Chapter 1
INTRODUCTION
Phosphorus is a vital nutrient element with respect to the growth of living
organisms including plants, animals and microorganisms. It is an essential
constituent of the adenosine triphosphate (ATP) which is often called the energy
currency of living cells and drives all energy-requiring metabolic processes. For
example, nutrients uptake by the plant roots, their translocation to other parts of
plant and assimilation into various biomolecules, all are energy requiring processes
in which the ATP plays a vital role. Phosphorus is also an integral part of
deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and phospholipids. It
enhances numerous components of plant development as adequate P stimulates
root development, strengthens the stalk and stem, improves flower and seed
formation, hastens crop maturity and increases nitrogen fixing capacity. It is
required particularly in large quantities in meristematic tissues where cell division
and enlargement takes place rapidly. For most plant species, phosphorus is the
second most abundantly required nutrient element after nitrogen and the total
phosphorus content of healthy leaf tissue ranges between 0.2 and 0.4% of the dry
matter (Taiz and Zeiger, 1998; Fuentes et al., 2006; Brady and Weil, 2008).
In soil, P exists in three main groups of compounds which include organic
phosphorus, aluminum- and iron-bound inorganic phosphorus and calcium-bound
inorganic phosphorus. Most soil organic P compounds are esters of orthophosphate
including inositol phosphates (10 to 50%), phospholipids (1 to 5%), and nucleic
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8
acids (0.2 to 2.5%). Of the inorganic P, aluminum- and iron-associated forms are
dominant in acid soils whereas calcium-associated compounds predominate in most
of the alkaline soils. The relative quantities of organic and inorganic P forms vary
widely among soils. In surface soils, organic fraction usually contributes 20 to 80%
of the total P while inorganic fractions may dominate in deeper soil horizons. All
three types of P compounds slowly release P into the soil solution but most of the P
in these compounds is of low solubility and is not readily available for plant
uptake. Thus, soil solution has usually very small proportion of the total P (0.01%)
in soil (Havlin et al., 2005; Brady and Weil, 2008). Plants take up P from soil
solution as primary and secondary orthophosphate ions (H2PO4- & HPO4-2) largely,
but some soluble low molecular weight organic phosphorus compounds like
nucleic acids and phytin are also absorbed by plant roots (Havlin et al., 2005). The
chemical forms of the P present in soil solution are primarily determined by pH of
the soil solution. Primary orthophosphate (H2PO4-) dominates in strongly acidic
soils having pH 4.0 to 5.5 whereas secondary orthophosphate (HPO4-2) dominates
in alkaline soils. However, the H2PO4- is considered relatively more available for
plant uptake (Ahmad and Rashid, 2003; Brady and Weil, 2008).
Despite its widespread distribution in nature, P is a limited resource and
deficient in most agricultural soils with respect to its availability to crop plants
(Shimamura et al., 2003; Fuentes et al., 2006). The seriousness of the problem may
be realized from the fact that an area over 2 billion hectares across the world and
about 80 to 90% of arid and semiarid soils is suffering from P deficiency problem
with respect to its P availability to plants (Fairhurst et al., 1999; NFDC, 2001).
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Basically the P problem in soil fertility is three-fold. First, the total P contents in
soils are low ranging from 200 to 2000 kg P per hectare-furrow slice (HFS) with an
average of about 1000 kg P per HFS. Second, the P compounds commonly present
in soils are mostly unavailable for plant uptake, often because they are highly
insoluble. Third, when soluble sources of P such as mineral P fertilizers and
manures are added to soils, they are largely (about 85-90% of the added inorganic
P) fixed and become unavailable to plants in the year of application due to complex
adsorption/ precipitation reactions with Fe, Al and Mn components in acidic and
Ca components in alkaline/ calcareous soils (Hinsinger, 2001; Khiari and Parent,
2005; Brady and Weil, 2008; Khan and Joergensen, 2009). Consequently farmers
are enforced to add two to four times as much P as is removed in the harvested
portion of crop to get optimum crop yields. The inorganic P fertilizers are very
expensive and unaffordable for majority of the resource poor farmers of developing
countries like Pakistan. In developed countries, fertilizer P use may be economical
and cost effective but its excessive use over crop requirements has adverse
implications on ecosystem in the form of eutrophication (Memon, 2008). Also, the
reserves of high grade rock phosphate used in the preparation of inorganic P
fertilizers, are depleting rapidly due to intensive mining across the world and
according to some estimates, global P resources are likely to be exhausted by 2050
(Vance et al., 2003) which poses a potential threat to the food security of the world.
