1 Introduction

Phosphorus (P) is the second most important nutrient for plants, just next to nitrogen (N), and makes up (0.05 -0.5)% dry weight of them (Malhotra et al. 2018). Though P is the 11th most abundant element of earth crust (Lian et al. 2019), 43% of the world arable land is P deficient (Liu et al. 2012). Due to very high reactivity, P gets fixed in soil. Only 0.1% of the total P in soil is available for plants (Rawat et al. 2021), which very often limits plant growth (Malhotra et al. 2018). Application of P fertilizer is inevitable for modern-day intensive cultivation (Maharana et al. 2021; Roy et al. 2015). The P fertilizers are produced from high-grade rock phosphates (HGRPs, contains > 30% P2O5) (Ditta et al. 2018; Sarkar et al. 2018). That is a finite and non-renewable source with ~ 60,000 million tonnes (Mt) world reserve (Kauwenbergh 2010). The predicted life time of this reserve is 75–175 years (Roy et al. 2018). Very soon we will suffer from scarcity of cheap and readily available good-quality P-fertilizers (Sarkar et al. 2020). In the absence of good-quality P-fertilizers intensive cultivation to feed the ever-increasing world population will be a superhuman task (Roy et al. 2018). Utilization of less expensive low-grade rock phosphate (LGRP) as P source to grow crops can reduce cost of cultivation, at the same time P fertilizer consumption may also reduce; as a result, life span of HGRP may increase. The LGRP (contains <20 % P2O5) is not used by the P fertilizer industries. Their low P2O5 and high CaCO3 content is not suitable for economic production of P fertilizers (Kumari and Phogat 2008). India has ~200 Mt of RP reserve, which is mostly LGRP (Basak 2019). The country annually imports ~7.5 Mt HGRP for meeting its P fertilizer demand (Sujata 2014), 95% of that is consumed by agricultural sector (Kumari and Phogat 2008). Imported HGRP increases the cost of P fertilizer that eventually declines P fertilizer consumption and results into imbalance fertilization (FAI 2014). Utilization of LGRP as replacement or supplement to the costly chemical P fertilizers can reduce the problems to a great extent. Rock phosphates (RP) as such are quite inert to be applied directly to grow crops. Those need to be modified or activated for their better utilization as P source to crops (Basak 2019). Organic acid treatment can increase P availability from LGRP (Roy et al. 2018). Oxalic acid performs comprehensively to solubilize P from RP (Basak 2019). Mendes et al. (2020) found oxalic acid even more efficient than sulfuric acid to solubilize P from RP. Thus, in search of a lucid method to utilize indigenous LGRP as a cheaper alternate to the costly chemical fertilizers, Udaipur RP (LGRP) was treated with oxalic acid and used as supplemental P source to grow wheat. Different forms of soil P are in complex dynamic equilibrium among them. When P is taken up from available P pool by the growing crops, other soil P pools replenish that (Roy et al. 2018). Thus for clear understanding the P-supplying capacity of the soil under different treatments, short-term P dynamics among different soil P fractions and soil enzyme activities were also studied at different growth stages of wheat. It was hypothesized that oxalic acid treatment may increase P solubility from LGRP; also, oxalic acid may be absorbed or adhered by the LGRP and persist for long time in soil after application. Later, this adhered oxalic acid can mimic the P-solubilizing mechanism of phosphorus-solubilizing microorganisms and solubilize P from LGRP as well as from fixed P pools of soil. The present experiment aimed i) to study the effect of oxalic-acid-treated LGRP on soil available P status by an incubation experiment; ii) to evaluate the effectiveness of oxalic-acid-treated LGRP as supplemental P source to grow wheat; iii) to study the effect of oxalic-acid-treated LGRP on short-term P dynamics among different soil P fractions (i.e., total P. inorganic P, organic P, microbial biomass P, available P, saloid P, Al-P, Fe-P, reductant soluble P, Ca-P) at different growth stages of wheat, and iv) to study the enzyme activities (dehydrogenase, acid phosphatase and alkaline phosphatase) at different growth stages of wheat in Alfisol.

