Increasing the maturity of compost used affects the soil chemical properties and the stability of microbial activity along a mediterranean post-fire chronosequence

• Increasing the maturity of compost used increased soil organic carbon content.
• Mature compost decreased microbial activity and stability 5 years after fire.
• Increase in compost maturity increased soil C-accretion from 5 to 18 years after fire.
• Increase in compost maturity used can impairs ecosystem services and sustainability.

Compost addition has been largely employed to improve chemical properties and microbial activities of several disturbed soils. However, few attempts have assessed the adequacy of compost quality considering the level of ecosystem recovery after frequent wildfires in combination with droughts. We investigated the suitability of the addition of 3 ages of compost (i.e. 3 weeks, 3 months and 9 months) crossing with 3 times since fire (i.e. 1, 5 and 18 years of recovery) to increase the soil organic and inorganic resources in frequently burned soils. We hypothesised that resource depending on quality (i.e. maturity) should improve microbial activity and its resistance and resilience against a drastic drought and could have some implication for SOM mineralisation. Our results showed that the more mature compost, richer in organic matter, increases TOC, total N, PO43−-P concentrations and pH but regardless the time since fire. Microbial activity weakly responded to this soil resource improvement whereas it was strongly depressed 5 years after the last fire. Mature compost resulted in a loss of resistance and resilience of the microbial activity in comparison with control soils depending on the time since fire, indicating that exogenous resource as compost affects microbial stability. The cumulative C-mineralisation clearly indicated that the loss of microbial activity and stability against drought with the more mature compost would result in an improvement of soil C-accumulation especially 5 years after the last fire.