⇦ Back to Soil Fertility and Plant Nutrition Home
¶ Introduction
Properly inoculated legumes have the capability to furnish significant amounts of nitrogen to a succeeding grain or grass crop. Legumes can convert inert atmospheric nitrogen (N2) into plant-available nitrogen through biological “fixation”. This capability is common to legume species, but (with rare exceptions) not found in other broadleaves or in grass species.
The amount of nitrogen that can be fixed varies among species due to soil conditions, amount of water available, and other seasonal factors during growth.
¶ A. Biological Nitrogen Fixation
- Refers to conversion of atmospheric nitrogen (N2) to ammonium-nitrogen (NH4-N)
- N2 is inert gas; cannot be used for plant nutrition.
- Fixation occurs inside root “nodules” of legume plants.
- Nodules are small irregular growths found on plant roots
- Contain colonies of nitrogen-fixing strains of bacteria
- Fixation most common with Rhizobia bacterial species
- Are some free-living, non-symbiotic microbes that fix nitrogen
- e.g., cyanobacteria (lichens), purple-sulfur bacteria, Azotobacter, Klebsiella, etc
- Fixation ability is limited
- Rhizobia are genus of “symbiotic” bacteria
- Symbiosis: relationship between organisms from different species, usually with benefits to one or both of the individual species
- Symbiosis: relationship between organisms from different species, usually with benefits to one or both of the individual species
- Rhizobia considered fast-growing species
- Bradyrhizobia considered slow growing species
- Are some free-living, non-symbiotic microbes that fix nitrogen
- Rhizobia are fairly specific to legume plants they will infect
- Are multiple species of Rhizobia and multiple strains within species
- Cross-inoculation group: legume species and strains that are effectively nodulated by same Rhizobia species (see Table 1)
- Nodulation process (refer to Figure 1)
- Legume roots exude flavonoids into rhizosphere immediately around root
- Flavinoids interact with bacterial protein
- Cause curling and deformation of root hairs
- Bacteria form infection thread affecting cortex (“skin”) of root tissue
- Cortex cell division is affected; forms “cancerous” growth called “nodule”
- Bacteria enter nodule via infection thread
- Form bacterial colony inside nodule
- Free-living bacteria switch to “bacteroid” form
- Plant surrounds bacteroid colony with membrane
- Colony produces nitrogenase enzyme and leghemoglobin that metabolically fix nitrogen
- Nitrogen fixation is symbiotic relationship between bacteria and legume
- Is infection that is mutually beneficial to both organisms
- Bacteroid colony is connected to legume vascular system (xylem, phloem) allowing exchange of nutrients
- Bacteria provide legumes with amino acids and ureides for protein formation
- Ureide = complex, protein-type molecule derived from urea
- Legume provides simple sugars (i.e., energy) for bacterial growth
- Bacterial infection may form nodules that are not able to fix nitrogen
- “Effective” nodules that can fix nitrogen produce leghemoglobin; gives pinkish or reddish color to interior of nodule
- Interior of “ineffective” nodules remain green due to lack of leghemoglobin
¶ Figure. 1. Soybean Root Nodule Formation.
