Download 10_chapter 1

INTRODUCTIO
N
INTRODUCTION
Nitrogen (N) is the nutrient element most frequently found limiting to the growth
of green plants. This results from the continual loss of nitrogen from the reserve of
combined or fixed nitrogen, which is present in soil and available for use by plants. It is
continually depleted by such processes as microbial denitrification, soil erosion, leaching,
chemical volatilization, and perhaps most important, removal of nitrogen-containing crop
residues from the land. The nitrogen reserve of agricultural soils must therefore be
replenished periodically in order to maintain an adequate (non-growth limiting) level for
crop production. This replacement of soil nitrogen is generally accomplished by the
addition of chemically fixed nitrogen in the form of commercial inorganic fertilizers or
by the activity of biological nitrogen fixation (BNF) systems. The significance of BNF as
the major mechanism of recycling of nitrogen from the unavailable atmospheric form to
available forms in the biosphere cannot be overemphasized.
Biological nitrogen fixation takes place through symbiotic and non-symbiotic
means. Symbiotic fixation occurs through the association of plant roots with nitrogen
fixing micro organisms. Many legumes form an association with the bacteria Rhizobium
while the symbionts of a few non-leguminous species belong to a genus of Actinomycetes
Frankia. Non symbiotic fixation is effected by tree living soil organisms and can be a
significant factor in natural ecosystem, which have relatively modest nitrogen
requirements from outside system. However, non-symbiotic N2 fixation is of minor
importance in agricultural systems that have far greater demands of nitrogen.
Presumably, this type of BNF varies according to the organic content and therefore the
micro biological activity of the soil.
Biological nitrogen fixation can take many forms in nature, including bluegreen
algae, lichens and free-living soil bacteria. These types of nitrogen fixation contribute
significant quantities of available N to natural ecosystems but not to most cropping
systems, with the exception of paddy rice. Their contributions are less than 5 lbs of
nitrogen per acre per year. However, nitrogen fixation by legumes can be in the range of
25-75 pounds of nitrogen per acre per year in a natural ecosystem, and several hundred
pounds in a cropping system.
Biological nitrogen fixation is the process that changes inert atmospheric N2 to
biologically useful nitrogenous substances. This process is mediated in nature only by
bacteria. Other plants benefit from nitrogen-fixing bacteria when the bacteria die and
release nitrogen to the environment or when the bacteria live in close association with the
plant. In legumes and some of non-legumenous plants, the bacteria live in small outgrowths on the roots called nodules and fix nitrogen. Nitrogen fixation by legumes and
non-legumes is a strong symbiotic relationship between a bacterium and a plant.
In most natural ecosystems, nitrogen (N) is the primary nutrient that limits crop
productivity (Vitousek et al., 1997). Human-induced activities and disturbance have
helped increase contributions to the N cycle, through energy and fertiliser production, and
mobilisation of N from long-term storage pools (Galloway 1998; Goulding et al., 1998).
This extra reactive N can affect ecosystem processes such as N deposition and soil
acidification, and microbiologically mediated soil processes such as mineralisation,
immobilisation, nitrification, denitrification and emission of nitrous oxide and methane
(Goulding et al., 1998; Fageria & Stone, 2006). Of all sources, biological N fixation is
the most ‘environmentally friendly’ process of supplying N to ecosystems (Jensen &
Hauggaard-Nielsen 2003). Leguminous and actinorhizal plants form nodules with their
respective host plants and fix N in association with rhizobia or Frankia (Franche et al.
2009). Rhizobium–legume symbioses represent the major mechanism of biological N
fixation compared with the N-fixing heterotrophs and associative bacteria and
actinorhizal plants (Zahran, 2001). Through symbiotic fixation, these N-fixing plants
provide high inputs of N to the ecosystem (Watt et al. 2003; Augusto et al., 2005). Such
N supply increases soil fertility and builds up the soil N pool through the decomposition
of N-rich litter, and the release of N from roots and nodules ( Ehrenfeld, 2003; Goldstein
et al. 2010) can contribute significantly to the global N cycle (Colebatch et al., 2002).
The quantity of symbiotic N fixation remains difficult to assess as it varies from species
to species and depends on environmental conditions ( Fageria and Stone, 2006; Jackson
et al., 2008).
A continuous tree growth depends upon the availability of nitrogen pool and its
efficient use. Nitrogen is a key element required for plant growth and its fixation is the
major route by which gaseous nitrogen is introduced into the ecosystem and the ability to
carry out this process is confined to a certain group of micro-organisms (Dalton, 1980).
Total global biological N fixation is approximately 17.2 x 107 tons/year. The relative
contribution of symbiotic or associate nitrogen fixing system to the total global level has
been assessed to be in the order of 70% symbiotic as compared to 30% non-symbiotic
nitrogen fixing system (Paul, 1988).
