Albic soil is a typical low-yield soil with obstacles in the Northeast China Black Soil Region. It has a thin layer of black soil on the surface, low nutrient content in the soil body, and a compact and dense albic layer that hinders root penetration of crops and water infiltration. Those characteristics result in poor soil permeability and make the surface soil vulnerable to drought and flood disasters, which severely restricts grain yield. Eliminating the albic layer obstacle and increasing crop yield have become the main goals. We systematically summarized the research progress of albic soil obstacle reduction, discussed the advantages and disadvantages of current technologies from three aspects of chemical, biological, and mechanical improvement. We further analyzed the mechanism and improvement effect of each improvement technology on the physical properties of albic soil such as hardness, bulk density, soil pore structure and aggregates, as well as the chemical properties such as organic matter content, nutrient composition, and pH. In view of the problems existing in the research on albic soil obstacle reduction, we suggested that future research should focus on four directions: innovation and optimization of soil improvement machinery, policy support and assistance, promotion and application of modern soil improvement technologies, and response to obstacle reduction technologies under climate change and post-effect sustainability assessment, in order to provide a reference for the quality improvement of albic soil.

Albic soil is a typical low yield soil with barrier layer in Northeast China. Due to the dense structure of the barrier layer and the difficulties in water and gas transmission, it restricts crop growth and yield formation. We set up a manipulative experiment, with no amendment (CK) as the control to explore the effects of three amendments: composite amendment (modified bentonite+modified humic acid+calcium oxide, GL), modified bentonite (PR), and modified humic acid (HA) on the physical structure and microbial diversity of albic layer. The results showed that the improvement agent significantly reduced soil bulk density of the albic layer by 3.6% to 5.4%, increased the porosity by 4.7% to 7.0%, and increased the stability of aggregates. Among them, GL treatment enhanced the connectivity of pores by loosening soil particles. PR treatment could improve soil aeration in the short term, but with insufficient stability of aggregate structure. HA treatment enhanced aggregate stability through colloid filling, but had limited improving effect on pore connectivity. Different amendments changed soil microbial community composition in the albic layer. GL treatment significantly increased bacterial diversity, with a 12.9% increase in richness, with the community under which being dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. The PR treatment increased bacterial richness by 17.4%, with the community under which being dominated by Actinobacteria, Proteobacteria, and Ascomycota. HA treatment significantly increased fungal species richness, with a 41.0% increase in Chao1 index. The dominant fungal communities were Ascomycota, Basidiomycota, and Zygomycota. Soil porosity was positively correlated with fungal abundance (r=0.62), and bulk density was positively correlated with actinomycete abundance (r=0.58). In summary, GL treatment had a good physical and biological synergistic effect and was more suitable for loosening the albic layer, improving soil permeability. PR treatment could improve the aeration and microbial activity of the albic layer in the short term, but its promotion effect on the stabi-lity of aggregate structure was limited. Long-term application may facilitate soil structure loosening. HA treatment was suitable for soils with high water retention requirements and the need to enhance aggregate stability.

