The role of soil pore structure on nitrate release from soil organic matter and applied fertilizer under three fertilization regimes

Soil pore structure is highly variable with soil management practices, and plays an important role in nutrient availability. However, the relationships between soil pore characteristics (pore connectivity and pore size distribution) and nitrate release from soil organic matter or applied fertilizer are still unclear. This study aimed to identify how soil pore structure affects nitrate release under three fertilization regimes: Control (no fertilization), NPK (mineral NPK fertilization), and OF (organic fertilization). Soil samples were subjected to three physical disturbances (Intact, Repacked, and Compacted) for each fertilization treatment to alter soil pore structure characteristics which were quantified using computed tomography (CT). Nitrate release derived from soil organic matter (Ndfs) and applied fertilizer (Ndff) was distinguished with 15N labelled urea and leached during an incubation. The results showed a decrease in cumulative Ndff in the NPK and OF treatments, compared to the Control, while an increase in cumulative Ndfs in the OF treatment, compared to the Control. Cumulative Ndff was higher in the Repacked treatment, but lower in the Compacted treatment, than in the Intact treatment. Correlation analysis showed that cumulative Ndfs was positively influenced by soil organic carbon content (SOC) and ammonia-oxidizing bacteria abundance, while cumulative Ndff was negatively influenced by SOC. Furthermore, cumulative Ndfs was not associated with soil pore characteristics; however, cumulative Ndff was positively associated with macroporosity, macropore connectivity, and the porosities of pores with diameters in the ranges of 100–500 μm and 500–1000 μm. Path analysis indicated that 100–500 μm pores indirectly influenced the potential and rate of Ndff by modulating water holding capacity and air permeability. Our findings provide a novel perspective, indicating that soil pore structure characteristics significantly influence nitrate release from applied fertilizer rather than that from soil organic matter, with porosity of 100–500 μm being particularly effective in influencing nitrate release from applied fertilizer.