2 Material and methods

2.1 Collection and processing of soil

On the basis of prior soil testing, P-deficient bulk soil (0–15 cm) of Alfisol order was collected from the research farm of Palli Siksha Bhavana (Institute of Agriculture), Visva-Bharti University (Central University), Sriniketan, Birbhum, West Bengal, India. It was air-dried under shade and sieved through a 2-mm sieve and used for incubation and pot experiments conducted at Division of Soil Science and Agricultural Chemistry, Indian Council of Agricultural Research-Indian Agricultural research Institutes (ICAR-IARI), New Delhi. The soil was loamy sand in texture with 81.2% sand, 8.9% silt and 9.6% clay (Bouyoucos 1962), salinity-free (EC(1:2.5 soil: water) 0.10), acidic in reaction (pH(1:2.5 soil: water) 5.9) and having 6.7 cmol(p+) kg-1 cation exchange capacity (Jackson 1973). The soil had organic C 0.37% (Walkley and Black 1934), mineralizable nitrogen 59 mg kg-1 (Subbiah and Asija 1956), plant-available phosphorus 2.09 mg kg-1 (Bray and Kurtz 1945) and available potassium 30.1 mg kg-1 (Hanway and Heidel 1952). The initial soil contained 134 mg kg-1 total P, out of that 102 mg kg-1 was inorganic P and 32 mg kg-1 organic P (Page et al. 1982). Inorganic P had 1.57 mg kg-1 saloid P, 13.8 mg kg-1 Al-P, 33.9 mg kg-1 Fe-P, 26.9 mg kg-1 Ca-P, 25.7 mg kg-1 Reds-P (Kuo 1996). Dehydrogenase activity (DHA) of the initial soil was 7.1 μg TPF g-1 soil h-1 (Klein et al. 1971), alkaline phosphatase activity (Alkaline PA) was 39.5 μg PNP g-1 soil h-1 and acid phosphatase activity (Acid PA) was 97.3 μg PNP g-1 soil h-1 (Tabatabai and Bremner 1969).

2.2 Rock phosphate (RP)

Udaipur rock phosphate was used in this experiment. The LGRP (100 mesh size) was collected from Udaipur district of Rajasthan, India, and it had 8.8% total P, 7.24% citrate-insoluble P, 1.57% citrate-soluble P and 0.003% water-soluble P (Roy et al. 2018).

2.3 Preparation of oxalic-acid-treated rock phosphate

To prepare oxalic-acid-treated RP, 1 kg LGRP was mixed with 1 L of 0.5 M oxalic acid solution. The mixture was kept for four days at room temperature to react. It was stirred for 5–10 minutes, twice a day. Then, the oxalic-acid-loaded LGRP was dried properly at 85°C, in a hot air oven. The oxalic-acid-treated LGRP was used in incubation and pot experiments after drying.

2.4 Incubation study

The incubation experiment was conducted in a completely randomized design (CRD) with three treatments and three replications. The treatment details are (T1) control (No P was added), (T2) 120 mg P2O5 kg-1 soil from LGRP and (T3) 120 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP. In all the treatments, 100 mg N kg-1 soil (from urea) and 60 mg K2O kg-1 soil (from potassium chloride) were applied. After treating as per the treatment details, soil samples (2-mm sieved, 50 g) were incubated in screw-capped plastic bottles at 30±1°C with moisture content around field capacity (FC). Five sets of samples were kept for five sampling dates. Incubated soils were analysed for Bray and Kurtz No. 1 solution (0.03 N NH4F and 0.025 N HCl) extractable P (Bray and Kurtz 1945) at 15, 30, 45, 60 and 90 days after incubation (DAI).

