Evaluating the benefits of legacy phosphate

doi:10.21203/rs.3.rs-1433952/v1

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Research Article

Evaluating the benefits of legacy phosphate

https://doi.org/10.21203/rs.3.rs-1433952/v1

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Aims

Many soils in the developed world have been fertilised with phosphate for several decades. Appreciable phosphate has accumulated in such soils. This is referred to as “legacy P”. The benefits of legacy P are not widely appreciated.

Methods

We produced five levels of legacy P by incubating the soil with added phosphate at high temperatures for 30 days. We then measured the effect on sorption/desorption and on plant growth in response to further additions of phosphate.

Results

Legacy P decreased soil buffering capacity, decreased hysteresis of desorption, and increased the amount of P in a similar state to that of the recently sorbed P. There were analogous effects of plant growth; effectiveness of P fertiliser increased; the decline in effectiveness with time decreased; and the amount of soil P accessible to plants increased. We think that soil tests reflect only the amount of soil P accessible to plants. Soil testing services which include estimates of phosphate buffering reflect its decrease and the consequent increase in fertilizer effectiveness. We propose a simple test to reflect the rate of decline in effectiveness with time.

Conclusions

Managing phosphate applications using soil tests alone underestimates phosphate status and leads to over application of phosphate. It is necessary to also include estimates of the change in phosphate buffering and the change in the rate of decline in effectiveness with time. A simple way estimating the rate of decline is suggested.

It is widely believed that it is necessary to apply more phosphate (P) than is needed by subsequent crops. This has led to large accumulations of P especially in soils of the developed world (Syers et al. 2008, Condron et al. 2013, Doydora et al. 2020, Pavinato et al. 2020). ) This accumulated P is commonly referred to as legacy P. Much thought has been devoted to ways in which this legacy P could be accessed, with the aim of decreasing further inputs of P (Menezes-Blackburn et al. 2018, Condron et al. 2013, Doydora et al. 2020, Pavinato et al. 2020). The methods suggested involve modification of root architecture or biochemistry or modification of the soil. Our approach is different; we think that the accumulated legacy P has changed soil properties in such a way that further applications of P are more effective; a better way to decrease further inputs is to understand these effects and develop ways of measuring them so that this understanding can be applied.

In order to explain this point, we need to rehearse the way that phosphate reacts with soil. It is initially adsorbed on heterogeneous variable-charge surfaces of soil particles; it then penetrates the soil particles by solid-state diffusion (Barrow 1999, Barrow et al. 2021). These reactions increase the negative charge on the soil particles (Barrow and Debnath 2014, Barrow et al. 2020a). One consequence of this is that it becomes more difficult for subsequent applications of phosphate to be adsorbed; the phosphate buffering capacity is therefore decreased (Barrow and Debnath 2014, Barrow 2015, Barrow et al. 2018); each further application of P therefor becomes more effective; and less is needed (Barrow et al. 2018). The second consequence is that the diffusive penetration becomes slower and eventually stops (Barrow et al. 2018). Once this happens, to maintain the status, it is only necessary to replace the P removed in the previous crop.

In this work we show how the effects of accumulated legacy P can be assessed by measuring soil sorption/desorption behaviour and that the characteristics of these effects so measured are closely related to the characteristics of the response of plants to further additions. We also show how these characteristics can be approximated by simple single-point measurements; we propose these would provide the tools needed to make better management decisions.

We established five levels of legacy P (including zero) by incubating soil with phosphate at 70°C for 30 days. In work with a similar soil (Barrow et al. 2018) it was estimated that the rate of the reaction at 70°C was about 65 times that at 20°C. That means that incubation for 30 days was equivalent to about five years at 20°C.

We then measured sorption and desorption properties by mixing samples of the treated soils with solutions containing several different levels of phosphate. After the designated period, we removed the solution and replaced it with a solution containing no phosphate and mixed for the same period. The following equations were used to describe the results.

S_s = a_s c_s^b - q_s (1)

S_d = a_d c_d^b – q_d (2)

where the subscript s indicates the sorption phase and subscript d indicates the desorption phase, S indicates phosphate sorbed by the soil, c indicates the phosphate concentration and solution and the other terms are parameters to be fitted. Because sorption is calculated from the observed change in concentration, the terms in the above equations are not independent. This problem is overcome (Barrow 2008) by treating the equations above as simultaneous with:

S_s = (ci – c_s) S_sr (3)

S_d = Ss – c_d S_sr (4)

The parameters of these equations were used to indicate the three benefits of legacy phosphate. Because the parameter b could be assumed to be common (see Fig. 1), the parameter a_s was used to assess the effect of legacy P on buffering capacity. The ratio of slopes a_s/a_d measures the hysteresis of sorption; when this is large it indicates that diffusive penetration is marked; the change of the ratio with increasing levels of legacy P reflects the extent to which the rate of diffusive penetration has decreased. Finally the parameter q_s reflects the amount of phosphate in a similar status to that of the recently sorbed phosphate. It might therefore be assumed to be proportional to the phosphate supplied by soil when plants are grown.

We also assessed the effect of the legacy P on response to fertiliser. At each level of legacy P, we prepared five sets of soil samples each containing 10 different levels of phosphate. The five sets were subjected to further incubations at 70°C for 0, 1, 3, 10, and 30 days. The samples so treated were then used as a source of phosphorus for plant growth. The responses were measured using the following equation.

Y = M_ax (1 – exp(- γ (x + d)) (5)

where Y indicates the weight of plants, x the level of phosphate applied, and the other terms are parameters to be fitted. The parameter γ indicates the slopes of the response curves. The values for the set of soils that had not been subject to a second incubation indicate the effect of legacy P on the soil P buffering capacity and therefore on phosphate effectiveness. The change in γ with increasing period of second incubation indicates the effect of legacy P on the diffusive penetration of phosphate and thus on its residual effectiveness. As there was little phosphate present in the seed, the parameter d reflects the effect of legacy P to which the plants have access. As it was infeasible to incubate all the soil to be used in the pot experiment at a constant temperature, we applied P to 200 g of soil for each treatment (one fifth of the pot soil).

We also measured the effects of legacy P on soil tests.

We used a soil from the Regional Research Station of Bidhan Chandra Krishi Viswavidyalaya at Jhargram in West Bengal, India (22°26´ 58.99´´N, 86°59´ 49.23´´E). It was low in P and had high buffering capacity for P. The site was well drained and uncultivated. This soil was previously used by Barrow et al. (2020, 2021). The average annual rainfall of this location is 1400 mm; 81% of this falls between June and early October. The average maximum summer temperature is 37°C and average minimum in winter is 15°C. We collected 50 kg soil in the dry season, in December 2019, to a depth of 20 cm; thoroughly mixed and ground well. A portion of soil was passed through a 2 mm sieve for laboratory experiments and stored at 25°C prior to use. The soil is classified as a coarse loamy Typic Haplustulf (USDA Soil Staff Survey, 1999) or an Orthic Acrisol (FAO WRB, 2006) in the red and lateritic soil zone. Some properties are: Colwell P, 11.36 mg kg^− 1 (Colwell, 1963); Bray P, 3.17 mg kg^− 1 (Bray & Kurtz, 1945); cation exchange capacity, 9.2 cmol (p⁺) kg^− 1 (Dewis & Freitas, 1984); water-holding capacity, 28% (Piper, 1966); and organic C content, 3.1 g kg^− 1 (Walkley & Black, 1934).

Legacy P

We established five different levels of legacy by adding to samples of soil 0, 1000, 2000, 3000, and 5000 mg P kg^− 1 in solutions containing KH₂PO₄. The samples were then incubated moist at 70°C for 30 days. As indicated above, this is equivalent to about five years at 20°C.

P sorption study

We mixed 5 g soil of different levels of legacy P in centrifuge tubes with 50 mL 0.01 M CaCl₂ solution containing the following concentrations of P: 0, 5, 10, 20, 30, 50, 70, 100, 150, and 200 mg P L^− 1 as KH₂PO₄. The tubes were gently shaken for 48 h in a reciprocal shaker at 25°C, centrifuged at 2146 g and P in the solution was measured by the method of Murphy and Riley (1962). To study phosphate desorption, the soil from the sorption run (after decanting the supernatant) was re-suspended with 0.01 M CaCl₂ and shaken for the same period as for the sorption run. Phosphorus in the solution was determined.

Cultivating plants

For each of the levels of legacy P, 10 levels of P were applied in solution to 200 g subsamples to give the following P concentrations: 0, 10, 20, 30, 60, 100, 200, 300, 500, and 1000 mg P kg^− 1 soil. These treatments were applied to five different sets of soils. The five sets were then incubated moist at 70°C for 0, 1, 3, 10, or 30 days.

The pots were prepared as follows. Bulk soil weighing 700 g was placed into pots with a surface diameter of 115 mm. The 200 g test soil samples were then added followed by 50 g of bulk soil; ten seeds of mustard (Brassica campestris L. ‘B9’) and a further 50 g of bulk soil. As the test soils comprised 20% of the total soil in the pot, these values may be divided by 5 to give mg pot^–1. The advantage of this method is that only a small amount of soil had been incubated at high temperature and therefore the bulk soil has not sustained any adverse effects from this treatment. After germination, five healthy plants were kept, maintaining equal spaces as far as possible. The weight per 100 seeds was 218 mg with a P concentration of 1.005 mg g^–1. Five seeds would therefore supply only 10.95 µg P. Seven days after emergence we added a basal nutrient solution containing the following nutrients (mg nutrient kg^–1 soil): Mn 4, Mo 1, Cu 1, B 0.2, Zn 10, K 100, Mg 6, and N 100. Plants were grown for 30 d in the open air on a rooftop. Night temperatures were as low as 18ᵒC and maximum day temperatures averaged 27ᵒC. All pots received the same amount of light The aboveground parts of plants were collected and washed with dilute H₂SO₄, This was to remove ant greasy material that might have left on the leaves. They were then washed with distilled water, air dried ,then oven dried at 60°C until a constant weight was reached.

Soil tests

Before cultivating plants, we extracted the soils that had been treated to give different levels of legacy P using three methods: Bray and Kurtz (1945), Colwell (1963) and Olsen et al. (1954.

With increasing levels of legacy P, the slope of the sorption curves decreased and thus buffering capacity decreased (Fig. 1). The amount of hysteresis also decreased; desorption curves approached the same slope as those for sorption curves (Fig. 1).This indicates that the penetration of P into the reacting surfaces decreased. The amount of phosphate in a similar status to that of the sorbed phosphate increased. This can be inferred from the data in Fig. 1f and is shown later (Fig. 4c) when comparisons are made with the plant growth experiment.

With increasing levels of legacy P, the slope of the plant response curves increased, indicating that the fertiliser had become more effective (Fig. 2). This result is consistent with the decreased buffering capacity. In the treatment with no legacy P (Fig. 2a) there was a large effect of period of second incubation on the response. As the amount of legacy P increased, this effect became smaller. This is consistent with decreasing penetration of P into the reacting surfaces. The amount of legacy P to which the plants had access increased. This can be inferred from the data in Fig. 2f and is shown later (Fig. 4c).

Figure 3 summarises the effects of amount of legacy P and of period of second incubation on the slopes of the response curves of Fig. 2. These data are used to generate a rate term (k) (Eq. 4) to summarise the effects. The values of this term are presented in Fig. 4b.

Figure 4 shows the remarkable correspondence between effects measured by sorption/desorption with those of plant growth.

The amount of phosphate that was extracted by three soil tests (Fig. 5) was larger than the amounts of legacy P to which the plants had access as indicated by the parameter d (Fig. 4c). The soil tests involve change in pH and also competition from anions; bicarbonate for the Olsen and Colwell tests, fluoride for the Bray and Kurtz tests. They therefore might be expected to be somewhat more effective at extracting P than plants. For the Olsen and for the Bray and Kurtz tests there was a curved relationship. This is consistent with competition for sorption from the anions involved. For the Colwell test curvature could not be detected. This is consistent with the larger solution soil ratio and therefore larger supply of competing anion.

This work emphasises the three components involved in the evaluation of legacy P. It shows that laboratory measurements of the components correspond remarkably closely with components of the response to P by plants.

Management of fertiliser programs places great emphasis on soil tests. We show that these correspond to just one component; that represented in Fig. 4c. A point to be debated is whether soil tests also take into account the decrease in buffering capacity brought about by the increase in the negative charge on the reacting surfaces. When differences in buffering capacity are caused by differences in the amount and nature of the reacting surface in soils, soil tests such as the Olsen test extract a larger proportion of added phosphate as the buffering capacity decreases (Barrow and Shaw 1976). They would therefore reflect the greater effectiveness of fertilisers. However, when decreases in buffering capacity are caused by increasing negative charge, it is debatable whether this would also be reflected by the Olsen test. This test brings the soils to a common pH of 8.5. We suggest that the changes in negative charge brought about by this change in pH would swamp differences in charge brought about by phosphate treatment. We think a similar argument would apply to other soil tests, all of which involve changes in pH. Reliance on soil tests alone would therefore underestimate the effectiveness of further fertilisers.

Especially in Australia, the importance of buffering capacity has been recognised since the early work of Ozanne and Shaw (1967) and soil tests are adjusted using a measure of phosphate buffering capacity (Moody and Bolland 1999). Single-point estimates of buffering capacity have been developed and the method of Burkitt et al. (2002) is widely used. This is designated as “PBI”. It is obtained by mixing a sample of soil with a phosphate solution which has a phosphate concentration sufficient to give phosphate sorption of 1000 mg kg^− 1. It is calculated as: phosphate sorbed, divided by solution concentration raised to the power 0.41. Figure 6 shows that PBI calculated from the present data indicates the effects of legacy P on buffering capacity in a very similar way to that of the slope term (a) obtained using all the data. This correspondence is obtained without adjusting the total sorption using the amount extracted by the three indicated reagents. This is consistent with the recommendations of Burkitt et al. (2008). Use of this, or similar measures, should enable the effects of previous phosphate application of buffering capacity to be taken into account. However, there is at present, no simple way to assess whether the diffusive penetration is still important.

The hysteresis ratio as used here is an effective measure of this property with ratios approaching unity indicating that little diffusive penetration is occurring. We wondered whether a single-point measure could be found using a process analogous to that for PBI. That is, we used the values of PBI for the sorption step; we then did a similar calculation for the desorption step; we tested whether the ratio PBI desorption/ PBI sorption would serve as a simple measure. We did a similar calculation on the much larger dataset of Barrow and Debnath (2014). Figure 7 shows that when the hysteresis ratio is large, the single-point measure does not reflect the range very well. However this is not a serious disadvantage; all we need to know for such soils is that it is large; the exact value is unimportant. The single-point measure does detect soils for which the hysteresis ratio is low. We suggest this would be a useful addition to soil testing services.

The levels of P applied here as pre-treatments were not unrealistically high. Consider the response shown in Fig. 2a for the treatment that had not been incubated with P. The P needed for nearmaximum yield was about 100 mg pot^− 1. This P was applied to 200 g of soil: thus the concentration in that soil was 500 mg kg^− 1. Reapplication at that level for 10 years would reach a total application equal to that of the highest legacy P. In practice, there would be removal of phosphate in produce, and levels of application might be decreased, but nevertheless such a level would be achieved over a period much shorter than that pertaining to P application on much land in the developed world. That is why similar declines in the hysteresis with P application have been observed in soils collected from tea plantations in northern India (Barrow and Debnath 2014).

Our work suggests that a phosphate fertiliser strategy, based on soil tests alone would underestimate the phosphate status of previouslyfertilised soils and lead to overapplication of phosphate fertilisers. Fertiliser strategies would be greatly improved by taking into account the decrease in buffering capacity and the decrease in the continuing reaction; both can be assessed by single-point methods.

Funding

No funds, grants, or other support were received during the preparation of this manuscript.”

Competing Interests

We have no relevant financial or non-financial interests to disclose

Author Contributions

AD designed the experiment, supervised the work, helped analyse the data and wrote parts of the manuscript. DR carried out the experiment. NJB completed the data analysis and wrote most of the paper.