It is, therefore, of paramount importance to develop some practicable and costeffective P management strategies to enhance P use efficiency in soil-plant system
(Gichangi et al., 2009).
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Various physical and chemical soil management techniques have been
evolved by the researchers to improve P use efficiency in soil. Such techniques are
meant to improve P availability, either by affecting P solubility and/ or fixation in
soil or by improving soil conditions for root growth and development. Among
these, improving soil structure, breaking of plough pan, loosening of compact soil,
band placement of P fertilizer, combined application of N and P fertilizers, use of
slow release P fertilizers, liming of acidic soils and acidification of alkaline soils
are more important. Although, individually each of these approaches might make
little contribution towards the improvement of P use efficiency in soil but their
combined effect may be reasonably large and noteworthy in some specific crop
production systems. A few adverse effects are also associated with the use of some
of these techniques e.g., increases in particulate P losses have been observed with
the application of slow release P fertilizers in grazed pastures on sloping lands
(Brady and Weil, 2008). Therefore, in spite of adoption of all above strategies, the
improvement in P use efficiency is not up to the mark and yet much more potential
exists to conduct research on this burning issue.
To address this critical issue, certain biological approaches such as crop
rotation, enhanced biological P cycling by addition of organic matter, enhancement
of mycorrhizal symbiosis, use of P-efficient plants and introduction of P
solubilizing microorganisms have been used widely. Among these, enhanced
biological P cycling through addition of different organic materials has shown
more potential to improving P use efficiency. Therefore, use of different organic
materials including manures, composts, crop residues and other rural and urban
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biogenic wastes has been commonly advocated by various researchers in order to
enhance P bioavailability in soil and to resolve the critical issue of low P use
efficiency (Schefe et al., 2008; Gichangi et al., 2009; Takeda et al., 2009).
Application of crop residues, composts and other organic materials enhance P
availability in soil because they contain P as well as they may also induce changes
in native soil P dynamics, e.g., decreasing P sorption capacity (Ayaga et al., 2006;
Schefe et al., 2008; Khan and Joergensen, 2009). The lower P sorption capacity
may be due to various mechanisms e.g., an improvement in soil pH, hiding of P
retention sites by organic acids released in consequence of decomposition of
organic materials, competition with phosphate ions for P sorption sites and
complexation of ions like soluble Fe, Al and Mn by organic anions (Erich et al.,
2002, Khan and Joergensen, 2009). Different organic materials vary in their quality
and P content and accordingly they have variable effects on P sorption capacity of
soils (Singh and Jones, 1976). Similarly the organic acids/ anions released during
decomposition of organic materials also vary in their effects on P retention (Hue,
1991; Reddy et al., 2005).
Another important mechanism that might be involved in increasing P use
efficiency is the assimilation of P in microbial cells and their associated
metabolites, preventing its fixation in highly insoluble and recalcitrant forms
(Gichangi et al., 2009; Khan and Joergensen, 2009). Addition of organic materials
in soil increases soil microbial biomass on account of addition of microorganisms
contained in them as well as easily mineralizable substrate-C and N, resulting in
stimulation of indigenous soil micro-biota which may increase P uptake into the
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microbial biomass (Ayaga et al., 2006; Gaind et al., 2006; Takeda et al., 2009).
Organic P bound in microbial cells is easily hydrolysable and turn into plants
available form after mineralization (Kouno et al., 2002; Takeda et al., 2009). The P
released on microbial turnover can be taken up by the plants more efficiently as
compared to a single large pulse of P added in the form of inorganic or organic
fertilizers (Ayaga et al., 2006; Gichangi et al., 2009; Khan and Joergensen, 2009).
