Effect of Soil Nutrient Management on P transformation under Protected Cultivation

Phosphorus, an essential element, plays a central role in cell metabolism and reproduction. It is a structural component of energy transferring molecules (ATP, ADP and AMP), nucleic acids, coenzymes, phosphor-proteins, phospholipids and sugar phosphates. In soils, the P does not occur abundantly as nitrogen or potassium. But only a proportion (~ 0.1-1.0%) of it is available for use by plants and microorganisms because phosphorous has poor solubility and gets fixed in soil (Bednarek, 1992).
       
Phosphorus exists in soil in various forms. These may be categorized into four groups (Barber 1984), namely, (i) P in ions and compound in the soil solution (solution P); (ii) P adsorbed on the surface of inorganic soil constituents; (iii) P minerals, both crystalline and amorphous; and (iv) P as a compounds of organic matter. Solution P is a form that occurs in minimal quantity yet directly affects the P availability to plants in the forms of dihydrogen orthophosphate (H₂PO₄^-) and mono hydrogen orthophosphate (HPO₄^-2).
       
Various P phases and fractions can be transformed to other fractions under certain conditions and have remarkable differences in mobility, bioavailability and chemical behaviors (Sharpley et al., 2000). Sequential extraction technique is used to define P fractions in soils qualitatively and quantitatively. Such information is useful for predicting P bioavailability, movement and transformations in agricultural soils. Thus, the main aim of this experiment were to find out how nutrient management influencing P fractions under protected cultivation.

The investigation area from which soil samples were collected i.e., Horticultural Farm, Assam Agricultural University, Jorhat-13, Assam is a part of old flood plain of the Brahmaputra river and located in between 26°44'N latitude and 94°12'S longitude and at an elevation of 91.0 m above mean sea level. The study was carried out under protected poly house condition in the year 2017-18. The test crop was Capsicum (Capsicum annum var. Swarna). The design of the experiment was Split-Split plot technique. The land was thoroughly prepared and divided into 3 blocks with 27 treatment plots per blocks Details of all the treatments are furnished in Table 1. Random soil samples were collected from different treatments under protected cultivation at a depth of 0-15 cm. The soil samples were collected at two different stages of the crop viz., flowering and fruiting stages at 45 and 115 DAP respectively. The soil were air dried, grounded and passed through a 2 mm sieve and analysed for different physico-chemical properties (soil texture (International pipette method), bulk density (Core method), particle density (Keen rackzowski box) along with Organic carbon (Walkey and Black (1934) method), Cation Exchange Capacity (NH₄OAc Method) and Initial N (Alkaline potassium permanganate method), P (Bray’s-I method (1945) and K (Neutral normal ammonium acetate method) content and details flourished in Table 2.
 

 

       
Fractionation of phosphorus was done by extracting the soil with different extracting agent. Various fraction of inorganic P (Pi) was sequentially extracted and the concentration of Pi in the extract was immediately determined by the phosphomolybdate colorimetric method of Murphy and Riley (1962) as described by Kuo (1996).
 
Statistical analysis
 
Data were subjected to analysis of variance using statistical package ‘MSTAT-C’ package (Freed, 2006). Whenever the F-test was significant (at 5% level) multiple comparison among the treatments were done with Duncan’s Multiple Range test (DMRT).

The results of the experiment that was carried out shows variations in the various P fractions upon nutrient management.
 
Loosely bound Phosphate (Pi-water)
 
