Agroforestry Chapter 5 (Part 1): Tree–Crop Interactions, Soil Productivity and Nitrogen Fixation
Agroforestry Notes · Chapter 5 · Part 1
Chapter 5: Tree–Crop Interactions, Soil Productivity and Nitrogen Fixation
The tree–crop interface, competition and complementarity, light and microclimate, allelopathy, how trees improve soil, litter quality and decomposition, biological nitrogen fixation and mycorrhizae (mainly from Nair).
1. Tree–crop interactions
Interface (Nair)The area where there is positive or negative interaction between two entities, such as between a row o trees and a row of crops.Source: Nair (1993), Glossary
Interaction is the effect of one component of a system on the performance of another component or of the whole system. In agroforestry the key question is whether the gain from trees (soil, microclimate, products) outweighs what they take away from crops (light, water, nutrients). Huxley (1985) introduced the term tree–crop interface (TCI); the tree–animal interface (TAI) is its counterpart in silvopasture.
| Interaction type | Effect on A | Effect on B | Agroforestry example |
|---|---|---|---|
| Competition | − | − | Tree roots and crop roots take the same water |
| Complementarity (facilitation) | + | + | Deep tree roots use water and nutrients crops cannot reach |
| Commensalism | + | 0 | Shade-loving crop under a tree that is not affected |
| Amensalism | − | 0 | Allelopathic tree suppresses crop without gain to itself |
| Neutralism | 0 | 0 | No measurable effect |
Positive effects at the TCI
- Microclimate amelioration (lower temperature, higher humidity, less wind)
- Soil fertility gain through litter and N-fixation
- Erosion control
- Weed suppression by shade and mulch
Negative effects at the TCI
- Competition for light, water and nutrients
- Allelopathy
- Trees as alternate hosts of pests and diseases
- Physical damage during tree felling
Allelopathy is the harmful (sometimes beneficial) effect of chemicals released by one plant on another. Classic examples: Juglans (walnut) releases juglone; Eucalyptus leaf leachates; mimosine in Leucaena; extracts of Gmelina and Acacia have been reported to inhibit crop germination.
Exam point: The net effect decides success: complementarity > competition means the system yields more than sole crops, which is shown by an LER above 1 (Chapter 6).
2. Light, photosynthesis and microclimate
CO₂ compensation point (Nair)The CO2 concentration at which photosynthetic fixation just balances respiratory and photorespiratory loss, being 50-100 ppm for C3 and 0-5 ppm for C4 plants.Source: Nair (1993), Glossary
LAI (leaf area index) (Nair)The ratio of leaf area (one surface only) of a crop to the ground area on which it grows.Source: Nair (1993), Glossary
| Pathway | First product | Examples | Shade response |
|---|---|---|---|
| C3 | 3-carbon (PGA) | Most trees, wheat, rice, legumes, coffee, cacao | Saturate at lower light; many tolerate shade |
| C4 | 4-carbon (oxaloacetate) | Maize, sorghum, sugarcane, many tropical grasses | Need full sun; yield falls most under shade |
| CAM | Malate at night | Pineapple, Agave | Water-saving; stomata open at night |
- Shade-tolerant understorey crops (coffee, cacao, ginger, turmeric, colocasia) are mostly C3; C4 cereals suffer most under trees.
- Under Inga shade, coffee had a maximum temperature about 5.4 °C lower than unshaded coffee (Nair).
- Trees raise relative humidity, cut wind speed and lower evaporation under the canopy.
- Light reaching the crop is controlled by pruning, spacing, row orientation (E–W) and choosing trees with light crowns.
3. Trees and soil productivity
Nair sums up the soil benefits of trees as the “soil-improving” hypothesis: trees maintain or increase soil fertility in ways that annual crops cannot.
Add to the soil
- Organic matter through litter, prunings and roots
- Nitrogen through biological fixation
- Nutrients from rain and dust caught by the canopy
Reduce losses
- Erosion control by litter cover and roots
- Safety net: deep roots catch nutrients leached below the crop root zone
- Less leaching under perennial cover
Improve properties
- Better structure, porosity and water-holding
- Moderated soil temperature
- More soil fauna and microbial activity
Nutrient pumping
Deep tree roots take up nutrients from lower layers and return them to the surface in leaf litter, where crops can use them.
Trees can also reduce fertility: they compete with crops for nutrients and water, export nutrients in harvested wood, and some produce acidifying or allelopathic litter.
