Net nitrogen mineralisation in a drained mollic gleysol under variable soil moisture conditions

Abstract

Mineralisation of organic matter can substantially add to the risk of nitrogen leaching from drained humus-rich soils under intensive agricultural use. The objective of this study was to investigate net nitrogen mineralisation (NNM) in a drained Mollic Gleysol over the course of a year and to separate contributions from the decomposition of fresh organic residues (‘litter’) and stabilised soil organic matter (‘humus’). Using NNM rates measured under different moisture conditions in two undisturbed subsoil monoliths and a moisture response function determined by incubation experiments with small soil cores from a humus-rich subsoil layer, a first-order rate model was calibrated to describe NNM of the ‘humus pool’. Standardised to the same moisture conditions by means of this response function, NNM rate constants were independent of depth and also did not differ between monoliths and small soil cores. The model predicted NNM rates determined in situ at a depth of 20–40 cm well. For the topsoil (0–20 cm) it was necessary to extend the model by a second pool of organic matter with a higher turnover rate (‘litter pool’). Assuming the same moisture and temperature dependence of NNM in the two pools, the basic NNM rate constant of the litter pool was determined by data fitting. With this calibration, the two-pool model predicted a net mineralisation of 322 kg N ha−1 during the experimental year in the upper 40 cm of the soil of which 55% occurred in the ‘litter pool’. While overall NNM predictions relating to the entire study period agreed well with the field data, predictions for individual incubation periods (3–6 weeks) and soil layers were much less reliable.