Should we burn or bust the biocrusts: an overview of biocrust management in the Australian rangelands

Abstract

The biocrust microbiome that occupies the surfaces of rangeland soils globally are key contributors to carbon sequestration, nutrient cycling and sustain vegetation cover. Previously, research in northern Australia has demonstrated N inputs from biocrusts of 5 kg/ha seasonally that accounts for approximately one sixth to half of the annual pasture N demand. Biocrusts are important indicators of rangeland health, and we address how management actions can facilitate their survival under climatic extremes.

We explored the resilience of biocrusts to fire and grazing at two long-term research sites at Kidman Springs (NT) and Wambiana (QLD), respectively. At Kidman Springs in our first DNA analysis (metabarcoding) we examined the recovery of biocrusts after one year of burning, on plots 2 and 4 year prescribed burning regimes, carried out late dry season. Biocrusts were resilient to fire and recovered in the first wet season with no grazing pressure, as there were no differences in composition between the control and late 2- and 4-year burns. However, DNA analysis from nearby grazing exclosures showed that after 60 years of no cattle grazing, biocrust composition had significantly diverged from grazed areas. Furthermore, in our second DNA analysis (metagenomics) we included samples from 2, 4 and 6 years prescribed fires, burnt early and late in the dry season, and we collected samples at the dry and wet season, demonstrating that there was significant variation in biocrust bacterial composition between all fire treatments and soil types. Bacterial genes responsible for nitrogen fixation were sensitive indicators, that responded to seasonal conditions. Biocrusts also had significantly more nitrogen and carbon than bare degraded soil.

At Wambiana, moderate stocking rates maintained good land condition and strengthened nitrogen fixation potential of biocrusts. Key indices of landscape function including biocrust cover were informed by land condition and climatic conditions. Moderate stocking rates combined with wet season rotational spelling on average every three years also facilitated nutrient cycling. Recently, we have shown that discrimination of biocrusts using satellite imagery is a feasible monitoring tool on a landscape scale. We can track changes in ground cover including biocrusts both spatially and temporally. Bare ground covered with biocrusts are sensitive indicators of landscape function.