Phosphorus (P) is an essential nutrient element for the growth of living organisms including plants, animals and microorganisms. It is a component of the adenosine triphosphate (ATP) that drives most energy-requiring biochemical processes, deoxyribonucleic acid (DNA) that is the seat of genetic inheritance, ribonucleic acid (RNA) that directs protein synthesis in plants and animals, phospholipids that compose cellular membranes and intermediate compounds of respiration and photosynthesis (Brady and Weil, 1996; Taiz and Zeiger, 1998; Fuentes et al., 2006).For most plant species, it is the second most abundantly required nutrient element after nitrogen and the total phosphorus content of healthy leaf tissue range between 0.2 and 0.4% of the dry matter (Brady and Weil, 2002).
Phosphorus in soil comes from both pedogenic and anthropogenic sources (Bolan et al., 2005). In spite of its wide distribution in nature, P is a limited resource (Adnan et al., 2003; Shimamura et al., 2003) and is deficient in most soils with respect to its availability to plants (Vassilev et al., 2001). The P problem in soil fertility is threefold. First, the total P level of soils is 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 those in fertilizers and manures are added to soils, they are fixed and only 10 to 15 percent of the P added through fertilizers is likely to be taken up by plants in the year of application (Brady and Weil, 2002). Hence, low P bioavailability limits crop production under most soil conditions.
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The research work carried out so far either at national or international level 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. 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. Microbial biomass is the most labile fraction of soil organic matter, and plays a vital role in sustainability of soil fertility and the functioning of soil ecosystem (Jenkinson and Ladd, 1981; Smith and Paul, 1990; Khan and Joergensen, 2006). The magnitude of microbial biomass pool directly affects the nutrients flux and their bioavailability in soil.
Studies on the dynamics of microbial biomass, microbial P and enzymes activities such as dehydrogenase and alkaline phosphatase 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 will therefore be conducted to achieve the following objectives:
To study different phosphorus fractions in soils of Potohar and their relationship with soil physical, chemical, and microbial parameters.
To evaluate dynamics of different P fractions in soil in response to various organic amendments and their relationship to soil microbial parameters.
REVIEW OF LITERATURE:
The literature pertaining to the proposed research is reviewed as under:
Stewart and Tiessen (1987) studied dynamics of soil organic phosphorus. They reported that microbial uptake of P and its subsequent release and redistribution play a central role in the soil organic P cycle. Interactions with soil minerals and stabilization of organic matter and associated P in organo-mineral complexes determine the persistence and build up of organic P through soil development, in different ecosystems and management conditions. They concluded that an understanding of organic P turnover in soils will be highly helpful in assessment of P fertility of many agricultural and native systems.
Lee et al. (1990) studied the influence of microbial activity in mobilizing P, maintaining it in a plant-available state, and preventing its fixation, and the effect of N and biocides on these processes in a highly weathered Ultisol. Exchangeable aluminum and soil moisture were also determined, since they interact with microbes and soil P. They found that increased microbial activity reduced sorption of dissolved and organic P by soil, maintained inorganic P in soluble and labile pools, increased microbial P, decreased mineral P, increased exchangeable Al, and water retention. Additions of N and biocides had variable effects probably due to complex interactions between N, degrading biocides and microbial populations.
Maguire et al. (2000) conducted a study to identify the effect of biosolids applications on the forms and release potential of P in agricultural soils. They collected samples from eight farms with a history of biosolids amendments, selecting fields that had setback areas (where biosolids applications were not permitted) to allow comparison of amended and unamended soils and analyzed them for P fractions (soluble P, Al-P, Fe-P, reluctant soluble P, and Ca-P; their sum equals total P), sequentially desorbable P (Fe-strip), oxalate P, Al and Fe, Mehlich-1 P, and the degree of P saturation. Results showed that following a N-based biosolids nutrient management plan could significantly increase total P (from 403 to 738 mg kg-1) and initially desorbable P (from 32 to 61 mg kg-1). The main soil components associated with P retention (Alox and Feox) also tended to be increased by biosolids amendment and this may help mitigate P release. Biosolids amendment significantly increased Fe-P (from 137 to 311 mg kg-1), probably due to Fe added to biosolids during production, and there was also a strong trend for higher Al-P where biosolids had been applied. Desorbable P was initially greatest from biosolids sites, but with increasing extractions, the release converged towards that from the setback areas. Mehlich-1 P and Pox were good predictors of desorbable P release, as measured by one and five sequential extractions with Fe-strips. Desorbable P, by both one and five Fe-strip extractions, was more closely correlated with Al-P than Fe-P, especially in setback areas, indicating that Al-P is probably the most important source of desorbable P independent of biosolids amendment.
