Long-term straw return with nitrogen fertilization enhances soil pore structure, POM accumulation, and their positive feedback in a Vertisol

Soil pore structure determines particulate organic matter (POM) decomposition by influencing water and gas transport. The accumulation of POM due to nitrogen (N) fertilization has been reported in many studies. However, the effects of N fertilization and straw management on POM and pores as well as the relationship between these two factors remain inconclusive. Therefore, a 15-year (2008–2023) N fertilization field experiment was conducted on a Vertisol, covering three N application rates (0, 360, and 540 kg ha⁻¹ year⁻¹, designated as N0, N360, and N540) and straw management (straw return and straw removal) in a wheat-maize cropping system. X-ray computed tomography (CT) was utilized to quantify the pore structure, and POM was classified into fresh POM and decomposed POM based on their morphological characteristics. The findings revealed that straw return treatment increased fresh POM by 3.08–3.77-fold at N0, N360 and N540 rates, along with enhancements in image-based porosity (>50 μm in diameter, Ø), connected porosity, connection probability at the N540 rate compared to straw removal (P < 0.05). Under straw return conditions, the N360 treatment notably increased fresh POM by 2.3-fold compared to the N0 treatment; the N540 treatment led to a 2.94-fold increase in fresh POM and a 1.16-fold increase in decomposed POM (P < 0.05). The N540 treatment also increased image-based porosity, connected porosity, surface area density, mean compactness, and decreased mean pore distance (P < 0.05). Furthermore, with straw return conditions, connected pores were identified as the primary site for fresh POM distribution, accounting for a distribution proportion of 26.9 %-77.9 %. Notably, POM exhibited a positive correlation with > 200 μm Ø porosity under straw return treatment (P < 0.05), whereas no significant relationship was observed between POM and pore structure under straw removal (P > 0.05). Overall, our findings indicate that long-term N fertilization (N360 and N540) coupled with straw return facilitates POM accumulation, particularly fresh POM, and enhances soil pore structure in Vertisol.