Overstory #140 - Nitrogen Fixing Plants (Temperate)
Nitrogen gas (N) constitutes four fifths of the world's atmosphere - a virtually inexhaustible supply, yet very few plants and no animals can assimilate nitrogen in its free form. Nitrogen is, though, the essential constituent of the proteins necessary for cell protoplasm, and all organisms are dependent on having it available in a form which they can utilize.
Most plants obtain their nitrogen from the mineralisation of soil organic matter and plant residues, and living organisms and ecosystems are organised to obtain and preserve usable nitrogen. The modern use of synthetic nitrogen fertilisers is fraught with long-term dangers (depleting soil nitrogen reserves, pollution of groundwater, rivers and lakes), and the fertilisers themselves will become increasingly expensive through increasing energy costs.
Biological nitrogen fixation, particularly of the symbiotic type, plays a crucial ecological role in maintaining adequate nitrogen reserves in the plant world. Two groups of plants in particular, the Legumes (Rhizobial plants) and Actinorhizal plants, can thrive without any fixed nitrogen or with a minimal supply in the soil. These plants, through the agency of specific bacteria (mostly Frankia and Rhizobium species) which invade the root hairs and establish a mutually beneficial association inside their root swellings (nodules), can convert free air nitrogen into fixed nitrogen for eventual plant protein assimilation and storage. These select groups of plants have thus obtained an evolutionary advantage over most other living organisms. Root infection and nodule development of both actinorhizal and legume symbioses are similarthe infection occurs via the penetration of deformed root hairs by bacteria, or by the bacteria gaining entry to the root through intercellular spaces.
Ecologically, most legumes and actinorhizal plants are pioneer species on open, nitrogen-poor sites. They improve the soil and enable the succession towards scrub or forest to begin. As shading of them increases, they decline. Since they are pioneers, there is scope for them to easily become naturalised and become somewhat weedy in the agricultural landscape - for example, Elaeagnus angustifolia is regarded as a weed by some in western North America. Several actinorhizal plants persist as understorey plants in open forest stands, and these plants are generally more shade-tolerant than the legumes.
Forest ecosystems are seldom at equilibrium, usually slowly accumulating Nitrogen during their life cycle and suffering periodic large losses when vegetation is naturally or artificially removed.
Apart from the legumes and actinorhizal plants, there are a number of other systems involving nitrogen-fixing cyanobacteria, notably of the bacterial genera Azotobacter, Anabaena, and Nostoc.
These systems involve the following:
- Gunnera-Nostoc. Probably all Gunnera species display a localised infection of the stem by Nostoc bacteria.
- Azolla-Anabaena. The aquatic plants of the Azolla family form a symbiosis with Anabaena bacteria.
- Liverwort-Nostoc. The liverwort genera Anthoceros, Blasia and Cavirularia all form associations with Nostoc bacteria.
- Lichen associations. About 7% of lichen species are not of the traditional fungi-algae symbiosis, but are instead formed of a fungi-cyanobacteria symbiosis. Nostoc in the bacteria genus is usually involved. The lichen genera Collema, Lobaria, Peltigera, Leptogium and Stereocaulon form this type of symbiosis. They are particularly important as nitrogen-sources in Arctic and desert ecosystems, where fixation rates may reach 10-20 Kg/ha/year.
- Leaf surfaces (the phyllosphere). There is increasing evidence that free-living N-fixing species of bacteria are abundant on wet and damp leaves in predominantly moist climates.
- Root zone (Rhizosphere). Free-living bacteria, for example Azotobacter species, may be more abundant in the areas immediately adjacent to plant roots and aid plant nitrogen nutrition.
- Free-living. N-fixing bacteria thrive where the Carbon:Nitrogen ratio is high and there is sufficient moisture, for example on rotting wood, in leaf litter, the lower parts of straw and chipping mulches etc.
Factors affecting nodule development
- Temperature. Depends on the bacteria species and the host plants, for example 4-6 deg C is adequate in Vicia faba, whereas 18 deg C or more is necessary for most sub-tropical and tropical species.
- Seasonality. For most species, fixation rates rise rapidly in Spring from zero, to a maximum by late spring/early summer which is sustained until late summer, then decline back down to zero by late autumn. In evergreen species, N-fixation occurs throughout the winter provided the soil temperatures do not fall too low.
- Soil pH. The legumes are generally less tolerant of soil acidity than actinorhizal plants. which is reflected by Rhizobium species being less acid-tolerant than Frankia species. Of the actinorhizal plants, Alders (Alnus spp) and Bayberries (Myrica spp) are most acid tolerant. Of Rhizobium species, acid-tolerance declines in the following ordercowpea group (most acid tolerant) - Soya bean group - Bean & Pea groups - Clover group - Alfalfa group (least acid tolerant).
In poor soils which are low in Nitrogen, the introduction of N-fixing plants usually leads to considerable acidification (e.g., a fall in pH of up to 2.0 in 20 years for a solid stand), which itself will in time start to affect nodulation efficiency.
- Availability of Nitrogen in the soil. If Nitrogen is abundant and freely available, N-fixation is usually much reduced, sometimes to only 10% of the total which the N-fixing plants use. In trials with Alders, at low soil N levels (under 0.1% total soil nitrogen), the majority of N used by the alder comes from N fixed from the air; when total soil nitrogen is as high as 0.5%, only 20% of the N used came from fixed N from the air.
- Moisture stress. in droughts, bacterial numbers decline; they generally recover quickly, though, when moisture becomes available again. Some species (usually actinorhizal), for example Alnus glutinosa and Myrica gale, are adapted to perform well in waterlogged conditions.
- Light availability. Nitrogen fixation is powered via sunlight and thus will be reduced in shady conditions. For most N-fixing plants, which are shade sensitive, N-fixation rates decline in direct proportion to shading, i.e. 50% shading leads to 50% of the N-fixed. The relationship for N-fixing species which are not so shade-sensitive is not so clearthey may well continue to fix significant amounts of nitrogen in shade.
Nitrogen availability and contributions
Nitrogen from N-fixers is made available to other plants by two main natural methods, litterfall which is high in nitrogen; and root turnover and leaching from roots, which is now believed to be a significant contributor to Nitrogen flow, returning at least as much as litterfall does. In addition to these, there may be interventions which aid the liberation of nitrogen, for example by regular coppicing or pruning, with the prunings left to decompose on the soil floor or shredded and used as a mulch.
All the evidence now indicates that Nitrogen-fixing trees and shrubs (both Actinorhizal and leguminous) can fix comparable amounts of nitrogen to the common horticultural crops (beans and peas, clover, alfalfa, etc.). Pure stands of Alders, Elaeagnus, Hippophae rhamnoides and others have all been recorded as making an extra 150 Kg N per Ha per year (or more) available in the soil. Fixing rates obviously start off much lower than this for young plants, but maximum N-production is reached within about 10 years.
Nitrogen accumulation rates can vary considerably, depending on the species-bacteria combination, age, growth conditions, time of year etc. They are also quite difficult to measure. Overall, rates for legumes and actinorhizal plants are similar. Some recorded N-accumulation rates (i.e. made available to other plants) for various genera are given below:
- Alders (Alnus spp); 60-360 Kg/Ha/year
- Ceanothus spp; 30-100 Kg/Ha/year
- Elaeagnus spp; 240 Kg/Ha/year
- Hippophae rhamnoides (Sea buckthorn); 180 Kg/Ha/year
- Lespedeza spp; 100 + Kg/Ha/year
- Lupinus arborens; 17 Kg/Ha in 10 weeks in summer
- Myrica cerifera; 120 Kg/Ha/year
- Ulex europaeus; 200 Kg/Ha/year
USES OF NITROGEN-FIXING PLANTS
The use of N-fixers to supply essential nitrogen to enable other plant growth has several advantages over the use of ordinary fertilisers:
- The supply of N is more regular and continues over a longer time.
- There is less leaching and volatilization of the Nitrogen.
- N-fixers also increase soil organic matter.
The disadvantages are:
- Slower in producing fertility increases.
- May be a source of competition.
- Produces a managerially more complex ecosystem.