During flowering stage of the crop, single factor effect of different level of nitrogen, phosphorus and potassium on loosely bound phosphate (Pi-water) was significant (Table 2a). The loosely bound phosphate (Pi-water) was significant with the interaction effect of NK (Table 4). The values varied between 1.821 and 2.900 mg kg^-1 and the highest value was measures under N₁₀₀K₆₀. Moreover, interaction effect of PK (Table 5) with loosely bound phosphate (Pi-water) was also found to be significant and the highest value (3.124 mg kg^-1) was found in the treatment P₈₀K₄₀. Loosely bound phosphate (Pi-water) was found to differ significantly with the interaction effect of NPK (Table 6) and the values varied from 1.336 (N₈₀P₆₀K₈₀) to 3.682 mg kg^-1 (N₁₂₀P₈₀K₆₀). During fruiting stage of the crop, single factor effect of different level of nitrogen, phosphorus and potassium on loosely bound phosphate (Pi-water) was also significant (Table 2). Besides, loosely bound phosphate (Pi-water) was significant with the interaction effect of NK (Table 4). The values varied between 3.637 and 4.553mg kg^-1 and the highest value was noted under N₁₀₀K₈₀. The interaction effect of PK with loosely bound phosphate (Pi-water) was also found to be significant and the highest value (4.728 mg kg^-1) was registered under P80K80. Loosely bound phosphate (Pi-water) differed significantly with the interaction effect of NPK (Table 6) and the values varied from 2.719 mg kg^-1 (N₁₀₀P₈₀K₄₀) to 5.766 mg kg^-1 (N₁₀₀P₈₀K₈₀).
 

       
Loosely bound phosphate (Pi-water) significantly differed with the application of different levels of N, P and K fertilizers and their interaction. The easily soluble phosphate forms, increased with the application of higher dose of nutrients, although their concentration was low as compared to other labile and non-labile fractions. Thus mineral fertilization significantly affected the easily soluble phosphate fraction in soil. However, this P fraction relatively prone to migrate to the 20-40 cm soil layer under the influence of acid rain (Bednarek, Kaczor 1994) but it is relatively easily taken up by plants. The loosely bound phosphate or the Resin-Pi increased during fruiting stage. This may be due to transformation of relatively unavailable forms of P into available forms. Similar results were also reported by Chmielews et al., (1980). This increase in the fractions followed by decrease in the other fraction (NaOH-Pi, Ca-Pi) confirms the occurrence of P transformation in soil.

Aluminum phosphate (Pi-Al)
 
During flowering stage of the crop, single factor effect of different level of nitrogen, phosphorus and potassium on aluminum phosphate (Pi-Al) was significant (Table 2). NP interaction was also significant with the aluminum phosphate (Pi-Al) and the values varied from 2.637 to 9.174 mg kg^-1 (Table 3). The highest value was noticed under N₁₂₀P₈₀. Besides, aluminum phosphate (Pi-Al) was also significant under interaction NK. The lowest (3.598 mg kg^-1) and the highest (6.258 mg kg^-1) values were obtained in the treatments N₁₂₀K₈₀ and N₈₀K₄₀ respectively. Additionally, interaction of PK with the aluminum phosphate (Pi-Al) was significant and the highest value (7.357 mg kg^-1) was recorded in the treatment P₆₀K₄₀ (Table 5). During fruiting stage of the crop, single factor effect of different level of nitrogen, phosphorus and potassium was found to be significant (Table 2) with the aluminum phosphate (Pi-Al). NP interaction was also significant with the aluminum phosphate (Pi-Al) and the values varied from 5.933 to 13.62 mg kg^-1 (Table 3). Aluminum phosphate (Pi-Al) was also significant under interaction of NK. Interaction PK with the aluminum phosphate (Pi-Al) was found to be significant. Aluminum phosphate (Pi-Al) also differed significantly under interaction of NPK (Table 6) and the values ranged between 2.954 mg kg^-1 (N₈₀P₄₀K₆₀) and 14.94 mg kg^-1 (N₁₀₀P₈₀K₄₀).
 

       
Aluminum phosphate (Pi-Al) also differed significantly under interaction of NPK and the values ranged between 1.135 (N80P40K40) and 11.79 mg kg^-1 (N₁₂₀P₈₀K₆₀). The aluminum phosphate (Pi-Al) also significantly differed with the different levels of NPK applied.The Pi-Al fraction increased with the increase in  level of nutrients. Similar results were also found by Chmielewska et al., (1980) and Bednarek (2011). At the same time, this fraction had a relatively high proportion in available phosphorus, as determined by different methods (Alexander and Robertson 1968). The Pi-Al content was ranged from 1.135-11.97 mg kg^-1 in the flowering stage to 2.94-14.94 mg kg^-1 of soil in the fruiting stage. The increase in the concentration of Pi-Al may be attributed to transformation of unavailable form into available form (Larsen S 1967).
 
