Changes in Soil Microbial Atributes and Nutrient Status in Response to Environmental Plantings of Acacia and Eucalyptus in South-Eastern Australia

Abstract

<p>Land clearing and agricultural activities in Australia have significantly disturbed and affected natural ecosystems, particularly soil and biodiversity. Therefore, restoring these landscapes can help improve and protect soil condition and biodiversity. Environmental plantings of native trees and shrubs have been established on agricultural lands in Australia to restore ecosystem functions and to restore and protect biodiversity degraded by agricultural activities. Although some work exists on the extent of the ecosystem recovery, assessment of soil microbial attributes (i.e., microbial activity and functional diversity) in these plantings may provide a useful indication of restoration status. The overall aims of this study were: 1) to understand how microbial activity and soil properties are spatially distributed under main tree species used in environmental plantings, 2) to expand the understanding of how environmental plantings, and time since their establishment, affects soil microbial activity, functional diversity and soil properties, 3) to determine if the recovery to soil conditions found under extant remnant woodland are achievable, 4) to determine if there are any predictable links between soil chemical properties and soil microbial attributes due to tree plantings, 5) to determine if the main plant genus used in environmental plantings (<i>Eucalyptus</i> and <i>Acacia trees</i>), and their age, affect soil microbial attributes and soil properties, and 6) to assess the extent to which manipulation of the soil microbial population can influence the rate and magnitude of microbial recovery under environmental plantings and pasture soil.</p> <p>To achieve these aims, a chronosequence of environmental plantings was examined which had been established between 1993 and 2005 adjacent to pasture and remnant woodland, at three separate study locations on contrasting soil types in New South Wales. Microbial activity and functional diversity were examined along with total organic carbon (TOC), total nitrogen (TN), extractable phosphorus (P), soil pH and electrical conductivity (EC) in the pastures, environmental plantings and remnant woodlands. Soil microbial activity was mostly confined to the zone under the tree canopy of A. pendula and E. camaldulensis and in the upper 20 cm of the soil. The relative activity of the microbes and levels of TOC, TN and P declined with increasing soil depth. Microbial functional diversity in soils under trees in the environmental plantings became more similar to that of the remnant woodland with increasing age of the environmental plantings. However, after more than two decades, microbial and soil conditions under the plantings had not achieved conditions observed in the remnant woodland soil. Mixed species plantings of <i>Eucalyptus</i> and <i>Acacia</i> were also found to have a more profound impact on soil microbial activity and functional diversity than it did on the range of other soil properties. By manipulating the microbial population in an environmental planting receptor soil, it was observed that either mixing with 10% - 20% remnant woodland soil, or inoculating soil with a 1:5 soil water extract derived from a remnant woodland soil, was effective in recovering microbial functional diversity in the soils of environmental plantings. However, neither the soil mixing treatments nor soil inoculation treatments had any significant effect on the microbial functional diversity in the adjacent pasture soil. For large scale field application, this study supports soil inoculation as a feasible approach to improving soil recovery under environmental plantings, but only after the plantings are established presumably due to the provision of suitable substrate to support the woodland microbial population. Overall, environmental plantings can be considered as an effective approach to enhance soil microbial recovery in degraded agricultural landscapes, and this can be further enhanced by manipulating and augmenting the soil microbial population under plantings.</p>