### 2.26.2.5. Other Features[¶](#other-features "Permalink to this headline")

#### 2.26.2.5.1. Physical Crop Characteristics[¶](#physical-crop-characteristics "Permalink to this headline")

Leaf area index (_L_) is calculated as a function of specific leaf area (SLA, [Table 2.26.2](#table-crop-phenology-parameters)) and leaf C. Stem area index (_S_) is equal to 0.1_L_ for temperate and tropical corn, sugarcane, switchgrass, and miscanthus and 0.2_L_ for other crops, as in AgroIBIS. All live C and N pools go to 0 after crop harvest, but the _S_ is kept at 0.25 to simulate a post-harvest “stubble” on the ground.

Crop heights at the top and bottom of the canopy, \\({z}\_{top}\\) and \\({z}\_{bot}\\) (m), come from the AgroIBIS formulation:

(2.26.16)[¶](#equation-25-16 "Permalink to this equation")\\\[\\begin{split}\\begin{array}{l} {z\_{top} =z\_{top}^{\\max } \\left(\\frac{L}{L\_{\\max } -1} \\right)^{2} \\ge 0.05{\\rm \\; where\\; }\\frac{L}{L\_{\\max } -1} \\le 1} \\\\ {z\_{bot} =0.02{\\rm m}} \\end{array}\\end{split}\\\]

where \\(z\_{top}^{\\max }\\) is the maximum top-of-canopy height of the crop ([Table 2.26.2](#table-crop-phenology-parameters)) and \\(L\_{\\max }\\) is the maximum leaf area index ([Table 2.26.3](#table-crop-allocation-parameters)).

#### 2.26.2.5.2. Interactive Fertilization[¶](#interactive-fertilization "Permalink to this headline")

CLM simulates fertilization by adding nitrogen directly to the soil mineral nitrogen pool to meet crop nitrogen demands using both industrial fertilizer and manure application. CLM’s separate crop land unit ensures that natural vegetation will not access the fertilizer applied to crops. Fertilizer in CLM5BGCCROP is prescribed by crop functional types and varies spatially for each year based on the LUMIP land use and land cover change time series (LUH2 for historical and SSPs for future) ([Lawrence et al. 2016](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#lawrenceetal2016)). One of two fields is used to prescribe industrial fertilizer based on the type of simulation. For non-transient simulations, annual fertilizer application in g N/m2/yr is specified on the land surface data set by the field CONST\_FERTNITRO\_CFT. In transient simulations, annual fertilizer application is specified on the land use time series file by the field FERTNITRO\_CFT, which is also in g N/m2/yr. The values for both of these fields come from the LUMIP time series for each year. In addition to the industrial fertilizer, background manure fertilizer is specified on the parameter file by the field `manunitro`. For perennial bioenergy crops, little fertilizer (56kg/ha/yr) is applied to switchgrass and no fertilizer is applied to Miscanthus. Note these rates are only based on local land management practices at the University of Illinois Energy Farm located in Central Midwestern United States [(Cheng et al., 2019)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#chengetal2019) rather than the LUMIP timeseries. For the current CLM5BGCCROP, manure N is applied at a rate of 0.002 kg N/m2/yr. Because previous versions of CLM (e.g., CLM4) had rapid denitrification rates, fertilizer is applied slowly to minimize N loss (primarily through denitrification) and maximize plant uptake. The current implementation of CLM5 inherits this legacy, although denitrification rates are slower in the current version of the model ([Koven et al. 2013](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#kovenetal2013)). As such, fertilizer application begins during the leaf emergence phase of crop development (phase 2) and continues for 20 days, which helps reduce large losses of nitrogen from leaching and denitrification during the early stage of crop development. The 20-day period is chosen as an optimization to limit fertilizer application to the emergence stage. A fertilizer counter in seconds, _f_, is set as soon as the leaf emergence phase for crops initiates:

(2.26.17)[¶](#equation-25-17 "Permalink to this equation")\\\[ f = n \\times 86400\\\]

where _n_ is set to 20 fertilizer application days and 86400 is the number of seconds per day. When the crop enters phase 2 (leaf emergence) of its growth cycle, fertilizer application begins by initializing fertilizer amount to the total fertilizer at each column within the grid cell divided by the initialized _f_. Fertilizer is applied and _f_ is decremented each time step until a zero balance on the counter is reached.

#### 2.26.2.5.3. Biological nitrogen fixation for soybeans[¶](#biological-nitrogen-fixation-for-soybeans "Permalink to this headline")

Biological N fixation for soybeans is calculated by the fixation and uptake of nitrogen module (Chapter [2.18](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/FUN/CLM50_Tech_Note_FUN.html#rst-fun)) and is the same as N fixation in natural vegetation. Unlike natural vegetation, where a fraction of each PFT are N fixers, all soybeans are treated as N fixers.

#### 2.26.2.5.4. Latitudinal variation in base growth tempereature[¶](#latitudinal-variation-in-base-growth-tempereature "Permalink to this headline")

For most crops, \\(GDD\_{T\_{{\\rm 2m}} }\\) (growing degree days since planting) is the same in all locations. However, for both rainfed and irrigated spring wheat and sugarcane, the calculation of \\(GDD\_{T\_{{\\rm 2m}} }\\) allows for latitudinal variation:

(2.26.18)[¶](#equation-25-18 "Permalink to this equation")\\\[\\begin{split}latitudinal\\ variation\\ in\\ base\\ T = \\left\\{ \\begin{array}{lr} baset +12 - 0.4 \\times latitude &\\qquad 0 \\le latitude \\le 30 \\\\ baset +12 + 0.4 \\times latitude &\\qquad -30 \\le latitude \\le 0 \\end{array} \\right\\}\\end{split}\\\]

where \\(baset\\) is the _base temperature for GDD_ (7th row) in [Table 2.26.2](#table-crop-phenology-parameters). Such latitudinal variation in base temperature could slow \\(GDD\_{T\_{{\\rm 2m}} }\\) accumulation extend the growing season for regions within 30°S to 30°N for spring wheat and sugarcane.

#### 2.26.2.5.5. Separate reproductive pool[¶](#separate-reproductive-pool "Permalink to this headline")

One notable difference between natural vegetation and crops is the presence of reproductive carbon and nitrogen pools. Accounting for the reproductive pools helps determine whether crops are performing reasonably through yield calculations. The reproductive pool is maintained similarly to the leaf, stem, and fine root pools, but allocation of carbon and nitrogen does not begin until the grain fill stage of crop development. Equation [(2.26.5)](#equation-25-5) describes the carbon and nitrogen allocation coefficients to the reproductive pool. In CLM5BGCCROP, as allocation declines in stem, leaf, and root pools (see section [2.26.2.4.2](#grain-fill-to-harvest)) during the grain fill stage of growth, increasing amounts of carbon and nitrogen are available for grain development.

#### 2.26.2.5.6. Tillage[¶](#tillage "Permalink to this headline")

Tillage is represented as an enhancement of the decomposition rate coefficient; see section [2.21.4](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Decomposition/CLM50_Tech_Note_Decomposition.html#decomp-mgmt-modifiers).