Litter quality and decomposition
Carbon/nitrogen ratio (Nair)The ratio of the weight of organic carbon (C) to the weight of total nitrogen (N) in a soil or organic material.Source: Nair (1993), Glossary
Mineralization (Nair)The conversion of an element from an organic form to an inorganic state as a result of microbial decomposition.Source: Nair (1993), Glossary
Decomposition speed depends on litter quality: C:N ratio, lignin and polyphenol content. Palm and Sanchez showed that the polyphenol:N ratio predicts N release from tropical legume leaves better than C:N.
| Prunings | Half-life (days) | C:N | Best use |
|---|---|---|---|
| Gliricidia sepium | 22 | 12 | Quick nutrient supply (green manure) |
| Leucaena leucocephala | 31 | 12 | Green manure and fodder |
| Flemingia macrophylla | 53 | 21 | Long-lasting mulch, weed control |
Exam point: Synchrony: nutrient release from prunings should match the time of crop demand. Fast litter (Gliricidia) feeds the crop; slow litter (Flemingia) protects the soil.
Memory trick: G-L-F = 22-31-53: Gliricidia fastest, Flemingia slowest.
4. Biological nitrogen fixation
Nitrogen fixation (Nair)The biological conversion of elemental nitrogen (N2) to organic combinations or to forms readily utilized in biological processes.Source: Nair (1993), Glossary
Rhizobia (Nair)Bacteria capable of living symbiotically with higher plants, usually in nodules on the roots of legumes, from which they receive their energy, and capable of converting atmospheric nitrogen to combined organic forms; hence, the term symbiotic nitrogen-fixing bacteria. (Derived from the generic name Rhizobium).Source: Nair (1993), Glossary
| Symbiont | Host plants | Remarks |
|---|---|---|
| Rhizobium (fast-growing) | Legume trees, e.g. Leucaena, Sesbania | Acid-producing in culture |
| Bradyrhizobium (slow-growing) | Legume trees, e.g. Acacia mearnsii, Faidherbia albida, Erythrina | Alkali-producing in culture |
| Frankia (actinomycete) | Actinorhizal non-legumes: Alnus, Casuarina, Myrica, Hippophae | About 200 spp. in 19 genera of 9 families |
- About 650 tree species are known to fix nitrogen (Nair).
- The Nitrogen Fixing Tree Association (NFTA) was founded in 1981 (Hawaii).
- Herbaceous legumes fix about 40–200 kg N/ha/yr.
- Methods of measurement: ¹⁵N isotope dilution, N-difference, and acetylene reduction (nitrogenase activity).
- Not all legumes fix N: many Caesalpinioideae (e.g. Cassia, Tamarindus in most reports) do not nodulate, while most Mimosoideae and Papilionoideae do.
| Species | N fixed (kg/ha/yr, as reported in Nair) |
|---|---|
| Leucaena leucocephala | 100–500 |
| Acacia mearnsii | about 200 |
| Erythrina poeppigiana | about 60 |
| Inga jinicuil | 35–40 |
| Faidherbia albida | low (under 20) |
| Gliricidia sepium | about 13 (one report) |
Exam point: The e-course nitrogen-fixer list includes Alnus and Casuarina, which are not legumes. A favourite MCQ: “Which of these is a non-leguminous N-fixer?”
5. Mycorrhizae
Ectotrophic mycorrhiza (Nair)A symbiotic association of the mycelium of fungi and the roots of certain plants in which the fungal hyphae form a compact mantle on the surface of the roots and extend into the surrounding soil and inward between cortical ells, but not into these cells. Associated primarily with certain trees.Source: Nair (1993), Glossary
| Type | Structure | Hosts |
|---|---|---|
| Ectomycorrhiza | Fungal mantle on root surface; Hartig net between cortical cells, not inside | Pines, oaks, eucalypts, dipterocarps |
| Endomycorrhiza (VAM / AM) | Hyphae enter cortical cells; form vesicles and arbuscules | Most crops and most tropical trees, incl. Leucaena |
- Main benefit: better uptake of phosphorus (and Zn, Cu) and water from a larger soil volume.
- Pine plantations in new areas fail without ectomycorrhizal inoculation of nursery soil.
- Mycorrhizae can link tree and crop roots, so they are part of below-ground interaction.
