Biochar amendment mitigates negative effects of controlled irrigation on paddy soil structure: Insights from micro-pore network analysis

With increasing rice production demands and water scarcity, developing water-saving irrigation techniques for paddy fields is a global priority. The impact of these techniques on soil structure remains unclear, especially under varying water-carbon conditions. From 2022–2023, field experiments examined four biochar rates and two irrigation methods (controlled irrigation (CI) and flooding irrigation (FI)), resulting in five treatments: CK (0t/ha+CI), CA (60t/ha+CI), CB (30t/ha+CI), CC (10t/ha+CI), and FK (0t/ha+FI). Compared to FK, CK decreased mean weight diameter (19.73–25.54 %), soil organic matter (4.64–9.79 %), total nitrogen (2.68–10.59 %), dissolved organic carbon (1.90–9.48 %), water content at saturation (0.23–15.83 %) and permanent wilting point (3.69–7.87 %), while it increased unstable aggregates index (6.29–15.11 %) and fractal dimension (1.59–1.88 %). Biochar treatments (CA, CB, CC) mitigated CK's adverse effects on soil aggregate stability, total nitrogen, and water retention capacity and significantly improved these indicators. CA increased porosity across various effective pore diameters, while CB and CC primarily increased the proportion of porosity for diameters > 250μm. Simulation results indicated that compared to CK (3.879μm²), the intrinsic permeability (K) of soil under CA, CB, and CC treatments increased by 106.69 %, 77.77 %, and 3.31 %, respectively, while FK showed a contrasting decrease of 3.58 %. K correlated well with > 250μm porosity and connected porosity representing microstructure, with correlation coefficients of 0.96 and 0.94. Overall, biochar improved chemical properties and micropore structure (porosity for diameters >250μm, connected porosity) of soil aggregates under CI, enhancing macroaggregate functions such as soil stability and hydraulic properties.