- The use of N-fixers may be more expensive than using chemical fertilisers on a per-unit N basis; however if the N-fixers themselves have uses, or long-term use of them is envisaged, or wider environmental benefits are taken into account, this short term economic gain may be irrelevant.
OVERVIEW OF MAIN USES
Nitrogen fixing plants by lakes, ponds and rivers can be a major source of Nitrogen to the aquatic ecosystem, one result being increased photosynthetic activity by phytoplankton which is at the start of the food chain for many fish.
Agriculture and horticulture
Interplanting N-fixers with other crops is perhaps the most important role these plants can play in agricultural and horticultural systems. The N they supply can allow for lower stocking rates of animals (overstocking is a major problem in many countries including Britain), and reduce or remove the need for external sources of Nitrogen. In agriculture the use of N-fixing annuals and perennials is common in green manures and pastures, with species likes clovers fixing up to 200 kg/ha/year.
It is important to realise that the use of N-fixing trees and shrubs can rarely achieve an N-supply equivalent to that which intensive agriculture or horticulture needs without utilising a large proportion of the total area. It is more important to look at the efficiency of these systems in terms of energy inputs and outputs, whereby if the outputs are halved but the inputs reduced to a quarter, the system is twice as efficient and more sustainable.
Forest and fruiting gardens
The use of nitrogen-fixing trees and shrubs to provide nitrogen for a cropping system has been rare to date, mostly because of a simple lack of knowledge of what they can contribute. Some use as nurse crops in forestry systems has been tried, and some species are known as soil improvers.
In practice, forest gardens (as opposed to orchards) may have ground cover layers and other shrubs to feed, so the proportion of N-fixers required may be somewhat higher. Nitrogen fixers in the understorey should be located both throughout the garden, and also concentrated near demanding species.
N-fixers are used in forestry as nurse trees, soil improvers and for erosion control for example, Alnus glutinosa and Elaeagnus umbellata dramatically improve the growth of Juglans nigra (black walnut) in North America. The benefits of interplanting N-fixers increase where soils are deficient of Nitrogen.
The two main areas for introducing N-fixers into forests are as crop trees and as ecosystem improvers:
- Crop trees
These can be introduced in three ways:
- Continuous cropping - for example on 25-30 year rotations for fibre and wood (e.g., Alnus glutinosa, A. rubra); or using 5-15 year short rotations for biomass production (e.g., Alnus glutinosa, A. rubra, Elaeagnus umbellata, Robinia pseudoacacia).
- Intercropping, using large N-fixing trees like the alders above between other species.
- Alternate cropping with other tree species, e.g., a 30-year crop of Alnus spp followed by a non-N-fixer, followed by another N-fixer etc.
- Ecosystem improvers
These can also be utilised in three ways:
- As a green manure crop before the non-N-fixing crop tree. Examples include herbaceous legumes such as Dryas spp. and Myrica spp.
- As an N-fixing understorey beneath the crop trees (see also about forest gardens above). The most promising species are shrubby Alnus spp, Comptonia spp, Dryas spp, Elaeagnus spp, Hippophae spp, Myrica spp, Purshia spp, Shepherdia spp and herbaceous legumes.
Most N-fixers are shade-intolerant and are shaded out as the main tree crop grows, provided the N-fixer does not grow faster than the main crop. If this is likely, then the N-fixing nurse can be cut back or coppiced. Alternatively, the main crop can be planted before the N-fixing nurse so that it gets a head startin one mixed planting of red alder and Douglas fir, a delay in alder establishment of 4-8 years was needed to prevent overtopping of the fir. Use of N-fixing shrubs avoids this problem of overtopping.
