Soil Inorganic N

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Abstract

Ammonium (NH4 ) and nitrate (NO3-) are the predominate forms of inorganic N in soils; their availability affects rates of N transformation, plant uptake, and N leaching. To measure the nutrient pool of NH4 and NO3- in soils, a subsample of a fresh, sieved composite sample is extracted, filtered, and the extract then analyzed using colorimetric techniques. Concentrations of NH4+ and NO3- are typically expressed as element mass of N per gram of dry soil, but conversion to an areal basis is possible if the soil bulk density and sample depth are known.


Sampling frequency: Depends on experiment, time of year, and research objective. Soil inorganic N is routinely measured as part of our LTER and GLBRC soil sampling programs.

Protocol

Triplicate subsamples (10 g fresh weight) from the fresh, sieved composite soil sample of each treatment plot are extracted with 100 mL of 1 M KCl. Extracting samples are shaken for 1 minute, allowed to sit overnight, and reshaken before filtering with a syringe equipped with a 1micron pore size, glass fiber filter. Filtrates are stored in plastic vials and frozen until analyzed for NH4 + -N and NO3 - -N. Both analyses are performed on a Flow Injector Analyzer (NH4 + -N by a sodium salicylate-based method and colorimetry and NO3 - -N by a sulfanilamide/NED cadmium reduction method and colorimetry). Note that this nitrate method is based on cadmium’s reducing nitrate to nitrite, which is then determined colorimetrically. Thus if samples contain nitrite in significant quantities (rare in our soils) nitrite must be analyzed separately (by bypassing the cadmium column) and subtracted out.

Materials

    • Labels for extraction cups and storage vials
    • 120 mL polyethylene extraction cups with lids (e.g. urinalysis cups)
    • 1 M Potassium chloride (KCl) solution (extractant), usually made in 20L carboys
    • Dispensette bottletop dispenser (100 mL capacity)
    • 7 mL polyethylene scintillation vials with screw-caps
    • 10 or 20 cc Luerlok polythylene syringes
    • Syringe filter holders, 25 mm diameter
    • Type A/E, 1um pore size, glass-fiber filters, 25 mm diameter
    • Plastic spoons
    • Balance
    • Cardboard sample storage tray (to store scintillation vials)

Procedure

  1. Subsample and weigh 10 g (±0.02 g) of the fresh, sieved composite soil sample into a tared, labeled, 120 mL extraction cup. Prepare triplicate samples from each composite sample. Cap the cups until all subsamples are ready for step 3.
  2. Weigh subsamples of each composite sample for gravimetric soil moisture content as described here.
  3. Using the Dispensette dispenser set at 100 mL, add 100 mL 1M KCl to each extraction cup with soil, and to each of three empty cups to serve as blanks. Recap the cups and shake for 1 minute.
  4. Allow the cups to sit for about 24 hours.
  5. Re-shake all cups for another minute.
  6. Allow the soil to settle for about 1 hour.
  7. Using a clean syringe, remove about 10 mL of the KCl solution from the extraction cup. Dispose of this solution; this step rinses the syringe.
  8. Remove another 10 mL of the KCl solution from the extraction cup. Attach a clean syringe filter holder loaded with a new filter to the end of the syringe.
  9. Filter a small amount of the solution into a labeled, scintillation vial. Cap the vial, shake to rinse, then discard the liquid. Filter the remaining solution in the syringe into the vial. Leave enough head space to allow for expansion when freezing.
  10. Use a clean syringe and new filter/clean holder for each extraction cup.
  11. Refrigerate or freeze samples until analysis.
  12. Analyze as per instrumentation protocol (see above). Values given in the datatables are the average of triplicate analyses.

Calculations

The Flow Injector Analyzer measures both ammonium and nitrate (+ nitrite) on the same sample and reports their concentrations in the extractant in mg N/L, which is converted to mass of N per mass of dry soil or per square meter using the following equations:

1. Determine soil N concentration for both ammonium and nitrate (Cs, in microgram N/g soil) as:
Cs = (Ce x V) / W
where:
Ce = concentration of N as ammonium or as nitrate (+ nitrite) in the extractant, in mg N/L
V = volume of extractant (e.g., 100 mL) + volume of water in soil sample, in mL
W = dry mass of soil (fresh weight of soil minus soil moisture) in g

2. Determine concentration of inorganic N in soil (Cn, in microgram N/g soil) as the sum of soil ammonium and nitrate concentrations:
Cn = Cs[ammonia] + Cs[nitrate]

3. Determine the areal soil N content (Ca, in g N/m2) as:
Ca = Cn x BD x SD
where:
Cn = as above, in microgram N/g soil
BD = bulk density (gravel-free), in g/cm3
SD = sample depth, in m
note: measuring sample depth in m simplifies the conversion of microgram N/g soil to g N/m2 according to the equalities 1 microgram N = 10-6 gram N and 1 cm3 = 10-6 m3

Date modified: Friday, Mar 01 2019

Authors:

Datatables