KBS021: N Mineralization Potential - In Situ Buried Bags

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Abstract

The nitrogen mineralization potential of soils is an estimate of the net production of inorganic N under given conditions and provides a means to assess soil N availability. To measure in situ N mineralization potentials subsamples of a sieved, composite soil sample are placed in polyethylene bags, reburied in the field, incubated for 21 days (short-term), retrieved and resampled for soil inorganic N. Polyethylene bags allow diffusion of oxygen but not moisture. The N mineralization potential is the difference between the final and the initial total inorganic N (nitrate + ammonium) concentration and is expressed on a gravimetric basis (microgram N/g dry soil/day), which can be converted to an areal basis (microgram N/m2/day) if the soil bulk density is known.

Sampling frequency: Depends on experiment and research objective. N mineralization potential is measured once annually (since 1996) as part of our core soil sampling of the LTER Main Cropping System Experiment, less frequently than earlier in the project.

Protocol

Two 100 g fresh weight subsamples from the sieved, composite soil sample of each plot (see Composite Soil Sampling – MCSE protocol) are placed in separate polyethylene bags, designated as T21A and T21B for 21-day field incubations. The composite soil samples are subsampled as per the Soil Inorganic N and Soil Moisture protocols to provide estimates of initial (T0) inorganic NH4+, NO3-, and moisture levels. The T21A and T21B sample bags are buried in the field in their respective plots and left in situ for 21 days. Sample bags are then removed; in the laboratory, bags A and B are composited, mixed, and subsampled for final (T21) soil inorganic N and moisture analysis. Net mineralization is determined as the difference between the final (T21) and initial (T0) soil NH4+ plus NO3- concentrations. Net nitrification and net ammonification can also be determined as the differences between the final (T21) and initial (T0) concentrations of NO3- and NH4+, respectively.

N-mineralization: T0

Procedure

  1. From each of the sieved composite soil samples (see Composite Soil Sampling – MCSE protocol for materials and procedure), take subsamples for soil inorganic N and moisture analysis:
    a. For soil inorganic N, take 3 subsamples of 10 g each (see Soil Inorganic N protocol for materials and procedure).
    b. For moisture analysis, take 1 subsample of 40-50 g (see Gravimetric Soil Moisture protocol for materials and procedure).

N-mineralization: T0 Field Incubations

Materials

    • Polyethylene bags (4 oz. Whirl-pak bags; ID# B00679; 3” W x 7 1/4” L x 0.57 mm thick)
    • Labels (1” x 2 5/8”)
    • Tape (secure overtop labels to protect from moisture)
    • Plastic spoons
    • Trowel
    • Wooden garden stakes (12” long x 1 ¼” wide)
    • Wire flags

Procedure

  1. Apply labels (labelled as T21A and T21B, along with sample site information) to polyethylene bags. Cover label with tape to protect label from moisture.
  2. Place a 100 g subsample of the fresh, sieved, composite soil sample into each of the 2 polyethylene bags per sample location (T21A and T21B). Tightly close the bags to seal.
  3. Bury each pair of bags in their respective treatment plot at about 5 m North and 5 m East of the southwest plot corner.
  4. Bury bags at approximately a 7 cm depth. Use a trowel to lift the soil. Place the soil bag on top of a garden stake and slide both the bag and stake into the ground under the lifted soil. Remove the trowel and gently pack down the soil.
  5. The garden stake should stick out of the ground slightly. Place a flag between the buried bags to help locate bags at the end of the incubation period.

N-mineralization: T21 Retrieval and Soil Sampling Processing

Materials

    • Trowel
    • Plastic bags for mixing/storing composite samples

Procedure

  1. Carefully remove buried sampling bags. Make sure the plot label on the bag is legible. If the plot label is not legible, re-write it on the bag.
  2. Note any bags that are unburied or damaged and discard.
  3. Return bags to the lab. Composite sample bags T21A and T21B from each plot by combining the two into a single bag and mixing thoroughly.
  4. From each composited plot soil sample (bags T21A + T21B), take subsamples for soil inorganic N and moisture analysis as follows:
    a. For soil inorganic N analysis, take 3 subsamples of 10 g each (see Soil Inorganic N protocol for materials and procedure).
    b. For moisture analysis, take 1 subsample of 40-50 g (see Gravimetric Soil Moisture protocol for materials and procedure).
  5. If sample processing for soil inorganic N and moisture cannot not be started until the next day, then refrigerate soil composites in the cooler overnight.

Calculations

Values are listed in datatables as the average of triplicate subsamples and are given in units of micrograms N/gram dry soil/day:

Net Nitrification = ([nitrate on T21] – [nitrate on T0]) / 21 days

Net Ammonification = ([ammonium on T21] – [ammonium on T0] / 21 days

Net Mineralization = ([nitrate + ammonium on T21] – [nitrate + ammonium on T0]) / 21 days

Authors:

Datatables