1. Root hairs form infection thread.
2. Bacteria penetrate the infection thread; form bacteroid.
3. Root cells grow around bacteroid; form nodule.
4. Nodule develops vascular tissue; permits exchange between bacteria and plant.
https://bio1903.nicerweb.com/Locked/media/ch37/bacteroids.html
¶ B. Nitrogen Fixation Rate
- Fixation rate depends on soil conditions
- Fixation is minimal while soils are cool in early spring
- Legumes will scavenge residual nitrogen carried over from previous organic matter mineralization or from fertilizer
- Fixation remains limited if soil nitrate is adequate for legume growth
- Legume will referentially utilize soil nitrate
- Nitrate taken up passively by mass flow as roots take up water
- Doesn’t require legume plant to expend additional energy for obtaining nitrogen
- Fixation requires plant to “trade” energy and food with the microbes for nitrogen
- Microbial activity increases as soils warm up
- Fixation rate increases as soil nitrogen is depleted
- Fixation is minimal while soils are cool in early spring
- Fixation stops if top growth is consumed , harvested, or terminated
- Fixation proceeds when plants have sufficient regrowth
¶ C. Nitrogen Credits
- Adjusting nitrogen rate for legume
- Legume may actually remove more nitrogen from soil than it leaves behind
- Legume forage and grain typically have high protein (nitrogen) content
- Roots, stems, and leaves decay over time
- Can release nitrogen for use by other plants
- Nitrogen fertilizer rate can often be adjusted downward when grass species follows legume in rotation or is grown concurrently with legume
- “Legume credit”: amount of nitrogen fertilizer adjustment for grass crop
- “Credit” typically due to increased soil nitrogen mineralization
- Increased conversion of organic nitrogen in plant tissues to inorganic nitrogen by soil microorganisms
- Plant tissue must first be broken down by soil microorganisms into ammonium and nitrate
- Credit depends on species, amount of biomass, growth stage at termination, etc
- Amount of above-ground biomass is important
- About half of total nitrogen potentially supplied by legumes is contained in leaves and stems
- Residue must be soil incorporated to supply maximum amount of nitrogen
- Residue remaining on the surface does not contribute nitrogen
- Soil microbes not in contact with residue; cannot decompose plant tissues
- If top-growth or grain is removed, roots are source of credit
- Legume root tissue more easily decomposed than grass root tissue
- Perennial legumes (e.g., alfalfa) may have about one ton of roots per acre after year of growth
- Short-season, annual legumes have smaller root systems
- About one-half ton of roots per acre for full-season growth of peas, annual clovers, etc
- About 30% to 50% of total nitrogen in terminated legume becomes available in first growing season
- Full nitrogen release does not occur during first growing season after termination
- Must be in contact with soil for decomposition by microbes
- May take several years for complete decomposition depending on soil and residue characteristics
¶ Table 1. Legume Species and Varieties
|
|---|
| ALFALFA GROUP Inoculant: Rhizobia meliloti |
|
| COWPEA & PEANUT GROUP Inoculant: Bradyrhizobium sp. (Vigna) |
|
| DRY BEAN GROUP Inoculant: Rhizobia phaseoli |
|
| LUPINE GROUP Inoculant: Rhizobia lupini |
|
| SOYBEAN GROUP Inoculant: Bradyrhizobium japonicum |
|
| TRUE CLOVER GROUP Inoculant: Rhizobia trifolii |
|
| VETCH & PEA GROUP Inoculant: Rhizobia leguminosarum |
|
| OTHER LEGUMES |
|
¶ References
Caddel, J., et. al. 2017. Forage Legumes and Nitrogen Production. OSU Fact Sheet PSS-2590. Oklahoma State Univ. Ext., Stillwater OK.
Duvauchelle, D. 2014. Understanding Inoculants. NRCS Technical Note #TN-PLANTMATERIS 10. accessed 23May2022 https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/hipmctn12340.pdf
D. L. Wright, D. L., et. al. 2018. Inoculation of agronomic and forage crop legumes. Pub. #SS-AGR-154. Univ. of Florida, Institute of Food and Agricultural Sciences, Gainsville, FL. accessed 23May2022 https://edis.ifas.ufl.edu/pdf/AA/AA126/AA126-15271867.pdf
Durst & Bosworth. 2008. Inoculation of Forage and Grain Legumes. Agronomy Facts 11. Penn State Extension, Penn State Univ., University Park, PA
Killpack, S. and D. Buchholz. Nitrogen in the Environment: Nitrogen Replacement Value of Legumes. Univ. of Missouri Ext., Columbia MO. https://extension2.missouri.edu/wq277, accessed 20May2022