Transformation of Different Fractions of N and P in Soil

A laboratory experiment was conducted to monitor the changes in different forms of inorganic and organic N and P in an alluvial and an acidic soil fertilized with inorganic N and P either alone or in combination. Results revealed that in both the soils exchangeable NH 4 + and soluble NO 3 - decreased with increase in the period of investigation. Irrespective of soils, addition of inorganic N alone causes faster rate of N loss from the soil than combined application of N and P fertilizers. Combined application of N and P fertilizers accentuates mineralization process of hydrolysable NH 4 + and amino acid-N fractions particularly, in acidic Jhargram soil. Irrespective of soils, the highest amount of decrease in active P (saloid-P + Al-P + Fe-P + Ca-P) was recorded in the treatment fertilized with inorganic P alone. Higher amount of decrease in organic P was recorded in alluvial Mohanpur soil than that of acidic Jhargram soil treated with inorganic P alone.

N and P are the major nutrients required by plants for growth and metabolic activities and are mostly taken up from the soil. Both N and P present in soil generally comes from natural and applied fertilizers. The amount of N and P supplied during a cropping season is influenced by many factors. Environmental conditions as well as management practices change the liberation process and also quantity of soil N and P to be released from its organic form for plant utilization.
A bulk of total nitrogen is present in organic form and only about 2 per cent in inorganic form, except where large quantities of inorganic nitrogen fertilizers have been added. The organic form of N, particularly the hydrolysable form, is slowly mineralized and is transformed to mineral N. As it is well established that native soil N-transformation is different from that of fertilizer N, it is necessary to understand the N-transformation process, which ultimately influences the N-uptake by plants.
Phosphorus is a vital component of the substances that are building blocks of genes and chromosomes. It plays an important role in virtually every plant process that involves energy transfer. It has already been established that quantity of total P has little or no relationship to the availability of P to plants. Thus understanding the relationship between various forms of P, their interactions in soils and various factors influencing P availability to plants is essential for efficient P management in soil. According to Chang and Jackson (1957) phosphorus bound to Al (Al-P), Fe (Fe-P) and Ca (Ca-P) constitutes the major active forms of inorganic P and are most available to plants. Another highly available form of P is the saloid bound 'P' defined by Peterson and Corey (1966). Organic phosphorus, which constitutes a substantial portion, of 21 to 74 per cent (Omotoso, 1971) of total soil phosphorus, mainly accumulates in soil from plant, bacterial, fungal and animal residues.
Application of N-fertilizer may influence the P transformation processes in soil. On the other hand, P-fertilization may also affect the transformation processes of different inorganic and organic forms of N in soils. However, very little investigations were carried out in this regard to the understand the relation ship among different forms of inorganic and organic N and P in soils fertilized with or without N and P fertilizers either alone or in combination.

Soil collection
Soils (0-15 cm) used for the present investigation were composite samples collected from the cultivated fields Instructional farm at Mohanpur in the district of Nadia and Jhargram farm in the district of West Midnapur that belongs to Bidhan Chandra Agricultural University in West Bengal, India. Before use, soils were air dried, ground and passed through 80 mesh sieve.
The physical and chemical properties of both the soils are presented in Table 1.

Experimental set up
Fifteen gram each of air dried soil samples were taken in 100ml beakers for the incubation study. There were four treatments replicated thrice. The 1 st treatment was control without addition of any inorganic N and P fertilizer. In the 2 nd treatment, inorganic N was added at 100 mg kg -1 N in the form of ammonium sulphate ((NH 4 ) 2 SO 4 ). In the 3 rd treatment, P was added as treatment material at 50 mg kg -1 P 2 O 5 as Single Super Phosphate (SSP). The 4 th treatment was both inorganic N and P added at 100 mg kg -1 and 50 mg kg -1 in the form of (NH 4 ) 2 SO 4 and SSP, respectively. All the treatment combinations were replicated thrice. Respective soil samples were moistened to 60 per cent of the Moisture Holding Capacity (MHC) by the addition of distilled water. After moistening, the soils were maintained at room temperature (30 ± 2 0 C) for a period of 105 days. Five separate sets were maintained for laboratory analysis on 0, 15, 45, 75 and 105 th day of incubation. Samples were collected on respective days for analysis of different fractions of N and P. Loss of moisture due to evaporation was replenished by the addition of distilled water on every alternate day by assessing the difference in weight.

Methods followed
Exchangeable NH 4 + and soluble NO 3 were determined according to the method of Bremner and Keeney (1966). Among the different fractions of organic N, total hydrolysable organic N, hydrolysable NH 4 + and amino acid-N, which are most susceptible to mineralization, were determined by the method of Stevenson (1996). Available phosphorus was determined using Bray and Kurtz extractant for the Jhargram soil and Olsen extractant for Mohanpur soil following the method of Jackson (1967). Fractions of soil P were determined according to the method of Chang and Jackson (1957). Saloid P was estimated by the method of Peterson and Corey (1966). Organic P was determined following the method of Jackson (1967). Total active P which is presented in the text is computed by adding Al-P. Fe-P, Ca-P and Saloid-P following the method of Prasad and Power (1997).

