Nitrogen-fixation by Acacia dealbata and changes in soil properties 5 years after mechanical disturbance or slash-burning following timber harvest

Abstract

Logging followed by slash-burning can result in large losses of Nitrogen from an ecosystem. Acacia dealbata (silver wattle) is the main N-fixing plant in Eucalyptus regnans (mountain ash) forest in Victoria, but its potential N-fixing capacity is uncertain. Losses of N and changes in other soil properties as a result of logging and slash-burning and inputs of N through N-fixation by A. dealbata were quantified in mountain ash forest near Tanjil Bren, Victoria. The study was based on four logging coupes harvested in 1988 (two burnt following harvest and two disturbed mechanically) and three areas of unlogged, 55-year-old regrowth. There was a 19% increase in concentration of potentially available N and a 48% increase in available P on burnt coupes relative to unburnt coupes, but a 12% decrease in total N and a 17% decrease in organic carbon. These differences were consistent with N and C being lost through burning and with an increase in fertility of surface soil due to the ash-bed effect. Wattles increased concentrations of potentially available N, total N and organic C and reduced bulk density of soil, but did not effect the amount of total N in soil. However, significant differences between the natural abundance of 15 N in A. dealbata and E. regnans and a significant inverse relationship between stocking of silver wattle and δ( 15 N) in soil indicated that N-fixation by A. dealbata trees had added substantial amounts of N to both vegetation and soil. At a stocking of 2500 stems ha −1 A. dealbata was estimated to contribute 40 kg ha −1 per year to total N in soil and 10 kg ha −1 per year to total N in vegetation through N-fixation over the first 5 years. This rate of N-fixation (50 kg ha −1 per year) is sufficient to replace an estimated loss of 430 kg N ha −1 due to timber harvesting and burning in 9 years.