The cementation mechanisms and processes of aggregate binding agents are important in understanding aggregate formation. However, the role of threshold values and the proportions of organic and inorganic binding agents in aggregate formation remain unclear. This research investigated the dominant aggregate binding agent dynamics in a sequence comprising buried ancient red soil unaffected by modern climate changes and human activities, alongside nearby exposed Quaternary ancient red soils subjected to different land use patterns influenced by these factors in northeastern China. By analyzing soil age, aggregate compositions, and organic/inorganic indicators of binding agents, including soil organic matter (SOM), free iron oxide (Fed), poorly crystalline iron oxide (Feo), crystalline iron oxide (Fed-Feo), and total clay particles (TCL), we determined the relative contributions of different binding agents using redundancy analysis (RDA). The results revealed that the buried ancient red soil did not contain dominant binding agents in the aggregate formation before 91.01 ka BP. Due to denudation, the buried ancient red soil was exposed at the surface and experienced the importation of soil organic matter, weathering of silicate-bound iron oxides, and crystallization of poorly crystalline iron oxides resulting from the effects of different land use patterns from 91.01 ka to the present. Under the influence of binding agent dynamics, dominant binding agents in the exposed Quaternary ancient red soils’ aggregate formation changed into SOM and Fed. When the C/(Fed-Feo) molar ratio was less than 2.13, Fed-Feo was the dominant aggregate binding agent. When the C/(Fed-Feo) molar ratio was greater than 2.13, SOM was the dominant aggregate binding agent. The results of this study improve our understanding of aggregate formation and the relationship between soil organic matter and iron oxides.

The Chaoyang profile represents a rare multi-period, continuous and complete sequence of aeolian paleo-deposits with a stable sedimentary origin and multi-stage paleoclimatic cycles. Benchmark profiles including soil types at different pedogenic stages can be used for the recognition and classification of paleosols and paleoclimate reconstruction. The loess–paleosol sequence benchmark profile (LBP) is also helpful in comparing the results of paleoenvironment reconstruction from different ecological regions. In this study, a loess–paleosol profile derived from thick loess in Chaoyang city of Liaoning province, Northeast China, was investigated as a well-preserved LBP that included various paleosol types. To determine the nature and origin of the Chaoyang profile, the geographic, stratigraphic and morphological characteristics were described in the field. Bulk samples from 42 horizons were collected for chemical and physical analysis, and sub-sampling of 946 samples at 2 cm intervals from the surface to the bottom were taken to measure grain size distributions and magnetic susceptibility. Results showed that the 19.85 m thick loess–paleosol profile had been continuously deposited since 423 ka BP. The upper part (0–195 cm), or UPP, was predominantly of aeolian loess deposition origin but was mixed with water-reworked materials from a nearby secondary loess source. The middle part (195–228 cm), or MIP, was also indirectly affected by the water-reworking process through the leaching of materials from the overlying UPP. The lower part (228–1985 cm), or LOP, was characterized by four reddish stratigraphic layers interbedded with five yellowish ones, indicating several types of paleosols developed under different ecological environments. The multi-stage paleoclimatic cycles as evidenced by morphological and physical characteristics as well as age dating and magnetic susceptibility correlated well with the Lingtai section and LR04 benthic δ¹⁸O. Because of these attributes, the Chaoyang profile can be deemed as a benchmark loess–paleosol profile for the recognition and classification of paleosols and paleoclimate reconstruction in Northeast China. The differences in morphological and physical properties between paleosols and loess suggest different soil fertility and agronomic properties and need further studies to assess their functionality with climate fluctuation.

Soil degradation has led to a continuous decline in the quality of phaeozem, which is seriously threatening the foundation of national food security. Therefore, precisely evaluating the health status of phaeozem resources and their spatial and temporal variations are crucial for ensuring the effective implementation of soil degradation prevention and control strategies. In this study, soil data from 140 sites were collected, and eight physical and chemical indices (sand content, silt content, pedon thickness, organic matter, total potassium, total nitrogen, total phosphorus, and pH) were utilized to assess the soil health status of phaeozem in Liaoning Province. The results showed the following: (1) The minimum data set is aligned with previous research findings and effectively mirrors the soil’s health condition. (2) The substantial coefficients of variation observed for pedon thickness, sand content, and silt content across regions suggest notable differences, likely influenced by climatic variations, parent material differences, and anthropogenic activities. (3) The study area’s average soil pH of 6.39 indicates an overall acidic nature, potentially attributed to intense soil weathering and suboptimal fertilization practices. (4) The soil health index (SHI) ranged from 0.33 to 0.91, with an average of 0.62, which indicates that the majority of the phaeozem in Liaoning Province are in a sub-healthy state (SHI < 0.6 = unhealthy, SHI 0.6–0.8 = sub-healthy, SHI ≥ 0.8 = healthy). These sub-healthy areas are primarily located in hills, mountains, and the Liaohe Plain, and are significantly impacted by rainfall and wind erosion. Meanwhile, unhealthy areas are concentrated in the south and center of the province, characterized by fragile ecological environments and challenging agricultural conditions.

