The enhancements of pore morphology and size distribution by straw return are mediated by increases in aggregate-associated carbon and nitrogen

The accumulation of soil organic carbon (SOC) and total nitrogen (TN) is easily accomplished by returning crop straw, which strongly affects the formation and pore structure of aggregates, especially in black soil. We returned maize straw at different rates (6,000, 9,000, 12,000 and 15,000 kg ha^–1) for nine years to investigate its influence on the SOC and TN contents in the SOC fractions of aggregates by combining size and density fractionation. Their subsequent influences on pore morphology and size distribution characteristics were examined using X-ray micro-computed tomography scanning (μCT). The results showed that returning straw significantly increased the contents of C and N in the SOC fractions of aggregates, especially at the return rates of 12,000 and 15,000 kg ha^–1, which in turn promoted aggregate formation and stability, and ultimately amended pore structure. The pore size>100 μm, porosity (>2 μm), and morphological characteristics (anisotropy, circularity, connectivity and fractal dimension) significantly increased, but the total number of pores significantly decreased (P<0.05). Our results indicated that the amendment of the pore morphology and size distribution of soil aggregates was primarily controlled by the higher contents of C and N in the density fractions of aggregates, rather than in the aggregate sizes. Furthermore, this pore network reconfiguration favored the storage of C and N simultaneously. The findings of this study offer valuable new insights into the relationships between C and N storage and the pore characteristics in soil aggregates under straw return.