The effect of lithology on leaf litter decomposition of Pinus pinaster forests along a Mediterranean precipitation gradient

Above-ground plant litter decomposition has a major influence on the global carbon (C) cycle by transferring 50% of net primary productivity to soil organic matter and releasing 60 Pg C annually into the atmosphere. Despite extensive research devoted to disentangling the main drivers controlling litter decomposition, the role of lithology remains understudied. Here, two studies were conducted to investigate the combined effects of lithology and climate on needle litter decomposition on three distinctive lithological substrates (calcareous, peridotite, and metapelite) along a precipitation gradient (ranging from 641 to 1097 mm yr-1) in the province of Málaga, south of Spain. Study one examined needle litter decomposition of Pinus pinaster (maritime pine) along the experimental gradient, and study two was a reciprocal transplant experiment established on calcareous and peridotite lithological substrates located in the centre of the precipitation gradient with litter of contrasting chemical recalcitrance obtained from P. pinaster and Abies pinsapo (Spanish fir) to assess the impact of lithology on the home field advantage hypothesis. Total litter mass loss during decomposition was highest in the calcareous substrate, exceeding metapelite and peridotite substrates by 24% and 50%, respectively. Decreased precipitation reduced litter mass loss only in calcareous soils (35%) but had little effect on metapelitic and peridotite sites indicating that more productive bedrock types are influenced to a greater degree by reducing precipitation, supporting the boom-bust hypothesis. On peridotite substrates, decomposition of the labile soluble cell fraction and cellulose-based crude fibre fractions of intermediate recalcitrance was delayed by one dry season whereas lignin decomposition ensued immediately highlighting physicochemistry-induced modification of substrate accessibility.  Moreover, study two demonstrated a pronounced home-field advantage for litter on calcareous substrates, contrasting with an away-field advantage for litter derived from peridotite substrates. These results underscore the significant role of lithology in dictating litter decomposition dynamics, directly influencing both litter quality and microbial substrate accessibility. Given that lithology directly impacts litter quality and its response to changing precipitation patterns—both critical variables in global ecosystem carbon models—incorporating lithological factors is essential for accurately predicting how plant litter decomposition will respond to climate change.