Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy [to the] Department of Animal, Plant and Soil Sciences, School of Life Sciences, College of Science, Health and Engineering, La Trobe University, Bundoora.
Application of lime to an acid soil with the aim of ameliorating soil acidity that limit plant growth is a common agricultural practice. However, its impact on soil organic carbon (SOC) sequestration is a growing concern. As areas of acidification increase, so do the amount of lime application around the globe. Therefore, it is very important to deeper understand the impact of liming on SOC dynamics for sustainable and environmentally friendly food production. The first experiment of this thesis explored the effect of long-term liming on changes in SOC content and soil aggregate stability in low-input acid soils. Lime application followed by annual cropping with above-ground residue removal for 34 years decreased SOC content and aggregate stability. However, maintaining native volunteer grass without cultivation for 5 years after initial lime application did not change SOC content, while increased soil aggregate stability. The results suggest that the effect of liming on SOC dynamics and soil aggregate stability largely depends on the accompanying agricultural practices such as tillage, residue management and nutrient management. The second experiment investigated the impact of lime-induced increases in soil pH on native SOC mineralization in response to C substrate supply, i.e. the priming effect, of two residues with differing in C:N ratio by applying stable isotope technique. Irrespective of residue type, overall decomposition of added residues and the priming effect increased with increasing initial soil pH. The study suggests that the rate of lime application to acid soils should be targeted to increase soil pH value which is high enough for optimal crop productivity but low enough to minimise SOC losses from the soil. The magnitude of native SOC losses was greater with lower C:N field-pea residue compared to the higher C:N wheat residue during a 90-day incubation period. However, the proportion of remaining added residues after 90 days was greater with field-pea residue, indicating that application of lower C:N residue has greater potential to contribute to long-term SOC storage relative to higher C:N residue. This also highlighted the important role of N availability to decomposer organisms in SOC mineralization. A further experiment scrutinized the interactive effect of soil pH and mineral nitrogen (N) on SOC mineralization in two soils with contrasting indigenous SOC content. Regardless of initial soil pH and SOC content, application of mineral N exhibited non-linear effects on SOC mineralization which greatly depends on the rate of N applied. Low rate of N application increased, the intermediate rate did not change and the high rate decreased SOC mineralization compared to the control. These findings pointed out the importance of balanced N fertilization for specific soils to improve crop productivity and soil health while minimizing extra SOC loss. The study also highlighted detrimental effects of excessive N fertilization on soil microorganisms, soil health and environment. The last experiment was conducted by applying 13C-labelled pure extract C sources, glucose and lignocellulose with or without N to address coupling effect of pH and N on C priming. Acidifying (pH 4.1) or liming (pH 6.6) increased priming effect compared to the control soils (pH 4.7). Labile C substrate, glucose yielded a greater priming effect compared to the more recalcitrant C compound, lignocellulose. The addition of mineral N decreased priming effect of both C substrates in all pH values, with this effect more pronounced in lignocellulose-amended soils. The study highlighted the importance of sufficient N application in agricultural field where there is concomitant supply of C substrates with various degree of biodegradability such as root exudates, decaying plant biomass and microbial metabolites to minimize extra SOC loss via the priming effect. In conclusion, native SOC losses and degrading soil structural stability due to liming acid soils can be minimized by adopting better agricultural practices such as minimum/no tillage, residue retention and balanced fertilization. In order to increase crop production while maintaining SOC in acid soils, net increase in primary production and biomass inputs due to liming should be able to compensate net increase in C respired by liming. Optimal amounts of N fertilization for specific soils would reduce extra SOC loss by priming effect in limed soils. Over-liming should be avoided to minimize extra SOC loss and micronutrient deficiency while maximizing the net profit in agricultural crop production.
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