The health status of Quaternary red soil is a comprehensive reflection of the production and ecological service functions, which directly affects agricultural productivity and ecosystem sustainability. Based on the Cornell Soil Health Assessment (CASH) system frame, a health evaluation system for Quaternary red soils was established including the soil’s physical, chemical, and biological indicators. The soil’s health status under different land use patterns (the buried Quaternary red soil, sparse forest and grassland, grassland, woodland, and arable land) was systematically diagnosed in the low hilly region of western Liaoning Province. The results showed significant differences in the soil health comprehensive index of the Quaternary red soils under different land use patterns (the whole soil), presenting a trend of woodland (0.64) > arable land (0.61) > grassland (0.49) > sparse forest and grassland (0.37) > buried Quaternary red soils (0.33). The woodland and arable land are at a healthy level, the grassland and sparse forest and grassland are at a sub-healthy level, and the buried Quaternary red soil is at an unhealthy level. The health status of the topsoil layer (A) under different land use patterns has a trend of woodland (0.86) > arable land (0.73) > grassland (0.70) > sparse forest and grassland (0.67). This is consistent with the overall health status of the profile, better than that of subsoil layer (B), which presents a trend of arable land (0.41) > grassland (0.40) > woodland (0.38) > sparse forest and grassland (0.34), with relatively poor soil health conditions. Overall, the soil health status of the four land use patterns is better than that of the buried Quaternary red soils, showing an evolution trend towards healthy soil. This indicates that at this stage, human land use activities have to some extent promoted the healthy development of Quaternary red soils. The Quaternary red soils of the woodland have a healthy status, and the land use pattern is suitable and can be scientifically recommended in low mountain and hilly areas.

The Quaternary red soil widely distributed in China is an important arable land resource. A quantitative understanding of nutrient changes of Quaternary red soils under different land-use patterns is the necessary premise for effective regulation, management, and sustainable utilization. In this study, five typical Quaternary red soil profiles under different land-use patterns were taken as the research object in Chaoyang City, Liaoning Province, China. The results showed that: (1) Buried Quaternary red soils were minimally affected by external disturbances. The contents of nitrogen (around 0.02%), phosphorus (ranging from 0.06% to 0.07%), and potassium (ranging from 3.12% to 3.50%) were at relatively low levels and homogeneously distributed with depth. (2) The total nitrogen content of red soils under each land-use pattern showed an increasing trend in the upper part of the profile (A and B horizons), and a sequence of woodland (CL-04) > grassland (CL-03) > arable land (CL-05) = sparse forest–grassland (CL-02). The nitrogen content in the lower part of different land-use patterns was about 0.02%. The phosphorus content of the topsoil layer remained unchanged (ranging from 0.05% to 0.06%), while the subsoil phosphorus decreased to varying extents. The potassium experienced leaching in both topsoil and subsoil layers, with the topsoil losses being lower than that in the subsoil. The range of total potassium content in the grassland (CL-03) ranged from 2.64% to 4.21%, from 3.91% to 4.44% for sparse forest–grassland (CL-02), from 2.41% to 2.63% for woodland (CL-04), and 2.85% to 2.92% for arable land (CL-05), respectively. The variation in nutrient content was related to the vegetation type, coverage rate, artificial fertilization method and species, etc. The accumulative mass change in the sparse forest–grassland increased by 384.16 g/100 cm², and the other land-use patterns showed a decreasing trend of arable land (83.71 g/100 cm²) > woodland (83.71 g/100 cm²) > grassland (83.71 g/100 cm²), with the topsoil leaching losses being smaller than those in the subsoil layer. The characteristics of windbreak, sand fixation, and soil and water conservation of the sparse forest–grassland could well hold the nutrient-rich loess sediments, resulting in increased nutrients in the Quaternary red soil, which is a reasonable land-use pattern for the Chaoyang area.

Understanding the spatial variability and driving mechanisms of humus horizon thickness (HHT) degradation is crucial for effective soil degradation prevention in black soil regions. The study compared ordinary kriging interpolation (OK), inverse distance weighted interpolation (IDW), and regression kriging interpolation (RK) using mean error (ME), mean absolute error (MAE), root mean square error (RMSE), and relative RMSE to select the most accurate model. Environmental variables were then integrated to predict HHT characteristics. Results indicate that: (1) RK was superior to OK and IDW in characterizing HHT with the smallest ME (11.45), RMSE (14.98), MAE (11.45), and RRMSE (0.44). (2) The average annual temperature (0.29), precipitation (0.27), and digital elevation model (DEM) (0.21) were the primary factors influencing the spatial variability of HHT. (3) The HHT exhibited notable variability, with an increasing trend from the southeast towards the central and northern directions, being the thinnest in the southeast. It was thicker in the northeast and southwest regions, thicker but less dense along the southern Bohai coast, thicker yet sporadically distributed in the northwest (especially Chaoyang and Fuxin), and thick with aggregated distribution over a smaller area in the northeastern direction (e.g., Tieling). These findings provide a scientific basis for accurate soil management in Liaoning Province.