The albic soil (profile stratification: black soil layer, albic layer, and illuvial layer) has low permeability and a compacted albic layer, which is a typical low-yielding soil with constraint factors in Heilongjiang Province. Subsoiling techniques can disturb the albic soil layer and improve water conditions in the rooting zone. In this study, we investigated the effect of two subsoiling techniques, i.e. surface soil interlayer mixing (mixing the albic layer with the illuvial layer) and subsoil interlayer mixing (mixing the black soil layer with the albic layer), on soil water distribution and storage through monitoring soil water dynamics during the maize growth period. Results showed that the albic layer of control exhibited a bulk density 23.6% higher than that of the black soil layer and an extremely low saturated hydraulic conductivity (0.0013 cm·h^-1). This impeded upward water movement during the seedling stage of maize when rainfall was scarce, resulting in the formation of dryness in top and albic layers. During the frequent rainfall period after the jointing stage, the low permeability of the albic layer caused water retained in the upper soil layers. After using the subsoiling techniques, the average saturated hydraulic conductivity for 0-60 cm soil layer reached 12.29 cm·h^-1 under surface soil interlayer mixing and 14.09 cm·h^-1 under subsoil interlayer mixing treatments, being 4.9 to 5.7 times of the control. Compared to the control, both techniques increased soil water content in the albic layer by 93.2% during the maize seedling stage. During periods of the jointing stage with frequent rainfalls (July 15 to July 19), the average soil water content in the 0-30 cm layer decreased by 10.3% and 8.3% respectively, and soil water infiltration was improved. Water storage capacity in the rooting zone (0-60 cm) increased by 5.5% during the growing season. Our results indicated that subsoiling techniques ameliorate the uneven soil water distribution within the albic soil profile during the maize growing season, with the subsoil interlayer mixing showing the better efficacy.

Obstacles in the albic layer of albic soil, such as hardening and acidification, are key factors restricting crop yield in albic soil areas. In this study, we conducted a field manipulative microplot experiment, included lime alone (900 kg·hm^-2, C), and combinations of lime with 4.5 t·hm^-2(B1) and 9 t·hm^-2 biochar (B2), control (no amendment, CK), to investigate effects of amendments on soil physical and chemical properties, maize growth characteristics, and yield. The results showed that lime application significantly increased soil pH value of the albic layer, with the three treatments increasing by 0.26-0.69 units compared to CK. Soil bulk density of treatment C increased by 7.6% compared to CK. The addition of biochar could effectively mitigate the risk of increased soil bulk density caused by lime addition, and soil bulk density of B1 and B2 decreased by 4.6% and 3.9% respectively compared to treatment C. Compared to CK, lime application increased soil total calcium content of the albic layer by 4.7%-22.7%, but it led to a decrease in soil organic matter content, which decreased by 17.6% in treatment C compared to CK. However, biochar application effectively reduced the risk of organic matter content decline and increase organic matter content of the albic soil layer by 15.0%-15.3% compared to treatment C. The application of all amendment formulations showed a tendency to increase maize plant height, but had no significant effect on leaf chlorophyll content. Compared to CK, grain yield of treatment C increased by 9%, but the difference was not significant, while that of B1 and B2 significantly increased by 19.3% and 18.0% respectively. There was no significant difference between B1 and B2. Mantel test analysis showed that grain yield was mainly regulated by soil pH of albic layer. In conclusion, the combined application of 4.5 t·hm^-2 and 9 t·hm^-2 biochar with 900 kg·hm^-2 lime could effectively alleviate the acidification of albic layer and significantly increase maize yield. Among them, the combined application of 4.5 t·hm^-2 biochar and 900 kg·hm^-2 lime demonstrates the highest application value.

Mixing the albic horizon with the illuvium horizon can break the poor soil structure of albic soil, while straw input is an effective way to enhance soil nutrient content. We conducted an incubation experiment to analyze the changes in soil liquid limit, plasticity index, mechanical composition, distribution of stable aggregates, and soil organic carbon content under different mixing ratios of the albic horizon and illuvium horizon (1:0, 2:1, 1:1, and 1:2), along with the addition of 1% corn straw. We further verified the impacts with a soybean field experiment. The results showed that with the increasing proportion of illuvium horizon soil, the liquid limit of the soil increased, and the plasticity index increased from 16.0% to 23.0%, 27.0%, and 31.0%, respectively. The suitability of albic soil for cultivation was enhanced. The stability indicators of soil aggregates, such as clay content, percentage of aggregates larger than 0.25 mm (R_>0.25), average weight diameter (MWD), geometric mean diameter (GWD), increased, with MWD and GWD increasing by 42.9%, 44.9%, 46.9% and 27.7%, 27.7%, 29.8%, respectively. At the same mixing ratio, straw addition increased soil R_>0.25 and enhanced aggregate stability. Soil mixing and straw addition, as well as their interaction, had significant effects on soil R_>0.25, MWD, and GMD. The treatment with soil mixing ratio of 1:1 and straw addition resulted in the highest R_>0.25, reaching 66.6%. Soil mixing reduced soil organic carbon content, while straw addition significantly increased soil total organic carbon and aggregate organic carbon content at various particle sizes. This indicated that the combination of soil layer mixing and straw addition synergistically promoted the formation and stability of soil aggregate structure by increasing soil clay and organic carbon content, thereby improving soil structure and fertility. Field results showed that the application of machinery could achieve approximately 1:1 mixing of albic soil horizon and illuvium horizon. Compared with conventional deep loosening control, the soybean yield increased by 7.2% after the mixing operation and full straw return. This model had significant promotion effect in the improvement of albic soil.

