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Crop-specific parameters for the Nitrogen Nutrition Index pipeline

Source: R/crop_params_NNI.R
crop_params_NNI.Rd

This function returns the default parameters used by the remote-sensing NNI pipeline (compute_NNI_from_S2) for a given crop. Values are taken from the peer-reviewed literature and represent first-guess defaults — they almost always require local recalibration before operational use (see the Calibration caveats section below).

Usage

crop_params_NNI(crop)

Arguments

crop

Character. Canonical crop name (case-insensitive). Any of "wheat", "maize", "rice", "barley", "rapeseed", "sorghum", "sunflower", "soybean". A helpful error is raised otherwise.

Value

A named list of seven numeric parameters plus the resolved canonical crop name.

Parameters

a, b

Coefficients of the critical-N dilution curve \(N_c(\%) = a \cdot W^{-b}\), with W in t DM ha\(^{-1}\). Lemaire & Gastal (1997); Justes et al. (1994) for wheat; Plenet & Lemaire (2000) for maize; Sheehy et al. (1998) for rice; Colnenne et al. (1998) for rapeseed.

w_min

Lower bound of validity of the dilution curve (t DM ha\(^{-1}\)). NNI is masked below this biomass.

alpha_leaf

Leaf-to-total-aboveground biomass allocation coefficient during the vegetative stages the NNI is meant for. W_total = W_leaf / alpha_leaf, so that typical partitioning of 0.30 (maize) / 0.35 (wheat) / 0.40 (rice) rescales the GPR-derived leaf biomass to aboveground biomass.

k_NChl

Canopy-level N : chlorophyll ratio used when converting CNC_Cab (g Chl m\(^{-2}\) ground) to canopy N (g N m\(^{-2}\)). Values in the 50-80 range; defaults are those reported by Houles et al. (2007) and Baret et al. (2007) as first-guess for Sentinel-2. Not used when the pipeline is fed with CNC_Cprot (protein-retrieval path, which already returns canopy N directly).

fvc_min

Minimum fractional vegetation cover required for the spectral signal to be dominated by canopy and not soil. Pixels below this threshold are masked out of the NNI retrieval.

Calibration caveats

  • The dilution-curve coefficients a and b are well established for the listed crops across European cropping systems; they rarely need recalibration.

  • alpha_leaf and k_NChl are the two locally sensitive parameters. Under-calibrated values commonly introduce a 15-30pct bias in the final NNI map. A minimal calibration dataset consists of 30-50 plot-level samples across the growing season, analysed for dry biomass (oven at 65 degC) and total plant N (Kjeldahl or Dumas), paired with the matching Sentinel-2 acquisition date.

  • NNI is only meaningful inside crop-specific phenological windows (e.g. GS31-GS55 for wheat, V6-V12 for maize). Outside these windows the critical-N curve is not valid.

  • When the scene is dominated by partially covered pixels (FVC < 0.5), the GPR retrievals degrade because the soil signal enters the BRDF. Use fvc_min to prune them.

References

Lemaire G, Gastal F. N uptake and distribution in plant canopies. In: Diagnosis of the Nitrogen Status in Crops, Springer, 1997. p. 3-43.

Justes E, Mary B, Meynard J-M, Machet J-M, Thelier-Huche L. Determination of a critical nitrogen dilution curve for winter wheat crops. Ann Bot 1994;74:397-407.

Plenet D, Lemaire G. Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Plant Soil 2000;216:65-82.

Sheehy JE, Dionora MJA, Mitchell PL, Peng S, Cassman KG, Lemaire G, Williams RL. Critical nitrogen concentrations: implications for high- yielding rice genotypes in the tropics. Field Crops Res 1998;59:31-41.

Colnenne C, Meynard J-M, Reau R, Justes E, Merrien A. Determination of a critical nitrogen dilution curve for winter oilseed rape. Ann Bot 1998;81:311-317.

Houles V, Guerif M, Mary B. Elaboration of a nitrogen nutrition indicator for winter wheat based on leaf area index and chlorophyll content. Eur J Agron 2007;27:1-11.

Baret F, Houles V, Guerif M. Quantification of plant stress using remote sensing observations and crop models. J Exp Bot 2007;58:869-880.