| dc.description.abstract | Heterotrophic nitrogen fixation is a key ecosystem process in unpolluted, temperate old-growth forests
of southern South America as a source of new nitrogen to ecosystems. Decomposing leaf litter is an energy-rich
substrate that favours the occurrence of this energy demanding process. Following the niche ‘complementarity
hypothesis’, we expected that decomposing leaf litter of a single tree species would support lower rates of
non-symbiotic N fixation than mixed species litter taken from the forest floor.To test this hypothesis we measured
acetylene reduction activity in the decomposing monospecific litter of three evergreen tree species (litter C/N ratios,
50–79) in an old-growth rain forest of Chiloé Island, southern Chile. Results showed a significant effect of species
and month (anova,Tukey’s test, P < 0.05) on decomposition and acetylene reduction rates (ARR), and a species
effect on C/N ratios and initial % N of decomposing leaf litter.The lowest litter quality was that of Nothofagus nitida
(C/N ratio = 78.7, lignin % = 59.27 4.09), which resulted in higher rates of acetylene reduction activity
(mean = 34.09 SE = 10.34 nmol h-1 g-1) and a higher decomposition rate (k = 0.47) than Podocarpus nubigena
(C/N = 54.4, lignin % = 40.31 6.86, Mean ARR = 4.11 0.71 nmol h-1 g-1, k = 0.29), and Drimys winteri
(C/N = 50.6, lignin % = 45.49 6.28, ARR = 10.2 4.01 nmol h-1 g-1, k = 0.29), and mixed species litter
(C/N = 60.7,ARR = 8.89 2.13 nmol h-1g-1).We interpret these results as follows: in N-poor litter and high lignin
content of leaves (e.g. N. nitida) free-living N fixers would be at competitive advantage over non-fixers, thereby
becoming more active. Lower ARR in mixed litter can be a consequence of a lower litter C/N ratio compared with
single species litter.We also found a strong coupling between in situ acetylene reduction and net N mineralization
in surface soils, suggesting that as soon N is fixed by diazotroph bacteria it may be immediately incorporated into
mineral soil by N mineralizers, thus reducing N immobilization. | |
