Abstract:
Soil aggregates can stabilize soil organic carbon (SOC) via physical and chemical protections and make SOC less susceptible to the priming effect: the new carbon input can stimulate further decomposition of old soil carbon existing in soils [1,2]. Irrigation management practices, such as continuous flooding (CF) or alternative wetting-drying (AWD) practice used in rice cropping systems, also influence soil aggregation by altering soil chemical properties (e.g., redox potential) and nutrient availability [e.g., iron (Fe) and phosphorus (P)]. These nutrients play a crucial role in the formation and stability of soil aggregation structure due to dynamic changes in microbial activities and biological processes [3]. For instance, the anaerobic condition likely improves the availability of soil Fe, acting as chelating and binding agents. The increased Fe2+ can facilitate aggregate formation via the flocculation of soil particles and organic matters (e.g., root and microbial exudates or other low molecular weight organic acids) and aggregate stability. Studies also showed that the SOC can be stabilized via the adsorption of soil organic matter (SOM) on Fe minerals and Fe-SOM co-precipitation (i.e., chemical protection for SOC), which is likely altered under different water management [3]. For rice fields, the AWD practice has shown its potential to increase water use efficiency and greenhouse gas (GHG) emissions, mainly methane (CH4), effectively; however, the AWD practice likely accelerates SOC loss by increasing microbial aerobic respiration. Also, the AWD practice alters the soil redox potential, substantially influencing the adsorption-desorption and/or precipitation-dissolution processes among minerals, SOC, and the available nutrients. Changes in these biogeochemical processes likely affect soil aggregation, SOC fractions, aggregate and SOC stabilities, and most importantly, the C sequestration.
Therefore, the overall goal of this study is to investigate the impacts of two irrigation management (CF vs. AWD) on 1) soil aggregate structures, SOC fractions (free and occluded POC, and mineral-C fractions) and stability using different fractionation techniques (e.g., wet and dry-sieving and density methods), and 2) the relationship between the changing nutrient availability and soil aggregation, and the SOC surface chemistry (i.e., physical protection via soil aggregate and chemical protection via adsorption on Fe oxyhydroxides and co-precipitation) using sequential extraction methods and Fe X-ray Absorption Near Edge Spectroscopy (XANES). So far, our results showed that the AWD practice resulted in the destruction of soil aggregates, likely resulting from the SOC loss. Under the CF conditions, the crystallinity of Fe species decreased, and the Fe species transformed from anhydrous Fe oxides to Fe hydroxides. Soil C sequestration is one of the important solutions for the mitigation of climate change. The sequestered C can be stabilized in soil via different mechanisms/protections; however, many of these mechanisms are still unclear due to the limitation of the current qualitative and quantitative methodologies, such as the distribution of C species and pore size on soil aggregates. The synchrotron-based spectroscopic techniques with high spatial-resolution characteristics, such as C (1s) XANES and near-edge X-ray absorption fine structure (NEXAFS), FTIR, and microcomputer tomography [4,5], will offer new opportunities to understand these protection processes to help the goal of Net-Zero policy.
Keywords – Soil aggregates, Soil Carbon Sequestration, alternative wetting-drying (AWD), Fe X-ray Absorption Near Edge Spectroscopy (Fe-XANES)
References
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