CARBON AND NITROGEN DYNAMICS IN SOILS AFTER REPEATED MANURE APPLICATIONS: MEASUREMENTS AND MECHANISTIC SIMULATIONS.

The first part of this Ph. D. Thesis (Chapters 1 and 2) is relative to the calibration of a soil organic matter simulation model (the CN-SIM model) by applying it to an experimental data-set, using two different methodologies, in order to verify its conceptual structure and obtaining parameters to describe the partitioning of C and N within the manures. Based on the optimization results, it emerged that the low model performance could be attributed to the lack of some processes involving mineral nitrogen transformations into the soil. In order to better understand the nitrogen dynamics following manure application to the soil, an incubation experiment was build up. In the experiment, two different manures were applied to two soils of different texture, and, in order to estimate the contribution of manure recalcitrant fractions to the mineralization dynamics of C and N in the soil (residual effect of added manure carbon and nitrogen), each manure was applied four times on the same soil (Chapter 3). In the incubation experiment were included measurements of non-exchangeable ammonium, with the aim of better understand if the lower recoveries of added manure ammonium, often found on the clay soils, were due to microbial biomass immobilization or ammonium clay-fixation. Measurements obtained in the incubation experiment were the used to perform a preliminary modification of the CN-SIM model to take into account the ammonium fixation and release processes (Charter 4).