Albic soil is a typical low-yield soil in China, characterized by low soil organic matter content. Exploring the impacts of maize straw return on the water-soluble organic carbon (WSOC) content of albic soils is of great significance for improving carbon sequestration and stabilizing soil fertility. In an experiment, we added maize straw (1% of soil mass) to two subtypes of albic soil (typical and meadow albic soils) at depths of 10 cm, 20 cm, and 30 cm, with no straw addition as the control, to analyze changes in WSOC content and fluorescence characteristics at 40, 100, and 290 days after straw addition. Results showed that maize straw addition significantly increased WSOC content at all depths in both subtypes, with a tendency to increase over time. After 290 days, WSOC content in typical albic soil increased by 6.4%, 15.7%, and 131.7% at 10 cm, 20 cm, and 30 cm depths, respectively, while meadow albic soil showed increases of 8.6%, 25.1%, and 54.0% at the same depths. Maize straw addition significantly decreased the water-soluble organic carbon/soil organic carbon (WSOC/SOC) ratio, indicating higher stability of SOC pool. The fluorescence index (1.15-1.79) and biogenic index (0.91-2.63) increased by 5.8% and 2.7%, respectively, while the humification index (0.25-0.62) decreased by 15.0%, suggesting that WSOC was influenced by both exogenous and endogenous substances and that straw addition led to a reduction in the humification of albic soils. Analysis of the WSOC fluorescence spectra identified four components belonging to three types of substances: soluble microbial products, humic acid like substances, and protein-like substances. After the addition of straw, the contents of humic acid like substances in typical albic soil decreased, while that of soluble microbial products and protein like substance increased. In meadow albic soil, the contents of humic acid like substances increased at depths of 10 cm and 30 cm but decreased at 20 cm; soluble microbial products increased at depths of 10 cm and 20 cm but decreased at 30 cm, while that of protein-like substances showed the opposite trend. Straw retuning increased the WSOC content in albic soil, promoted the formation of low-molecular-weight fluorescent components in WSOC solution, simplified the structure and enhanced the nutrient-supplying capacity.