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Qualls and Richardson (2000) studied the influence of P additions on the elevation of microbial biomass P in the soil. In order to isolate the effects of P enrichment, they placed bags containing cattail (Typha domengensis Crantz) and sawgrass (Cladium jamaicense Pers.) litter into two sets of experimental channels into which controlled inputs of five different phosphate concentrations were added continuously. After one year of incubation, litter was analyzed for C, P, N, Cu, Ca, and K content. Loss of C at the end of one year increased linearly with increasing average PO4 content in the channels with a similar slope for both species of litter. Immobilization caused an absolute increase in P content of the litter up to approximately nine fold across the range of water P concentrations, while immobilization of N, Ca, and K did not vary with water P concentrations. The microbial biomass P was up to nine times higher in the surface soil of the most enriched channel compared with the control, but this elevation in concentration was restricted to the upper 12 cm of soil.
Cleveland et al. (2002) tested the effects of P availability on the decomposition of multiple forms of C, including dissolved organic carbon and soil organic carbon (SOC) using natural gradients in P fertility created by soils of varying age underlying tropical rain forests in southwestern Costa Rica, combined with direct manipulations of carbon (C) and P supply. Results from a combination of laboratory and field experiments suggested that C decomposition in old, highly weathered oxisol soils is strongly constrained by P availability. In addition, P additions to these soils (no C added) further revealed that microbial utilization of at least labile fractions of SOC was also P limited. This was regarded to be the first direct evidence of P limitation of microbial processes in tropical rain forest soil. They suggested that P limitation of microbial decomposition might have profound implications for C cycling in moist tropical forests, including their potential response to increasing atmospheric carbon dioxide.
Kabba and Aulakh (2004) conducted an experiment to examine the effect of climatic conditions and crop residue quality on N, P, and S mineralization in soils with contrasting P status. They observed the effect of three temperatures (15 °C, 30 °C, and 45 °C) and two moisture regimes (60% and 90% water-filled pore space (WFPS)) on the mineralization-immobilization of N, P, and S from groundnut (Arachis hypogea) and rapeseed (Brassica napus) residues (4 t ha-1) in two soils with contrasting P fertility. Crop residue mineralization was differentially affected by incubation temperature, soil aeration status, and residue quality. Only the application of groundnut residues (low C: nutrient ratios) resulted in a positive net N and P mineralization within 30 days of incubation, while net N and P immobilization was observed with rapeseed residues. The initial P content influenced the mineralization of N and P, which was significantly higher in the soil with a high initial P fertility (18 mg P (kg soil)-1) than in soil with low P status (8 mg P (kg soil)-1).
Saleque et al. (2004) conducted the experiment to evaluate the effect of different nutrient management in wetland rice on the changes of soil P fraction at different depths. Soil samples from five depths (0-5, 5-10, 10-15, 15-30, and 30-50 cm) were collected from a long-term experimental field. The field received six treatments for 10 year: absolute control with no fertilizer applied (T1), one-third of recommended fertilizer doses (T2), two-thirds of recommended fertilizer doses (T3), full doses of recommended fertilizers (T4), T2 + 5 Mg cow dung (CD) and 2.5 Mg ash ha-1 (T5), and T3 + 5 Mg CD and 2.5 Mg ash ha-1 (T6). The apparent balance of P compared with the initial P status after 10 years varied from -115 kg ha-1 under T1 to 348 kg ha-1 under T6. The P fractionation study was conducted over the treatments and soil depth. Treatment and depth had no significant effect on solution P. Larger concentrations of NaHCO3 soluble P, NaOH extracted inorganic P (Pi), and acid P were observed under treatments with organic fertilizers (T5 and T6) than with other treatments at 0 to 5, 5 to 10, and 10 to 15cm depths. The concentrations of NaHCO3-P, NaOH-Pi and acid P fractions were lowest under T1 and T2 treatments. At 15 to 30 cm or lower soil depths, none of the P fractions were affected by treatments. The change in NaOH organic P (Po) and residual P (extracted with HNO3 + HClO4) with soil depth was not significant, and the differences in these P fractions under the tested P treatments were not large.
