J. R. Simpson

A closed ammonia cycle within a plant canopy

Abstract Ammonia losses to the atmosphere from a grass-clover pasture were measured by a combined chemical-micrometeorological technique. Losses from the pasture when grazed were quite considerable (~13g N ha −1 h −1 ) but when ungrazed they were comparatively small (2 g N ha −1 h −1 ). Measurements within the canopy of the ungrazed pasture at maturity indicated a large production of NH 3 near the ground surface and almost complete absorption of it by the plant cover. The amounts of NH 3 absorbed appeared to be too large for stomatal uptake alone. This closed NH 3 cycle has important consequences for the field assessment of N 2 fixation and gaseous N losses.

Ammonia flux into the atmosphere from a grazed pasture.

A micrometeorological technique has been used to measure the flux of ammonia and related gaseous nitrogen compounds into the atmosphere from a pasture grazed by sheep. During 3 weeks in late summer, the average daily flux density of nitrogen in these forms was 0.26 kilogram per hectare. This is a substantial part of the nitrogen turnover in grazed pastures.

A sampler for measuring atmospheric ammonia flux

Abstract This paper describes the design, construction and testing of a simple sampling device for determining ammonia fluxes in the atmosphere. Sampler performance was predicted theoretically and tested in a wind tunnel, the laboratory and in the field. The results showed that air flowed through the sampler at a rate linearly proportional to the external wind speed and that the ammonia it contained was absorbed quantitatively by the sampler. The mass of ammonia, M , collected by the instrument during a sampling period, t , is thus proportional to the mean convective flux density of ammonia, ( uϱ N ) since M = ( uϱ N )t , where u is the wind velocity, ϱ N the ammonia density, A is the effective cross-sectional area of the sampler, and the overbar represents a time-mean.

Nitrous oxide emission from soils at low moisture contents

Abstract The production of nitrous oxide by soils was studied over short periods at a range of moisture contents up to field capacity with a highly-sensitive gas Chromatographic method. Nitrous oxide (N 2 O) was emitted from all soils studied at all soil moisture contents, which ranged from air dry to field capacity. The rate of emission increased with increasing moisture content and with increasing temperature up to 37°C. The evolution of N 2 O was not due to displacement of soil air during wetting. It was inhibited by HgCl 2 and toluene, and was prevented by formaldehyde and autoclaving. Thus it appeared to be due to microbiological processes. The results of experiments with nitrification and denitrification inhibitors suggest that a considerable part of the N 2 O was produced by the oxidation of ammonia. Production by denitrification of nitrate cannot be ruled out. The relative importance of these two mechanisms probably depends on the moisture and oxygen content of the soil. It is concluded that the microbial production of N 2 O is continuous in soil at all moisture contents. The process at low moisture contents constitutes an important component in the cycle which maintains the N 2 O concentration in the atmosphere.

Emission of nitrogen oxides (NO x ) from a flooded soil fertilized with urea: Relation to other nitrogen loss processes

Emissions of nitric oxide and other odd nitrogen oxides (NOx) from a flooded rice field were studied after urea had been broadcast into the floodwater.The NOx flux from the fertilized area was very low (0.2×10-9 g N m-2 s-1) for the first few days after application of urea and was high (0.95×10-9 g N m-2 s-1) in the subsequent period when significant nitrite and nitrate were present in the floodwater. At night, little if any NOx was exhaled but ambient NO2 was absorbed by the floodwater. An uptake velocity for NO2 of 3×10-4 m s-1 was measured during one night. Maximum NOx losses were observed near 1300 h when temperature and solar ultraviolet light were maximum.While the amounts of nitrogen oxides emitted are of little agronomic importance (∼2×10-3 per cent of the fertilizer nitrogen was lost as NOx during the 10-day study period), they may well be of significance as a source for some gas reactions in the atmosphere and for the global nitrogen cycle.Of the fertilizer nitrogen applied (as urea) approximately 30% was lost to the atmosphere by NH3 volatilization, 15% by denitrification, presumably as N2, and the remainder, less minor losses of NO and N2O, remained in the plant/soil/water system.

Processes of ammonia loss from shallow floodwater

Abstract Flux densities of ammonia, water vapour and sensible heat were measured over a flooded rice field following the application of urea fertilizer. These measurements provided estimates of transfer velocities for NH 3 and water vapour in the air and for NH 3 in the water ( g Nw ). Ammonia fluxes were controlled by transport processes in both the atmosphere and the water. Diurnal variations of g Nw appeared to be caused by changes in the relative strength of mechanical and buoyancy mixing forces in the turbid water. Stable stratification of the subsurface water layer until early afternoon was attributed to absorption of shortwave radiation, which caused g Nw to decrease relative to that for neutrally stratified water.