2.5 Pot experiment

During winter (Rabi) season of 2017–2018, the pot experiment was conducted taking wheat (Triticum aestivum) as the test crop. The experiment was conducted in CRD design, with four treatments and three replications. The treatment details are (T1) control (no P was applied); (T2) LGRP (120 mg P2O5 kg-1 soil); (T3) Di-ammonium phosphate (DAP) (60 mg P2O5 kg-1 soil) and (T4) DAP+ oxalic-acid-treated LGRP (30 mg P2O5 kg-1 soil from DAP+ 60 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP). Pots were filled with five kg air-dried soil (4.75 mm sieved) after treating as per the treatment details. In each pot, 100 mg N kg-1 soil (from urea) and 60 mg K2O kg-1 soil (from potassium chloride) were added. In DAP-treated pots, N added through DAP was calculated and N addition from urea was adjusted. Five plants were grown in each pot. Three separate sets of pots with same treatment combinations were maintained for collecting soil and plant samples at three different growth stages of wheat.

2.6 Soil and plant sampling and analysis

At maximum tillering (40 days after sowing), flowering (90 days after sowing) and maturity stage of wheat soil and plant samples were collected. Total soil from each pot was taken on a clean plastic sheet and mixed well. Nearly 500 g root-free soil sample was collected from each pot of first second and third set of pots at maximum tillering flowering and maturity stages, respectively. Collected soil was air-dried and sieved through 2-mm sieve. Different soil P fractions and enzyme activities were analysed in the soil samples at all the three different growth stages. At each growth stage, total plant sample from each pot was collected, washed and air-dried in shed. The air-dried plant samples were finally dried in hot air oven at 65 ± 2°C for 72 h. Dry shoot biomass was recorded at maximum tillering and flowering stage of wheat. Straw and grain biomasses were recorded at maturity stage of wheat. The dried plant samples were grinded in Wiley mill followed by digestion with di-acid mixture (HNO3/HClO4 = 9:4). Total P in the digested plant sample was analysed spectrophotometrically by vanadomolybdo-phosphate yellow colour method (Jackson 1973).

2.7 Soil P fraction analysis

The total and inorganic P in soil were analysed by following the procedure outlined by Walker and Adams (1958). Inorganic P was subtracted from total P to get the estimate of organic P. Soil inorganic P was fractioned into saloid P (sal-P), aluminium P (Al-P), iron P (Fe-P), reductant soluble P (Reds-P) and calcium P (Ca-P). Different fractions of inorganic P were extracted following the sequential extraction method outlined by Kuo (1996). The sal-P, Al-P, Fe-P, and Ca-P contents in the aliquots were determined by ascorbic acid blue colour method (Watanabe and Olsen 1965). For determination of Reds-P, isobutyl alcohol and ammonium molybdate were added in a separatory funnel along with aliquot. The intensity of blue colour developed by the solution mixture in the separatory funnel was measured by spectrophotometer at 660 nm (Page et al. 1982). Plant-available P was extracted from soil by using Bray and Kurtz No. 1 solution (Bray and Kurtz, 1945), and P in the extract was determined by ascorbic acid blue colour method (Watanabe and Olsen 1965). The microbial biomass P (MBP) was estimated by following the procedure outlined by Wu et al. (2000).

2.8 Soil enzyme activities

Soil dehydrogenase activity was estimated by monitoring the production rate of triphenyl formazone (TPF) from triphenyl tetrazolium chloride (TTC) as per the standard protocol of Klein et al. (1971). The acid PA and alkaline PA were monitored as per the standard procedure described by Tabatabai and Bremner (1969).

2.9 Statistical analysis

Analysis of variance (ANOVA) of the data obtained from incubation and pot experiments was carried out with SAS version 9.3 software. At P ≤ 0.05, the least significance difference (LSD) between the treatment means was calculated (Gomez and Gomez 1984).