6. Ecophysiology and interactions: Nair’s details
C3 plants (Nair)Species with the photosynthetic pathway in which the first product of CO2 fixation is a 3-Carbon molecule (3-phospho glyceric acid), (see Chapter 11)Source: Nair (1993), Glossary
C4 plants (Nair)Species that have 4-C acids (malate and asparate) as primary CO2 fixation products, (see Chapter 11)Source: Nair (1993), Glossary
CAM plants (Nair)Species with stomata that open primarily at night, and organic acids, especially malic, as the primary CO2 fixation products, (see Chapter 11)Source: Nair (1993), Glossary
| Feature | C3 | C4 | CAM |
|---|---|---|---|
| CO₂-fixing enzyme | RuBP carboxylase (Calvin cycle) | PEP carboxylase (Hatch–Slack) | PEP carboxylase at night |
| Max. photosynthesis (mg CO₂/dm²/h) | about 30 | about 60 | about 3 |
| Water used (g H₂O per g CO₂) | about 600 | about 300 | about 50 |
| CO₂ compensation point (ppm) | about 50 | about 5 | about 2 |
| Photorespiration | High | Very little | – |
- PAR (photosynthetically active radiation) is 400–700 nm, about 44–50% of solar radiation. One einstein = one mole of photons.
- C3 leaves saturate at about 25% of full sunlight, so shade-tolerant understorey crops are mostly C3. C4 crops need open sun.
- Net photosynthesis = gross photosynthesis − respiration; respiration uses about one-third of what is fixed.
- Highest crop growth rate recorded: about 77 g/m²/day (Loomis and Williams, 1963).
- Photosynthetic efficiency is 2–2.5% in advanced agriculture but under 1% as a world average; Pantnagar crops reached 1.7–2.38% (Nair et al., 1973).
- Net primary production of tropical rain forest: 10–35 t/ha/yr.
CGR = NAR × LAI Beer–Lambert law: I = I₀ e−kL (k = extinction coefficient, L = leaf area index)
Design rule from the Beer–Lambert law: put species with small, erect leaves (low k) in the top layer and species with large, horizontal leaves (high k) below, so that light is shared.
Interactions in more detail (Chapter 13)
| Interface | Positive effects | Negative effects |
|---|---|---|
| Tree–crop (TCI) | Shade that lowers stress, biomass added to soil, water and soil conservation | Competition for light, nutrients and water; allelopathy |
| Tree–animal (TAI) | Shade for animals, manure deposited | Toxins in fodder, browsing damage, trampling, trees as pest hosts |
- Vapour pressure deficit (VPD) = saturation vapour pressure − actual vapour pressure; it is the “drying power” of the air. Under Inga jinicuil (205 trees/ha, 14 m tall) in Mexico, coffee had a lower VPD and a minimum temperature 1.5 °C higher (Barradas and Fanjul, 1986).
- A 0.5 m deep root barrier between Leucaena hedges and millet removed the yield loss, proving it was due to water competition (Corlett et al., 1989).
- Eucalyptus tereticornis (3.5 years old) cut mustard and wheat yields by over 30% within 10 m of the trees (Malik and Sharma, 1990).
- Cacao yields more under shade when it is short of nutrients (Alvim, 1977). Fruit and seed crops tolerate shade less than leaf crops.
- Heavy shade worsens black pod of cacao (Phytophthora palmivora) but reduces witches’ broom (Crinipellis perniciosa).
| Allelopathic tree | Affected plant (Nair, Table 13.2) |
|---|---|
| Alnus nepalensis | Soybean |
| Casuarina, Eucalyptus tereticornis, Leucaena | Cowpea, sorghum, sunflower |
| Eucalyptus | Potato |
| Gliricidia, Leucaena | Maize and rice seedlings |
| Grevillea robusta | Its own seedlings (autotoxicity) |
| Toxin in fodder | Plant |
|---|---|
| Mimosine | Leucaena |
| Cyanogenic glucosides, fluoroacetate | Some Acacia spp. |
| Robitin | Robinia |
| Tannins | Calliandra, Gliricidia, Prosopis |
| HCN | Cassava leaves |
To increase tree growth
Better microclimate, fertiliser, mulch, irrigation, tillage, adapted species, supplementary feeding of animals.
To reduce tree competition
Pruning, pollarding, root pruning or trenching, shading, herbicides, controlled grazing.
7. Nitrogen fixation in depth (Nair, Chapter 17)
Chapter 17 of Nair is adapted from Dommergues (1987). Nitrogen fixation occurs in over 90% of Mimosoideae and Papilionoideae but only 34% of Caesalpinioideae.
| Rhizobial partner | Tree species (Nair) |
|---|---|
| Rhizobium only (fast-growing) | Leucaena (mostly), Sesbania grandiflora, Gliricidia, Acacia senegal, A. nilotica, A. tortilis, Mimosa scabrella |
| Bradyrhizobium only (slow-growing, “cowpea miscellany”) | Acacia mearnsii, Faidherbia albida, Erythrina spp. |
| Both (promiscuous) | Acacia seyal |
- Faidherbia albida nodulates as a seedling but nodules are seldom found in the field; its fixation is low.
