Enhancing soil biophysical condition for climate-resilient restoration in mesic woodlands

Practitioners of ecological restoration must increasingly consider likely impacts of climate change, requiring reassessment of limits to ecological recovery and persistence. Overcoming soil nutrient enrichment and exotic invasions are common restoration targets in mesic ecosystems, but in a drying climate, the focus may shift towards more fundamental ecosystem functions such as capacity to capture and store water. We investigated opportunities for enhancing the climate-resilience of restoration efforts in temperate grassy eucalypt woodlands, by evaluating benefits after two years of soil amelioration techniques in characteristic sites that had become compacted and nutrient-depleted as a result of past land use. Our goal was to restore soil biophysical functioning without exacerbating exotic invasion. High C:N soil amendments including a green-waste biochar and a mixed-size green-waste mulch facilitated recovery of soil biophysical attributes within two years. Both led to lower bulk density, softer soil surfaces, occasionally higher soil moisture and better persistence of a sown, large-seeded native forb. Biochar also led to improved water infiltration and total C, while mulch led to higher labile C and higher microbial and invertebrate activity (but limited establishment of small-seeded forbs). Benefits were achieved without promoting exotic species, and for attributes with comparable reference data, benefits after two years were on average 25% of that required to reach reference condition. By contrast, P fertilization promoted exotic species at the expense of native grasses, with minimal improvement to soil biophysical condition after two years. Potential benefits of re-introducing a native C₄ grass were not evident within the experimental timeframe. We conclude that bulky, high C:N soil amendments have the potential to facilitate recovery of functions associated with soil water availability and hence resilience to a drying climate. However, longer-term evaluation is needed to assess whether benefits reflect once-off small increments, or transitions across ecological thresholds that will facilitate ongoing improvement towards reference condition.