Iron phosphate (Pi-Fe)
 
Data indicates that during flowering stage of the crop, single factor effect of nitrogen, phosphorus and potassium on Iron phosphate (Pi-Fe) was significant (Table 2). Interaction effect of NP on iron phosphate (Pi-Fe) was also significant and the highest value (90.89 mg kg^-1) was recorded under N₈₀P₈₀ (Table 3). Iron phosphate (Pi-Fe) was also significant under interaction of NK and the highest value (84.19 mg kg^-1) was recorded under N80K80. Besides, interaction effect of PK was also significant with the iron phosphate (Pi-Fe) and the values ranged between 70.40 and 87.81 mg kg^-1 (Table 5). The highest value was obtained in the treatment receiving the highest level of phosphorus. Iron phosphate (Pi-Fe) was also differed significantly under interaction of NPK (Table 6) and the highest value (102.9 mg kg^-1) was obtained in the treatment N₈₀P₈₀K₈₀. Data showed that during fruiting stage of the crop, single factor effect of nitrogen was significant (Table 2) with the iron phosphate (Pi-Fe) and the values ranged from 55.84 to 64.28 mg kg^-1. Further, different level of phosphorus and potassium on iron phosphate (Pi-Fe) was also significant. Interaction effect of NP on iron phosphate (Pi-Fe) was also significant and the highest value (71.51 mg kg^-1) was found in the treatment N₈₀P₈₀. Beside this, iron phosphate (Pi-Fe) was also significant with the interaction effect of NK and the highest value (66.56 mg kg^-1) was recorded under N₈₀K₈₀ (Table 4). Moreover, interaction of PK was also significant with the iron phosphate (Pi-Fe). The highest value was obtained in the treatment receiving the highest level of phosphorus and lowest dose of potassium (P₈₀K₄₀). Iron phosphate (Pi-Fe) was also differed significantly with the interaction of NPK (Table 6) and the highest value (73.61 mg kg^-1) was obtained in the treatment N₈₀P₆₀K₄₀.
 

 

 

       
The iron phosphate (Pi-Fe) also differed significantly with the nutrient management and these fractions constitute a larger pool of P as compared to labile forms of phosphate (Resin-Pi and NaHCO₃-Pi). The soils of Assam contain high amount of exchangeable Al₃+  (Sahu et al., 2001). The acidity of the soils of north eastern region was attributed to the presence of Al3+ in clay complex. This explains the dominance of either Pi-Al or Pi-Fe fraction in acid soils of Assam (Halder et al., 1981). This pool is considered to be important to plant P nutrition and important in P transformation (Kumoyo et al., 2005). The iron phosphate fraction decreased at fruiting stage and this may be because of transformation of iron bound phosphate to more labile forms of phosphate and thereby increasing the availability of labile P fractions (Chao et al., 2015). In this way, hydroxide or acid extractable Pi (NaOH-Pi or HCl-P) may act as the quantity factor that buffers the more labile P forms.
 
Calcium phosphate (Pi-Ca)
 
Data revealed that during flowering stage of the crop, single factor effect of different levels of nitrogen, phosphorus and potassium on calcium phosphate (Pi-Ca) was significant (Table 2). Interaction effect of NP on calcium phosphate (Pi-Ca) was also significant (Table 3) and the values varied between 9.992 and 15.76 mg kg-1and the highest value was obtained under N₁₂₀P₄₀. Calcium phosphate (Pi-Ca) also differed significantly under interaction of NK and PK. Calcium phosphate (Pi-Ca) differed significantly under interaction of NPK (Table 6). The lowest value (5.098 mg kg^-1) and the highest value (21.50 mg kg^-1) were recorded under N₈₀P₆₀K₆₀ and N₈₀P₈₀K₈₀ respectively. Data revealed that during fruiting stage of the crop, single factor effect of different levels of nitrogen, phosphorus and potassium was significant (Table 2) with calcium phosphate (Pi-Ca). Interaction of different levels of NP on calcium phosphate (Pi-Ca) was also significant and the values varied between 9.23 and 17.07 mg kg-1 and the highest value was obtained under N₁₂₀P₄₀. Besides, calcium phosphate (Pi-Ca) was also significant under interaction effect of NK (Table 4). Interaction of PK was also significant under calcium phosphate (Pi-Ca) and the values ranged from 10.24 mg kg^-1 (P₄₀K₈₀) to 17.79 mg kg^-1 (P₄₀K₆₀).  Calcium phosphate (Pi-Ca) was differed significantly under the interaction of NPK (Table 6). The lowest value (3.928 mg kg^-1) and the highest value (22.45 mg kg^-1) were recorded under N₈₀P₆₀K₆₀ and N₈₀P₈₀K₈₀ respectively.
       
