Change of ergosterol content after inorganic N fertilizer application does not affect short-term C and N mineralization patterns in a grassland soil

•Investigation of N fertilizer on microbial biomass and mineralization in grassland.•Effects of root residue presence on mineralization processes were evaluated.•No effects of fertilizer type or level on soil microbial biomass and activity.•The microbial community of the grassland soil has a high level of resistance.•Roots and inorganic fertilizer resulted in relative increase of saprotrophic fungi.

N released from soil organic matter and plant residues is an important source of available N in permanent grassland. Inorganic fertilizer application alters root biomass and quality and therefore changes the input and turnover of soil organic matter. Little is known about how intensification of N fertilization affects C and N mineralization processes and thus contributes to N losses and C emissions. It remains unclear if N fertilizer addition positively or negatively affects soil microbial biomass carbon and soil organic C stored in the short- and long-term. Our research investigated whether increased N fertilization of permanent grassland soil affects the microbial community and if this is reflected by a change in mineralization processes from soil organic matter and Lolium perenne roots. We performed an aerobic incubation experiment on samples from a field trial on permanent grassland with varying factors: fertilizer type and N level, and presence of roots. Inorganic fertilization and root presence caused a shift in microbial community structure with an increase of saprotrophic fungi, as indicated by ergosterol content and ergosterol-to-MBC ratio. However, no significant effects of organic or inorganic fertilizer application on mineralization processes and soil microbial biomass were observed, reflecting a strong resistance of the present grassland soil system. This is probably caused by the high level of microbial biomass and clay content, providing a large chemical and biological buffer capacity. Overall, our results indicate, that the microbial community and mineralisation processes are not affected by intensification of N fertilization.