Decomposition of black pine (Pinus nigra J. F. Arnold) deadwood and its impact on forest soil components

Highlights: • Distinct pools, structural and functional diversity of deadwood, soil, and litter • Dual role of deadwood as pool and source of C throughout the decomposition process • Increased CO₂ and CH₄ emissions and cellulolytic enzymes with deadwood decay • Deadwood biomass at the final decay class was fragmented and transferred to litter • Decrease of fungi and increase of bacteria over the deadwood decomposition process Deadwood decomposition is a complex and dynamic process with large implications for biogeochemical cycling of carbon (C) and nitrogen (N) in forest soil and litter. Moreover, it affects functional and structural diversity of fungal and bacterial communities in these components. Mesocosms with deadwood blocks at progressive decay classes were set in a black pine forest and incubated for 28 months in the field with the aim to assess the impact of deadwood decomposition on i) CO₂, CH₄ and N₂O fluxes; ii) C and N pools and allocation among deadwood, litter and soil; iii) the fungal and bacterial structural diversity and activity. CO₂, CH₄ and N₂O fluxes from deadwood were monitored throughout the field incubation; deadwood biomass loss and decay rate for each decay class were calculated. The stock of C and N, enzyme activities, fungal and bacterial communities in deadwood, litter fractions (fresh, fragmented and humified) and soil at two depths were measured. Emissions of CO₂ and CH₄ increased over the deadwood decomposition advancement and the decay reached the maximum rates in the last decomposition classes. N₂O fluxes were low and showed either production (prevalent in the first year) or consumption. Independent of the decay class, 20% of C stored in deadwood was lost as CO₂ in the atmosphere, whereas 32% was transferred to the fragmented and humified litter fractions in the last decay class. A corresponding increase of cellulose and hemicellulose degrading enzymes was found in deadwood, also favored by substrates accessibility through fragmentation and successional changes in fungal and bacterial communities. Deadwood, litter fractions and soil components were clearly distinguished in terms of chemical and microbiological properties and activities. Fragmented and humified litter fractions were the only components responsive to the advanced stage of deadwood decomposition, being directly affected by the physical redistribution of fragmented organic matter.