Litter quality modulates the effects of environmental drivers on microbial decomposition and home‐field advantage in headwater streams

We investigated how microbial decomposition in headwater streams is influenced by environmental factors and litter quality, including the potential adaptation of microbes to decompose more efficiently leaf‐litter species from their native range (HFA, ‘home‐field advantage’ hypothesis). We conducted a leaf‐litter decomposition experiment in streams from four subregions contrasting in water chemistry and temperature regime, using cross‐subregion reciprocal incubations of the dominant riparian plant species in each subregion. Low‐quality litters decomposed faster at their native site, supporting the HFA hypothesis for low‐quality litter species. Moreover, temperature sensitivity of decomposition of low‐, but not high‐, quality litters was close to that predicted by the metabolic theory of ecology. Among litter traits, nitrogen content and toughness were the major predictors of decomposition. Temperature and nitrate concentration in stream water enhanced decomposition but, these two extrinsic factors did not act synergistically. Together our results indicated that leaf‐litter traits have a leading role as drivers of the decomposition process in headwater streams. However, this role is indirect, and occurs because low‐quality litters modulate the decomposition effects of environmental drivers (temperature and N in water). Consistently with the above finding, HFA occurred only in low‐quality litter. Thus, if climate change leads to minor local changes in litter quality, microbial decomposition would be accelerated additively by warming and stream‐water nitrate enrichment, with more pronounced effects on low‐quality litters.