The nitrogen present in the soil is mostly organically bound and unavailable to
plants. After mineralization a very small fraction (3-7%) is converted into inorganic
forms (nitrate ammonium) and is readily available to plants. The atmosphere rich in
elementary N2 (78.09%) is the major source of N2 for plants. The plants gain this N2
through soil by various processes viz., biological fixation, non-biological fixation,
atmospheric precipitation and absorption of combined N2 from the atmosphere (Mandal
and Chhonkar, 1984).
Biologically fixed N is used directly by the plant and so it is less susceptible to
volatization, denitrification and leaching losses. Legumes are the components of various
cropping systems, grazing system, plantation systems and agroforestry system.
Legume nitrogen fixation starts with the formation of a nodule. A common soil
bacterium, Rhizobium, invades into the root and multiplies within the cortex cells. On the
other hand the plant supplies all the necessary nutrients and energy for the bacteria.
Within a week after infection, small nodules are visible with the naked eye. In the field,
small nodules can be seen 2-3 weeks after planting, depending on legume species and
germination conditions. When nodules are young and not yet fixing nitrogen, they are
usually white or gray inside. As nodules grow, they gradually turn pink or reddish in
color, indicating nitrogen fixation has started. The pink or red color is caused by
leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to the bacteria.
Nodules on many perennial legumes, such as alfalfa and clover, are fingerlike in
shape. Mature nodules may actually resemble a hand with a center mass (palm) and
protruding portions (fingers), although the entire nodule is generally less than 1/2 inch in
diameter. Nodules on perennials are long-lived and will fix nitrogen through the entire
growing season, as long as conditions are favorable. Most of the nodules (10-50 per large
alfalfa plant) will be centered around the tap root. Legume nodules that are no longer
fixing nitrogen usually turn green and may actually be discarded by the plant. Pink or red
nodules should predominate on a legume in the middle of the growing season. If white,
grey or green nodules predominate, little nitrogen fixation is occurring as a result of an
inefficient Rhizobium strain, poor plant nutrition, pod filling or other plant stress.
The nodulated legumes contributed a good deal to the amount of nitrogen fixed in
the biosphere.
Leguminous plants belong to the family leguminaceae which is sub
divided into three sub familities – Caesalpineadeae, Mimoseadeae and Paipllionoideae. It
is known that certain tree legumes do not possess nodules. Hardly 16% of the legumes
have been examined for nodulation, so far. It was found that 95% of Mimosoideae , 26%
of Caesalpinoideae and 90% of Paillionoideae posses root nodules.
The total N2 fixed by the legumes in the field is difficult to assess accurately (La
Rue and Patterson, 1981) and vary from <1 to >7500 kg/ha/year depending upon the
species and the environmental conditions (Laurie, 1981; Nutman, 1976). However, since
the capacity to fix N2 depends on several factors, such as physical, environment,
nutritional and biological, it can not be assumed that any N2 fixing system will
automatically make large contribution to the N cycle.
The basic reason for the poor growth of various species growing in the degraded
land is the deficiency of available nutrients and nitrogen in particular. Importance of
nitrogen fixing herb, shrub and climber species during the primary succession is well
known. These species are generally more adaptive and have potential to grow on poor
soil with low moisture regime. Introduction of nitrogen fixing herb, climber and shrub
species may provide a suitable means for biological amelioration and rejuvenation of site
as a prelude to introduction of tree species, because direct introduction of tree species on
the rocky barren, severely eroded areas and cliffs is very difficult and expensive at the
initial stage.
Though literature is available on various aspects of BNF, but most of it deals with
N2 fixation by herbaceous species. Although several tree and shrubs are known to have
N2 fixing qualities, efforts to exploit these in productive land use system started only
recently, concurrent with the interest in agroforestry.
Agroforestry system offers a
unique opportunity for exploiting the nitrogen fixing qualities of multipurpose trees and
shrubs Gastav Dalberg (Parrotta, 1989, Lapido, 1991).
Formation of symbiotically effective root nodules initiated by signaling between
host and microsymbiont. Flavinoids and isoflavinoids released from the root of the
legume, host induce transcription of nodulation genes in compatible rhizobia, leading to
the formation of lipochito oligosaccharide molecules that, in turn signal the host plant to
begin nodule formation.
The uptake of nitrogen by the plant species is known as nitrogen assimilation. N2
is the most important element of all living organisms after C, H and O. The acquisition
and assimilation of N is second only to photosynthesis in terms of importance for plant
growth and development. On an average 15% of protein and 13% of nucleic acid is
contributed by this element (Inokuchi et al., 2002). It has been estimated that 6400 kg of
elemental N2 is present above every 1 hactare of land (Food and Agriculture
Organisation, 1984).
