Summary
Intensive land-use practices often alter soil microbial communities, nutrients, and other soil properties in native ecosystems. As these properties shape the development, function, and composition of vegetation communities, they should be considered in restoration practices. While previous studies have investigated the potential of whole-soil inoculants from undisturbed areas in ecosystem restoration, few have examined their effects on native and non-native invasive plants (weeds) and changes in the underlying soil.
Using a glasshouse experiment, we measured seedling emergence and morphological traits of two co-occurring natives (Acacia harpophylla (Brigalow) and Casuarina cristata (Belah)) and two weeds (Cenchrus ciliaris (Buffel grass) and Harrisia martinii (Harrisia Cactus)) under inoculation with whole soil from a remnant site of Brigalow woodland. This site, one of the largest undisturbed fragments of Brigalow woodland in the Brigalow Belt Bioregion South, was chosen as a representative case study area. A cleared adjacent site acted as a control.
Inoculation with the remnant whole soil resulted in a higher proportion of seedling emergence for both native species, as well as a lower seedling emergence for Harrisia martinii and a slower emergence rate for Cenchrus ciliaris. By illustrating some inhibitory effects on weeds and remedial effects on native species, this study builds upon previous research on the potential use of whole-soil inoculants for restoring diversity in degraded areas.
Background
The Brigalow Belt Bioregion stretches over 1,500 km in eastern Australia. Both soil and vegetation communities have experienced considerable modification through land-use change, and soil legacies persist in the landscape, with studies showing changes in the soil’s physicochemical and microbial characteristics. As soil history can determine the success of plant development, soils from remnant communities with native legacies may inhibit the growth of invasive species by providing natives with a competitive advantage (Kulmatiski et al. 2006).
Using the threatened Brigalow woodland ecological community as a case study, we inoculated whole-soil communities from adjacent cleared and remnant sites on native, weed, and co-occurring native and weed species in a glasshouse experiment.
The aims of this study were to:
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Investigate whether the soil inoculant from the remnant site would promote native seedling emergence and growth in the presence of invaders.
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Discern any inhibitory effect on invaders.
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Assess the effects of the inoculated soil on the chemical and microbial soil properties for the different plant communities.
Methods for the Glasshouse Study
Whole-soil inoculum (remnant) was collected from the Brigalow Nature Reserve in Narrabri, NSW. The reserve is surrounded by an agricultural matrix of cropping and grazing land that has been predominantly cleared. Soil for the control treatment was collected from cleared fields adjacent to the Brigalow Nature Reserve.
Inoculation ‘pellets’ were formed using 30 g soil from remnant or control sites and 8 ml of autoclaved deionised water mixed to a pliable consistency. The mixture was then formed into round 6 x 6 cm pellets and placed in a sterile laminar flow workstation for 24 hours until completely dry. The pellets were stored in a controlled cool room until use in the glasshouse experiment.
Mesocosms consisting of plastic pots (100 mm diameter x 100 mm height) were filled with 150 g of sterilised sand at the bottom and 350 g of soil from the control site on top. Pellets for each treatment (remnant or control) were distributed on the mesocosms. Seeds of each species were added according to the experimental design indicated below.
Seedling emergence rate, proportion emerged, height, and biomass were measured for all species in each pot. Seedling emergence was defined as the point of cotyledon leaves opening and scored each day for the first 28 days and then once per week to calculate the rate of emergence over time. The experiment was terminated after 68 days, and a final count of seeds was taken for each replicate to calculate the total proportion of emerged seedlings. At completion of the experiment, seedling height was measured to determine whether whole-soil inoculation affected plant morphological development.
The experimental design included soil collection, a glasshouse experiment, and microbial and physicochemical analyses, comparing control and whole-soil inoculated treatments across native and weed plant species from a threatened Brigalow community. The categories were: Natives (N), weeds (W), natives and weeds (NW), and no seed (NS). Baseline subsamples were collected to determine soil properties of the initial inoculant.
Results and Discussion
Overall, the results showed that seedling emergence and timing were primarily driven by soil treatment, with a higher proportion of native seedlings emerging from soils treated with the remnant inoculant. In addition, fewer seedlings of the weed species Harrisia martinii emerged under the remnant inoculant, and emergence timing was slower for Cenchrus ciliaris; however, these effects were generally weaker in the mixed plant community.
Soil treatment also drove changes in seedling traits for some plant species, with lower total biomass for Cenchrus ciliarisgrown with the remnant inoculant and reduced height for Harrisia martinii. Increasing the proportion of native seedling emergence may be critical in providing a competitive advantage against weeds during early stages of establishment (Firn et al. 2010).
Using a threatened ecological community from the Brigalow Belt Bioregion as a case study, we demonstrate that whole-soil inoculation from a remnant site can promote native species while inhibiting weed growth. Of particular significance was the decreased emergence of the cactus Harrisia martinii and the slowed emergence of Cenchrus ciliaris, species that have caused significant negative impacts on natural ecosystems and agricultural areas in the Brigalow Belt Bioregion and other parts of Australia, resulting in large economic losses.
While these results may have direct outcomes for the study species, replication at the site level is needed to make broader ecological assertions. Nevertheless, given that the case study area sits within a heavily cleared landscape, the results of this research will have meaningful outcomes for the restoration of native Brigalow species and the control of competing invasive weed species.
References
Firn J, MacDougall AS, Schmidt S, Buckley YM (2010) Early emergence and resource availability can competitively favour natives over a functionally similar invader. Oecologia 163, 775-784.
Kulmatiski A, Beard KH, Stark JM (2006) Soil history as a primary control on plant invasion in abandoned agricultural fields. Journal of Applied Ecology 43, 868-876.
This research was funded by the Australian Flora Foundation, established in 1981 with the aim of fostering scientific research on the biology and cultivation of Australian plants. Funds for research are raised through the membership of the Foundation, donations from various sources and the income from bequests.