Summary

This project investigated the impacts of a range of cropping practices on soil biological activity across different agricultural regions of SA. A microbial functional approach was used to examine the role and activity of microbes rather than just investigating the type or number. This was combined with a laboratory incubation assay to assess changes in soil biological functional capacity, diversity and resilience.

Background

Resilient soils have a greater ability to resist or bounce back from drought, wet-dry cycles or agrochemical inputs. Along with physical and chemical properties, soil biological activity is essential to soil resilience and functions like nutrient cycling and carbon sequestration that maintain healthy and productive soils. Looking at microbial diversity and activity of soil microbes can indicate if a soil is building or losing carbon. When this information is combined with resilience testing via incubation assays, it provides an indication of the capacity of a soil to build organic carbon under cropping practices.

Research Aims

The core objectives of the project were to:

• Determine the impact of cover crop systems on soil biological functional resilience and its relationship with crop performance.
• Quantify the impact of soil and crop management practices such as stubble management, tillage, fertilisers and rotations on soil functional resilience.

In The Field

The functional microbial approach was combined with a three-month laboratory incubation assay to test soil biological activity at:

• Six sites across SA where trials had been run across multiple years using mixed cover crops.
• Calcareous soils trial sites on Eyre Peninsula (GRDC investment) that use various soil and crop management practices.

This testing approach aimed to get a clearer understanding of soil health by looking at how well soil biology is working, how robust it is, and the genetic potential of the soil microbes. It also examined physical and chemical factors like soil organic carbon and soil structure. The testing links back to key functions like carbon turnover (sequestration), nutrient supply and soil biodiversity which all influence soil health, crop productivity and sustainability.

Results

Results from soil samples from the six locations indicated that incorporating summer or winter cover crops with multiple plant species (between 4 and 12 species) did not always lead to improvements in how well the soil biology functioned, the diversity of bacteria present, or the ability of the soil to bounce back from stress.

When these soils were exposed to repeated wet-dry conditions, results suggested that biological resilience to stress was not determined by soil type alone. In the sandy soil with low soil organic carbon (SOC) from Waikerie, multi species cropping increased microbial diversity, microbial biomass, and nitrogen mineralisation. Microbial activity resilience benefits were also recorded with multi-crop species in the chromosol soils (Ungarra and Parilla). However, benefits were not seen with multi-crop species in the high SOC soil (Stokes Bay).

Similarly, analysis of soil samples from field trials on calcareous soils showed that treatments involving fertiliser + fungicide and phosphorus-enriched biochar did not consistently show measurable improvements in soil microbial activity, diversity, N mineralization or resilience. However, some improvements in crop yields were observed.