What are the 3 major reservoirs of Nitrogen?
1. N2 atmosphere is ~ 72% N
2. soil organic matter
3. plants and organisms
What are the processes in the N cycle?
1. symbiotic N fixation (air N2 to plants due to rhizobium - aerobic heterotrophs- )
2. plants die and decompose/mineralize/ammonification due to aerobic heterotrophic bacteria to form NH4+
3. NH4+ is then absorbed by plants or converted to NO3- via nitrification
4. NO3- is then either leached, absorbed by plants, or converted back to N2O then released into atmosphere via denitrification via anaerobic heterotrophic bacteria
What is nitrification?
NH4+ to NO2- due to nitrosomonas and then NO2- to NO3- due to nitrobacter (both aerobic autotrophs)
When is NO3- application not recommended?
do not apply NO3- in the fall
What do nitrification inhibitors do?
slows the reaction of NH4+ to NO2-
What are some ways you can keep N in the soil?
1. N inhibitors
2. do not apply NO3- in the fall
3. inoculating legume seed with N-fixing bacteria (rhizobium)
4. Drain soil to prevent denitrification
5. residue management to control C/N ratio
What is the importance of soil organic matter?
1. improves water holding cap of soils
2. provides CEC
3. improves soil structure
4. contains nutrients (esp. N)
H2O quality improves
What happens as soil organic matter decays?
1. they lose mass, mostly C
2. become more stable
3. become "humus" - important to soils (tilth, H2O Cap, CEC)
How does cultivation affect soils org. matter?
cultivation lowers soil org matter (stablilizes at lower % over the years)
What contributes to the high CEC of humic acid?
the COOH breaks off to COO- giving it a higher CEC = 200 cmolc/kg
What are organic soils?
1. residues of plants and animals
2. >30% org. matter, >20% for sandy soils
3. deeper than 18", 24" for peat
4. peats and mucks (histosols)
What are are the classifications for histosols?
1. fibrists - lots of plant parts visible (peats)
2. hemists - some recognizable fibers
3. sparists - highly decayed, amorphous (shapeless) - muck (usually black)
What are organic soil characteristics?
1. mostly organic, dark in color
2. low weight, low BD, often <0.2 g/cc
3. subject to wind erosion when dry
4. shrink and decay (subsidence)
5. high water holding capacity
6. high CEC
7. nutrient content available (high source of N)
What is subsidence?
loss of org. matter from draining and decay
What is the C/N ratio of stable humus?
plant residues have a much high C/N ratio
then when they decay they lose a lot of C due to CO2 loss
as C/N ratio goes down with decay, more N is available to plants
What is the critical C/N ratio?
if C/N ratio>25 soil N is used by organisms (wood chips, straw)
if C/N ratio<25 then N will be released (plant and animal residues)
What is immobilization and mineralization?
1. immobilization - available soil N decreases after high carbon residues are added (C/N ratio increases), no immobilization if low carbon residues are added
2. CO2 (c) is released during immobilization and mineralization with decay
3. mineralization - release of N (available N increases- decay and C/N ratio decreases)
What is bioremediation?
the use of organisms to clean up (remediate) a contaminated site
1.plants absorb toxins
2.stabilize temp, O2, and moisture , add substrates to enhance microbial degradation of toxins
3. aeration and stirring to speed volitization, encourages aerobic activity, and exposes fresh surfaces to activity