Burning effects on the distribution of organic N compounds in a 15N labelled forest soil

Nitrogen distribution was studied, by successive 1 M (H1) and 3 M HCl (H2) hydrolyses, on a natural soil before (NS) and after 15N labelling (LS) in an incubation chamber and burning (BLS) in a furnace simulating an intense fire (385 °C, 10 min). The labelling increased the organic-N of H1 (H1-N) by 4.7%, due to the increase in hydrolyzable unidentified-N (HU-N, 66.3%) and amino acids (AA-N, 11.2%), which counterbalanced the reduction of amides (AM-N, 33.2%) and amino sugars (AS-N, 68.0%). After labelling, H2-N decreased by 7.5%, mainly due to the reduction of AA-N (12.2%) and AS-N (14.9%); conversely, ammonium-N (A-N) and non-hydrolyzable-N (NH-N) did not vary and total organic-N increased slightly (2.4%). In LS, the 15N labelling decreases as follows: H1-N (with AM-N>AS-N>AA-N≈HU-N)>H2-N (with HU-N>AA-N≈A-N>AS-N)>NH-N. The added 15N was mainly incorporated in organic forms (92.2%), following the distribution of the endogenous organic-N; nevertheless, the higher proportion of recently incorporated 15N in hydrolyzable fractions, and lower in NH-N, showed that it is more labile than endogenous N. The added 15N undergoes similar, but stronger, transformations and losses due to burning than the native N: (1) 18.1% of endogenous-N and 22.4% of exogenous-N were lost; (2) H1-N, H1-15N, H2-N, H2-15N, AA-N, AA-15N, HU-N and HU-15N decreased by 69.7%, 74.1%, 76.6%, 82.9%, 96.5%, 96.8%, 92.1% and 98.3%, respectively; (3) NH-N, NH-15N, A-N and A-15N increased by 81.0%, 314%, 81.3% and 78.2%, respectively; (4) AM-N increased (51.2%) whereas AM-15N decreased (1.7%). Therefore, soil burning reduces the soil organic N reserves, through N volatilization (especially of labile N), and decreases N bio-availability, through an important net transfer of N from the labile to the recalcitrant pool; jointly, both processes will increase the negative effects of wildfires on the N cycle. In spite of the previous 15N labelling process, LS could be considered as a representative forest soil, which undergoes similar changes during burning than unlabelled soils, leading to a representative burnt labelled soil. Neither in LS nor in BLS the distribution of the added 15N was uniform among the N fractions; nevertheless, as the reference levels of 15N enrichment in the organic N fractions are accurately known, both LS (as control treatment) and BLS will be useful for further studies on the efficiency of several techniques on the post-fire restoration of the soil N distribution.