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 (mu CT). 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 mu m, porosity (>2 mu m), 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.

Soil compaction has been found to deform soil structures and alter water flows. Although previous studies have suggested that a load exceeding the critical stress, determined by static load application, can be applied for a short duration without causing substantial damage to the soil structure, the immediate consequences of short loading times on structural integrity and the subsequent influence on soil water flow remain relatively underexplored. The principal objective of this research was to explore the effects of loading intervals, ranging from 0.1 to 2.5 s, commonly used by vehicles and machinery in the agricultural sector, on the changes in water-stable aggregates and saturated hydraulic conductivity (K-sat) associated with soil compaction, thereby enhancing our understanding of how transient external forces could affect the soil properties. Four distinct soils with varying soil organic matter (SOM) contents (13, 43, 77, and 123 g/kg) were collected from a typical Mollisol area in Northeast China, each characterized by different initial gravimetric soil water contents of 11%, 15%, 19%, and 24%, respectively. Under an applied load of 4.0 kg/cm(2), the short loading time resulted in an increase in small macroaggregates (SMAs) and a decrease in microaggregates within the distribution of water-stable aggregates, whereas it did not affect aggregate stability. K-sat decreased significantly (p < 0.05) as the loading time increased from 0.1 to 2.5 s. The effects of loading time and SOM on water-stable aggregates with particle sizes exceeding 0.25 mm, mean weight diameter, geometric mean diameter, and K-sat were identified as statistically significant or highly significant (p < 0.05 or p < 0.01). Notably, the initial soil water content remained unchanged during the short compaction period. A significant negative correlation was identified between SMAs and K-sat for each soil, with the loading time and initial soil water content (correlation coefficients ranging from -0.834 to -0.622). The results, combined with the structural equation modeling analysis, indicated that both a short loading time and SOM could directly increase SMA and decrease K-sat, with both factors influencing K-sat through SMA during the soil compaction process. This suggests that the loading time and SOM during a short duration under the same external force, rather than initial soil water content, can determine the potential degradation of the soil.

Soil microplastics (MPs) are a substantial threat to soil health, particularly by disrupting soil aggregation. Additionally, MPs undergo aging processes in the soil, which may significantly alter their long-term impacts on soil structure. To investigate these effects, we conducted an eight-month soil incubation experiment, examining the influence of MPs and their aging on soil aggregation. The experiment utilized a factorial design with various combinations of MPs and biochar additions: 1% by weight of 1000-mesh polyethylene and polypropylene MPs, and 5-mm biochar, resulting in six treatment groups: [CK], [PE], [PP], [Biochar], [PE + biochar], and [PP + biochar]. Our findings revealed that both MPs and biochar underwent aging throughout the incubation, evidenced by the formation of oxygen-containing functional groups on their surfaces. Microplastics, particularly polyethylene, primarily affected the 0.5-1 mm and >2 mm aggregate fractions, with average reductions of 21% and 77%, respectively. These adverse effects intensified with the aging of MPs. Contrary to expectations, the addition of biochar was found to exacerbate the negative impacts of MPs on the 0.25-0.5 mm aggregates, with a decrease of 11% associated with PE MPs. The influence of biochar on mitigating the damage caused by MPs to soil aggregation is dependent on aggregate size.