Factors influencing nitrogen derived from soil organic matter mineralisation: Results from a long-term experiment

Mineralised nitrogen (N) from soil organic matter (SOM) is a crucial source of N for both natural ecosystems and agroecosystems. Therefore, accurate estimation of the amount of N available to crops from SOM mineralisation is necessary to correctly manage N addition. For application in an N budget, a field-scale assessment of the main factors affecting SOM mineralisation is required. The objective of this study was to quantify the influence of meteorological conditions and soil properties on N mineralised by SOM in an agroecosystem. The N mineralised from the SOM was calculated as the N uptake of the unfertilised plot minus the N derived from atmospheric deposition and irrigation. This study analysed 29 years of crop, agrometeorological, and soil data from three maize cropping systems (maize for grain, maize for silage, and maize-It. ryegrass double cropping) in a long-term experiment conducted in NW Italy. A Linear Mixed Model (LMM) was developed for the purpose of this study. The average of N derived from SOM mineralisation predicted by the model was 96 kg N ha⁻¹ yr⁻¹, with a root mean square error of 22 kg N ha⁻¹ yr⁻¹. The fixed factors of LMM, which are soil organic carbon (SOC), carbon-to-nitrogen ratio (C/N) and the sum of rainfall and irrigation (R.I.), were responsible for 19 % of the annual variations in mineralised N. SOC and R.I. had a positive effect and greater weight on the process, whereas C/N had a negative effect and lower weight. The explanatory power of the model increased to 52 % when cropping systems and interannual variability were included as random factors. This study highlights the importance of weather conditions and SOC content in determining the amount of N derived from soil mineralisation and can contribute to plant nutrition. In a future climate scenario characterised by increased aridity, N mineralisation could decrease, thus increasing the demand for fertilisers.