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.