Dalbergia sissoo is native to the Himalayan foothills in India, Pakistan and Nepal.
It is a deciduous tree, often with crooked trunk and light crown. Under favourable
condition, it attains a height of about 30m, a girth upto 3m and clear bole up to 15m; it
has long thick tap and ramifying lateral roots. Both tap and lateral roots are commonly
covered with nodules in soil infected with nitrogen fixing Rhizobium spp.(Troup, 1921).
Traditionally plantations of Dalbergia sisso are raised through seeds, which show
considerable variations in growth, yield, seed setting, and disease resistance resulting in
unpredictable yields, which is not conducive for successful plantation. Therefore, tree
improvement programme for this species is very essential.
Dalbergia sisoo, a multipurpose tree crop, has been recognized for the production
of fuel wood, timber and forage in plantation programmes and forestry systems. Apart
from its ability to fix atmospheric nitrogen, this spp. is able to release a considerable
amount of nitrogen rich organic biomass, which decomposes quickly and improves the
nutrient status of the soil. Selection of the higher nitrogen fixing plants would therefore
greatly increase the usefulness of D.Sissso in plantation programes, particularly on
degraded wastelands. Presently Dalbergia sissoo plants are prepared though cutting as
well as from seedlings for propagation in the forest nurseries. Both the methods are good
but the vegetative method is superior over the later due to the maintenance of character of
parent plants. In case of seed raised plants there are chances of segregation of characters.
However, there will be some differences in their growth behaviour, and biomass and
nodulation pattern. In both the cases no comparative studies have been conducted in
respect to growth, biomass and other enzymatic behaviour.
In general nitrogen fixing plants require no extra nitrogenous fertilizer for their
growth and development and can be beneficial in management practice that is alternate,
mixed and under planting etc. To improve the nitrogen status of soil, it is necessary to
select the indigenous nitrogen fixing plants in a particular agro climatic zone so that
symbiotic association can be useful for rejuvenating the sites for different planting
combinations. There is need for the screening of compatible nitrogen fixing plants
available at different drought regions in the northwestern India.
Vegetative propagation is generally considered as an important method for
multiplication, genotypes and thus maintained its parental character. In case of
propagation by seed there is chance of segregation of characters, which may not give the
identical plants. The goal is to get the best planting stock with highest genetic quality
material through vegetative propagation (Nanda, 1970; Wright, 1976; Hartmann and
Kester, 1985). Historically vegetative propagation in forest trees was originally used for
multiplication of superior multipurpose species, which reproduce vegetatively under
natural conditions, such as Poplars, Eucalyptus and Cryptomeria etc. It was adopted by
tree breeders to multiply the selected trees; mostly through grafting for use in seed
orchards and also to preserve germplasm in clonal banks. Recently techniques have been
developed in many forest tree species for mass propagation through cuttings of cost
similar to those of seedlings.
Low fertility is one of the major problems in establishing vegetation on the
degraded and barren land. Since nitrogen is generally deficient in these soils, therefore
the reforestation can be accomplished either by increasing the input or using such species
which can utilize atmospheric nitrogen apart from soil nitrogen for its growth,
development and regularly enrich the soil nitrogen status. Recently fast growing
multipurpose woody legumes have received increasing attention in tropical fertility.
They are used in timber, pulpwood, fuel wood, forage production and reclamation of
degraded land in great variety of agroforestry system.
Dalbergia sissoo can be vegetatively propagated by branch, stem, root, juvenile
shoot cutting and root suckers in order to reduce the phenotypic variation and to achieve
increased productivity. Selection of population, clonal study has been conducted by
several workers in different place.
The clonal planting stock can be accessed on the basis of higher biomass
production as well as high nitrogen fixing and high nitrogen assimilating ability for future
plantation to get improvement in forest productivity with quality timber.
Studies
involving certain physiological parameters will be useful for identification of productive
and adaptive genotype of Dalbergia sissoo tree, which can judiciously utilize the
available nitrogen from the soil as well as atmosphere for maximizing production in short
rotation under various afforestation programmes.
Therefore, in the present study an attempt has been made to investigate the
seasonal effects on nodulation and nitrogen assimilation behaviour of seedlings vis-à-vis
cutting raised plants in Shisham (Dalbergia sissoo).
Keeping in view the above facts, the main objectives of the present study are as
follows.
1. To compare seasonal growth behaviour and biomass production of cutting and
seedling raised plants.
2. To study the seasonal valuation effects on nitrogen assimilation behaviour between
seedlings and cuttings raised Dalbergia sissoo.
3. To study the effect of nitrogen fertilizer on seedling and cutting raised plants.