Bacterial and fungal growth in burnt acid soils amended with different high C/N mulch materials

• Different high C/N mulch materials were added to a recently burned forest acid soil.
• Wheat straw, coconut fibre, Eucalyptus bark and wood chips were used in the assay.
• Fungal and bacterial growth responded in different ways to the additions.
• Fungi were favoured by addition of mulch materials with higher C/N ratios.
• Eucalyptus residues appeared suitable as effective mulching agents.

Wildfire has become a major disturbance agent in Mediterranean area, with Galicia in the north-west of Spain having the most frequent fires. In order to combat soil erosion, different post-fire stabilization treatments can be applied, like adding different mulch materials. In a laboratory experiment we evaluated the effect of adding plant residues, commonly used in post-fire rehabilitation (wheat straw, coconut fibre, Eucalyptus bark and wood chips), at two different rates on the microbial response (respiration, bacterial and fungal growth, using leucine and acetate-in-ergosterol incorporation, and biomass and community structure by means of phospholipids fatty acid analysis) in an acid forest soil affected by a high intensity wildfire. We expected fungal growth to be favoured compared to bacterial growth after adding plant material with high C/N content, especially at the higher rate. Compared to unburnt soil, burnt soil had higher pH (increasing from 4.4 to 5.8) and more extractable (available) C, resulting in a relatively higher increase in bacterial compared to fungal growth. Adding Eucalyptus residues to burnt soil, especially wood chips, favoured fungal growth, especially at the higher rate. Straw addition favoured both fungal and bacterial growth and was the substrate most available for decomposition, as indicated by the higher soil respiration rates. Coconut fibre contained less available C. Adding Eucalyptus residue, at both rates, and coconut fibres at the high rate, initially inhibited bacterial growth, resulting in at most a 14 day lag period. Aiming at high fungal to bacterial growth in order to maximize stabilization of soils through fungi suggest Eucalyptus residues to the prime choice for further studies on effective mulching agents, although coconut fibres, with low degradation rates, may also be a suitable choice. Nevertheless, besides aspects related to soil quality, additional information concerning the efficiency of these materials to control post-fire soil erosion should also be considered before their implementation under field conditions.