3 Result

3.1 Phosphorus release pattern from RP and oxalic-acid-treated RP

Outcome of incubation experiment confirmed the superiority of oxalic-acid-treated LGRP over untreated LGRP (Figure 1). Oxalic-acid-treated LGRP treatment (T3) maintained ~101, 83.9, 83.7, 83.9 and 84.1% higher available P over untreated LGRP treatment (T2) at 15, 30, 45, 60 and 90 DAI, respectively. That indicates, application of oxalic-acid-treated LGRP can supply more P to the crops as compared to inert raw LGRP. During the incubation period, P release pattern showed alternate trough and crest.

Figure 1
figure 1

Soil-available P during 90-day incubation period. T1= No P was added (control), T2=120 mg P2O5 kg-1 soil from untreated low-grade rock phosphate (LGRP); and T3= 120 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP. Treatment means with at least one common letter among them are statistically at par as per Duncan multiple range test at LSD (P < 0.05). Error bars indicate SD.

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3.2 Wheat yield and P uptake

P-untreated control (T1) and untreated LGRP (T2) treatments produced at par wheat yield and P uptake at all the crop growth stages. Wheat grain and straw yields under DAP+ oxalic-acid-treated LGRP treatment (T4) were significantly higher as compared to control (T1) and untreated LGRP (T2) treatments (Table 1). Grain and straw yields from DAP+ oxalic-acid-treated LGRP treatment (T4) were ~541 and 697% higher, as compared to those from untreated LGRP treatment (T2). Similarly, grain and straw P uptakes under DAP+ oxalic-acid-treated LGRP treatment (T4) were ~608 and 822% higher, respectively, over those from untreated LGRP treatment (T2). In DAP+ oxalic-acid-treated LGRP treatment (T4), 50% less fertilizer P was added as compared to DAP treatment (T3), and an attempt was made to supplement it by oxalic-acid-treated LGRP. Nevertheless, DAP+ oxalic-acid-treated LGRP treatment (T4) and DAP treatment (T3) maintained statistically at par grain and straw yields and P uptakes throughout the crop growth stages. Thus, wheat yield and P uptake data proved that oxalic-acid-treated LGRP successfully supplemented 50% DAP.

Table 1 Yield and P uptake at different growth stages of wheat during pot experiment
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3.3 Different forms of soil P

Soil total P and inorganic P were found significantly higher (Table 2) under untreated LGRP (T2) and DAP+ oxalic-acid-treated LGRP (T4) treatments, as compared to DAP treatment (T3). The DAP+ oxalic-acid-treated LGRP treatment (T4) maintained ~18, 19 and 20% higher total P, 21, 20 and 22% higher inorganic P as compared to DAP treatment (T3), at maximum tillering, flowering and maturity stages, respectively. Total P as well as inorganic P was maximum at maximum tillering stage and minimum at maturity stage of wheat. The DAP+ oxalic-acid-treated LGRP treatment (T4) provided ~174, 138 and 157% higher available P over LGRP treatment (T2) at maximum tillering, flowering and maturity stages, respectively (Table 2). Available P content under DAP+ oxalic-acid-treated LGRP (T4) and DAP (T3) treatments was statistically at par at all the crop growth stages. For all the treatments, available P was maximum at maximum tillering stage, minimum at flowering stage; interestingly, at maturity stage soil-available P content increased to higher level as compared to flowering stage of wheat.

Table 2 Total, inorganic and available P status of soil at different growth stages of wheat during pot experiment
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To clearly understand the short-term P dynamics in Alfisol, different fractions of inorganic P were studied at three different growth stages of wheat (Table 3). Under control treatment, Fe-P contributed most to the inorganic soil P followed by Ca-P, Reds-P, Al-P and Sal-P. But LGRP treatments increased the content of Ca-P fraction of the soil even above Fe-P. The Sal-P and Ca-P contents significantly increased under DAP+ oxalic-acid-treated LGRP treatment (T4) as compared to DAP treatment (T3). Under DAP+ oxalic-acid-treated LGRP treatment (T4), Sal-P was ~20, 28 and 28% higher and Ca-P was ~21, 20 and 18 % higher over those under DAP treatment (T3) at maximum tillering, flowering and maturity stages, respectively, whereas Al-P and Fe-P build up in soil were more from DAP treatment (T3) as compared to DAP+ oxalic-acid-treated LGRP (T4) and untreated LGRP treatments (T2).

