To examine changes in the N mineralization potential over the growing season, samples are collected three times a year: 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). Typically two soil cores are collected from each sampling station (e.g., 3 stations per BCSE treatment) and are composited into a single sample; for smaller plots two cores are collected at each of three randomly selected sites and composited as above. Ideally soil cores should be the same depth and diameter (e.g., 10 cm long and 2 cm diameter) throughout the study.

Materials

• Soil core (2 cm diameter)
• Plastic bags
• Labels or sharpies
• 1 M KCl
• Specimen cups with threaded tops

Procedure

1. Collect soil from plots, as directed above, and place in plastic bags.
2. Sieve the fresh soil through a 4-mm mesh screen and weigh 40 g into a tin to determine the gravimetric moisture content as described here. Refrigerate the remaining soil until ready to prepare incubations.
3. Determine gravimetric soil moisture and use value to calculate soil water-filled pore space (WFP) of each sample according to:
   
   %WFP = [Pw x (Db/St)] x 100

   where:
   Pw = gravimetric moisture content expressed as a percentage; [(g water/g dry soil) x100]
   Db = bulk density; use 1.46 g/cm3 for KBS soils and 1.33 g/cm3 for Arlington soils (mean bulk density in surface soils)
   St = total porosity; use 45% for KBS soils and 50% for Arlington soils (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 for KBS soils = 0.1875 x dry wt of remaining soil
   Target g of water for Arlington soils = 0.222 x dry wt of remaining soil
   where:
   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 to achieve 60% WFP= (Target g of water) – (g of water in remaining soil)
   where:
   g of water in remaining soil = fresh wt – dry wt
5. Add the amount of deionized water needed to each refrigerated composite sample. Mix the sample to evenly distribute the added water.
6. Weigh 10 g of soil into each of 6 specimen cups or flasks for each composite sample.
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.
9. Place containers 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. For example, if soil fresh weight decreases to 9 g, then add 1 mL water.
11. After 28 days, extract the nutrients in all sample cups, as in step 7.

Calculations

Convert NO3- and NH4+ values to a gravimeteric (microgram N per gram dry soil) or an areal basis (gram N per square meter), as described here.

Net mineralization rate:

where:
Nitrate_T28 = nitrate concentration at the end of the 28-day incubation
Ammonium_T28 = ammonium concentration at the end of the 28-day incubation
Nitrate_T0 = nitrate concentration at the beginning of the incubation
Ammonium_T0 = ammonium concentration at the beginning of the incubation

Net nitrification rate: