High-resolution & high-throughput methods for soil carbon research

Soil organic carbon plays a crucial role in agricultural sustainability and climate regulation; however, its dynamics under various soil management practices remain poorly understood. The role of nutrient supplementation of crop residue input on soil organic carbon dynamics is often not well understood, and it is also unclear how it interacts with minimum tillage practices. There are high-resolution and high-throughput spectroscopic and imaging methods that can help with estimating and understanding soil organic matter and structure, which are not rigorously tested for field experimentation. This thesis aims to examine the impact of minimum tillage and nutrient supplementation on incorporated crop residue, utilising mid-infrared (MIR) spectroscopy and micro-computed tomography (µCT) to estimate soil organic carbon concentration, soil physical structure, and soil organic matter quality. Complementary methods, including X-ray fluorescence (XRF) scanning and hyperspectral imaging, are further developed for potential application in field studies. Results show that the soil organic carbon concentration increased by 0.36 wt% under minimum tillage between the first and second years, whereas conventional ploughing showed no change. MIR spectroscopy indicated contributions from lignin-derived aromatic and aliphatic compounds, suggesting enhanced fungal networks and activity under minimum tillage, as supported by improvements in soil macroporosity. Although nutrient supplementation did not affect soil organic carbon concentration during the study period, positive changes in mid-infrared spectra indirectly suggest a hypothesised change in the adsorption of aliphatic organic compounds to clay minerals. XRF scanning successfully calibrated elements Ca, P, Zn, Sr, and S, achieving high R² values of 0.94, 0.93, 0.93, 0.92, and 0.91, respectively. Hyperspectral imaging accurately predicted soil organic carbon concentration with an R² of 0.96 for a higher-resolution test set.
These findings underscore the importance of integrating chemical and structural analyses to comprehend soil organic carbon (SOC) dynamics under conservation agriculture. The successful application of XRF and hyperspectral imaging methods underscores their potential for high-resolution soil analysis in field experiments.