Regenerative opportunities for building soil biological resilience – a case study in the low-rainfall zone in Southern Australia

Take home messages

• Soil improvement is at the core of regenerative agriculture, with a strong focus on ecosystem and environmental sustainability.
• Management is the key to maintaining and enhancing soil biological functional capacity in lower organic matter soils in southern Australia, which is integral for productivity, C sequestration, soil and ecosystem health and sustainability of agriculture.
• Management practices that reduce the amount of plant C inputs, such as grazing crops/stubble and hay removal, generally resulted in lower soil biological capacity and overall multi-functional biological index.
• Practices relevant to regenerative agriculture have direct consequences on soil multi-functionalities and resilience in the lower fertility agricultural fields of southern Australia, suggesting potential for some customisation of regenerative agriculture application in this region.

Background

To meet the ever-increasing global food demand, new approaches for sustainable agriculture with reduced environmental impact have been proposed. These include sustainable intensification, climate-smart agriculture, organic and regenerative agriculture, all of which rely on soil’s health or capacity to support production and other ecosystem functions. While there are many definitions of regenerative agriculture (RA), it is usually seen to have a central emphasis on the state and trajectory of the natural capital base (soil, water and biodiversity), including soil resilience (Robertson et al. 2022). Although there are many versions of what RA is, enhancing and improving soil health, optimising resource use and management, tackling climate change and improving water availability are agreed as core themes (Schreefel et al. 2020, Dempster et al. 2021).

Maintaining and enhancing a resilient soil biological functional capacity is integral for productivity, soil and ecosystem health and sustainability of Australian agriculture, especially on lower fertility/organic matter soils in the low rainfall regions in southern Australia. In these soils, management is the key to maintaining and improving soil biological functions and C sequestration (Gupta et al. 2019), including their ability to withstand and recover from environmental, physical or chemical stresses (resilience). In the low rainfall, winter-dominant rainfall environments, establishing perennial pastures is typically not a feasible option.

Soil microbial communities mainly determine the soil functional capacity relevant to nutrient supply and availability, microbial C turnover and C sequestration. The status of soil biological functional capacity in agricultural systems is influenced by soil, environment (rainfall and temperature), plant type and management practices through changes in the diversity, composition, population level and activity of soil (micro)biota. Hence, understanding the effect of crop and management factors on soil biological capacity and functional resilience is essential to maintain and improve the soil resource base in the low rainfall regions where microbial C turnover plays a key role in providing functions essential to sustainable production and overall system health.

Adoption of conservation agriculture (CA) practices during the last two decades has helped to improve production, as well as water and resource use efficiency in low rainfall regions of South Australia and Victoria. However, recent attention to the regenerative agriculture approach has raised interest in understanding the relevance and importance of various management practices that are part of CA systems, in terms of their influence on soil biological functional capacity and resilience across different low rainfall agroecological regions. Understanding how to improve both resistance and resilience of soil systems to stress and stress thresholds is important for land managers and policy makers to optimise management decisions.

Regenerative agriculture practices have been developed for particular environments in different climates around the world, so the applicability and feasibility of the different farming practices need to be adapted for other agricultural regions. Currently, there is a need for greater context-specific evidence of the profitability of regenerative farming systems (Francis 2020) and agri-environmental impacts. In this study, we use a grower field-based assessment approach for science-based evidence, focussing on the potential soil impacts of management practices considered relevant to the practice of regenerative agriculture, specifically in the light textured soils of the low rainfall zone in south-eastern Australia.

The aim of this work was to determine the status of soil biological capacity and resilience as influenced by practices relevant to regenerative agriculture in the low rainfall region in southern Australia.

Methods

Surface 10cm soils were collected from 35 grower fields from across the low rainfall cropping regions in South Australia and Victoria during the in-crop season in 2021 and summer period in 2022. Fields were selected to provide sufficient contrast with crop/soil management practice categories generally considered relevant to the RA philosophy (Dempster et al. 2021). These included tillage, stubble management, crop diversity, ground cover/cover crops, grazing during pasture phase, use of pesticides and fertilisers and manures (Figure 1).

Using a functional microbial ecology approach that integrates responses and changes in microbial biomass (MB), C turnover, N mineralisation, catabolic diversity and enzyme activities, representing functional microbial groups involved in C, N, P and S cycling, were measured. Additionally, resilience in biological properties when exposed to wet-dry cycles simulating changing rainfall patterns was also determined (Gupta et al. 2008). In view of the complexity of soil functionality and microbial properties, a multi-functionality (MF) index was calculated to compare across fields. The MF index determines the average level of a suite of functions by standardising each function to a common scale by taking their mean across all the soils tested in this study using a z-score transformation (Bradford et al. 2014).

Figure 1. Key categories of management practices commonly followed in broadacre agricultural systems and their relevance to accepted regenerative agricultural (RA) practices (Dempster et al. 2021). Practices are ranked on a 0–2 scale where ‘0’ represents one of the accepted RA principles.