Fluctuations in soil moisture, and plant uptake of surface applied phosphate

The mechanisms leading to decreased plant uptake of surface-applied phosphatic fertilizers during short periods of drying were investigated in controlled glasshouse experiments with subterranean clover.Phosphate and water were added differentially to surface and subsurface layers of reconstituted soil profiles. The surface layers were subjected to brief periods of drying and to partial or complete remoistening at frequencies ranging from daily to fortnightly.Phosphorus uptake from surface applications and shoot yield were proportional to the frequency of remoistening of the soil surface to field capacity, and the response to additional subsurface phosphate diminished as the amount of surface water increased. In treatments where moistening did not achieve field capacity, yield was linearly proportional to the amount of water applied to the surface.Watering the surface to field capacity twice a week led to a 50 percent reduction in phosphorus uptake compared with daily watering. Watering twice as frequently with half the amount each time did not affect plant yield or phosphorus uptake. Thus it appeared that rapid phosphorus uptake occurred only at high moisture contents, and that uptake was proportional to the volume of soil brought close to field capacity and the length of time that it remained moist. This indicates that desorption of soil-adsorbed phosphate and its diffusion to plant roots can occur rapidly only at high moisture contents. Similar results were obtained with different soil types when the extractable phosphate was concentrated near the soil surface.

Control of gaseous nitrogen losses from urea applied to flooded rice soils

This paper reports field experiments designed to determine whether the two main processes responsible for nitrogen (N) loss from flooded rice (ammonia volatilization and denitrification) are independent or interdependent, and glasshouse studies which investigated the effect of soil characteristics on gaseous nitrogen loss.In the first field experiment ammonia (NH3) loss from the floodwater was controlled using algicides, biocides, frequent pH adjustment, shade or cetyl alcohol, and the effect of these treatments on total N loss and denitrification was determined. Most treatments reduced NH3 loss through their effects on algal growth and floodwater pH. Total gaseous N loss (54% to 35%) and NH3 loss (20% to 1.2%) were affected similarly by individual treatments, indicating that the amount lost by denitrification was not substantially changed by any of the treatments.In a subsequent field experiment NH3 and total N loss were again affected similarly by the treatments, but denitrification losses were very low. In control treatments with different rates of urea application, NH3 and total N loss were each a constant proportion of the urea applied (NH3 loss was 17% and total N loss was 24%). These results indicate that techniques which reduce NH3 loss can be expected to reduce total gaseous N loss.The glasshouse experiment showed that gaseous N losses could be reduced by draining off the floodwater, and incorporating the urea into the 0–0.05 m soil layer before reflooding. Even with this method, losses varied widely (6–27%); losses were least from a cracking clay and greatest from a coarse sand which allowed the greatest mobility of the applied N. Incorporation of applied urea can therefore be expected to prevent losses more successfully from clay soils with high ammonium retention capacity.

The evaluation of urease inhibitors to improve the efficiency of urea as a N-source for flooded rice

Abstract The effectiveness of phenylphosphorodiamidate (PPD) and N-( n )-butyl) thiophosphorictriamide (BTPT) as urease inhibitors in moist and flooded soils was evaluated in laboratory, glasshouse and rice-field experiments. The effectiveness of the inhibitors varied markedly, depending on type of soil, cultural conditions and algal growth. In flooded soils in the dark, PPD was a more effective inhibitor than BTPT. However, in the presence of light and algae, BTPT was the more effective inhibitor of urea hydrolysis. BTPT was completely ineffective in one flooded soil. Addition of ethylenediaminetetraacetic acid (EDTA) together with BTPT greatly improved urease inhibition in this soil. Addition of BTPT increased the depth of penetration, immobilization and retention of urea N in a flooded soil in the glasshouse, but these effects were not translated into yield increases in field plots.

The mineralization of nitrogen from some organic fractions in soil

Abstract The mineralization of labelled and indigenous nitrogen was studied in soils of different total nitrogen contents after long periods of equilibration with added labelled ammonium nitrogen but without the addition of a carbohydrate energy source. The changes during incubation were followed in a number of organic nitrogen fractions. In one soil, the effects of leaching, drying and heating treatments before incubation were also determined on each fraction. A substantial mineralization of labelled nitrogen occurred in all soils during incubation. The non-distillable acid-soluble fraction decomposed by a greater percentage than did the other labelled nitrogen fractions in the first stages of incubation and probably contributed most to labelled mineral nitrogen. In the indigenous nitrogen, the acid-insoluble fraction appeared to be the most active as determined by its percentage change, but it could not be identified as the main contributor to mineral nitrogen. The removal of mineral nitrogen by leaching, and drying the soil at 50° or 100°C produced some small effects on the rate of mineralization of the labelled and indigenous nitrogen during subsequent incubation. The proportion of the labelled nitrogen which was mineralized was greater than that of the indigenous nitrogen during the same period, and thus there appeared to be an ‘active’ fraction within the soil nitrogen. In addition, in all the organic fractions the labelled nitrogen underwent a greater percentage change than the indigenous nitrogen and thus it appears that there was an ‘active phase’ within each organic nitrogen fraction.