Conclusions and recommendations for future work
7.1 Conclusions
The review of literature revealed the potential to produce low cost reactive layer polyurethane coated urea and nitrification inhibitor DCD to assist in the mitigation of nitrate leaching and nitrous oxide emissions in pastoral farming. This requires
furthering of current scientific understanding in terms of:
1. The release mechanism of the reactive layer polyurethane coated urea (UCU) and nitrification inhibitor (PDCD) to assist in the explanation of observed release of urea and DCD in field and repacked core studies, respectively. 2. In the development of the reactive layer polyurethane coated nitrification inhibitor, PDCD, for the treatment of potential urine affected soils. The understanding of the movement of both the inhibitor DCD from the surface application of PDCD and urine was required. In addition to the interactions of the inhibitor, soil and urine N, which were measured and characterised in the modelling of the fate of urine N within the soil profile.
7.1.1 Mechanism of release from UCU
A new mechanism of urea release from urethane coated granules was modelled from an assumption that the coating was water repellent and micro-porous. Under these
conditions only water vapour diffusion though gas filled pores may enter the coated granule and dissolve the internal core. The increase in volume within the coated granule then allows solution to be expelled from the granule into the surrounding soil. The rate of urea and DCD release, was found to be significantly affected by the coating thickness (lo) implying the dependence of water vapour permeability (W’) on coating
thickness. The relationship between W’ and lo was experimentally estimated and
modelled as an exponential decay to a minimum value (Equation 2.37). The
distribution of coating thicknesses within the population of granules was determined gravimetrically and the model applied. This showed that these two factors played a
161 significant role in the release characteristics of the coated urea, as a granule with
coatings less than 0.0026 cm released rapidly while above this value granules released slowly. The addition of palmitic acid to the coating was found to increase the initial release rate, interpreted as an increase in minimum water vapour permeability (W’min)
indicating an increase in porosity of the coating. In addition to the W’(lo) relationship, it
was expected that the change in volume of the coated granule, due to morphological changes would have resulted in an increased delay in urea release, this was not observed in the release data, possibly due to the high tensile strength and low elasticity of the coating. In water extraction trials it was found the model, without a release lag time, fitted the observed release of urea with an R2 0.93. The main deviations of observations from the modelled data occurred within the first few days, when the model had
underestimated the urea release.
7.1.2 Field trials of modified RLP coated urea 5UCU and 7UCU
It was concluded from the field trials of 5UCU and 7UCU in winter applications to pasture that a single application of 150 kgN could be safely applied without risk of nitrate leaching and decreased N use efficiency. Pasture herbage N content and DM production for the 5UCU and 7UCU treatments were used to model urine N returns, that were 5 to 10 kgN ha-1 less, over the 150 day trial, than those predicted for un-coated urea fertilised treatment.
At the end of the trial significant quantities of urea (39 and 52% of applied N for 5UCU and 7UCU, respectively) remained unreleased or unaccounted for. The fate of this urea was shown to be related to the soil moisture with no observed release of urea at 7% moisture content, while at 18.3% release of urea was observed in both 5UCU and 7UCU. These results indicate that the unreleased urea is likely to become available in the subsequent autumn, when the soil moisture increases. A longer term trial is required to investigate this hypothesis.
7.1.3 Evaluation of PDCD in repacked soil core studies
The movement of DCD from both uncoated and the coated DCD (PDCD) by diffusion showed that DCD is capable of rapidly diffused from the soil surface to effect inhibition
162 of nitrification. The results of the absorption isotherm showed that DCD was weakly bound by soil organic matter and degraded rapidly in non-urine affected soil. The application of urine (600 kgN ha-1) appeared to have stalled or reduced the degradation rate of DCD. The effectiveness of DCD inhibition of nitrification was found to be related to DCD soil concentration by an inhibitor response constant (K), which varies between soils making the assessment of K important in the determination of effective DCD application levels.
The modelling of DCD movement from the surface applied PDCD showed that to achieve rapid inhibition of nitrification a mixture of uncoated DCD and PDCD was required at a ratio 2:8. This combination applied at a rate of 26kg DCD ha-1 showed the potential to inhibit nitrification for up to 270 days at soil temperatures of 20oC,
increasing the longevity of uncoated DCD at the same rate by 120 days.
The results of the repacked core studies and modelling have indicated that the combined DCD: PDCD mixture should go to field trials to confirm the modelled effects.
7.2 Further work
Further work is required to extend the release rate modelling of coated products to allow accurate predictions of release based on polymer coating and granule properties. The initial 20 day period of release requires further investigation with focus on the volume change and re-equilibrium processes of release, which may result in the higher than predicted release rates. These processes may require the measurement of granule volume, internal pressure and release rate on individual coated granules using micro- sensors; possibly attached via a capillary tube allowing pressure measurements. Under field conditions without irrigation the 5UCU and 7UCU product produced significant reductions in both direct losses of nitrate-N via leaching and estimated urine N return to pasture. This work requires to be validated under irrigation with grazing and the addition of herd urine and milk urea testing to confirm the reduction in urine return.
163 The initial modelling study of PDCD as a nitrification inhibitor for urine affected soils suggests that the combination of DCD and PDCD may increase the inhibition of nitrification in high temperature areas. This would require a combination of lysimeter and drainage plots to determine the infield effect of the product. The modelling also showed the significance of both the inhibitor response constant K and maximum nitrification rate on the potential of DCD to reduce nitrate accumulation in soil. This work requires further development to determine spatial variability within farm and regional areas to identify optimal areas and regions for DCD and PDCD application.
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