Higher rates of decomposition in standing vs. surface litter in a Mediterranean ecosystem during the dry and the wet seasons

Aims: Plant litter decomposition in drylands is not well understood, and even less is known about decay of the abundant standing dead residues. Here, we followed decomposition of standing and surface litter, and assessed the underlying drivers and mechanisms. Methods: In a field experiment during contrasting seasons, litterbags were suspended at 0.05 and 1 m above ground (standing litter) and were placed on the ground (surface litter). We also quantified the moisture content of free-standing litter. Results: During nighttime in the dry, rainless season, minimum temperature was 2–3 °C lower in standing litter, leading to higher litter moisture and a doubling of microbially-driven CO₂ emissions from standing compared with surface litter. Free-standing litter moisture increased linearly with height to almost 2 m above ground. Ultimately, mass loss was higher in standing than in surface litter during the dry season (11–12% vs. 7%) and over both the dry and the wet seasons (27–34% vs. 23%), and was positively related to potentially active microbial biomass. Conclusions: Our results suggest that standing litter decomposed faster than surface litter because of enhanced microbial degradation, and possibly photodegradation, all-year-round. Therefore, carbon turnover in drylands and beyond may be underestimated by only considering surface litter decay.