Nitrogen Mineralization


In use from 2013-05-01


In 2013, samples were taken three times a year for N mineralization potential. Sampling occurs prior to planting and fertilizer application (approximately late April or early May), during the middle of the growing season (approximately July), and just before harvest (approximately October). Samples are taken in both the main plot and microplots. In the main plot, collect 2 soil cores from each of 3 stations and composite the samples, resulting in one sample per plot. In the microplots, randomly choose three sites, collect 2 cores from each, and composite, as in the main plot. Each core should be 10 cm long, with a 2-cm diameter corer. In May 2013, the cores were 25 cm deep.



Soil core (2 cm diameter)
Paper bags
Labels or Sharpies
1 M KCl
Specimen cups with threaded tops


1. Collect soil from plots and place in bags.
2. Sieve the fresh soil and weigh 40 g into a tin for gravimetric moisture content as described here. Refrigerate the remaining soil.
3. Determine gravimetric soil moisture and use to calculate soil water-filled pore space (WFP) of each sample.

WFP = [Pw x (Db/St)] x 100
Pw = water content [(g water/g dry soil) x100]
Db = bulk density; use 1.46 g/cm3 at KBS and 1.33 g/cm3 at Arlington (mean bulk density in surface soils)
St = total porosity; use 45
at KBS and 50% at Arlington (St = [1-(Db/particle density)]*100, assume particle density of 2.65 g/cm3)

4. Determine amount of water needed to raise the WFP to 60% in each sample.

Target g of water at KBS = 0.1875 x dry wt of remaining soil
Target g of water at ARL = 0.222 x dry wt of remaining soil

dry wt of remaining soil = fresh wt of remaining soil – fresh wt of remaining soil x (g water in subsample/fresh wt of subsample)

mL of water to add = (Target g of water) – (g of water in remaining soil)

g of water in remaining soil = fresh wt – dry wt

5. Add the amount of deionized water needed to each sample. Mix the sample to evenly distribute the added water.
6. Weigh 10 g of soil into each of 6 specimen cups or flasks.
7. Set aside three of the cups for nutrient extraction with 1M KCl, as described here.
8. Cap the remaining flasks loosely, to allow oxygen and carbon dioxide exchange, but not entry of water vapor.
9. Place in a dark place with constant temperature.
10. Re-weigh a sub-set of containers each week. If weight decreases by >10%, then add water accordingly (e.g., if decreases to 9 g, then add 1 mL water).
11. After 28 days, extract the nutrients in all sample cups, as in step 7.


Convert NO3- and NH4+ values to an areal basis and gravimetric basis, as described here.

Net mineralization rate

Nmineralized = [(NitrateT28 + AmmoniumT28)-(NitrateT0 + AmmoniumT0)]/28 days

where NitrateT28 = nitrate concentration at the end of the 28-day incubation
AmmoniumT28 = ammonium concentration at the end of the 28-day incubation
NitrateT0 = nitrate concentration at the beginning of the incubation
AmmoniumT0 = ammonium concentration at the beginning of the incubation

Net nitrification rate

Nnitrified = (NitrateT28-NitrateT0)/28 days