Atividade enzimática no limite altitudinal de crescimento das árvores sob aquecimento do solo e acréscimo de CO₂ atmosférico

Abstract

Soils in high mountains are characterized by low availability of nutrients, particularly nitrogen (N), as a result of low-temperature constraints on decomposition and mineralization. Global warming and changes in allocation of photosynthates in an elevated CO₂ environment can lead to shifts in soil microbial communities, and, consequently, in the extracellular enzymes they produce, which are the first-limiting step of decomposition. We aimed to study the impact of warming and the residual impact of CO₂ enrichment, which terminated 3 years before our study, on soil extracellular enzyme activities (EEA) at the treeline. We studied enzymes responsible for the decomposition of labile and recalcitrant carbon (C) substrates and for the release of N and phosphorus (P) from soil organic matter. Enzyme activities were measured once after five years of the warming treatment (+4 °C, 2007-2012), which was nested within a CO₂ enrichment experiment (2001-2009) at a treeline site with Pinus mugo ssp. uncinata and Larix decidua in the Swiss Alps (2180 m a.s.l.). We measured the potential activity of eight hydrolitic and oxidative extracellular enzymes in different soil layers under optimum conditions and simulating natural conditions, by using fluorimetric and calorimetric microplate assays. The phosphatase activity increased in the litter (L) layer in Pinus plots due to warming. The chitinase activity were higher in the fermentation (F) horizon in Larix plots in response to warming. Warming also induced cellobiohydrolase, β-glucosidase and β- xylosidase activities in the F horizon, implying a shift in the potential decomposition of plantderived C from cellulose and hemicellulose. Some of the enzyme activities' responses were tree species-dependent, indicating that climate change could eventually impact differently tree species growth and composition in the treeline. We found significant increases in β- glucuronidase activity in warmed soils beneath Pinus under elevated CO₂ in the F horizon. Activities of β-xylosidase in the humus (H) horizon were more sensitive to incubation temperature (IT) in the control compared to warmed plots. Unexpectedly, chitinase activity in the F horizon was more sensitive to IT in warmed plots than in the control. The shifts in the EEA profiles along the soil layers can indicate varying impacts of warming and elevated atmospheric CO₂ on decomposition, which could eventually decrease soil organic matter in this alpine ecosystem.