Bioavailability of N released from N-rich pyrogenic organic matter: An incubation study

In Mediterranean ecosystems, natural wildfires and prescribed burning play an important role. In order to obtain a better understanding of the impact of charcoal on the nitrogen (N) cycling in soil, ¹⁵N-enriched pyrogenic organic material (PyOM) obtained from “Lolium perenne” charred for 4 min at 350 °C was mixed with a typical Mediterranean agricultural soil and incubated for 72 days under controlled conditions. The main objectives were to analyze the availability of N from this material and to obtain more insight into the recalcitrance of PyOM in soils. Addition of artificially produced ¹⁵N-PyOM increased the biomass production and N retention. After 72 days of incubation time, 10% of the ¹⁵N added to the soil (¹⁵N_add) was incorporated into new grass biomass. Solid-state ¹⁵N NMR spectroscopy revealed that at least some of this N derived from the degradation of pyrrole-type structures. Increase of the amide-N intensity in the solid-state ¹⁵N NMR spectrum of the incubated soils indicated further that some ¹⁵N from the PyOM was also incorporated into new microbial biomass. Our results confirmed a relatively low recalcitrance of N-rich PyOM. Since during its degradation, N is only slowly transferred into a plant-available form, it may contribute to the observed improvement of soil fertility by avoiding fast N losses due to leaching and volatilization as a slow N-release fertilizer.