Nutrient dynamics in an Andean forest region: a case study of exotic and native species plantations in southern Ecuador

Information about nutrient dynamics is of upmost importance in order to contribute to the restoration of degraded forest environments in the Andes of southern Ecuador. This study aims to investigate the differences of nutrient dynamics between a native alder (Alnus acuminata) and an exotic pine (Pinus patula) tree species in this region. Based on litterfall, forest floor and mineral topsoil (0-20 cm) of two pine and two alder plantations, we studied the litterfall production and its seasonality; temporal variations of nutrient concentrations, stoichiometric ratios and potential nutrient return (PNR) of leaf-litterfall; mean residence times (MRT) of nutrients in the forest floor; and assessed soil biogeochemical properties. Our results showed that total litterfall production in pine was twice as high as in alder. Litterfall biomass seasonality was similar for both species and highly associated to periods with less precipitation. Pine exhibited the highest seasonality of nutrient concentrations and stoichiometric ratios. PNR of N, K, Ca, and Mn exhibited the major differences between the species. The annual PNR of N and Ca were higher in alder, while those of K and Mn were higher in pine. Pine exhibited higher MRT values for C, N, P, S, Cu, and Zn, while alder showed the higher for Mg, K, Mn, and Ca. In soils, alder exhibited higher concentrations and stocks of nutrients, but not for C. Although, the soil microbial biomass was similar under both species, microbial activity was different. C and net N mineralization were higher in alder, and nitrification dominated over ammonification processes. In general, our findings show a faster cycling of nutrients in alder than in pine.