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Another means of assessing active carbon is through measurements of the short term release of CO2 from the soil. The amount released and compared among treatments has recently been associated with forms of C in the soil more readily decomposed than those assessed through POXC (Hurisso et al., 2016). It seems to be associated with recent additions of organic matter to the soil and should thus be an appropriate measure (along with POXC) for comparing soils under different management treatment (Culman et al., 2013). Again, these comparisons should be made for treatments or different management situations on farmer’s fields on the same soil type. The CO2 evolved can be measured several ways, if a gas analyzer is available the method below can be used. One of the most widely used methods is based on an alkali trap that is placed in the container with the soil sample for the incubation period, then removed and titration used to measure the amount of CO2 trapped. Recently, a convenient Solvita method has become available that relies on a gel for detection of CO2 in lab or field-based incubations (Haney et al., 2008).
The challenge with all soil respiration and MINC methods is that for reproducible and comparable results, the soil moisture status needs to be consistent, preferably 50% Water-Filled Pore Space (wfps). This can be calculated as shown in the appendix, based on the Haney and Haney 2010 method. Or, a simplified approach that is promoted by Solvita (https://solvita.com/) is to use the equation 50% wfps = (volume of the soil - 40/2.65) x 50%. An assessment of how to use Solvita is available from USDA, see respiration for educators information on this website: https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/health/assessment/ .
How to operationalize the metric
Method of data collection and data needed to compute the method:
CO2 Evolution from Wetting a Dried Soil (capped IRGA method) Snapp Lab
This procedure describes a technique for the determination of CO2 respired from air-dried soil that has been rewetted. The method is described in detail by Franzluebbers et al. (2000).
The procedure described below uses an infrared gas analyzer. Incubation times can vary from 1 to 3 day, or longer. Snapp lab has found treatment sensitivity and decreased analytical variation with 1 day incubations that are closely correlated with results from longer-term mineralization (3,7 and 24 day), so for convenience we recommend a one-day incubation.
- Mason canning jars (half-pint or quart size)
- Lids and rings for canning jars
- Rubber septa for jar lids
- Plastic specimen containers (100 mL) or glass beakers (100 mL)
- 5 mL pipettor
- Syringes (1 and 3 mL)
- Syringe needles (25G, 1 and 1.5 in)
- Gas tank – 1% CO2 (99% N2) + regulator
- Gas tank – Helium or N2 + regulator
- Septa for IRGA – SS174 Teflon Faced Septa 11mm (100/pkg) from Supleco Catalog #2-2731
- Balance for weighing soil
- LI-820 infrared gas analyzer (IRGA)
- LI-820 software
Prior to set-up:
Drill holes into mason jar lids the side of the septa (or rubber stoppers) and insert septa into the holes. Make sure they are airtight. This can be accomplished by sealing with vacuum grease around the septa if necessary.
- If not known, determine water holding capacity and moisture content of air-dried, sieved soil (Haney and Haney, 2010).
- Weigh 10g air-dried soil into 100 mL containers and place containers in mason jars. Label mason jar lids (containing septa). (This can be done ahead of time. Rewetting the soils and baseline IRGA measurement should be done one jar at a time.)
- Adjust moisture to 50% water holding capacity (50% WFPS) by adding determined volume of deionized water evenly over the surface of the soil with a 5 ml pipettor, or if a pipettor isn’t available, use a syringe.
- Seal jar with labeled lid (containing septa) and ring
- Determine a baseline CO2 reading with the IRGA:
- Insert emptied syringe through the septa of incubation jar, drawing air in and out repeatedly (~ 5 times) to mix.
- Expel air in syringe until there is 0.5ml of air exactly in syringe.
- Remove syringe of incubation jar and inject in IRGA septa port.
- Record injection time.
- Record starting weight for jar + beaker + wetted soil.
- Place jars into 25°C incubator in the dark.
Measurements (0 and 1 day):
- Turn on gas tank containing IRGA carrier gas (N2 or helium).
- Calibrate to zero after 10 minutes of running the carrier gas.
- Create IRGA standards in septa jars labeled A, B, and C, insert syringe needle into jar’s septa to act as a vent, then inject CO2/N2 mix from gas tank into jar for 1 minute. Remove standard jar from gas tank, then remove vent needle after jar has depressurized (listen for the air escaping from the vent needle, then remove as air stops venting).
- Start logging measurements.
- Take CO2 measurements by removing 0.5 mL air from the jar with a 1 mL syringe and injecting it into the IRGA. Standards should be run first and last (2-3 times each).
- Record the injection time for each sample. Wait until the ppm returns to zero before entering the next sample.
- After taking CO2 readings, stop logging. Return sample jars to the incubator.
- Note: on the last day of measurements, record the weights for the jars (+ beaker + soil). Any substantial weight difference from day 0 to the last day may indicate leakage.
Unit of analysis:
Limitations regarding estimating and interpreting: