WO1996019546A1

WO1996019546A1 - Nitrification inhibitor

Info

Publication number: WO1996019546A1
Application number: PCT/AU1995/000867
Authority: WO
Inventors: John Raymond Freney; Jonathon Howard Hodgkin; Kevin James Harrington
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1994-12-20
Filing date: 1995-12-20
Publication date: 1996-06-27
Also published as: AUPN015994A0

Abstract

Compositions for the prolonged slow release of acetylene in the presence of water comprise a hydrophobic polymer matrix having dispersed throughout it finely ground calcium carbide. The compositions are useful for treating soil to reduce loss of soil nitrogen or for increasing the efficiency of utilisation of fertiliser nitrogen.

Description

NITRIFICATION INHIBITOR

Field of the Invention

This invention relates to a novel system for the prolonged release of a low cost, easily manufactured and effective nitrification inhibitor into the soil.

Throughout the world millions of metric tons of nitrogen fertiliser are used annually in agriculture but a high proportion (average 40-50%) of this fertiliser nitrogen is lost before it can be taken up by the target plants due to bacterial oxidation (nitrification) to nitrate which may then leach or be reduced by other microbes and lost as gaseous oxides of nitrogen or nitrogen gas. One way to reduce these losses is by the use of a low cost nitrification inhibitor to minimise the rate at which the ammonium nitrogen is oxidised to nitrate. The benefits would be not only more efficient fertiliser use but also the environmental benefits of the reduction in emission of nitrogen oxides (potent greenhouse gases) and in pollution of water systems by leachable nitrate.

The biological conversion of fertiliser nitrogen to ammonium and then to nitrate and nitrous oxide is a complex process with various intermediate steps (Scheme 1).

nitrification denitrification

Fertiliser nitrogen → ammonium — > nitrite → nitrate → nitrite - (e.g. urea) i (leaching) nitric oxide -» nitrous oxide → N2

Scheme 1

If the oxidation of ammonium could be prevented (by a nitrification inhibitor) then leaching of nitrogen would not occur, because ammonium is strongly held by the cation exchange complex in soil, and nitrous oxide would not be emitted. Numerous compounds have been proposed as nitrification inhibitors, but the few that are available commercially all have limitations to their usefulness. It has been established in laboratory studies that acetylene (C2H2) is a very good inhibitor of nitrification, but because it is a gas there are problems in introducing it into soil and maintaining it during the growing season at the concentration required to limit nitrification. This problem may be overcome by adding calcium carbide (CaC2).

If uncoated calcium carbide is added to soil it quickly reacts with water (Equation 1) and the acetylene soon dissipates.

CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂ (Equation 1)

Calcium carbide + water -> acetylene + calcium hydroxide

For calcium carbide to be a suitable means of providing acetylene as an effective nitrification inhibitor it needs to be coated in some way so that it acts -as a slow release source of acetylene.

Description of the Prior Art

A number of nitrification inhibitors are in use or in development [Juliette, L. et al. App. Environ. Microbiol. 59 (II), 3718-27 (1993)] but all have limitations. The organic chemicals in use, for example nitrapyrin (2-chloro-6-trichloromethyl pyridine), are generally ineffective because of sorption on soil colloids, hydrolysis to 6-chloropicolinic acid or loss by volatilisation. Acetylene (C2H2) is, however, known to be a very effective inhibitor of ammonium oxidation. It acts by combining with the enzyme ammonia mono oxygenase in the Nitrosomonas bacteria responsible for oxidation of ammonia to nitrate.

The advantages of C2H2 as a nitrification inhibitor are its gaseous nature which allows it to readily permeate the soil matrix and its independence of the variation in soil properties which affect other nitrification inhibitors [Keever, Special Publication of the Society for General Microbiology, pp 99-115 (1986)]. However, this gaseous nature is a disadvantage in trying to maintain a useful concentration in the soil for long periods. A number of higher molecular weight organic compounds with acetylene groups in the molecule such as phenylacetylene, have been proposed as alternatives but these would be considerably more expensive. A much lower cost alternative was proposed by Banerjee and Mosier [US Pat. Appl. 07/229,386, 8 Aug 88] who used encapsulated large lumps of calcium carbide (CaC2) which in the presence of soil moisture were designed to release C2H2 gas into the soil at a slow, controllable rate. Unfortunately the encapsulating resin systems and production methods disclosed by Banerjee and Mosier are not practical for large scale, low cost production. These controlled release lumps were manufactured using large quantities of solvent which then had to be removed. As pointed out by these workers the encapsulating substances must not be reactive with CaC2 nor be toxic to soil and aquatic flora and fauna. They favoured the use of natural resins as encapsulants and used large lumps of calcium carbide. The encapsulated product did not show any inhibitory effect after about ten days. We have observed that such large lumps as described by Banerjee et al. burst after contact with soil moisture rather than provide a slow release of acetylene. Cost, product variability and difficulty of scale up make this system impractical.

In an attempt to overcome these problems paraffin wax was used as a coating medium. Wax-coated calcium carbide added with the nitrogen fertiliser blocked nitrification in irrigated wheat for more than 10 weeks, thus preventing denitrification and loss of nitrogen (J R Freney, C J Smith, & A R Mosier Fertiliser Research 32 1-11, 1992). Wax-coated calcium carbide also markedly decreased emission of nitrous oxide from irrigated maize, increased lint yield in irrigated cotton, and increased grain yield in maize (J R Freney, D L Chen, A R Mosier, I J Rochester, G A Constable & P M Chalk, Fertiliser Research 34 37-44, 1993). In addition, loss of applied nitrogen from flooded rice was reduced from 56% to 13% and methane production was markedly reduced (D G Keerthisinghe, J R Freney, & A R Mosier Biology and Fertility of Soils 16 71-75, 1993). Unfortunately the wax coatings melt if left in the sun and the product becomes unusable.

Summary of the Invention

It has now been discovered that powders of calcium carbide (CaC2) encapsulated in inert hydrophobic polymers, in the presence of soil moisture, release C2H2 gas into the soil at a slow, controllable rate over many months and thus serve as an extremely effective nitrification inhibitor in solid form without the handling difficulties of the wax coated product. Such a product can be readily manufactured by the methods used for the production of filled thermoplastics.

Accordingly in one aspect this invention provides a composition for the prolonged slow release of acetylene in the presence of water which composition comprises a hydrophobic polymer matrix having dispersed throughout it finely ground calcium carbide.

One object of the invention is to provide a simple, single step, low cost means of reducing loss of soil nitrogen due to denitrification and nitrate leaching over prolonged periods in a variety of environments.

In accordance with another aspect of the invention there is provided a method for treating soil to reduce loss of soil nitrogen in which an encapsulated calcium carbide composition in accordance with the invention is applied to and/or mixed with the soil, optionally together with a nitrogenous fertiliser.

In accordance with a further aspect of the invention there is provided a composition for the treatment of soil to provide sustained levels of soil nitrogen which comprises an encapsulated calcium carbide composition in accordance with the invention, together with a nitrogenous fertiliser.

A further object of the invention is to provide a simple, cost-effective means of increasing the efficiency of utilisation of fertiliser nitrogen (ammonia and ammonium salts) in soil in many widely different environments.

In accordance with a still further aspect the invention provides a method for increasing the efficiency of utilisation of fertiliser nitrogen which comprises treating soil to which a nitrogenous fertiliser has been, or is to be, added with an encapsulated calcium carbide composition in accordance with the invention.

The hydrophobic polymer matrix may be composed of any thermoplastic polymer that does not significantly transmit water from the atmosphere, and does not react with calcium carbide, and which is solid at the temperatures normally encountered in agricultural practice. Preferred hydrophobic polymers are homo- or co-polymers of olefins, acrylic or styrenic monomers. Preferred polyolefins are the homo- or co- polymers generally called polyethylene or polypropylene. Preferred acrylic polymers are homo- or co-polymers of methyl methacrylate, ethvl methacrylate, ethyl acrylate or butyl methacrylate. Suitable styrenic polymers are the various grades of polystyrene.

Particularly suitable as encapsulating materials are synthetic resins of the hydrocarbon type such as polypropylene or polyethylene as they are low cost, inert, water impermeable and readily melted and mixed with mineral fillers on a large scale. Even low quality or recycled samples of these polymers could be used. These materials do not release volatile or toxic products either during production or in degradation unlike solvent-based encapsulating materials or many thermoset polymers.

The majority of the best, low cost, encapsulating materials are too water impermeable to give a useful, controlled rate of release of acetylene on their own. While calcium carbide or its decomposition products can provide a water- conducting path, optionally one or more water-permeable fillers may be added to provide slow controlled generation and release of acetylene in environments ranging from completely aqueous situations to dry land conditions over periods from days to years.

In this case, sufficient of the water absorbing adjuvant mineral filler is added to enhance the rate of acetylene generation and release from the thermoplastic encapsulated calcium carbide without allowing the encapsulated material to burst even in liquid water. A wide variety of adjuvant fillers are suitable for use in this invention. Preferred mineral fillers are calcium carbonate, calcium sulphate, barium sulphate, alumina, aluminosilicate minerals such as kaolinite, montmorillonite, etc. The particle size of the mineral filler should be less than 200 μm.

It is preferred that sufficient adjuvant filler be added to cause adequate microbridging in the encapsulated materials and hence controlled water penetration and acetylene generation. This critical filler level will depend on the use conditions and the filler particle size and density but is generally up to 60% filler by weight. The ratio of calcium carbide to filler can also be easily controlled to vary the rate of release in a particular application.

If the quantity of calcium carbide used is sufficient to give adequate microbridging then the adjuvant filler is not necessary.

Fine grinding of the calcium carbide to the sizes normally encountered in the standard fillers in thermoplastic processing (10-150μm) leads to a uniform rate of release of acetylene. Equipment that can do this and keep the resulting powder in a dry explosion proof container until use is well known in the plastic processing industry. The calcium carbide should form at least 5 % by volume of the composition; its particle size should be in the range 1 - 200 μm.

The mineral filler and calcium carbide taken together should constitute at least 20% by volume of the composition.

The amounts and type of adjuvant filler and thermoplastic that need to be mixed with the powdered calcium carbide can be adjusted to suit the agricultural application, ranging from flooded rice to dry land wheat. The ways of variation in the amount and type of both the encapsulating material and the filler will be known to those skilled in the art, and more than one way of application of the same or different substances may be utilised to impart the desired characteristics to the product.

This invention also provides a process for the manufacture of an encapsulated calcium carbide composition according to the invention which process comprises addition of powdered calcium carbide, optionally with one or more mineral fillers, to molten thermoplastic polymer in a suitable mixer/disperser at temperatures above the boiling point of water. Following mixing the molten blend may be further processed by extrusion through a die and subsequently formed into pellets or any other form suitable for convenient handling and application.

This invention additionally provides a process for the manufacture of an encapsulated calcium carbide composition according to the invention which comprises first combining powdered calcium carbide and any fillers, lubricants or processing aids with solid hydrophobic polymer particles by tumbling or any other appropriate means and then introducing the resulting mixture into a plastics extruder or similar melt-mixing apparatus with subsequent treatment of the molten intimate mixture to produce the composition of the invention in particulate form.

The even encapsulation of the powdered calcium carbide in a form that will prevent the absorption of water can be accomplished in large scale plastic processing equipment such as a standard screw extruder using many of the standard available hydrophilic thermoplastics.

The addition of internal lubricants and processing aids may aid the encapsulation of calcium carbide in such equipment.

Soil may be treated in any convenient fashion with encapsulated compositions of this invention. For example, small pellets of these formulations may be added to the soil at the same time as the plant seed to the depth required. These compositions may be applied to the soil alone or admixed with fertilisers.

It is also an object of the invention to provide a nitrogen inhibitor package that is not toxic to soil and aquatic flora and fauna or leave residues that have other environmentally deleterious effects. To achieve this, the matrix polymers in small pellets or other forms should be selected to be very inert and beneficial to delicate plants, such as nursery plants, and so that after both the filler and calcium carbide have been exhausted, they will be highly porous, help to improve soil structure and degrade slowly by mechanical abrasion and normal hydrocarbon oxidation. Their eventual breakdown can even be accelerated by agriculturally beneficial heavy metal ions such as iron, copper and magnesium using current available technology, if required.

Other objects and advantages of the invention will be readily apparent from this description. The following examples are intended only to further illustrate the invention and are not intended to limit the scope of the invention.

EXAMPLE 1

Calcium carbide lumps (BDH Laboratories UK, 2mm in diameter) were pulverised in a high speed mixer under a stream of dry nitrogen and then mixed with the anhydrous calcium carbonate in a ratio of 50:50 by weight. The filler mixture was combined with polypropylene pellets (GMY-45 ICI Australia, 50% by weight) in a Brabender batch mixer through a 10mm die aperture. The resulting extrusion was pelletised to yield encapsulated pellets. The grey pellets were tested for controlled release ability by placing in a beaker of water. Fine bubbles of acetylene slowly formed on the surfaces and continued to do so for more than ten weeks. Even in highly accelerating conditions of strong acid solutions, acetylene bubble formation continued for at least four weeks.

EXAMPLE 2

The encapsulation process described in Example 1 was repeated with 21 parts of calcium carbide and 21 parts of dry calcium carbonate to 58 parts of polyethylene (ICI Australia ET6062). The water test indicated that the product was not only effectively encapsulated but provided prolonged controlled acetylene release.

EXAMPLE 3

The encapsulation process described in Example 1 was repeated with 30 parts of calcium carbide and 30 parts of dry calcium carbonate to 40 parts of polyethylene. The water test again indicated that this was an effective controlled release system.

EXAMPLE 4

Preparation of pellets containing Polyethylene/CaC2 /paraffin oil, nominal composition

42/53/5 w/w. Ingredients: Calcium carbide 965 g

Paraffin oil 91 g

Polyethylene 765 g

1821 g

The raw material CaC2 was ground to pass through a 200 mm mesh screen. Active carbide content after grinding was 81%.

To an empty 4 litre paint can was added the required weight of polyethylene (PE) and half the oil. This was tumbled for 5-30min on a mechanical tumbler. Then about half the required weight of carbide was added and the tumbling continued. The remainder of the oil was added and the mixture tumbled again, followed by the balance of the carbide with more tumbling until use. This procedure gave a slurry of carbide/oil coated polymer beads. Scoops of this material were fed into the hopper of a Brabender® Plast- Corder PL2000 blender set up for Twin Screw mixing with counter rotating screws. It was important not to add too much material to the hopper as this made it very difficult to overcome any bridging problems at the throat of the hopper.

The zone temperature settings on the Brabender Plast-Corder which were found optimal were (zone 1) 145°C, (zone 2) 145°C, (zone 3) 150°C, and (zone 4) 160°C. Nitrogen was run through the hopper at all times and also applied at a point under the entry block. Only the temperature at zone 4 was monitored. Throughput was 2-2.5kg per hour. With the speed setting at 80rpm the torque range for producing good material was 800-1200mkg. The torque increased as the proportion of PE in the composition is increased. This is probably due to more complete filling of the screw cavity. The mixed material was extruded through a 5-6mm rod die. The extruded rod was of glossy grey appearance and was quite flexible.

It was important to continually bag the extrusion as soon as it came off the machine as considerable acetylene can be produced. Bags of the extrusion were heat sealed until chopping which usually occurred within 24h. Chopping was done on a JSW chopping machine and 8kg could be put through in about 30min with two operators. Material made in this way was of glossy grey appearance analyzing at 35.8% available CaC₂-

Using this method and blending apparatus CaC2 contents above 60% tend to produce an incompletely mixed product which is brittle. Also the surface of the extruded rod appears to be porous so making these pellets too reactive.

EXAMPLE 5

Laboratory Evaluation in soil.

Three batches of inhibitor in pellet form containing 21%, 25% and 30% w/w calcium carbide and equal amounts of dry calcium carbonate were supplied for initial testing in the laboratory. In the first set of experiments they were evaluated as nitrification inhibitors in a fine textured grey clay, classified as Ug 5.25 or Typic Pellustert, with pH 8.2, total N 0.10%, organic carbon 0.9% and a clay content of 62% from the Narrabri Agricultural Research Station, NSW. This soil was chosen because of its capacity to rapidly oxidize ammonium to nitrate.

One gram of finely ground ammonium sulfate was added to lOOOg of air-dried soil and the two were well mixed. 200g of the mixture was placed in a polythene bottle (50mm radius and 200mm high) and the required amount of inhibitor was added to the top of the soil. The remainder of the mixture was added and 270 mL water was added to adjust the soil moisture to field capacity. The bottles were placed in an incubator maintained at 25°C. Sufficient of each of the three matrices was added to supply 10, 25, 50, 100 or 200 mg of calcium carbide per bottle and each treatment was studied in duplicate. Appropriate controls, without calcium carbide were conducted concurrently.

After 1, 2, 4, 6, 8, 10, 12, 14 and 16 weeks the equivalent of 10 g of air dry soil were removed and extracted with 2M potassium chloride. After filtration through a Whatman No 42 filter paper the extract was analysed for ammonium and nitrate. At intervals the bottles were stoppered with a lid fitted with a rubber septum. At the end of the covered period a gas sample was removed through the rubber septum with a gas tight syringe and the acetylene content determined by gas chromatography. The results indicated that this was an effective controlled release system for acetylene and this inhibited nitrification. As can be seen from Figure 1 which is typical, in the absence of the inhibitor of this invention the ammonium content of the soil had decreased substantially in 14 days and completely after 28 days, while in the presence of inhibitor more than half the ammonium remained after 100 days. Figure 2 which directly shows the rate of release of acetylene as a function of time demonstrates that acetylene release continues for at least 70 days from a composition containing 21% calcium carbide and 21% calcium carbonate in moist soil.

EXAMPLES 6-12

Evaluation of Compositions Containing Calcium Carbide as Inhibitors of Nitrification in Soil.

1. Evaluation procedure

The procedure was essentially the same as described in Example 5. Sufficient of each of the compositions was added to supply 200 mg of calcium carbide per bottle and each treatment was studied in duplicate. Appropriate control trials, without calcium carbide were conducted concurrently.

After 7, 21, 31, 50, 60, 77, 91, 114, 141 and 178 days the equivalent of 10 g of air dry soil was removed and extracted with 2M potassium chloride. After filtration through a Whatman No. 42 filter paper the extract was analyzed for .ammonium and nitrate. At intervals the bottles were stoppered with a lid fitted with a rubber septum. At the end of a covered period a gas sample was removed through the rubber septum with a gas tight syringe and the acetylene content determined by gas chromatography; this provided information on the capacity of the compositions to provide acetylene.

2. Compositions The compositions were prepared by the method of Example 4 using the ratios of ingredients shown in Table 1. Results

In these experiments the nine compositions containing calcium carbide were compared with uncoated calcium carbide and a prior art composition of calcium carbide coated with shellac, beeswax and paraffin wax. The latter was previously demonstrated to be a slow release source of acetylene and inhibited nitrification for long periods in field crops.

The results are given in Table 1 below which shows time taken for the ammonium level in the soil to be reduced to 70% of the original value, ie to lose 30% of added fertilizer, and the period for which detectable acetylene emission occurs.

Table 1

Inhibition of Nitrification in Soil by Polyethylene Based Compositions

As would be expected no acetylene was emitted from the control treatments without calcium carbide. Acetylene was emitted from the wax coated calcium carbide treatments

2 at a slow rate overall (< lpg/m / min) up to day 60.

Trace amounts of acetylene were emitted from the treatments containing the 21% compositions and the 41% composition but thereafter no emission of acetylene was detected from any treatment. However, the fact that acetylene emission could not be detected does not mean that acetylene was not released from the compositions within the soil.

All compositions produced acetylene in quantities sufficient to saturate the soil and allow some excess acetylene to be emitted from the soil up to day 60. The rate of acetylene production from each of the compositions and wax coated calcium carbide was sufficient to inhibit the oxidation of ammonium to nitrate for at least 80 days. Consequently all of the compositions were at least as good as wax coated calcium carbide, and some were considerably better.

