Non-symbiotic BNF (NSBNF) is performed by free-living soil bacteria (e.g. Azotobacter, Clostridium), associative bacteria in the rhizosphere (e.g. Azospirillum), cyanobacteria in paddy soils, and endophytic bacteria (e.g. Gluconacetobacter in sugarcane).
The model calculates: $$NSBNF = NSBNF_{base} \times f_N \times f_T \times f_W \times f_{SOM} \times f_{pH} \times Area / 1000$$
Where \(NSBNF_{base}\) comes from the BNF data table (crop-specific: rice 33, sugarcane 25 kg N/ha/yr) or a default rate for general cropland.
Usage
calc_nonsymbiotic_bnf(
x,
nsbnf_default_kgha = 5,
k_n_ns_synth = 0.005,
k_n_ns_org = 0.0025,
t_opt_ns = 25,
t_sigma_ns = 10,
k_som = 2,
som_ref = 2.5,
ph_opt = 6.8,
ph_sigma = 1.5,
k_clay = 20,
clay_ref = 25
)Arguments
- x
Data frame. Required columns:
- Area_ygpit_ha
Harvested area in hectares.
Optional columns (if absent, modifier = 1):
- N_synth_kgha
Synthetic N fertilizer (kg N/ha).
- N_org_kgha
Organic N inputs (kg N/ha).
- TMP
Mean temperature (degrees C).
- WaterInput_mm
Precipitation + irrigation (mm).
- precip_mm
Precipitation (mm).
- irrig_mm
Irrigation (mm).
- PET_mm
Potential evapotranspiration (mm).
- SOM_pct
Soil organic matter content (percent).
- soil_pH
Soil pH.
- kgNha
Crop-specific NSBNF base rate from BNF table. If absent and Name_biomass exists, joined automatically.
- nsbnf_default_kgha
Numeric. Default NSBNF base rate for crops without a specific value in the BNF table (default 5 kg N/ha/yr). Based on Cleveland et al. (1999) for temperate agricultural soils.
- k_n_ns_synth
Numeric. Rate constant for N inhibition of NSBNF by synthetic N (default 0.005). Stronger than symbiotic inhibition since free-living fixers avoid the energy cost of fixation when mineral N is available.
- k_n_ns_org
Numeric. Rate constant for N inhibition of NSBNF by organic N (default 0.0025). Weaker than synthetic because organic N mineralizes slowly.
- t_opt_ns
Numeric. Optimal temperature for non-symbiotic fixation (default 25).
- t_sigma_ns
Numeric. Temperature Gaussian width (default 10). Broader than symbiotic (8) because diverse microbial communities provide thermal buffering.
- k_som
Numeric. SOM half-saturation constant (default 2.0 percent).
- som_ref
Numeric. Reference SOM for normalization (default 2.5 percent).
- ph_opt
Numeric. Optimal pH (default 6.8).
- ph_sigma
Numeric. pH Gaussian width (default 1.5).
- k_clay
Numeric. Half-saturation constant for clay effect on NSBNF (default 20 percent clay).
- clay_ref
Numeric. Reference clay content for normalization (default 25 percent, typical loam).
Value
Data frame with added columns:
- NSBNF_base_kgha
Base rate before adjustment (kg N/ha).
- f_N_ns
N inhibition factor.
- f_temp_ns
Temperature factor.
- f_water_ns
Water factor.
- f_SOM_ns
SOM factor (normalized at som_ref).
- f_pH_ns
pH factor.
- f_clay_ns
Clay texture factor (normalized at clay_ref).
- f_env_ns
Combined environmental factor.
- NSBNF
Non-symbiotic BNF in Mg N.
Details
Estimates free-living and associative BNF in agricultural soils using crop-specific base rates and environmental modifiers for temperature, water availability, N inputs, soil organic matter, and soil pH.
**Base rates** from literature:
Rice paddies: 33 kg N/ha/yr (Ladha et al. 2016) - cyanobacteria and heterotrophic anaerobic fixation
Sugarcane: 25 kg N/ha/yr (Urquiaga et al. 2012) - endophytic fixation (Gluconacetobacter diazotrophicus)
General cropland: 5 kg N/ha/yr default (Cleveland et al. 1999)
**N inhibition** (Dynarski & Houlton 2018): At 100 kg N/ha, NSBNF reduces to ~61 percent of base. Free-living fixers avoid the energy cost of fixation when mineral N is available.
**SOM effect** (Reed et al. 2011; Dynarski & Houlton 2018): Heterotrophic fixers require carbon as energy source. Michaelis- Menten kinetics normalized at typical agricultural SOM (2.5 percent). High-SOM soils show enhanced NSBNF.
**pH effect** (Belnap 2002): Nitrogenase activity and microbial communities are sensitive to soil pH. Gaussian around optimal 6.8 with moderate decline in very acidic (pH < 5) or alkaline (pH > 8) soils.
References
Cleveland CC et al. (1999) Global Biogeochemical Cycles 13:623-645.
Dynarski KA, Houlton BZ (2018) New Phytologist 217:68-85.
Ladha JK et al. (2016) Scientific Reports 6:19355.
Reed SC et al. (2011) Annual Review of Ecology, Evolution, and Systematics 42:489-512.
Urquiaga S et al. (2012) Plant and Soil 356:5-21.
Examples
if (FALSE) { # \dontrun{
load_general_data()
# Basic: only area required, uses defaults
df |> calc_nonsymbiotic_bnf()
# With environmental data:
df |>
dplyr::mutate(
N_synth_kgha = 120, TMP = 22,
precip_mm = 700, PET_mm = 850,
SOM_pct = 3.2, soil_pH = 6.4
) |>
calc_nonsymbiotic_bnf()
} # }