In vitro anaerobic incubation: a reliable method to predict the potential of nitrogen mineralization after grassland ploughing

Abstract

Description of the subject. Large amounts of nitrogen may be released after grassland ploughing, and no reliable method is available for routine laboratories to help farmers managing these cultural practices. Objectives. The objective of this study was to assess whether short-term anaerobic potentially mineralizable nitrogen (PMN) is a reliable estimator of the N amount actually mineralized in the field during the growing season after grassland ploughing. Method. PMN was determined in vitro using a 7-day anaerobic incubation on three plots of grassland that were tilled and monitored for two years. The mineral nitrogen (NO3- -N and NH4+-N) amounts were measured at different times in the three plots and used to estimate the actual in situ mineralization with the LIXIM calculation model (ISM). Results. Our results indicated that in vitro anaerobic incubation of grassland soil samples provided estimations very close to the actual data of ISM observed during the monitoring (less than 10% of relative difference). A close relationship (p = 0.0029) was found between PMN and ISM with a high correlation (R2 = 0.91). Additionally, our study revealed very high levels of nitrogen mineralization during the first and second year after grassland ploughing, even after temporary grassland of short duration (3 years). Conclusions. PMN estimated by short-term anaerobic incubation is suitable for a reliable application in grasslands of all ages. This method should be used only if the mineral N content of the soil is not too high (< 100 kg N·ha-1 in our experiment) and the mineralization depth taken in account in the calculation should be defined rigorously in each situation.