Nitrogen release from soil organic matter and from different kinds of nitrogen containing materials added to the soil for fertilising is calculated in a module that hosts the following routines:

• Nitrogen mineralisation from both soil bound and freshly added organic matter
• Nitrogen volatilisation from added manures and slurries
• Hydrolysis of urea and volatilisation of ammonia

Nitrogen mineralisation from organic matter is based on the routines used in the DAISY model (Hansen et al., 1990). Three pairs of conceptual pools (soil organic matter, soil microbial biomass and added organic matter), each representing a rapidly decomposable and a slowly decomposable class of nitrogen containing organic substances, respectively, describe carbon dynamics in the soil. Decomposition rate coefficients are temperature and moisture dependent and reflect the environmental conditions of the simulated site. Decay and maintenance rates of soil microbial biomass are additionally influenced by soil clay content. Efficiency parameters determine the loss of carbon dioxide during the single turn-over processes. N release as ammonium is a consequence of carbon lost as carbon dioxide from the system that maintains fixed carbon to nitrogen ratios in the different pools. Processes of nitrification and denitrification are implemented to complete the turn-over model.

Residues of crops simulated with the crop growth model enter the mineralisation routine with a dynamic carbon to nitrogen ratio, which reflects the growth conditions of the crop during season with respect to N supply. The variable carbon to nitrogen ratio is assigned to the rapidly decomposing part of the material, while the remaining part is considered to decompose slowly, having a fixed carbon to nitrogen ratio. Decomposition rate coefficients of both pools are also fixed (Abrahamsen, 2000). Carbon to nitrogen ratios and partitioning coefficients for crop residues are derived from stepwise chemical digestion experiments (Jensen et al., 2005). Manure and slurry properties are taken from DAISY parameterisations (Abrahamsen, 2000).

Nitrogen volatilisation from soil applied manures and slurries are described using an empirical relation implemented in the ALFAM model (Søgaard et al., 2002). A soil pH dependency factor was introduced by fitting data from He et al., (1999) to Michaelis-Menten kinetics and subsequently normalising the relation between pH and volatilisation half life time to pH 7.0.

Gaseous nitrogen loss from applied urea fertiliser is calculated using the routines of the AMOVOL model (Sadeghi et al., 1988), taking into account the temperature dependent equilibrium between ammonium ions, solved and gaseous ammonia as well as the effect of soil organic matter, soil temperature and soil water potential on the hydrolysis process itself. An atmospheric resistance parameter finally governs the loss of gaseous ammonia from the top soil.

Literature

Abrahamsen,P., Hansen,S., 2000. Daisy: an open soil-crop-atmosphere system model. Environmental Modelling & Software 15, 313-330.

Hansen, S., Jensen, H.E., Nielsen, N.E., Svendsen, H., 1990. Daisy - A Soil Plant Atmosphere System Model. NPO Research from the National Agency of Environmental Protection No. A 10. 272 pp.

He, Z.L., A.K. Alva, D.V. Calvert und D.J. Banks (1999): Ammonia volatilization from different fertilizer sources and effects of temperature and soil pH. Soil Science, 164 (10): 750-758.

Jensen,L.S., Salo,T., Palmason,F., Breland,T.A., Henriksen,T.M., Stenberg,B., Pedersen,A., Lundström,C., Esala,M., 2005. Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil. Plant and Soil 273, 307-326.

Sadeghi, A.M., K.J. McInnes, D.E. Kissel, M.L. Cabrera, J.K. Koelliker und E.T. Kanemasu (1988): Mechanistic model for predicting ammonia volatilization from urea. In: B.R. Bock und D.E. Kissel (Hrsg.), Ammonia volatilization from urea fertilizers. National Fertilizer Development Centre, Tennessee Valley Authority, Muscle Shoals, Alabama, 67- 92.

Søgaard, H.T., S.G. Sommer, N.J. Hutchings, J.F.M. Huijsmans, D.W. Bussink und F. Nicholson (2002): Ammonia volatilization from field-applied animal slurry - the ALFAM model. Atmospheric Environment, 36 (20): 3309-3319