Field studies of ammonia volatilisation potentials of unsaturated soils fertilised with granular urea

The potential for ammonia, NH₃, loss from granular urea applied to soil can be influenced by various soil, environmental and agronomic factors. A series of field studies were conducted to investigate the soil factors influencing the volatilisation potentials of a wide range of undisturbed New Zealand soils (12 soils) under different environmental conditions. Sample blocks of surface soil (0-7 cm depth) were transported from various parts of New Zealand to the experimental site (Lincoln University, Canterbury) and buried at ground level in enclosures configured for active trapping of volatilised NH₃. Urea granules (2-3 mm diameter), with or without 15N labelling were surface-applied to the soils at a rate equivalent to 100 kg N ha⁻¹. Ammonia volatilisation was measured by using 50 mL of 0.05 M H₂SO₄ as the trapping medium and an air flow rate of 16 air exchange min⁻¹. This trapping system was found to be efficient in preliminary laboratory trials conducted prior to the field experiments. Field measurements of changes in soil surface temperature, soil moisture and microsite pH (0-1 mm soil depth) and rates of urea hydrolysis, ammoniacal-N accumulation, nitrification and immobilisation were also made following application of urea. Each soil was characterised for texture, urease activity, mineralisable- and hydrolysable-N contents, organic-C and total-N contents, native soil pH, buffered CEC, H⁺ buffer capacity and exchangeable plus soluble Ca²⁺ and Mg²⁺ contents. In most cases volatilisation was essentially complete 7 days following urea application. Losses ranged from 4.4 to 53.9% of the applied-N (mean, 32%). Simple correlation tests were performed between percentage cumulative (7 days) NH₃-N losses (i.e. 'volatilisation potential') and each soil property. Each relationship was viewed using scatter diagrams for possible curvilinearity and the influence of outliers. The two major environmental conditions encountered during the studies were (i) warm, rapid soil drying and (ii) cool slow-drying conditions. Statistically significant relationships were found between volatilisation potentials and several of the many soil properties and urea-N transformation processes studied. However, only the microsite pH measured 8 hours following urea-N application ('initial microsite pH') showed highly significant linear relationships (r ≥ 0.94; P ≤ 0.05) with volatilisation potential under all environmental conditions. This observation was further examined by combining all the data sets and a correlation test was performed. The volatilisation potentials of all the soils studied were highly correlated to the 'initial microsite pH' and the following relationship was obtained: NH₃ loss (7 days) = -123 + 19.8 Initial microsite pH (R² = 85.4%; P < 0.01) It was proposed that a soil which has a high H⁺ buffer capacity is capable of resisting the alkalinity build-up during the initial stage of urea hydrolysis. Such a soil should have a greater rate and extent of protonation of NH₃(aq) released from urea and consequently have a lower volatilisation potential than that of a soil with a lower H⁺supplying capacity. The Significant overall relationship obtained between NH₃ losses and sand content (%) (R² = 65.5%; P < 0.01) also supported this proposal by indicating that soils with a high abundance of chemically inert components, such as sand, can sustain high NH₃ losses when broadcast with urea. The measurement of 'initial microsite pH' following urea application provides a more reliable and simpler technique than the standard titration method used to assess the H⁺ buffer capacity and/or the volatilisation potential of soils. The immobilisation of ¹⁵N labelled urea was monitored in enclosed and non-enclosed soils throughout the volatilisation experiment carried out under cool, slow-drying conditions. Immobilisation of the applied urea-N ranged from 13 to 19% after 14 days in the non-enclosed soils and was significantly less (P ≤ 0.01) in the enclosed soils (5 to 12%). Similarly, nitrification was more rapid in the non-enclosed soils (P ≤ 0.01). Between 2.0 to 9.9% of the applied-N was present as NO₃⁻ after 14 days in the non-enclosed soils. The comparable figures for NO₃⁻accumulation in the enclosed soils were 0.2 to 5.5%. Nevertheless, the total-N accounted for in each of the enclosed and non enclosed soils on each sampling occasion were not significantly different (P ≤ 0.05). Moreover, when the measured NH₃ losses were added to the amounts of applied-N recovered in both the enclosed and non-enclosed soils, quantitative recovery was shown for 5 of the 6 soils studied. It was concluded that immobilisation and nitrification of applied urea-N have very little influence on the volatilisation potentials of soils fertilised with broadcast granular urea. It was also concluded that the enclosure method, provided it adequately simulates external soil temperatures, is an effective and accurate method for measuring the volatilisation potentials of soils under field conditions. However, a micrometeorological (mass balance) experiment using both active and passive NH₃ samplers showed that the volatilisation potential of soil receiving broadcast urea can be markedly reduced (from 35 to 7%) by various environmental factors including rainfall, initial soil moisture and the presence of vegetative cover.