KBS021: MCSE: Short-term N Mineralization Potential

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Abstract

Sampling Frequency: bimonthly (1989 to 1995); 1996 to present: minimum of once during the growing season (April – October)

N mineralization and nitrification are measured by estimated changes in accumulated and in situ nitrogen pool sizes, the difference equaling the assumed net mineralization potential. Two methods of estimating mineralization and nitrification rates are used: a short-term buried bag (in field) incubation that accounts for the daily and seasonal temperature fluctuations and a long-term incubation that assesses soil organic matter quality with respect to mineralizable N.

Two 100 g fresh weight subsamples from the composite soil sample of each plot are placed in separate polyethylene bags and designated T21A and T21B. The composite soil samples (T0) are immediately brought to the laboratory and sub sampled for inorganic N analysis and soil moisture. The T21A and T21B samples are buried in the field in their respective plots. In-field incubations are for 21 days after which time the soil cores are brought to the laboratory, and both A and B samples are composited and subsampled for moisture and inorganic N analysis. Net mineralization is determined by subtracting the original soil NH4+ and NO3- concentration (T0) from the incubated soil NH4+ and NO3- concentration (T21). Net nitrification is determined by subtracting the initial NO3- concentration (T0) from incubated NO3- concentration (T21).

Protocol

N-mineralization T0

Materials

  1. labels
  2. 120 ml screw-cap containers
  3. 1 M KCl solution
  4. 7 ml scintillation vials
  5. 10 cc Luerlok syringes
  6. filter holders
  7. type A/E glass-fiber filters
  8. plastic spoons
  9. cardboard sample tray (to store filled vials)
  10. balance

Procedure

  1. From each of the LTER soil composite samples, 4 subsamples are taken for inorganic NH4+ and NO3- and moisture analysis.
  2. The subsamples are as follows
    1. For inorganic NH4+ and NO3- analysis, 3 analytical replicates of 10 g each (see Soil Inorganic N Protocol).
    2. For moisture analysis, 1 subsample of 40 g (see Gravimetric Soil Moisture Protocol).

N-mineralization T21 In Field Incubations

Materials

  1. polyethylene bags
  2. plastic spoons
  3. trowel
  4. wooden garden stakes
  5. wire flags

Procedure

  1. Two 100 g subsamples of fresh sieved soil are place into polyethylene bags. Tightly close the bags and designate T21A and T21B.
  2. In each plot, respective soil bags are buried at about 7 m North and West of the southwest corner post at approximately a 7 cm depth.
  3. A trowel is used to lift the soil. The soil bag is placed on top of a garden stake and the two are slide into the ground under the lifted soil. The trowel is removed and the soil gently packed down.
  4. The garden stake should stick out of the ground slightly and a flag should be placed between the buried bags. This will help in locating the bags after the incubation period.

N-mineralization T21 Soil Sampling Processing

Materials

  1. labels
  2. 120 ml screw-cap containers
  3. 1 M KCl solution
  4. 7 ml scintillation vials
  5. 10 cc Luerlok syringes
  6. filter holders
  7. type A/E glass-fiber filters
  8. plastic spoons
  9. cardboard sample tray (to store filled vials)
  10. balance

Procedure

  1. When collecting soil bags make sure that the plot number on the bag is legible. If the plot number is not legible, it needs to be written on the bag.
  2. Note any unburied or damaged bags.
  3. The bags are brought to the lab, where bag A and bag B from each of the plots is composited. This is done by combining the two bags into a single bag and mixing thoroughly.
  4. From each composited plot soil sample (bag A + bag B), 4 subsamples are taken for inorganic NH4+ and NO3- and moisture analysis.
  5. The subsamples are as follows.
    1. For inorganic NH4+ and NO3- analysis, 3 analytical replicates of 10 g each (see Soil Inorganic N Protocol).
    2. For moisture analysis, 1 subsample of 40 g (see Gravimetric Soil Moisture Protocol).
  6. If sample processing for soil moisture and inorganic N will not be started until the next day, then soil composites are to be left in the cooler overnight.
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