- Acacia mearnsii fixes about 200 kg N/ha/yr but needs pH 4.5 or above; it failed to nodulate in the acid, high-aluminium soils of the Burundi highlands.
- Leucaena is “not a miracle tree” (Brewbaker, 1987): it is sensitive to acidity and needs a lot of nutrients. A 3-year stand took up 11–27 kg P, 174–331 K, 138–305 Ca and 31–62 Mg per ha.
- Sanginga et al. (1989), IITA, ¹⁵N method: Leucaena fixed 98–134 kg N/ha in 6 months. Inoculation with strains IRc 1045 and IRc 1050 gave a gain equal to 150 kg urea/ha.
- Coriaria arborea under Pinus radiata in New Zealand fixes up to 192 kg N/ha/yr; C. sinica in Hunan (China) is a green manure and silkworm feed. All 15 Coriaria species nodulate.
- Casuarinaceae: 82 species in four genera (Casuarina, Allocasuarina, Gymnostoma and one undescribed). There is little cross-inoculation between Casuarina and Allocasuarina.
- Alnus acuminata with pasture is a popular system in Costa Rica.
| Method | Principle |
|---|---|
| N difference | Total N of the fixing plant minus that of a non-fixing plant |
| Acetylene reduction | Nitrogenase reduces acetylene (10% C₂H₂, 1–2 h) to ethylene, which is measured; theoretical C₂H₂:N₂ ratio 3:1 |
| ¹⁵N enrichment (isotope dilution) | Soil labelled with ¹⁵N; fixing plants are less enriched because part of their N comes from air |
| Natural ¹⁵N abundance | Fixed N has slightly less ¹⁵N than soil N; needs a mass spectrometer. Prosopis in the Sonoran desert was shown to fix N though no nodules were found |
| Xylem sap analysis | Ureide exporters (soybean, cowpea: allantoin, allantoic acid) can be assessed; amide exporters (lupin, clover) cannot. Casuarina sap has citrulline |
Lulandala and Hall (1986), Tanzania: Leucaena 2,500 trees/ha, 63 kg nodules/ha, ratio 3:1 → about 197 kg N/ha/yr
| Species | N fixed (kg/ha/yr, Nair Table 17.1) |
|---|---|
| Casuarina equisetifolia | 60–110 |
| Sesbania rostrata | 83–109 |
High fixers (100–300 kg N)
- Demanding: Leucaena, Calliandra (need P, K, Ca)
- Tolerant: Acacia mangium (acid, poor soils)
- Also Casuarina equisetifolia
Low fixers (under 20 kg N)
- Faidherbia albida
- Acacia raddiana (tortilis)
- Acacia senegal
NFP vs ANF
Nitrogen-fixing potential (NFP) is fixation with no limiting factor (Halliday, 1984); actual N fixation (ANF) in the field is much lower.
Practical tips
- Mineral N fertiliser inhibits nodulation
- Pellet seeds with calcium carbonate or rock phosphate on acid soils
- Frankia, like VAM, does not move in soil: mix the inoculum into the potting mix
- Use at least two measurement methods
8. Quick revision box
- TCI coined by Huxley (1985); TAI for animals.
- Competition (−/−), complementarity (+/+), commensalism (+/0), amensalism (−/0).
- Juglone = walnut; mimosine = Leucaena.
- CO₂ compensation: C3 50–100 ppm, C4 0–5 ppm. CAM: pineapple, Agave.
- Coffee under Inga: max. temperature 5.4 °C lower.
- Nutrient pumping and safety net by deep roots.
- Polyphenol:N (Palm and Sanchez); half-life Gliricidia 22, Leucaena 31, Flemingia 53 days.
- Frankia: ~200 spp., 19 genera, 9 families; ~650 N-fixing trees; NFTA 1981.
- Leucaena 100–500 kg N/ha/yr; herbaceous legumes 40–200.
- Mycorrhiza → phosphorus; ecto = mantle; VAM = arbuscules.
- C3 enzyme RuBP carboxylase; C4 PEP carboxylase (Hatch–Slack); PAR 400–700 nm (44–50%).
- CGR = NAR × LAI; Beer–Lambert I = I₀e^(−kL); max CGR 77 g/m²/day.
- 0.5 m root barrier removed millet loss (water competition). Grevillea: autotoxicity. Robitin = Robinia.
- Bradyrhizobium: A. mearnsii, Faidherbia, Erythrina; both: Acacia seyal. Acetylene → ethylene, 3:1.
- Coriaria arborea 192 kg N; Leucaena 98–134 kg N in 6 months (¹⁵N); NFP > ANF.
9. Interactive MCQ Quiz
29 questions. Tap an option to see the answer and explanation, then go to the next question.
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