The tree lupin (Lupinus arboreus) is much used in New Zealand, interplanted with Pinus radiata on sandy soils; the lupins rapidly establish and supply nitrogen to the pines, which are planted 3 years after the lupins have been sown, after the lupins have been crushed to set them back.Various annuals and perennials have been interplanted with trees to improve their growth and establishment, including White clover (Trifolium repens) via seed sown in the compost of containerised trees; Crown vetch (Coronilla varia), Blue lupin (Lupinus angustifolius), Bokhara clover (Melilotus alba) and Birdsfoot trefoil (Lotus corniculatus) with poplars; Alaskan lupin (Lupinus arcticus) with larch in Iceland; Lespedeza cuneata with pines; and Subterranean clover (Trifolium subterraneum) with Pinus radiata in Australia. Interplanting Lupinus polyphyllus with spruce and Scots pine resulted in improved nitrogen, calcium and potassium nutrition of the trees, faster litter decomposition and nutrient cycling. Most of these N-fixing species are shade-sensitive, thus the N fixed by them falls as the tree canopy closes, and is near zero at canopy closure; however, most of them leave long-lived seeds in the soil and when trees are thinned or harvested, the N-fixing layer regenerates.
- Alone or mixed for soil stabilisation and amelioration. Useful species here include Ceanothus spp, Myrica spp, Purshia spp and herbaceous legumes.
Many N-fixers are excellent mineral accumulators, as befits their role as pioneer plants which improve soil conditions for future successions of plants. They achieve this by finding mineral sources in the subsoil with their deep taproots, and raising the minerals so gained to their upper parts, which may be eaten or die off in winter, releasing their minerals into the soil or fauna. There is also increasing evidence that accumulated minerals may leach out of the roots into the surrounding soil, where the root systems of other plants may be able to utilise them. A second method of accumulation is via mycorrhizal associations, where phosphorus is made available to the plants via these symbiotic fungi.
Several N-fixers are excellent windbreak trees, e.g., Caragana arborescens (Siberian pea shrub) and Hippophae rhamnoides (Sea buckthorn). Maritime exposure (salt spray) is tolerated by several species as well.
Bee (honey) plants
A large number of legumes are excellent bee (honey) plants, from perennials like clovers and vetches to trees like the black locust.
Using N-fixing plants increases soil organic matter by up to 20%. Soil organic matter is the primary storage medium for soil Nitrogen and an increase in it improves soil tilth, mineral levels, water retention, soil porosity and aeration, and soil structure. There is now evidence that N-fixing understorey plants can recycle greater amounts of phosphorus in litter and thus influence phosphorus cycling on sites where this mineral is in limited supply.
Another effect often found is the suppression of fungal diseases, notably with alders. The reasons are not clear, but the alders may release substances from their roots which suppress these fungi, and the increased nitrification of soils may encourage pests of the fungi.
Many of the legumes and actinorhizal plants, because they are pioneer plants, synthesise various poisons which are held in the aerial parts of the plant to deter animals from browsing on them. However, others are nor poisonous and many have edible products, for example in the pea (Pisum) and bean (Phaseolus) species.
An extensive bibliography, too large to reprint here, is given in the original source.
This article is excerpted with the gracious permission of the author and publisher from:
Crawford, M. 1995. Nitrogen-fixing Plants for Temperate Climates. Agroforestry Research Trust, Dartington, Totnes, Devon, UK.
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About the author
Martin Crawford is director of the Agroforestry Research Trust in Devon, England, a research charity he set up in 1992. The A.R.T. undertakes research in temperate agroforestry systems, with particular interest in unusual fruiting trees and nut producing trees. One of the projects Martin Crawford manages is a 2 acre temperate forest garden, now 10 years old. He has also written numerous books on useful temperate species, fruits, nuts etc, and edits the quarterly journal Agroforestry News. In addition to his work with the A.R.T., Martin is also a director of the Gaia charity (founded by Prof. James Lovelock) which facilitates research into earth systems science. The A.R.T. website is: http://www.agroforestry.co.uk.
Related editions to The Overstory
- The Overstory #131--Microsymbionts
- The Overstory #107--Tropical Forage Legumes
- The Overstory #91--Fodder Tree Establishment
- The Overstory #81--Soil Food Web
- The Overstory #78--Reforestation of Degraded Lands
- The Overstory #70--Rhizosphere
- The Overstory #65--Biological Nitrogen Fixation
- The Overstory #61--Effects of Trees on Soils
- The Overstory #42--Improved Fallow
- The Overstory #4--Nitrogen Fixing Trees -- A Brief Introduction