Direct benefits of rhizobial inoculation to Acacia mearnsii De Wild, as tubed stock and in a plantation, and synergistic benefits to interplanted Eucalyptus nitens (Deane & Maiden) Maiden

Abstract

Acacia mearnsii De Wild., inoculated (soil enrichment inoculation) with three effective strains of Bradyrhizobium or uninoculated, was grown as tubed stock in a nursery. After 33 weeks, the seedlings were outplanted into a plantation as monocultures (monocrops) or interplanted with Eucalyptus nitens (Deane & Maiden) Maiden (polycrops). Observations on root nodulation, tree growth and symbiotic nitrogen fixation (N2 fixation) were made during the nursery phase and for 5 y (1998–2003) after outplanting. Four benefits of inoculation were identified. First, it led to the production of tubed stock that was significantly (P<0.05) superior to uninoculated plants in abundance of nodulation, in the proportion of total shoot nitrogen (N) derived from atmospheric N2 and in shoot dry matter. Second, after outplanting, survival of inoculated plants was significantly higher than survival of uninoculated tubed stock. Third, after outplanting, the difference in size between inoculated and uninoculated acacias that had been observed in the nursery was maintained in the plantation. (However, differences were not significant from 2002, i.e. 4 y after outplanting.) Fourth, E. nitens interplanted with inoculated A. mearnsii grew significantly better than eucalypts grown with uninoculated acacias. Acacias that were interplanted with eucalypts had consistently greater diameters than monocropped acacias; by 2003 (5 y after outplanting), the difference was statistically significant. In addition, interplanting of inoculated acacias with eucalypts (polycropping) conferred a growth benefit on the E. nitens by comparison with eucalypts grown as a monocrop. In terms of total timber production, the treatment in which E. nitens was interplanted with inoculated A. mearnsii was more productive than any other. These differences were attributed to the improved N economy within the inoculated treatments stemming from the nitrogen-fixing ability of the three original inoculant strains. Eucalypts also benefitted from interplanting with uninoculated acacias. It is concluded that the use of well-nodulated, N2-fixing nursery-grown tubed stock provides opportunities for efficient establishment of acacias in plantations and farm forestry and for revegetation, especially in riparian situations. © 2011 Taylor & Francis Group, LLC.