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Kaur et al. (2005) conducted studies to compare the soils receiving organic manures with and without chemical fertilizers for the last 7 years with pearl millet-wheat cropping sequence for soil chemical and biological properties. The application of farmyard manure, poultry manure and sugarcane filter cake alone or in combination with chemical fertilizers improved the soil organic C, total N, P, and K status. The increase in soil microbial biomass C and N was observed in soils receiving organic manures only or with the combined application of organic manures and chemical fertilizers compared to soils receiving chemical fertilizers only. Basal and glucose-induced respiration, potentially mineralizable N, and arginine ammonification were higher in soils amended with organic manures with or without chemical fertilizers, indicating that more active microflora is associated with organic and integrated system using organic manures and chemical fertilizers together which is important for nutrient cycling.
Khan and Joergensen (2006) conducted a study to analyze the amounts of microbial-biomass C, biomass N, and biomass P in 11 rain-fed arable soils of the Potohar plateau, Pakistan, in relation to the element-specific total storage compartment, i.e., soil organic C, total N, and total P. Average contents of soil organic C, total N, and total P were 3.9, 0.32, and 0.61 mg per g soil, respectively. Less than 1 percent of total P was extractable with 0.5 M NaHCO3. Mean contents of microbial biomass C, biomass N, and biomass P were 118.4, 12.0, and 3.9 µg per g soil, respectively. Values of microbial biomass C, biomass N, biomass P, soil organic C, and total N were all highly significantly interrelated. The mean crop yield level was closely connected with all soil organic matter and microbial biomass-related properties. The fraction of NaHCO3-extractable P was also closely related to soil organic matter, soil microbial biomass, and crop yield level. This revealed the overwhelming importance of biological processes for P turnover in alkaline soils.
Smith et al. (2006) conducted an experiment to determine the effect of different stages of sewage sludge treatment on phosphorus (P) dynamics in amended soils using samples of undigested liquid (UL), anaerobically digested liquid (AD) and dewatered anaerobically digested (DC) sludge. Sludges were taken from three points in the same treatment stream and applied to a sandy loam soil in field-based mesocosms at 4, 8 and 16 t ha-1 dry solids. Mesocosms were sown with perennial ryegrass (Lolium perenne cv. Melle), and the sward was harvested after 35 and 70 days to determine yield and foliar P concentration. Soils were also sampled during this period to measure P transformations and the activities of acid phosphomonoesterase and phosphodiesterase. Data showed that the AD amended soils had the greatest plant-available and foliar P content up to the second harvest, but the UL amended soils had the greatest enzyme activity. Characterization of control and 16 t ha-1 soils and sludge using solution 31P nuclear magnetic resonance (NMR) spectroscopy after NaOH-EDTA extraction revealed that P was predominantly in the inorganic pool in all three sludge samples, with the highest proportion (of the total extracted P) as inorganic P in the anaerobically digested liquid sludge. After sludge incorporation, P was immobilized to organic species. The majority of organic P was in monoester-P forms, while the remainder of organic P (diester P and phosphonate P) was more susceptible to transformations through time and showed variation with sludge type.
MATERIALS AND METHODS:
EXPERIMENTS:
The study will consist of three experiments, details of which are described below:
Study-1:
Study of phosphorus fractions in Pothwar soils and their relationship with soil microbial and biochemical parameters.