Table 3 Inorganic P fractions of soil at different growth stages of wheat during pot experiment
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Organic P content was maximum at maximum tillering stage and minimum at flowering stage of wheat (Table 4). Maximum organic P was observed under DAP+ oxalic-acid-treated LGRP treatment (T4). Organic P from DAP+ oxalic-acid-treated LGRP treatment (T4) was ~7, 10 and 14% higher over those from DAP treatment (T3) at maximum tillering, flowering and maturity stages, respectively. Microbial biomass phosphorus (MBP) was maximum at flowering stage and minimum at maturity stage of the crop (Table 4). DAP+ oxalic-acid-treated LGRP treatment (T4) and DAP treatment (T3) maintained at par MBP throughout the crop growth stages.

Table 4 Organic P and MBP of soil at different growth stages of wheat during pot experiment
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3.4 Soil enzymes

For all the treatments, soil DHA (Figure 2) was maximum at flowering stage and minimum at maximum tillering stage, whereas acid PA (Figure 3) was maximum at flowering stage and minimum at maturity stage of wheat. DAP+ oxalic-acid-treated LGRP treatment (T4) and DAP treatment (T3) maintained at par DHA between them. Both the treatments provided significantly higher DHA over control (T1) and untreated LGRP (T2) treatments, at all the crop growth stages. For every treatment, alkaline PA (Figure 4) was least at flowering stage and maximum at maturity stage. Control (T1) and untreated LGRP (T2) treatments maintained maximum alkaline PA throughout the crop growth stages, which were at par with the alkaline PA under DAP+ oxalic-acid-treated LGRP treatment (T4). As compared to untreated LGRP treatment (T2), the DAP+ oxalic-acid-treated LGRP treatment (T4) produced ~29.2, 37.2 and 29.9% higher acid PA ~8.03, 9.0 and 5.68 % lower alkaline PA at maximum tillering, flowering and maturity stages, respectively.

Figure 2
figure 2

Soil dehydrogenase activity at different growth stages of wheat during pot experiment. T1= No P was added (Control), T2=120 mg P2O5 kg-1 soil from untreated low-grade rock phosphate (LGRP); T3= 60 mg P2O5 kg-1 soil from di-ammonium phosphate (DAP) and T4= 30 mg P2O5 kg-1 soil from DAP+ 60 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP. Treatment means with at least one common letter among them are statistically at par as per Duncan multiple range test at LSD (P < 0.05). Error bars indicate SD.

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Figure 3
figure 3

Soil acid phosphatase activity at different growth stage of wheat during pot experiment. T1= No P was added (control), T2=120 mg P2O5 kg-1 soil from untreated low-grade rock phosphate (LGRP); T3= 60 mg P2O5 kg-1 soil from di-ammonium phosphate (DAP) and T4= 30 mg P2O5 kg-1 soil from DAP+ 60 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP. Treatment means with at least one common letter among them are statistically at par as per Duncan multiple range test at LSD (P < 0.05). Error bars indicate SD.

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Figure 4
figure 4

Soil alkaline phosphatase activity at different growth stages of wheat during pot experiment. T1= No P was added (Control), T2=120 mg P2O5 kg-1 soil from untreated low-grade rock phosphate (LGRP); T3= 60 mg P2O5 kg-1 soil from di-ammonium phosphate (DAP) and T4= 30 mg P2O5 kg-1 soil from DAP+ 60 mg P2O5 kg-1 soil from oxalic-acid-treated LGRP. Treatment means with at least one common letter among them are statistically at par as per Duncan multiple range test at LSD (P < 0.05). Error bars indicate SD.

