Abstract

Nitrogen input to evergreen températe forests of Chiloé Island, Chile occurs predominantly via non-symbiotic fixation (NSF). Because this is a bacterial-mediated process (diazotrophs), in addition to environmental factors (e.g., temperature and moisture), phosphorous availability and energy supply from carbón in the substrate may influence the rates of N fixation. Our hypothesis is that if both phosphorous and carbón are limiting NSF, this limitation would be greater in logged forests, where additions of labile P and C would stimulate microbial activity. Our objectives are to assess the effects of inorganic phosphorus and labile carbón (as glucose) additions (0 mmol P/L, 0.645 mmol P/L, 3.23 mmol P/L y 6.45 mmol P/L and 0 mmol P/L, 23.3 mmol C/L, 46.6 mmol C/L y 70 mmol C/ L, respectively) on the rates of NSF measured in the litter layer of each forest in laboratory assays, under controlled temperature and moisture and using homogeneous litter samples. We studied lowland evergreen rainforests (100-200 m of altitude), located in the Chonchi district, in Chiloé Island. Two forest stands were logged, subjected to industrial and non-industrial selective logging, and the third stand was unlogged (control). The NSF of nitrogen was assessed by the acetylene reduction assay. Two-way ANOVAs showed that phosphorous addition had no effect on acetylene reduction rates (ARR) in the litter of logged or unlogged forests, but the addition of labile carbón in the form of glucose negatively affected ARR when applied at the máximum level to the litter of unlogged forest. In all treatments the factor forest accounted for the differences in ARR, which was higher in unlogged forest. These differences were not explained by any of the variables experimentally manipulated in this study. The main difference among forests was floristic composition of litterfall, but litter did not differ among forests in its C/N ratio or total P and N contents. © 2008 Sociedad de Biología de Chile.