KBS021: Soil Inorganic N

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Abstract

Sampling Frequency: LTER – Monthly (1989 to 1995); bimonthly (1996 to present); GLBRC – every two weeks (2010 to 2012); 3 times per year (2013 to present)

Location: LTER – all plots in MCSE; GLBRC – all plots in intensive sites, KBS and Arlington.

Triplicate subsamples (10 g fresh weight) from the composite, sieved soil sample of each plot are extracted with 100 mL of 1 M KCl. Soil extracts are shaken for 1 minute and allowed to sit overnight and reshaken before filtering. Soil extracts are filtered with a syringe filter using a Type A/E, 1um pore size, glass fiber filter. Filtrates are stored in 7 mL scintillation vials and frozen until analysis for NH 4 + -N and NO 3 - -N are completed. Both analyses are performed on a Flow Injector Analyzer (NH 4 + -N via diffusion colorimetry technique and NO 3 - -N via cadmium reduction and colorimetry).


Negative numbers that are close to zero are set to zero.

Protocol

Materials

  • labels
  • 120 mL polyethylene extraction cups with lids (e.g. urinalysis cups)
  • 1 M Potassium chloride solution
  • 7 mL polyethylene scintillation vials with screw-caps
  • 10 or 20 cc Luerlok polythylene syringes
  • syringe filter holders, 25 mm diameter
  • Type A/E glass-fiber filter paper, 25 mm diameter
  • plastic spoons
  • balance
  • cardboard sample tray (to store filled vials)

Procedure

  1. Soil samples sieved through a 4 mm sieve should be weighed in triplicate into three extraction cups. Cap the cups until ready for step 2.
  2. Weigh a subsample for moisture content as described here.
  3. Add 100 mL 1M KCl to each extraction cup with soil, as well as three empty cups (blanks). Recap the cups and shake for 1 minute.
  4. Allow the cups to sit for about 24 hours.
  5. Reshake all cups for another minute.
  6. Allow the soil to settle for about 1 hour.
  7. Fill a syringe with the soil and KCl solution from the extraction cup. Dispose of this solution and refill the syringe. This serves as a rinse for the syringe.
  8. Attach a syringe filter holder loaded with filter paper to the end of the syringe.
  9. Filter a small amount of solution into a sample vial. Cap the vial, shake, then discard that rinse liquid. Use the solution remaining in the syringe to filter and fill the vial. Leave enough head space to allow for expansion when freezing.
  10. Label the vials or caps. Refrigerate or freeze samples until analysis.

Calculations

Gravimetric soil N
Cg=(C x V)/W
where:
Cg is expressed as ugN/g soil
C=concentration of element, expressed in mg/L
V=volume of extract + water in soil sample, expressed in mL
W=dry mass of soil, i.e., fresh weight of soil minus water content

Areal soil N
g element/m2 = Cg x BD x SD
where:
Cg is expressed as ugN/g soil
BD = bulk density, expressed as g/cm3
SD = sample depth in m

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