In this study, representative soil samples from 12-15 prominent soil series of the Pothwar plateau will be collected from the agricultural fields. The soil samples of 1.5 kg will be taken at 0-15 cm depth from 4 different locations of each of the selected site. The samples will be properly labeled, packed in polyethylene bags, brought to the laboratory. The field moist samples will be hand picked to remove stones, larger plant residues and soil animals (earth worms etc.), passed through a 2-mm sieve, mixed thoroughly and stored in polyethylene bags at 5 °C prior to biological analysis.
A portion of each soil sample will be air-dried, ground to powder form and analyzed for selected physico-chemical properties such as particle size analysis, EC, pH, CEC, CaCO3, organic C, total N, total P, P fractionation and water soluble cations (Na, K, Ca, Mg) and anions (CO3, HCO3, Cl, SO4).The soil samples prepared and stored for biological analysis will be equilibrated to room temperature, incubated at 30 °C for 7 days after moisture adjustment to 50 percent of their water holding capacity (WHC) and analyzed for microbial biomass C, biomass N, biomass P, soil respiration, and the activities of enzymes like dehydrogenase and alkaline phosphatase. The data obtained will be analysed statistically to evaluate the relationship of different P fractions with soil physical, chemical, microbial and biochemical properties.
Study-II:
Dynamics of phosphorus fractions in soils amended with organic manures and their relationship with soil microbial and biochemical parameters.
In this study, two soils deficient in plant available phosphorus with variable physico-chemical properties such as clay content, pH, or organic C will be selected on the basis of the results of study-I. The soils will be adjusted to 50 percent of their water holding capacity and incubated at 30 °C for 7 days prior to amendment addition. The treatments will include: 1) Control; 2) Farmyard manure (FYM); 3) Poultry litter (PL) and 4) Biogenic waste compost (BWC), each applied to 600 g (oven dry basis) soil separately at the rate of 1 percent of the oven dry soil weight. All the treatments will be quadruplicated according to completely randomized design (CRD).
After amendment addition, soil samples will be transferred to 2 litre capacity incubation jars and incubated at 30 °C for a period of 72 days. The CO2 evolved will be absorbed into 2M NaOH solution in 100 ml beakers. The NaOH solution will be changed after 1, 2, 3, 5, 7, 10, and 14 days and thereafter weekly. Soil samples of 50 g oven dry weight will be taken at 0, 14, 28, 56, and 72 days of incubation for the determination of microbial biomass C, biomass N, biomass P, and 0.5M NaHCO3-extractable P. Different P fractions and activities of enzymes like dehydrogenase and alkaline phosphatase will be measured in samples collected at 0 and 72 days of incubation.
Study-III:
Relationship between microbial biomass, enzyme activities and P availability in soils amended with organic manures under wheat crop.
A greenhouse experiment will be conducted in completely randomized design (CRD) to evaluate the effect of organic amendments on the relationship between soil microbial biomass, enzyme activities and P availability in soil under wheat crop. For this purpose, two soils used in study-II will be collected, passed through 2-mm sieve and amended with organic manures. The treatments will include: 1) Control; 2) Farmyard manure (FYM); 3) Poultry litter (PL) and 4) Biogenic compost (BC), each applied to 5 kg (oven dry basis) soil separately at the rate of 1 percent of the oven dry soil weight. All the treatments will be quadruplicated and the moisture contents will be adjusted to field capacity gravimetrically.
Seeds of wheat will be sown after 2 weeks of organic amendments addition. Soil samples will be collected at 0, 14, 28, 42 and 56 days after the sowing of seeds and analyzed for microbial biomass C, biomass N, biomass P, enzyme activities like dehydrogenase and alkaline phosphatase, and 0.5M NaHCO3 extractable P. After 56 days, plants will be harvested and data on plant growth parameters such as plant height, oven dry weight etc. will be recorded. Plant samples will be washed properly with distilled water, oven dried at 60 oC, ground in Wiley Mill and analyzed for important macro and micronutrients and phosphorus uptake will be calculated.
ANALYTICAL METHODS:
Analyses of Soil Microbial Biomass:
Soil Enzymes Analysis:
Plant Analyses:
Following plant analyses will be carried out:
REFERENCES:
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