Soil organic carbon (SOC) dynamics with and without residue incorporation in relation to different nitrogen fertilisation rates

Crop residue incorporation is recognised as a simple way to increase C input into the soil, with positive effects on Csequestration from the atmosphere. However, in some long-term experiments, a lack of response to soil C input levels has been observed as a consequence of saturation phenomena and/or interactions between C input and fertilisation. This paper analyses the outcomes of a long-term experiment in north-eastern Italy that started in 1966 and is still ongoing, where residue incorporation is compared with residue removal, over a range of mineral N fertilisations. A general decrease of SOC content was observed in the first 10 years of the experiment, followed by an approach to a steady state. However, SOC content differed markedly according to residue management and, in plots with residue incorporation, to N fertilisation. Considering 20 years as a compromise period for reaching a new equilibrium after a land-use change, the sequestration rate of residue incorporation in comparison with removal resulted as 0.17 t ha−1 of C per year. The measured data were then simulated with Century, a model based on first-order decomposition kinetic, to evaluate if the data could be interpreted by this kind of decomposition process. Model performances were good in most cases, but overestimated SOC decomposition in the more limiting situations for C and N inputs. A possible explanation is given for this behaviour, involving a feed-back effect of the microbial community.