Non-symbiotic nitrogen fixation during leaf litter decomposition in an old-growth temperate rain forest of Chiloé Island, southern Chile: Effects of single versus mixed species litter

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.