Results and Discussion
Irrespective of soils and treatments, the amount of exchangeable NH 4 + decreased with increase in the period of incubation ( Table 2). The decrease in exchangeable NH 4 + with time might be due to its conversion to NO 3 form or loss through denitrification (Groffman et al., 1987) and/or volatilization (Freney and Black, 1988). Irrespective of soils, addition of  Table 2 clearly pointed out that irrespective of soils, addition of inorganic N alone causes faster rate of N loss from the soil, than combined application of N and P fertilizers because of the priming effect of N-fertilization (Saha and Mukhopadhyay, 1984). The results also showed that addition of inorganic P fertilizer slowed down the N-mineralisation rate. This is perhaps due to the decrease in activities of ammonifying bacteria in the presence of added inorganic P (Kitayama, 1996). Duncan's statistical test of the results in Table  2 also reveal that, irrespective of sampling days, addition of inorganic N either alone or in combination with inorganic P fertilizer significantly affect exchangeable NH 4 + -N in both the Mohanpur and Jhargram soils.
Irrespective of soils and treatments, the amount of soluble NO 3 --N (Table 3) decreased with increase in the period of investigation. The amount of decrease was more pronounced in Mohanpur than that of Jhargram soil because of the variation in Table 2. Changes in the amount (mg kg -1 ) of exchangeable NH 4 + in soils treated with or without inorganic N and P either alone or in combination total N content of the respective soil under investigation. The results also clearly state that absence of N and P or presence of N alone or in combination with P did not affect the NO 3 accumulation process to a great extent in both the soils.   fraction. It is important to mention that both the hydrolysable NH 4 + -N and amino acid-N fractions of organic N are highly susceptible to mineralization in presence of both the inorganic N and P fertilizers particularly, in Jhargram soil, which is acidic in reaction. This is perhaps because of fixation of added inorganic P by Fe and Al present in acid soil forming Fe and Al-phosphates, respectively. Duncan statistical test reveal that changes in the amount of amino acid N did not behave in a similar way as was found for other inorganic and organic forms of N. Addition of inorganic P encouraged conversion of available P to other forms. The lowest amount of decrease in available P (Table 6) was recorded in both the soils, in the treatment which received combined application of inorganic N and P. This is due to interaction effect (Kitayama, 1996). Duncan's results revealed that addition of inorganic P alone as treatment significantly influences the changes in the amount of available P in Mohanpur soil throughout the experimentation period.  Table 6. Changes in the amount (mg kg -1 ) of Available-P in soils treated with or without inorganic N and P either alone or in combination S 1 = Mohanpur Soil ; S 2 =Jhargram Soil; (a,b,c,d) and (p,q,r,s) define Duncan's results for Mohanpur and Jhargram soil respectively.

Table 4. Changes in the amount (mg kg -1 ) of hydrolysable-NH 4 + -N in soils treated with or without inorganic N and P either alone or in combination
Irrespective of soils, the highest amount of decrease in active P (Saloid-P + Al-P + Fe-P + Ca-P) was recorded on 105 th over 0 th day of investigation where, addition of inorganic P alone was done (Table  7). Again, irrespective of treatments and sampling days, comparatively higher amount of active P is Table 5. Changes in the amount (mg Kg -1 ) of Amino acid-N in soils treated with or without inorganic N and P either alone or in combination Table 7. Changes in the amount (mg kg -1 ) of active-P in soils treated with or without inorganic N and P either alone or in combination accumulated in Jhargram over that of Mohanpur soil. Thus it is clear from the results that the amount of active P in the soil is not the only factor but also the physical and chemical properties of soil is of great concern for transformation of this fraction of inorganic P in a soil system (Zhao et. al, 2010). Duncan test reveals that addition of inorganic P fertilizer alone significantly influences the changes in active P over the whole experimentation period in both the soils under study. Furthermore, it is shown that the addition of both N and P fertilizer also significantly affected the transformation process of active P in Jhargram but not in Mohanpur soil. This is due to presence of higher amount of active P in Jhargram soil.
Comparatively higher amount of decrease in organic P was recorded in Mohanpur than that of Jhargram soil treated with inorganic P alone ( Table  8). Addition of inorganic P alone increased the Table 8. Changes in the amount (mg kg -1 ) of organic P in soils treated with or without inorganic N and P either alone or in combination organic P content due to temporary tying up of the P by microorganisms. Prasad and Power (1997) also found that the rate of organic P mineralization is of lower order in acidic soils. Data in Table 8 further reveal that combined addition of inorganic N and P showed a completely reverse trend of results in both the soils. Addition of inorganic N and P makes a balanced nutrition for growth and activities of microorganisms resulting higher amount of immobilization of inorganic P in both the soils (Powell et. al, 1999). Results of the Duncan's test revealed that addition of inorganic P alone, significantly influence the changes in the amount of organic P in Jhargram soil but not to that extent in Mohanpur soil particularly, at the later stage of incubation.

Conclusion
Combined application of inorganic N and P helps to retain comparatively higher amount of N in available form in soils. This is due to faster rate of N mineralization of organic N. Application of inorganic P decreased active P content in soil. The content of organic P in a particular soil plays an important role with respect to its mineralization.