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4 Discussion

Oxalic acid (an organic acid) treatment can solubilize P from LGRP (Mendes et al. 2020). Organic acid anions (Jones, 1998) and protons (Calvaruso et al. 2006) associated with organic acids help them to solubilize P from RP as well as from soil. Complexation (Fox et al. 1990) or chelation (Sagoe et al. 1998) of Ca2+ from LGRP as well as from soil by oxalate and formation of calcium oxalate (Welch et al. 2002), proton-promoted P dissolution (Calvaruso et al. 2006, Hammond et al. 1986) are the mechanisms of P solubilization from LGRP by oxalic acid. During incubation experiment, oxalic acid treatment increased P solubilization from LGRP. As a result, oxalic-acid-treated LGRP treatment (T3) maintained significantly higher soil-available P over LGRP treatment (T2). During incubation period, P release pattern followed alternate peak and fall. Roy et al. (2018) also reported to have similar unique pattern of P release from LGRP in soil. Difference in P release and uptake by soil microorganisms (Rodríguez and Fraga 1999), P fixation by soil and its subsequent release by ligand exchange of phosphate (Otani et al. 1996) or anion exchange of PO43- by oxalate ions, formation of organic P compound by oxalate and their subsequent utilization by soil inherent microbes (Illmer and Schinner 1992) may be the probable reasons behind this unique P release pattern.

During pot experiment, untreated inert rock phosphate failed to supply significant amount of P (Basak 2019) to increase wheat yield and P uptake over control. As a result, P-untreated control (T1) and untreated LGRP treatments (T2) provided statistically at par yield and P uptake, whereas oxalic-acid-promoted P solubilization from LGRP increased P availability to the growing crop. The solubilized P may have supplemented 50% of the chemical P fertilizers; thus, in spite of having 50% less DAP addition, DAP+ oxalic-acid-treated LGRP treatment (T4) maintained at par yield and P uptake with DAP treatment (T3). The outcome of our study is in accordance with Roy et al. (2015); they also reported that oxalic-acid-treated LGRP treatment increased wheat yield and P uptake. The P in LGRP (Udaipur RP) is Ca-bound inorganic P (Roy et al. 2018). Higher amount of P was added through Udaipur RP in untreated LGRP (T2) and DAP+ oxalic-acid-treated LGRP (T4) treatments as compared DAP treatment (T3). Higher addition of Ca-bound inorganic P increased total P, inorganic P and Ca-P content in LGRP-treated soils (i.e., under T3 and T4 treatments) over DAP treatment (T3). P uptake by wheat removed P from soil in significant quantity, which may have decreased total P and inorganic P from maximum tillering to maturity of wheat. Oxalic acid treatment solubilized P from LGRP in substantial amount (Basak 2019, Mendes et al. 2020) and increased soil-available P. Thus, in spite of applying 50% less DAP; DAP+ oxalic-acid-treated LGRP treatment (T4) maintained at par available P with DAP treatment (T3) without reducing the yield and P uptake of wheat. P added in soil as chemical fertilizer under DAP treatment (T3) immediately got fixed by the soil or got converted to initial reaction products (Tiwari 2009). Whereas in DAP+ oxalic-acid-treated LGRP treatment (T4), oxalic acid solubilized P from LGRP and soil (Basak 2019), it also chelated and complexed phosphate-bounding Al3+, Fe3+ and Ca2+ cations (Mendes et al. 2020). Oxalate anions also competed with phosphate for common bounding sites in soil (Biswas et al. 2021). Thus presence of oxalic acid may have reduced P fixation in soil also. As a result DAP+ oxalic-acid-treated LGRP treatment (T4) maintained significantly higher Sal-P as compared to DAP treatment (T3). Maximum P removal occurred during the period of maximum tillering to flowering stage of wheat, which may have brought available P and saloid P level to minimum at flowering stage, and disturbed the equilibrium between soil solution P and fixed P. After maturity of wheat, soil P removal by the crop stopped. The equilibrium between soil-fixed P and solution P was restored with time. As a result, P from other soil P fractions was added to soil-available P and Sal-P and increased their concentration (Biswas et al. 2021; Roy et al. 2018). Phloem transport also translocates back some plant nutrients from shoot to root and root to soil after maturity of the plant (Biswas et al., 2017; Mengel and Kirkby 2012), which can increase soil available P and Sal-P, content after maturity stage as compared to flowering stage of wheat. Biswas et al. (2021) also reported that soil-available P was minimum at 70 days after transplanting rice and that increased after maturity of rice in an Inceptisol. Soil biological parameters like organic P, MBP, DHA, acid PA and alkaline PA were also altered by external application of P and crop growth stages. During the period of maximum tillering stage to flowering stage, shoot and root growth rate of wheat become maximum (Biswas et al. 2017); thereby, at flowering stage amount of root exudates in soil upsurges, which act as food and energy sources for soil microorganism and increase the growth and activity of soil microbes (Biswas et al. 2019); these microbes immobilized soil P in their body and increased MBP to maximum at flowering stage. Soil dehydrogenase is an oxido-reductase enzyme that indicates total oxidative activity by soil micro-flora (Nannipieri et al.2017). Thus increased microbial activity may have increased soil DHA. Our findings are in agreement with Bera et al. (2017), Biswas et al. (2019), where they found maximum DHA at maximum vegetative growth period of wheat and those were less at maturity stage and earlier growth stage. Along with soil microbes, plant roots also produce acid phosphatase, and alkaline phosphatase is solely produced by soil bacteria (McLachlan 1980), thus, at flowering stage increased root growth and microbial activity increased the level of acid PA to maximum, and at the same time rhizosphere acidification may have reduced the alkaline PA to the minimum (Eivazi and Tabatabai 1977). Alkaline PA was maximum in the P-limited condition, i.e., under control (T1) and untreated LGRP (T2) treatments. Cotner and Wetzel (1991) also found higher alkaline PA in P-limited conditions. Under DAP+ oxalic-acid-treated LGRP treatment (T4), oxalic acid may have acidified soil micro-environment; as a result, acid PA increased and alkaline PA decreased under this treatment as compared to untreated LGRP (T2) treatment (Kamran et al. 2018). At flowering stage, higher root growth and augmented microbial activity increased acid PA; this subsequently mineralized and broke down soil organic P (Zhu et al. 2017) and decreased its level to minimum.

