Seasonally freeze-thaw (FT) processes affect soil salinisation in cold and arid regions. Therefore, understanding the mechanisms behind soil salinisation during winter and spring is crucial for management strategies effectively alleviating this. This study aimed to explore the soil FT characteristics and their influences on soil water and salt migrations to clarify the underlying mechanism of the springtime soil salinisation in the western Songnen Plain, China. The spatiotemporal distributions of soil water and salt, frozen depths and soil temperatures were examined at depths of 0–200 cm in three typical landscapes (farmland, Leymus chinensis (Trin.) Tzvel (LT) grassland and alkali-spot (AS) land) from October 2015 to June 2016. Results indicated that the strongest freezing process occurred in AS land, which was characterised by the deepest frost depth (165 cm) and highest freezing rate (3.58 cm/d), followed by LT grassland, and then farmland. The freeze-induced upward redistribution and enrichment of soil water and salt caused the rise and expansion of the soil salification layer, which was the main source of explosive accumulations of surface salt in springtime. Therefore, the FT processes contributed to the surface soil salinisation and alkalisation. Landscapes also affected soil water and salt migrations during FT processes, with the trend being AS land > LT grassland > farmland.

Since the 1950s, heavy plowing of Mollisols, combined with a lack of organic matter intake, has resulted in severe soil degradation in northeast China. The use of biochar in combination with fertilizer is a sustainable method of improving soil quality. In this paper, we conducted field experiments to explore the response of the stability mechanism of the soil aggregate, the dynamic properties of organic carbon, and changes in the microbial community structure to biochar. The biochar input levels were C1, C2, and C3 (9.8, 19.6, and 29.4 Mg C ha⁻¹, respectively), while the nitrogen (N) fertilizer rates were N1/2 (300 kg N ha⁻¹) and N (600 kg N ha⁻¹). Results indicated that biochar combined with N fertilizer effectively increases soil carbon storage and aggregates stability (P<0.05). And C2N treatment increased the aggregate contents of the >2 mm and 0.25–2 mm fractions by 56.59 % and 23.41 %, respectively. The phospholipid fatty acid (PLFA) analysis revealed that microbial community structure was effectively improved with biochar combined with N fertilizer application (P<0.05). The $\mathrm{F}/\mathrm{B}$ ratio increased by 25.22 % and the gram-positive (Gm⁺) to gram-negative (Gm⁻) ratio by 4.65 % under the C2N1/2 treatment. This study concluded that the response of Mollisols to biochar is primarily determined by the interplay of aggregate, organic carbon, and microorganisms. Therefore, the use of biochar combined with N fertilizer might mitigate soil degradation of Mollisols under an optimal application ratio, but the underlying mechanism still requires further exploration. This study will provide a scientific basis for the conservation and sustainable utilization of Mollisols resources.

The purpose of this study was to identify the fractal dimension and their relationships with alkalinity properties of soils, and to evaluate the potential of fractal dimension as an indicator of alkalinity properties of soil. Six soils with an increasing salinity (electrical conductivity was 0.09, 0.18, 0.62, 0.78, 1.57 and 1.99 dS m⁻¹, respectively) were selected from the western part of the Songnen Plain (China). Salt content, exchangeable sodium percentage, sodium adsorption ratio and other properties of the soils were determined and the soil particle-size distribution (0–2000 μm) was measured using a laser diffraction particle size analyser. Our results show that the overall fractal dimension of the selected soils ranged from 2.35 to 2.60. A linear regression analysis showed a significant negative correlation between fractal dimension and the amount of coarse sand and fine sand (r = − 0.5452, P < 0.05 and r = − 0.8641, P < 0.01, respectively), and a significant positive correlation with silt and clay (r = 0.9726, P < 0.01 and r = 0.9526, P < 0.01, respectively). Thus, soils with higher silt and clay content have higher fractal dimension values. Strong linear relationships between fractal dimension and salt content (P < 0.05), in particular a very significant positive relationship with HCO₃⁻ (P < 0.01), also exist. It is therefore possible to conclude that a soil’s fractal dimension could serve as a potential indicator of soil alkalization and the variability in alkaline soil texture.

Biochar and nitrogen fertilizers are known to increase soil carbon storage and reduce soil nitrogen loss as amendments, suggesting a promising strategy for highly effectively increasing soil productivity. However, few studies have explored the mechanisms of their effects on crop yield in terms of active carbon fraction and enzyme activity, which ultimately limits the potential for the application of biochar in combination with nitrogen fertilizers. To evaluate the effect of biochar and nitrogen fertilizer on the improvement of black soils in northeast China, a field experiment was conducted in the black soil to compare and analyze the application methods on total organic carbon (TOC), total nitrogen (TN), enzyme activities, and maize yields. Biochar rates: CK, C1, C2, and C3 (0, 9.8, 19.6, and 29.4 Mg·ha⁻¹); N fertilizer rates: N1/2 and N (30 and 60 kg·ha⁻¹). Results indicated that biochar and N fertilizer amendments significantly ameliorated soil fertility, such as TOC and TN, compared to the unamended soil. The TOC levels in the C3 treatment increased by 35.18% and the TN levels by 23.95%. The improvement in TN is more significant when biochar is blended with N fertilizer. Biochar blended with N fertilizer increased maize cellulase, urease, and invertase activities by an average of 53.12%, 58.13%, and 16.54%, respectively. Redundancy analysis showed that TOC, TN, and MBN contributed 42%, 16.2%, and 22.2%, respectively, to the maize yield indicator. Principal component analysis showed that reduced N fertilizer was more effective in improving yields, with a maximum yield increase of 50.74%. Biochar blended with N fertilizer is an effective strategy to improve the fertility and productivity of black soils in northeast China, while nitrogen fertilizer reduction is feasible and necessary for maintaining grain yield.