To reveal if the input of pyrogenic organic matter (PyOM) affects the nitrogen (N) cycling in soil and its N can be used for the synthesis of microbial biomass, we investigated the incorporation of ¹⁵N from ¹⁵N-enriched grass residues (OM) or their PyOM into extractable amino acids (AAs) of soil organic matter (SOM) from an unburnt and a burnt soil amended with those residues.

Materials and methods

Pots seeded with Lolium perenne and filled with soil from a burnt and an unburnt Cambisol from southern Spain were topped either with ¹⁵N-enriched grass residues (¹⁵N-OM), its ¹⁵N-PyOM, mixtures of KNO₃ (N_i) and ¹⁵N-OM or ¹⁵N-PyOM, as well as K¹⁵NO₃ mixed with non-enriched OM or PyOM. After incubation of the pots for up to 16 months under controlled conditions, the AAs, extracted from the litter-free soil, were quantified by gas chromatography mass spectrometry (GC/MS). The fate of the added ¹⁵N (¹⁵N_add) was followed by isotopic ratio mass spectrometry (IRMS) and analyzed by statistical means.

Results and discussion

The contribution of extractable AAs to SOM of the non-amended burnt soil was twice of that for the unburnt soil. After amendment, their yields and the percentage of ¹⁵N_add recovered in AAs were always higher for the burnt soil. Stabilization of proteinaceous residues during the incubation was indicated. Already after 2 weeks, ¹⁵N_add from ¹⁵N-PyOM was recovered within the AAs.

Conclusions

Our experiment confirmed that N from PyOM is incorporated into the peptidic fraction of SOM of post-fire soils. The efficiency of this incorporation is not altered by the presence of N_i and vice versa. We demonstrated further a short-term and medium-term impact of fire on N cycling in soils, expressed by alteration of the contribution of acid-extractable AAs to the soil organic N (SON) pool.