The poor tillage properties in low-yielding albic soil caused by physical structural obstacles are unfavorable for mechanical tillage operations. To address this problem, we conducted indoor simulation experiments with three types of treatments: tillage layer mixing (TLM, where 10%-50% of the albic soil was incorporated into the surface soil), subsoil mixing (SM, where 10%-50% of the illuvial soil was incorporated into the albic soil), and subsoil fertilization (SF, with gradients of 2%-10% organic fertilizer, straw, and biochar addition, respectively). We analyzed the differential regulatory mechanisms of various soil improvement measures on the plasticity (indicated by liquid limit, plastic limit, and plasticity index), swelling-shrinkage behavior (indicated by shrinkage limit), and optimal tillage period (indicated by shrinkage index) of albic soil. Results showed that TLM improved soil homogeneity and plasticity, shortened the optimal tillage period, and degraded tillage characteristics. SM improved the tillage performance of albic soil by increasing clay content and optimizing the sand-clay ratio, which together extended the optimal tillage period. SF, leveraging the synergistic effects of organic matter, specific surface area, maximum hygroscopic water, and organic carbon, demonstrated superior improvement effects on tillage characteristics compared to TLM and SM. Among these, organic fertilizer enhanced soil water retention capacity (plastic limit increased by 37.5%-58.1%), optimizing soil tillage characteristics. Straw exhibited the most significant regulatory effect on soil swelling-shrinkage behavior (shrinkage limit decreased by up to 41.1%), with the optimal effect achieved at a 6% straw addition. Biochar improved soil tillability by optimizing the hierarchical pore structure, where a 10% addition increased the liquid limit and plastic limit by 7.3% and 14.9%, respectively, while reducing the plasticity index by 7.6% and increasing the shrinkage index by 12.6%. These findings indicated that all three soil fertilization materials could improve the tillage characteristics of low-yielding albic soil. This study would provide theoretical support for the remediation of albic soil barriers and the construction of sustainable tillage systems.

Deep incorporation of organic materials is an effective practice for soil improvement. We conducted a two-year experiment in typical albic soil with five treatments: conventional tillage (CT), deep tillage (DT), straw deep incorporation (DS), organic fertilizer deep incorporation (DM), and straw-organic fertilizer deep incorporation (DSM). We investigated the effects of deep incorporation of different organic materials on microbial biomass, extracellular enzyme activities, and their stoichiometric ratios in albic soil. The results showed that deep incorporation of organic materials (DS, DM, and DSM) significantly increased microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass phosphorus (MBP) in both the 0-15 cm and 15-35 cm soil layers, with the DSM treatment showing the strongest effect. MBC, MBN, and MBP under DSM increased by 72.8%, 61.6%, and 147.7% in the 0-15 cm soil layer, and 78.7%, 136.7%, and 248.4% in the 15-35 cm soil layer, respectively. Extracellular enzyme activities decreased significantly with increasing soil depth. The addition of organic materials significantly enhanced enzyme activities in each soil layer with DSM again exhibiting the largest promotion. The activities of C-acquiring enzyme (β-1,4-glucosidase), N-acquiring enzymes (β-1,4-N-acetylglucosaminidase and leucine aminopeptidase), and P-acquiring enzyme (acid phosphatase) under DSM increased by 11.6%, 65.3%, and 50.4% in the 0-15 cm layer, and 68.9%, 37.4%, and 31.1% in the 15-35 cm layer compared to CT, respectively. The enzyme-based vector model revealed that soil microbial communities across all layers of the albic soil were generally co-limited by carbon and phosphorus. Deep incorporation of organic materials significantly alleviated the nutrient limitation. Correlation analysis and random forest modeling further indicated that soil extracellular enzyme activities were regulated by MBC, MBP and MBN. Deep incorporation of organic materials could enhance microbial and enzymatic activities in albic soil and promote nutrient cycling, with the combined application of straw and organic fertilizer having the highest improvement.