5 Conclusion

Higher available P under oxalic-acid-treated low-grade rock phosphate (LGRP) treatment over control and untreated rock phosphate treatment, throughout the incubation period, indicates oxalic acid treatment can increase P availability from LGRPs. It also indicates oxalic-acid-treated LGRP can maintain higher available P in soil for long duration. This statement was supported by the outcome of pot experiment. There di-ammonium phosphate (DAP)+ oxalic-acid-treated LGRP treatment maintained similar crop yield and P uptake as compared to DAP treatment. Though the earlier treatment was having 50% less added P via DAP. That indicates oxalic-acid-treated LGRP can supplement chemical P fertilizer to grow wheat in Alfisol. Application of oxalic-acid-treated LGRP (@60 mg P2O5 kg-1 soil) can reduce 50% of the chemical P fertilizer consumption by wheat in Alfisol. Study of different soil P fractions and enzyme activities at different growth stages of wheat has given proper insight into short-term P dynamics and enzyme activities under wheat in Alfisol. Beside supporting wheat yield and P uptake, oxalic-acid-treated rock phosphate treatment can improve the status of soil saloid P, total P, inorganic P and calcium P as compared to chemical fertilizer treatment, those can exert beneficial residual effect on the next crop. Soil-available P, saloid P, organic P and alkaline phosphatase activity reduce to minimum at flowering stage and increase after maturity of wheat. Soil dehydrogenase activity, acid phosphatase activity and microbial biomass phosphorus increase to maximum at flowering and reduce after maturity of wheat. Further, a field experiment on this topic can more consolidate the fact of using oxalic-acid-treated low-grade rock phosphate as supplemental P source to grow wheat in Alfisol.