Apatite phosphate (Pi-Ca) differed significantly with the different levels of NPK and the values ranged between 5.09-21.50 mg kg^-1 at the flowering stage and 3.928-22.45 g kg^-1 at fruiting stage. This fraction also constitutes a small proportion as compared to the total Pi fraction in soil. This is because as the soil is acidic, they do not contains much calcium as most of the cations are leached down due to rainfall or application of irrigation (Kumoyo et al., 2005). Talashikar et al., (2006) in some lateritic soils of Konkan region of Maharashtra found that Total-P, Ca-P and Occluded-P increased with the increased in acidity.
 
Residual phosphate (Residual-Pi)
 
Data indicated that during flowering stage of the crop, the single factor effect of different level of nitrogen on residual phosphate (Residual-Pi) was non-significant (Table 2). The effect of levels of phosphorus and potassium on residual phosphate (Residual-Pi) was also non-significant.  Interaction effect of NP with the residual phosphate (Residual-Pi) was also non-significant (Table 3). Besides, residual fraction of P was also non-significant with the interaction of NK. Moreover, interaction of PK and NPK was also non-significant with the residual phosphate (Residual-Pi).
       
Residual phosphate (Residual-Pi) was non-significant with the interaction effect of NPK. The concentrated H₂SO₄ digested P (Residual P) is the P which is not readily removed by 0.5 M NaHCO₃, 0.1M NaOH or 1M HCl extracting solution and is considered to be a recalcitrant P form of very low solubility and availability, with the residual P as the most resistant fraction (Tiessen and Moir, 1993). It was found that residual P was very high indicating that most of the soil P was found in residual form. The nutrient management on residual-P was non-significant as it has very low solubility and do not had any effect on application of different levels of fertilizers (O’Halloran, 1993; Richards et al., 1995; Tran and N’dayegamiye, 1995; Zhang and MacKenzie, 1997a, 1997b).
 
Effect of nutrient management on yield and yield prediction by various P fractions
 
Data indicated that nutrient management does influence the yield of capsicum. Single factor effect of different nitrogen levels on yield was significant (Table 7) and the highest value obtained at N₁₂₀ (4288 g/plant). Phosphorus also significantly influence the yield of capsicum and the highest value was recorded on P₈₀ (3577 g/plant). Different levels of potassium were not significantly influence the yield of capsicum.
 

       
Using Artificial Neural Networking method (Fig 1), we tried to find out which P fraction is able to contribute more to the yield of capsicum and we found that Pi-water and Pi-Fe have an impact on yield of capsicum indicating that these two fractions contribute more to the yield of capsicum as compared to other P fractions.
 

The various fractions of P in the plow layer (0-15 cm) of acidic soil under 27 different treatments of nutrient management were evaluated using a sequential extraction method. Results revealed that, different soil nutrient management has a significant influence on various P fractions viz., Pi-water, Al-P, Fe-P, Ca-P, except for residual-P fractions. Soil nutrient management also influences the transformation of P. The availability of readily available P tends to increase in the fruiting stage as compared to the flowering stage of the crop which may be due to transformation of P from unavailable form to available form, which in turn helps in growth and development of the plant. Resin-Pi and NaOH-Pi contribute more to the yield attributes as compared to the other P fractions.