Thus incorporation of P from less available P forms to microbial cells can be a
useful tool for enhancing P use efficiency in soils with high P fixing capacities.
However, microbial biomass P sustains the P supply at lower levels as it usually
ranges 2-5% of total P in soil with substantially variable absolute concentrations
(Kouno et al., 2002, Takeda et al., 2009). Achat et al. (2010) reported the stock of
microbial biomass P up to 21.6 kg ha-1 in wet soils which could be more than the
requirement of a number of crop plants. Anderson and Domsch (1980) observed
relatively much higher quantities of P (83 kg ha-1) in microflora of the soils in
upper 12.5 cm layer.
Microorganisms can play an important function in P cycling by excreting
enzymes like dehydrogenase and phosphatase. Addition of organic amendments
stimulates microbial population to produce these enzymes resulting in enhancement
of organic matter decomposition and organic P mineralization (Takeda et al.,
2009). The dehydrogenase enzyme activity is an index of oxidative activities in
soil (Trevors, 1984) and usually considered as an indicator of microbial activity
(Burns, 1978). Dehydrogenase performs a vital task in the oxidation of soil
organic matter through transfer of protons from substrates to acceptors. This
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enzyme is deemed as an essential part of intact cells and thus, its extracellular
accumulation in the soil does not take place. Phosphatases represent a wide group
of enzymes that act as a catalyst to hydrolyze the ester-phosphate bonds resulting
in release of phosphate (Tabatabai, 1994; Quiquampoix and Mousain, 2005) and
have broad substrate specificity. The phosphatase activities indicate the biological
activity and organic P mineralization potential in soils (Kramer and Green, 2000).
Higher microbial activity after organic amendments application increases
phosphatase activity and thus releases P during the decomposition of organic
matter.
Plants are fed by soil solution P which is replenished by different P pools in
soils including microbial biomass P (Mengel and Kirkby, 2001; Saleque et al.,
2004; Gichangi et al., 2009). These pools affect P availability to plants by affecting
desorption/ release patterns of P in soil (Ahmad et al., 2006). Therefore,
information about different P fractions in soils, their dynamics, and relationship
with available P is of potential significance to increase P use efficiency and better P
management (Mostashari et al., 2008; Halajnia et al., 2009). Sequential extraction
of various P pools has proved more helpful in P management as compared to the
conventional methods used for the determination of available P because it
partitions the soil P into different organic and inorganic P pools of varying plant
availability. The information on organic P pools which constitute 20-80% of the
total soil P (Brady and Weil, 2008) and contributes significantly to the plant
available P in most soils is very important. It provides information about NaOH
and HCl extractable P pools in a given soil in addition to resin or NaHCO3
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extractable P pools. These P fractions are considered very important as they
replenish the depleting available/ labile P pool (Saleque et al., 2004). Furthermore,
it furnishes information about the redistribution of soil P after organic or inorganic
P additions.
In view of above it is obvious that all the processes involved in the
transformation
of
organic
P
are
biologically
mediated
and
therefore,
microorganisms execute a key function in the recycling of P contained in organic
amendments and the soil organic matter. The research work carried out so far to
improve P nutrition of crop plants has been mainly confined to exploring the
physical and chemical processes related to improving P availability in soil, whereas
the microbial and biochemical processes which are the key elements behind almost
all biochemical transformations leading to nutrients bioavailability in soil have in
general been less explored. Therefore, studies on the dynamics of microbial
biomass, MBP and enzymes activities such as alkaline phosphatase and
dehydrogenase in relation to different P fractions in soil are important for
understanding the role and contribution of different microbial/ biochemical
parameters to P bioavailability in soils. The proposed research work was therefore
conducted to achieve the following objectives:
1.
To study different phosphorus fractions in soils of Potohar and their
relationship with soil physical, chemical, and microbial parameters.
2.
To study the dynamics of different P fractions in soil in response to various
organic amendments and their relationship to soil microbial parameters.
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3.
To elucidate the organic amendments effect on phosphorus availability in
soil and its uptake by wheat plants.
4.
To evaluate the effect of different organic and inorganic P sources on P
pools, microbial biomass and microbial community structure in a P
deficient soil.