Ryegrass-derived pyrogenic organic matter changes organic carbon and nitrogen mineralization in a temperate forest soil

Pyrogenic organic matter (PyOM) is considered as a technique to improve soil fertility and store carbon (C) in soil. However, little is known regarding soil organic C and nitrogen (N) mineralization in PyOM-amended soils. To investigate the relationship between the C and N mineralization rates and the possible consequences in terms of C storage and N availability, we incubated ryegrass-derived PyOM (pyrolyzed at 450 °C) enriched in 13C (4.33 atom %) in a forest Cambisol for 158 days with and without mineral N addition. We determined PyOM and native soil organic C mineralization, NH4+ and NO3− contents in the soil, gross N mineralization, phenol-oxidase and protease activities, and microbial biomass throughout the incubation experiment and the incorporation of PyOM in microbial biomass at the end of the experiment (158 days). We determined that 4.3% of the initial PyOM-C was mineralized after 158 days. Moreover, PyOM induced a strongly positive priming effect within the first 18 days; a negative priming effect was observed from Days 18 to 158. The initial increase in organic matter mineralization corresponded to a higher gross N mineralization and NH4+ content in the PyOM-treated soil than in the untreated soil. Ammonium was rapidly transformed into nitrate and stored in this form until the end of the experiment. We conclude that the presence of PyOM affected the mineralization pattern of native soil organic matter mineralization and increased mineral N content, while N addition did not influence PyOM or soil organic matter mineralization.