As wax coated calcium carbide has been shown to inhibit nitrification in fields of irrigated wheat (Freney et al. 1992), maize (Bronson et al. 1992; Zhang et ai. 1992), cotton (Freney et al. 1993; Chen et al. 1994), and flooded rice (Banerjee et al. 1990; Keerthisinghe et al. 1993) the results suggest that all of the compositions tested would be effective in preventing nitrification of fertilizer and soil nitrogen in agricultural soils and result in increased efficiency of fertilizer use.

References

Banerjee, N. K., Mosier, A. R, Uppal, K. S. and Goswami, N. N. (1990). Use of encapsulated calcium carbide to reduce denitrification losses from urea-fertilized flooded rice. Mitteilungen. der Deutschen. Bodenkundlichen. Gesellschaft, 60:245-248.

Bronson, K. F., Mosier, A. R, and Bishnoi, S. R. (1992). Nitrous oxide emissions in irrigated corn as affected by nitrification inhibitors. So/7 Scence Society America Journal, 56:161-165. Chen, D. L., Freney, J. R, Mosier, A, R, and Chalk, P. M. (1994). Reducing denitrification loss with nitrification inhibitors following presowing applications of urea to a cotton field. Australian Journal of Experimental Agriculture, 34:75-83. Freney, J. R., Smith, C. J., and Mosier, A. R. (1992), Effect of a new nitrification inhibitor (wax coated calcium carbide) on transformations and recovery of fertilizer nitrogen by irrigated wheat. Fertilizer Research, 32:1 -1 1.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A composition for the prolonged slow release of acetylene in the presence of water characterised in that it comprises a hydrophobic polymer matrix having dispersed throughout it finely ground calcium carbide.

2. A composition as claimed in Claim 1, characterised in that the hydrophobic polymer is a homo- or co-polymer of an olefinic, acrylic or styrenic monomer.

3. A composition as claimed in Claim 2, characterised in that the polymer is polyethylene or polypropylene.

4. A composition as claimed in Claim 2 characterised in that the polymer is a homo- or co-polymer of methyl methacrylate, ethyl methacrylate, ethyl acrylate or butyl methacrylate.

5. A composition as claimed in Claim 2 characterised in that the polymer is a polystyrene.

6. A composition as claimed in any on of Claims 1 to 5, characterised in that at least one water permeable filler is dispersed in the matrix, together with the calcium carbide.

7. A composition as claimed in Claim 6, characterised in that the filler is a mineral filler.

8. A composition as claimed in Claim 7, characterised in that the filler is calcium carbonate, calcium sulphate, barium sulphate, alumina, or an aluminosilicate mineral.

9. A composition as claimed in Claim 7 or Claim 8, characterised in that the filler has a particle size of less than 200μm.

10. A composition as claimed in any on of the preceding claims, characterised in that the calcium carbide has a particle size in the range 1 to 200μm.

11. A composition as claimed in any one of the preceding claims characterised in that the calcium carbide and the filler, if present, constitute at least 20% by volume of the total composition.

12. A process for the manufacture of an encapsulated calcium carbide composition comprising a hydrophobic polymer matrix having dispersed throughout it finely ground calcium carbide, characterised in that powdered calcium carbide, optionally mixed with one or more mineral fillers, is added to a molten thermoplastic polymer in a suitable mixer/disperser at a temperature above the boiling point of water.

13. A process for the manufacture of an encapsulated calcium carbide composition characterised in that it comprises first combining powdered calcium carbide and optionally any fillers, lubricants or processing aids with solid hydrophobic polymer particles by tumbling or any other appropriate means and then introducing the resulting mixture into a plastics extruder or similar melt-mixing apparatus with subsequent treatment of the molten intimate mixture to produce the composition in paniculate form.

14. A composition for the treatment of soil to reduce loss of soil nitrogen characterised in that it comprises a calcium carbide composition as claimed in any one of Claims 1 to 11, together with a nitrogenous fertiliser.

15. A method for treating soil to reduce loss of soil nitrogen characterised in that a calcium carbide composition as claimed in any one of Claims 1 to 11, is applied to and/or mixed with the soil, optionally together with a nitrogenous fertiliser.

16. A method for increasing the efficiency of utilisation of fertiliser nitrogen characterised in that soil to which a nitrogenous fertiliser has been, or is to be, added is treated with a calcium carbide composition as claimed in any one of Claims 1 to 11.