White slurry layer of albic soil has low fertility and poor nutrient availability. Organic material return to fields is a primary agricultural management practice for enhancing soil fertility. We examined the variations of soil nitrogen, phosphorus and potassium nutrient contents and pH of the topsoil layer (0-15 cm) and subsoil (15-35 cm) layers of albic soil under different treatments, including deep tillage at 35 cm without organic materials (T₃₅), straw deep mixing at 35 cm (T₃₅+S), organic fertilizer deep mixing at 35 cm (T₃₅+M), combined straw and organic fertilizer deep mixing at 35 cm (T₃₅+M+S), and conventional tillage at 15 cm without organic materials as the control (CK). Compared to CK, the T₃₅+S treatment significantly increased the contents of total N (TN), total P (TP), total K (TK), available N (AN), available P (AP), and available K (AK) in both the topsoil and subsoil layers. Compared to the T₃₅ treatment, the T₃₅+S treatment showed significant increases in AN, AP, and AK in both soil layers. The T₃₅+M and T₃₅+M+S treatments had significantly higher total and available nutrient contents in both the topsoil layer and subsoil layer than other treatments, with the T₃₅+M+S having the highest total and available nutrient contents. Compared with CK, the contents of TN, AN, AP, and AK in the topsoil layer soil increased significantly by 34.8% and 36.4%, 20.5% and 29.0%, 112.3% and 118.0%, and 39.5% and 49.5% under the T₃₅+M and T₃₅+M+S treatments, respectively. Correspondingly, these parameters in the subsoil layer increased significantly by 68.6% and 72.3%, 39.4% and 46.3%, 124.9% and 131.2%, and 50.4% and 64.9%, respectively. The treatments T₃₅, T₃₅+S, T₃₅+M, and T₃₅+M+S increased the abundance indices (subsoil layer nutrient content/topsoil tillage layer nutrient content) of TN, AN, AP, and AK in subsoil layer by 5.3%-12.4%, 4.8%-5.5%, 11.4%-14.4%, and 18.0%-20.7%, respectively. Our results indicated that deep mixing of organic materials induced by deep tillage effectively mitigated the decrease in TN, AN, AP, and AK content in the topsoil layer, while simultaneously increased nutrient content in the subsoil (white slurry layer). The pH values in the 0-35 cm soil layer increased by 0.01, 0.09, 0.17, and 0.14 under the four treatments, respectively. In summary, deep mixing of organic materials into the soil enhances soil fertility in both the topsoil and subsoil layers of albic soil, and the treatments with organic fertilizer alone or in combination with straw would be the most effective practice.

Soil organic matter serves as a crucial indicator for soil quality. Albic soil, characterized by a barrier layer, exhibits limitations in organic matter content, which can adversely affect crop growth and development. To elucidate the impact of deep mixing of various organic materials on the redistribution of organic matter in the surface soil of albic soil could provide theoretical and technical insights for establishing suitable plough layers for albic soil in Northeast China. We conducted a two-year positioning experiment in Shuangyashan, Heilongjiang Province with five treatments, conventional shallow tillage (0-15 cm, CK), inversion tillage (0-35 cm) without or with straw return (T₃₅ and T₃₅+S), inversion tillage with cattle manure (T₃₅+M) and cattle manure plus maize straw (T₃₅+S+M). The results showed that soil fertilization via deep mixing of organic materials to a depth of 35 cm significantly increased maize yield in albic soil, with the T₃₅+S+M treatment demonstrating the most pronounced effect, yielding an average production of 2934.76 kg·hm^-2. Compared to CK, the T₃₅ treatment resulted in a significant 8.4% decrease in organic matter content in the tillage layer, a significant 7.6% increase in organic matter in the sub-tillage layer, and a relative richness degree of soil organic matter in the sub-tillage layer increased by 17.5%. Deep mixed return of organic materials following deep ploughing markedly increased organic matter content of the plough layer, with organic matter conversion ranging from 16.3% to 31.0%. In comparison to the T₃₅ treatment, there was no significant increase in soil organic matter content in the T₃₅+S tillage layer and sub-tillage layer. Conversely, soil organic matter content increased by 4.6% and 6.9% in the T₃₅+M and T₃₅+S+M treatments, with corresponding increase of 11.2% and 15.4% in sub-tillage layer, respectively. Additionally, the soil organic matter richness index in sub-tillage layer increased by 2.5% and 5.1%, respectively. There was a significant positive correlation between organic matter content in the entire plough layer and maize yield, with a contribution rate of 17.5%. Therefore, the utilization of organic fertilizer or a combination of organic fertilizer and straw deep mixing can quickly fertilize albic soil by increasing soil organic matter content in both the whole tillage layer (0-35 cm) and the sub-tillage layer (15-35 cm).