N Mineralization Potential – Short-term Lab Incubations
In use from 2013-05-01
The nitrogen (N) mineralization potential of soils is an estimate of the net production of inorganic N under given conditions and provides a means to assess soil N availability. To measure in vitro N mineralization potentials subsamples of a sieved, composite soil sample are brought to 60% water-filled pore space, placed in loosely capped specimen cups or flasks, incubated at constant temperature in the dark for 28 days, analyzed for soil inorganic N, and compared to subsamples prepared on day 0. The N mineralization potential is the difference between the final and the initial total inorganic N (nitrate + ammonium) concentration divided by days of incubation and is expressed on a gravimetric basis (microgram N/g dry soil/day), which can be converted to an areal basis (microgram N/m2/day) using soil bulk density. The net nitrification potential can also be determined as the difference between final and initial nitrate concentrations divided by days of incubation and expressed as above.
Frequency: Depends on experiment and research objective. For example, N mineralization potential was measured in all treatments of the GLBRC Biofuel Cropping System Experiment (BCSE) three times over the 2013 growing season.
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.
- Soil core (2 cm diameter)
- Plastic bags
- Labels or sharpies
- 1 M KCl
- Specimen cups with threaded tops
- Collect soil from plots, as directed above, and place in plastic bags.
- 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.
- 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
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)
- 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
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)
g of water in remaining soil = fresh wt – dry wt
- Add the amount of deionized water needed to each refrigerated composite sample. Mix the sample to evenly distribute the added water.
- Weigh 10 g of soil into each of 6 specimen cups or flasks for each composite sample.
- Set aside three of the cups for nutrient extraction with 1M KCl, as described here.
- Cap the remaining flasks loosely, to allow oxygen and carbon dioxide exchange.
- Place containers in a dark place with constant temperature.
- 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.
- After 28 days, extract the nutrients in all sample cups, as in step 7.
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:
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:
Date modified: Tuesday, Oct 24 2023