Modelling water, carbon and nitrogen dynamics in soil covered with decomposing mulch

Summary A decomposing mulch of residues on the soil affects strongly the water, carbon and nitrogen cycling in the soil. The mulch generates complex and interacting effects that remain difficult to quantify and model. An original mulch module was developed and implemented in the PASTIS model to account for the main biological and physical effects of decomposing mulch. In the extended model, PASTIS mulch , the formalism splits the mulch into a decomposable layer in contact with the soil and an overlying not decomposable layer that ‘feeds’ the former. The model was calibrated on laboratory data derived from mulched soil columns experiments. Two types of residues were tested: rape residues made of large elemental particles (length, 10 mm; width, 3–10 mm; thickness, 0.1–8 mm) with a C:N ratio of 29, and rye residues consisting of smaller particles (length, 10 mm; width, 2 mm; thickness, 0.1 mm) with a smaller C:N ratio of 16. Calibration showed that mulch parameters were dependent on the type of residue. The model run with calibrated parameters provided good simulations of the water, carbon and nitrogen dynamics, with global efficiencies greater than 0.8. The sensitivity analysis carried out on seven key parameters showed that the total mulch dry mass and the proportion of this dry mass in contact with the soil are decisive parameters. PASTIS mulch highlighted that mulch decomposition was not a continuous process but occurred in the form of successive pulses that correspond to favourable hydric conditions.