the cooler end. Insert the flushing line into the eight vials. Remove the flushing line after a minute and immediately cap each vial.

Record the exact closing time in the spreadsheet. While the first batch is flushing, load the next batch of samples into the block and insert the flushing line into this next batch. Repeat the loading and flushing procedure until the temperature gradient block is fully loaded.

Close the temperature gradient block and wait 3.5 hours from the moment the last vial was closed. To measure the carbon dioxide, turn on the flushing system and place the gas analyzer onto the movable table. After 3.5 hours of incubation, begin measuring samples, starting with those at the highest temperatures.

Connect the injection needle of the analyzer to a flushing line between samples. In the gas analyzer software, create a unique and simple remark before each measurement using the remark function. Insert the analyzer needle into a 60 milliliter sample vial for 20 seconds, then remove the needle from the vial.

Start the remark at the zero seconds. Insert the needle at 10th seconds. Then remove the needle at the 30th second.

And stop the remark at the 45th second. Replace the 0.45 micrometer filter connected to the injection needle every 16 samples. To unload samples, remove each vial one by one from the temperature gradient block and place it into a box while continuing carbon dioxide measurements.

Most of the eight temperature sensors reach their target temperature within one hour after switching the cooling system and heater on and remained stable after an initial overshoot during the incubation period. After stabilization, the temperatures recorded by the eight sensors showed a perfectly linear distribution along the length of the aluminum block. Soil samples placed inside the stabilized block reached the target temperatures recorded by the sensors within approximately 12 minutes.

After 12 minutes of incubation, soil temperatures displayed a strong linear relationship with the sensor temperatures across the block. With an average incubation time of 3.5 hours, higher incubation temperatures resulted in greater soil moisture loss regardless of soil texture. An optimal measuring time of 20 seconds was identified, allowing the carbon dioxide signal to reach equilibrium for approximately five seconds.

When the gas analyzer was used as a closed system, a pressure drop occurred. Soil respiration rates measured across 22 discrete temperatures produced consistent temperature response curves with small standard deviations. Soil respiration data collected with the temperature gradient block were successfully fitted using Arrhenius and macromolecular rate theory models.

Some soil samples exhibited altered respiration patterns associated with fungal growth and were excluded from further analysis. Our protocol uses an advanced temperature gradient block that measure soil respiration at 22 temperatures, enabling a detailed diverse response curves for soil organic matter decomposition rates. This method improves mechanistic understanding of the temperature effects on soil organic matter decomposition, revealing roles of soil properties and land management.