Modelling the Nitrogen Balance in CropSyst

Apresentação em tema: "Modelling the Nitrogen Balance in CropSyst"— Transcrição da apresentação:

1 Modelling the Nitrogen Balance in CropSyst
Marcello Donatelli CRA-ISCI, Italy Claudio Stockle BSE, Washington State University, USA

2 Modelling the Nitrogen Balance
Nitrogen is an important nutrient whose availability is primordial for crops to reach their potential growth To model N acquisition by crops, many components of the soil root zone N balance must be considered

3 organic N inorganic X.NH4+ NH4+ NO3- N inorg. fertilizer crop residues
mineralization immobilization denitrification crop uptake (- fixation) nitrification leaching l volatilization

4 Mineral N Balance Components
Input: Output: Fertilization Mineralization N fixation Crop Residues Irrigation water Atmospheric N Volatilization Denitrification Immobilization Plant uptake Leaching

5 Fertilization Mineral or organic N Many sources available
Quantity, main N species involved (nitrate or ammonium), form (organic or inorganic; solid or liquid), and application method must be specified for simulation purposes

6 N Symbiotic Fixation Symbiotic N2 fixation converts atmospheric N2 gas into plant N The quantification of the process is not well developed The fraction of crop N demand supplied by N fixation increases with crop development, reaching a maximum at flowering, and decreasing again thereafter

7 N Symbiotic Fixation (cont.)
N fixation is reduced as the soil dries, becoming negligible when the soil water content is below the middle range of the water holding capacity The rate of bacterial fixation is reduced when N is already present (about 50 kg ha-1), becoming negligible at high soil N content (300 kg ha-1)

8 N in the Irrigation Water
This quantity can vary greatly from site to site (10 to 150 kg ha-1 year-1) It is determined as the product of the irrigation volume and the N concentration of the water Atmospheric N This quantity can vary greatly from site to site (10 to 150 kg ha-1 year-1)

9 N transformations in the Soil
Net mineralization (mineralization - immobilization) Nitrification (no direct effect on balance) Denitrification Microbiologically-mediated processes Usually modelled assuming that they obey irreversible first-order kinetics

10 Nitrogen Transformations
ND t = N0 [1 - e (-K D t)] NDt = Amount transformed during time interval Dt (kg m-2 t-1) N0 = Amount available for transformation (kg m-2 t-1) K = Rate constant (t-1)

11 Nitrogen Transformations
Note that K = f ( Temp, SWC)

12 Nitrogen Volatilization
Important when N is applied as ammonium and is not incorporated to the soil Can be simulated mechanistically based on gas concentration gradients and resistances A simpler approach subtracts a fixed fraction which depends on application conditions

13 Nitrogen Leaching Related to the movement of water in the soil and the mobility of the N species of interest (nitrate or ammonium) N transport can be simulated for both the cascading and the finite difference approaches N transport in the soil is also important to determine where the N is available in the soil profile

14 Nitrogen Leaching N transport in the soil may be obtained by multiplying water fluxes between layers by the N concentration of the water in the layer originating the flow The effect of diffusion and hydrodynamic dispersion may also be added While nitrates are not retained by the soil, the movement of ammonium is restricted due to its absorption by the solid matrix The amount retained and the amount in concentration are related by a Langmuir relation

15 Ammonium Sorption K Q [NH4 ] X.NH4 = 1 + K [NH4 ]
Soil.NH4 = [X.NH4 + w NH4] r b X.NH4 = Ammonium in exchage sites (kg kg-1) Soil.NH4 = Total amount in the soil (kg m-3) w = Gravimetric soil water content (kg kg-1) r b = Bulk density (kg m-3) K, Q = Constants (kg kg-1)

16 Crop Nitrogen Uptake N uptake is the component of the balance that relates directly with the simulation of crop growth Usually is calculated once the concentration of nitrate and ammonium in the soil solution is known for each soil layer

17 Crop Nitrogen Uptake bulk soil roots rizosphere uptake mass flow
diffusion

18 Potential N Crop Uptake
Nup = Numax [Nr ] K + [Nr ] Nup = Potential N uptake per unit root length Numax = Maximum N uptake per unit root length Nr = N concentration in the rizosphere K = Half-rate constant

19 Potential N Crop Uptake
Nup = Numax Navail PAW 2 Nup = Potential N uptake per unit root length Numax = Maximum N uptake per unit root length Navail = Availability factor [ e fn(N in bulk soil)] PAW = Plant available water

20 Growth Limited by Nitrogen
B = growth limited by radiation and water Npcrit = critical plant N concentration Np = plant N concentration Npmin = minimum plant N concentration

21 Crop N uptake: Demand Maximum plant N concentration
Critical plant N concentration Minimum plant N concentration

22 Crop N uptake: Demand Nd = (Npmax- Np) Bc + Npmax Bt
Nd = Plant N demand Npmax = Maximum plant N concentration Np = Current plant N concentration Bc = Current cumulative biomass Bt = Potential biomass to be produced today

23 Actual Crop N Uptake Nact = MIN ( Nup , ND )
Note that Nup for the entire soil profile is determined by the sum of the product of the potential N uptake and the root length for each layer

24 Modelling the Nitrogen Balance
Other N input/output components exist. For example, some of the plant N (2 to 8%) is lost to the atmosphere (as ammonia and volatile amines) during plant senescence. The N balance has more uncertainties and it is more complex to study and model than the water balance