Southeastern Australian agroecosystems: linking carbon to nutrient dynamics & biotic responses

Soil is one of the most valuable of common goods. It provides essential ecosystem services including food, nutrient and water cycling, waste treatment, carbon sequestration and greenhouse gas regulation to support plant and human life. However, soils around the world are severely degraded with immense costs to human wellbeing. Addition of organic amendments, in the form of natural organic matter, to soil may allow us to simultaneously improve soil carbon, recycle and replenish nutrients, improve structure, retain water and increase plant productivity. To achieve these improvements, it is necessary to understand the regulation of organic matter transformation. The main aim of this thesis is to examine the chemical and biotic interactions in the transformation of organic inputs in agricultural soils and the impacts of these organic amendments on them. This aim is addressed by examining the interaction between soil carbon, nitrogen and phosphorus cycling and soil microbiota, which play a major role in determining the transformation of organic matter. The findings of the two main experiments are summarised below. The knowledge developed will contribute to managing aagroecosystems for the supply of multiple benefits to the wellbeing of society and agricultural sustainability. The first experiment focuses on examining the effects of organic input stabilisation via composting or pyrolysis on its transformation by soil biota. The key findings from this experiment are (i) soil microbial composition changes with organic amendment but (ii) overall stoichiometric invariance provides a mechanistic framework for understanding the transformation of organic amendment, and (iii) carbon composition strongly influences microbial community composition and soil functions. Results from this experiment are presented in two chapters. The second experiment builds on the first experiment by increasing the ecosystem complexity using a terrestrial model ecosystem to better reflect the soil ecosystem in nature. It seeks to determine the interaction between altered rainfall (drying and rewetting) and organic input. In the context of global change, the bigger question asked was, “Does organic input improve the resistance and resilience of a grassland ecosystem to altered rainfall?” The key findings from this experiment are (i) grassland soil microbial communities are generally resistant and resilient to fluctuations in rainfall regardless of compost amendment, and (ii) these properties of the soil microbial community were translated to resilience but not resistance in soil functions, (iii) Plant growth responses were sensitive to rainfall and compost amendment, but interaction effects were rarely detected. Finally, in terms of elemental stock content, (iv) compost amendment increased soil carbon, nitrogen and phosphorus stocks regardless of rainfall regimes but, (v) these subtle improvements can only be detected by careful consideration of depths in the soil profile. Results from this experiment are presented in two chapters. These results supply further weight to the argument that it is necessary to take multiple measures and examine interactions to effectively determine the impact of organic amendments on soil community structure and function, particularly in a world experiencing rapid global change.