Subsoil Microbial Processes Under Australian Rotational Cotton Systems

Abstract

<p>Soil microbiota have been well characterised in agricultural topsoils (0 - 30 cm depth) globally. However, understanding of microbial functioning below 30 cm is in its infancy, particularly in the Southern Hemisphere. Increasing pressure on developing sustainable agricultural practices has instigated a shift to investigating subsoil (>30 cm depth) microbial dynamics. In the Australian cotton industry, interest in subsoils has increased due to the physical properties of Vertosol soils, on which most Australian cotton is grown. Commonly referred to as 'cracking clays', Vertosols have the unique characteristics of self-mulching and cracks extending well below the topsoil. These properties, along with frequent irrigations, are thought to facilitate the movement of organic matter and other nutrients, into deeper soil horizons and hypothesised to be utilised by subsoil microbiota.</p> <p>This project represents an initial study into microbial processes down the soil profile (0 – 100 cm) under cotton crops grown in Vertosols at the Australian Cotton Research Institute (ACRI), New South Wales. The overarching PhD aims were to i) Evaluate the potential for subsoil microbial activity to perform ecosystem services and ii) Determine the influence of crop rotation (continuous cotton and cotton-maize rotations) and sample time (3 sample points over two cropping seasons) on subsoil microbial processes. Microbial activity and biomass were assessed by respiration and stable oxygen isotope methodologies, whilst microbial diversity was measured utilising high throughput sequencing.</p> <p>Microbial biomass and diversity analyses followed the expected trend of decreasing measurements with increasing depth, following the trends observed in international studies. Microbial activity was just as prevalent in subsoils for both field fresh and long-term (isotopic) assessments, which has not been observed in other studies. A lack of significant differences in microbial processes down the soil profiles under continuous cotton and cotton-maize systems was also observed. This research has led us to believe that the physical properties of Vertosols (self-mulching and formation of deep cracks) facilitate microbial activity in subsoils, thus having the potential to contribute to ecosystem services. It would also appear that the physical properties of Vertosols exert a greater influence than system management.</p>