Results and discussion

Results indicated that surface 10cm soils in the study area had a wide range of soil organic C (SOC, 0.33–1.73%) and total N (0.04–0.16%) and significant differences between fields reflected the influence of variations in cropping history within the sandy and sandy loam texture soils in the region. Since changes in SOC from crop management practices generally take long periods (decades), differences in SOC levels observed are likely the result of long-term management. Average microbial biomass C and N levels were 338±39mg C/g soil and 49±5mg N/g soil, with significant variation between fields within the sub-regions of SA and Victoria, indicating the influence of different crop management practices. Differences in MB levels showed a significant relationship with active soil C levels confirming previous reports that in the lower organic matter soils in this region, MB levels are regulated by the availability of C (Gupta et al. 2019). Also, the generally lower microbial quotient (MB per unit SOC), for example, ~50% of fields showed microbial quotient (MQ) values <3.5%, suggests the necessity to implement management practices that would increase MB levels and associated benefits. Similarly, data for microbial catabolic diversity and C turnover related processes (for example, average metabolic response and C mineralisation potential) showed significant variation between fields but the differences between in-crop and summer collected soils were seen only in some soils. For the in-crop soils, enzyme activities related to C, N, P and S cycling were significantly different between fields. The variation in C-cycling enzymes showed a correlation with MB and/or total SOC, stoichiometric ratio of C to N and S cycling enzymes indicated N and S limitation for C cycling and nutrient availability. Nitrogen mineralisation potential (PMN) was generally higher in the in-crop samples (0.54kg N/ha/day) compared to the summer samples (0.46kg N/ha/day) for the majority of fields and PMN showed a significant positive relationship with MB, catabolic properties and total N (R2=0.43, 0.41 and 0.42, respectively; P<0.05).

Figure 2. Effect of different management practices on soil biological properties and multi-functionality index for soil samples from selected SA soils collected during in-crop 2021. Measurements are categorised in groups relevant for (i) microbial biomass and turnover (red box), (ii) catabolic diversity and activity (green box), (iii) N mineralisation and C, N, P, S cycling (purple box) and (iv) active carbon levels (pink box).

Overall, the use of MF index that aggregated responses in various microbial and functional properties provided an integrated metric or index reflecting the soil biological functional capacity as influenced by management practices (Figure 2). This was also made possible by the selection of measures using a functional microbiology approach. In general, the differences in the MF index for different fields were explained mainly by the differences in a range of biotic properties, for example, MB, C turnover, catabolic diversity and PMN. In general, differences in MF index could be explained based on differences in the amount of C inputs returned to the soil under various practices. Higher amounts of MB were mostly seen in fields under management practices that added larger amounts of plant C inputs (Figures 2 and 3). Management practices such as stubble retention, no-till and crop rotation are generally common across these fields but some of them with low MF index have some forms of crops/pasture grazing and/or stubble or hay removal practice, all of which would have reduced the amount of C inputs returned to the soil. Additionally, fields SA 7779 and VIC-5961 were also exposed to lower C inputs, either due to recent repeated droughts or fallow as part of rotation. Conversely, fields showing a higher MF index (for example, SA-6870, SA-9557, VIC-101103, VIC-5658, VIC-4143) didn’t practice grazing as part of the management practice (Figures 2 and 3). For example, fields with grazing as a management practice had a lower MF index (-4.06±1.67) compared to those with no grazing practice (2.01±1.84).

Figure 3. Effect of different management practices on soil biological properties and multi-functionality index for soil samples from fields in Victorian low rainfall Mallee during in-crop 2021. Measurements are categorised in groups relevant for (i) microbial biomass, (ii) catabolic diversity and activity, (iii) N mineralisation and C, N, P, S cycling and (iv) active carbon levels. Grazed = -4.008+1.684 MF index; No grazing = 1.121+3.40 MF index.

Variations in the use of fertilisers and herbicides did not show any measurable relationship with differences in the MF index. Since the majority of fields tested were exposed to no-till practices, effects of tillage/disturbance could not be evaluated. Results from the resistance and resilience of biological properties and functions when exposed to repeated wet-dry events indicated that sandy and sandy loam soils have limited stable soil structural components (that is, micro- and macro-aggregation due to very low clay content) and habitat conditions for highly stable biological functions are limited. These results also confirm that long term practice of grazed volunteer pastures, stubble grazing and fallow-crop rotations can cause a decline in the biological functional capacity resilience of microbial populations and processes.

Table 1: Summary of previous knowledge and potential effects of management practices on soil biota and their processes in agricultural soils in Australia.

Conclusions

• A framework categorising the various crop and soil management practices for their relevance to regenerative agricultural (RA) philosophy is proposed for a systematic interrogation of practices currently adopted as part of conservation agriculture-based farming systems.
• Management practices that reduce the amount of plant C inputs, such as grazing crops/stubble and hay removal were associated with generally lower soil biological capacity and overall multi-functional biological index.
• Resistance and resilience of soil biological functional capacity in the sandy and sandy loam soils is generally low, hence management practices that include pastures, stubble retention and reduced till systems are required to maintain and improve soil biological health and build soil C in the long-term.
• The functional microbial ecology approach used in this study clearly demonstrated potential effects of some of the regenerative agriculture practices on soil biological health and resilience.
• With the adoption of locally appropriate management practices that promote biological activity, it is possible in low rainfall environments to achieve soil improvement relevant to the objectives of regenerative agriculture.
• The MF index should also be linked to productivity, and whether RA management practices can be adopted to maintain improve resilience without compromising farm sustainability.

Acknowledgements

This collaborative project was supported by the Murraylands and Riverland Landscape Board with funding from the Australian Government's Future Drought Fund: Natural Resource Management Drought Resilience Program Grant ID: G37237W, South Australia. Authors acknowledge the help provided by Eliza Rieger (MRLB, Murray Bridge, SA), Jamie Wilson (Wilcol Pty Ltd, SA) and Alison Frischke (Birchip Cropping Group, Vic) for field sampling.

References

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Gupta V, Roper M, Thompson J (2019) Harnessing the benefits of soil biology in conservation agriculture. In ‘Australian Agriculture in 2020: From Conservation to Automation’. (Eds J. Pratley, J. Kirkegaard) pp. 237-253. (Agronomy Australia and Charles Sturt University: Wagga Wagga, NSW).

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Contact details

Gupta Vadakattu
CSIRO Agriculture and Food, Waite Campus, SA 5064
08 8303 8579
Gupta.Vadakattu@csiro.au
@LifeInSoil5