Sophie Génermont

A mechanistic model for estimating ammonia volatilization from slurry applied to bare soil

The loss of ammonia by volatilization after slurry application may lead to large losses of soil-plant nitrogen. It is also a major source of atmospheric ammonia. These fluxes must therefore be accurately measured. However, volatilization depends on many features of the soil, climate and slurry, so that it is difficult to predict, or even to interpret and compare data from different experiments. We have developed a mechanistic model that simulates the influence of the various factors on volatilization, accounting for the transfers and equilibria in the topsoil and between the soil and the atmosphere. The model uses readily available input data, including soil, meteorological and slurry data. It includes energy balance and advection submodels, which make it suitable for field scale applications using simple meteorological data. Sensitivity analysis showed that soil pH has a large influence on volatilization. The model is also sensitive to soil adsorption capacity and some hydraulic characteristics (saturation water conductivity, water content at field capacity). It has been calibrated under real field conditions using experimental data for ammonia fluxes measured over two weeks after slurry application, with a micrometeorological method giving data at 15-min intervals. The soil, climate and slurry factors were also measured. The model provides a fair picture of the ammonia fluxes throughout the volatilization, including the total ammonia volatilized, the decrease in daily loss and their short-term (< 1 h) variations due to the influence of meteorological conditions on soil surface temperature and atmospheric diffusion. Lastly the model has been used to simulate the influence of various meteorological conditions and agricultural techniques on ammonia volatilization. This model will make it easier to interpret data from different experiments and will help to improve the emission submodel of atmospheric ammonia deposition models.

Measuring ammonia fluxes after slurry spreading under actual field conditions

Abstract A large fraction of atmospheric ammonia is emitted during the application of slurry to fields. Two micrometeorological methods were used to obtain a consistent estimate of emissions under actual field conditions. The mass balance method (MBM) was used to quantify the very large emissions that occur during slurry spreading and for the first few hours. Wind speed was measured at 5 heights, and ammonia was sampled at these heights by trapping it in dilute sulphuric acid. A two-height aerodynamic gradient method (2AGM) was used for later automated hourly monitoring of the long-term flux over a large surface (over a hectare). The ammonia concentration gradient was measured continuously with a chemiluminescence analyser. The hourly estimates of ammonia fluxes were similar to the data from a labelled nitrogen recovery method (15NRM). The MBM gave reliable flux estimates using only two measurement heights. Thus, ammonia fluxes could be determined directly in real time using the chemiluminescence analyser, and two air temperature measurements by two anemometers, beginning from the first minutes after the start of slurry spreading, and continuing until several weeks later.