Data Availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Barrow NJ (1978) The description of phosphate adsorption curves. J Soil Sci 29:447–462
Barrow NJ (1999) The four laws of soil chemistry: The Leeper lecture 1998. Aust J Soil Res 37:787–829. doi:10.1071/SR98115
Barrow NJ (2008) The description of sorption curves. Europ J Soil Sci 59:900–919
Barrow NJ (2015) Soil phosphate chemistry and the P–sparing effect of previous phosphate applications. Plant Soil 397:401–409. doi:10.1007/s11104-015-2514–5
Barrow NJ, Debnath A (2014) Effect of phosphate status on the sorption and desorption properties of some soils of northern India. Plant Soil 378:383–395. doi:10.1007/s11104-014-2042–8
Barrow NJ, Shaw TC (1976) Sodium bicarbonate as an extractant for soil phosphate. III. Effects of buffering capacity of a soil for phosphate. Geoderma 16:273–283
Barrow NJ, Barman P, Debnath A (2018) Three residual benefits of applying phosphate fertilizer. Soil Sci Soc Am J 82:1168–1176
Barrow NJ, Debnath A, Sen A (2020a) Effect of pH and prior treatment with phosphate on the rate of reaction of soils with phosphate. Europ J Soil Sci 72:243–253. https://doi.org/10.1111/ejss.12968
Barrow NJ, Debnath A, Sen A (2020b) Measurement of the effects of pH on phosphate availability. Plant Soil. https://doi.org/10.1007/s11104-020-046475
Barrow NJ, Debnath A, Sen A (2022) Effect of phosphate sorption on soil pH. Europ J Soil Sci. DOI: 10.1111/ejss.13172
Barrow NJ, Sen A, Roy N, Debnath A (2021) The soil phosphate fractionation fallacy. Plant Soil 459:1–11
Bray RH, Kurtz LT (1945) Determination of total, organic and available forms of phosphorus in soils. Soil Sci 59:39–46
Burkitt LL, Moody PW, Gourley CJP, Hannah MC (2002) A simple phosphorus buffering index for Australian soils. Aust J Soil Res 40:497–513
Burkitt LL, Sale PWG, Gourley CJP (2008) Soil phosphorus buffering measures should not be adjusted for current phosphorus fertility. Aust J Soil Res 46:676–685
Colwell JD (1963) The estimation of the phosphorus fertilizer requirements of wheat in southern New South Wales by soil analysis. Aust J Exp Agric Anim Husb 3:190–197
Condron LM, Spears B, Haygarth P et al (2013) Role of legacy phosphorus in improving global phosphorus-use efficiency. Environ Develop 8:147–148. 10.1016/j.envdev.2013.09.003
Dewis J, Freitas F (1984) Physical and chemical methods of soil and water analysis. Oxford and IBH Publishing Company, New Delhi, pp 51–106
Doydora S, Gatiboni L, Grieger K et al (2018) Opportunities for mobilizing recalcitrant phosphorus from agricultural soils: a review. Plant Soil 427:5–16. https://doi.org/10.1007/s11104-017-3362-2
Moody PW, Bolland MDA (1999) Phosphorus. In Soil Analysis An Interpretation Manual. ed Peverill KI, Sparrow LA, Reuter D. eBook
Murphy J, Riley JP (1962) A modified single solution method for the determination of phosphate in natural waters. Anal Chim Acta 27:31–36
Olsen SR, Cole CW, Watanabe F, Dean LA (1954) Estimation of available phosphorus in soils by extraction with sodium bicarbonate. US Dep. of Agric. Circ.939. USDA, Washington, DC
Ozanne PG, Shaw TC (1967) phosphate sorption by soils as a measure of the phosphate requirement for pasture growth. Aust J Agric Res 18:601–602
Pavinato PS, Cherubin MR, Soltangheisi, A etal. (2020) Revealing soil legacy phosphorus to promote sustainable agriculture in Brazil. SciRep 10:15615. H
Piper CS (1966) Single value physical constants. Soil and plant analysis. Hans Publishers, pp 80–111
Syers JK, Johnston AA, Curtin D (2008) Efficiency of soil and fertilizer phosphorus use: Reconciling changing concepts of soil phosphorus behaviour with agronomic information. Fertilizer and Plant Nutrition. Food and Agriculture Organization, Rome, Italy, Bulletin
Walkley AJ, Black IA (1934) Estimation of soil organic carbon by the chromic acid titration method. Soil Sci 37:29–38

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You are reading this latest preprint version

Evaluating the benefits of legacy phosphate

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Version 1

Abstract

Figures

Introduction

Outline Of The Experiment

Details Of Methods

Legacy P

P sorption study

Cultivating plants

Soil tests

Results

Discussion

Conclusions

Declarations

Data Availability

References

Status:

Version 1