Garden with Insight v1.0 Help: Nitrogen  Mineralization
There are three types of organic N kept track of here:
fresh organic N (crop residue and microbial biomass)
active organic N in the humus
stable organic N in the humus
They add up to the total organic N.
The N mineralization model is a modification of the PAPRAN mineralization model (Seligman and van
Keulen, 1981). The model considers two sources of mineralization: fresh organic N pool, associated with
crop residue and microbial biomass, and the stable organic N pool, associated with the soil humus.
Mineralization from the fresh organic N pool is estimated with the equation [Equation 173] where RMN
is the N mineralization rate in kg/ha*day for fresh organic N in layer l, DCR is the decay rate constant for
the fresh organic N, and FON is the amount of fresh organic N present in kg/ha.
Equation 173
RMN = DCR * FON
Code:
same
Variables:
RMN = FreshNMineralizationForLayer_kgPha
DCR = decayRateConst
FON = organicNFresh_kgPha
The decay rate constant is a function of C:N ratio, C:P ratio, composition of crop residue, temperature and
soil water: [Equation 174] where CNP is a C:N and C:P ratio factor and FC is the soil water content in
mm at
field capacity.
Equation 174
DCR = 0.05 * CNP * sqrt(SW / FC * TF(n))
Code:
Variables:
DCR = DecayRateConstForLayer
CNP = cNPCompositeRatio
SW/FC = soilWaterOverFieldCapacity
TF(n) = nutrientCyclingTempFactor
The value of CNP is calculated with the equation [Equation 175] where CNR is the C:N ratio and CNP is
the C:P ratio in layer l.
Equation 175
CNP = min(exp(0.693 * (CNR  25) / 25), exp(0.693 * (CPR  200) / 200), 1.0)
Code:
if CNR > 25 resultWithN = exp(0.693 * (CNR  25) / 25)
else resultWithN = 1
if CPR > 200 resultWithP = exp(0.693 * (CPR  200) / 200)
else resultWithP = 1
CNP = min(resultWithN, resultWithP)
same, because in either case if the else conditions is met it will produce
exp(ve number) which will be > 1.
Variables:
CNP = CNPCompositeRatioForLayer
CNR = cNRatioCropResidue
CPR = cPRatioCropResidue
The C:N and C:P ratios of crop residue are computed for each soil layer with the equations [Equation 176]
and [Equation 177] where FON is the amount of fresh organic N in kg/ha, FOP is the amount of fresh
organic P in kg/ha, and AP is the amount of labile P in kg/ha for layer l.
Equation 176
CNR = 0.58 * FR / (FON + WNO3)
(the 0.58 is the inverse of 1.72, which is the ratio of organic carbon to soil humus)
Code:
same
Variables:
CNR = CNRatioCropResidueForLayer
FR = flatCropResidue_tPha
FON = organicNFresh_kgPha
WNO3 = nitrate_kgPha
Equation 177
CPR = 0.58 * FR / (FOP + AP)
Code:
same
Variables:
CPR = CPRatioCropResidueForLayer
FR = flatCropResidue_tPha
FOP = organicPFresh_kgPha
AP = labileP_kgPha
In the code, an adjustment is made to organic matter when fresh and/or humus mineralization take place:
humus changes in the same proportion that organicNActiveHumus_kgPha is changed by these processes.
(no equation in publication)
ON(a) = HMN
ON(a) += 0.2 * RMN
So OM = OM + OM * (HMN) / ON + OM * (0.2 * RMN) / ON
Organic N associated with humus is divided into two pools  active and stable  by using the equation
[Equation 178] where ON(a) is the active or readily mineralizable pool in kg/ha, RTN is the active pool
fraction, ON is the total organic N in kg/ha, and the subscript l is the soil layer number.
Equation 178
ON(a) = RTN * ON
Code:
same
Variables:
ON(a) = OrganicNActiveHumusForLayer_gPt
RTN = organicNActiveHumusFraction_frn
ON = organicNTotalHumus_gPt
The active pool fraction in the plow layer depends on the number of years the soil has been cultivated and
is estimated with the equation [Equation 179] where YC is the period of cultivation before the simulation
starts in years. The concepts expressed in equation 179 are based on work of Hobbs and Thompson
(1971). Below the plow layer the active pool fraction is set at 40% of the plow layer value, based on work
of Cassman and Munns (1980).
Equation 179
RTN = 0.4 * exp(0.0277 * YC) + 0.1
layers below plow layer are 40% of upper layers
Code:
same equation, layers below layer 2 are 40% of upper layers
Variables:
RTN = OrganicNActiveHumusFractionForLayer_frn
YC = cultivationBeforeSimulationStarts_yr
Organic N flux between the active and stable pools is governed by the equilibrium equation [Equation
180] where RON is the flow rate in kg/ha*day between the active and stable organic N pools, BKN is the
rate constant (~~1 x 10(5) /day), ON(s) is the stable organic N pool, and subscript l is the soil layer
number. The daily flow of humus related organic N (RON) is added to the stable pool and subtracted from
the active pool.
Equation 180
RON = BKN * (ON(a) * (1.0 / RTN)  ON(s))
Code:
RON = BKN * (ON(a) * (1.0 / RTN  1.0)  ON(s))
Variables:
RON = OrganicNFromActiveToStableInHumusForLayer_kgPha
BKN = kRateConstantForOrganicNFlowInHumus_Pday = 0.00001
ON(a) = organicNActiveHumus_kgPha
RTN = organicNActiveHumusFraction_frn
ON(s) = organicNStableHumus_kgPha
Only the active pool of organic N is subjected to mineralization. The humus mineralization equation is
[Equation 181] where HMN is the mineralization rate in kg/ha*day for the active organic N pool in layer
l, CMN is the humus rate constant (~~0.0003 /day), BD is the settled bulk density of the soil in t/m3, and
BDP is the current bulk density as affected by tillage in t/m3.
Equation 181
HMN = CMN * ON(a) * sqrt(SWF * TF(n)) * sqr(BD) / sqr(BDP)
Code:
same
organicNActiveHumus_kgPha is used before some of it moved out into
organicNStableHumus_kgPha; the reason this is done is because the movement is so slow
that mineralization can occur on all the organic n active humus, not just the part
left behind. adding it back on in a local variable makes it so that calculation doesn't
have to be upset by waiting to subtract the active n moving until now.
Variables:
HMN = ActiveHumusNMineralizationForLayer_kgPha
CMN = kHumusMineralizationConst_Pday = 0.0003
ON(a) = organicNActiveHumus_kgPha
SWF = soilWaterOverFieldCapacity
TF(n) = nutrientCyclingTempFactor
BD = plowDepthSettledBulkDensityAtInput_tPm3
BDP = bulkDensity_tPm3
To maintain the N balance at the end of the day, the humus mineralization is subtracted from the active
organic N pool. The residue mineralization (fresh) is subtracted from the FON pool, 20% of RMN is
added to the active ON pool, and 80% of RMN is added to WNO3 pool.
The crop residue is reduced by using the equation [Equation 184] where FR(o) and FR are the amounts of
residue in soil layer l at the start and end of a day in kg/ha.
Equation 184
FR = FR(o)  DCR' * FR(o)
Code:
same
Variables:
FR = FlatCropResidueAdjForMinAndImmob_tPha
FR(o) = flatCropResidue_tPha
DCR' = adjustedNDecayRateConst
Finally, the immobilized N is added to the FON pool and subtracted from the WNO3 pool.
