Straw Deep Burial and Returning to Farmland: Mechanistic Study on Enhancing Albic Soil Fertility

Abstract

This study developed an innovative model integrating straw subsoil deep burial (SD) and mixing plow to mitigate albic soil’s physical and chemical constraints and enhance crop yield. A field experiment with four treatments, including conventional tillage (CT), straw mulching (SM), straw subsoil deep burial (SD), and straw burning (SR), was conducted to assess impacts on soil enzyme activity, nutrient dynamics, crop yield, and soil physical properties. Results showed that SD treatment significantly improved albic soil properties compared to conventional tillage: catalase activity in the albic horizon decreased by 13.51%, reducing peroxide toxicity. In the albic horizon, alkaline hydrolysis nitrogen, total nitrogen, available phosphorus, total phosphorus, available potassium, total potassium, and organic matter increased by 29.98%, 58.70%, 36.86%, 20.46%, 5.00%, 21.70%, and 40.46%, respectively. Correspondingly, maize and soybean yield under SD reached 8686.6 kg/ha and 2245.3 kg/ha, increasing by 15.39% and 19.94% compared to CT, respectively. Additionally, SD treatment improved physical properties of the albic horizon: soil hardness reduced by 43.56%, with enhanced water-holding capacity, permeability coefficient, porosity, and hydraulic conductivity. Its findings not only boost agronomic productivity by improving crop yields but also support environmental sustainability by enhancing soil fertility, which is of great significance for ensuring food security.

1. Introduction

Soil degradation severely threatens global agricultural sustainability. Albic soils have physical constraints and chemical infertility, and they are a major concern across multiple continents. According to the Food and Agriculture Organization (FAO), such soils are distributed across 32 countries worldwide, predominantly in Eastern North America, Atlantic-influenced regions of Europe, and small patches in Southeastern Australia [1,2,3]. In China, albic soils exhibit extensive distribution, forming complex soil zones alongside black soils, brown soils, and dark brown soils, spanning latitudes 30° N to 55° N [4,5,6]. Their core distribution lies in Northeastern China, covering approximately 5.27 million hectares across the Eastern Jilin Province, Heilongjiang Province, and the Jilin-Liaoning border. Notably, the Sanjiang Plain in Heilongjiang alone accounts for 67.3% of the province’s total albic soil area. Critically, albic soils constitute 9.4% to 10.07% of arable land in Jilin and Heilongjiang, and play a pivotal role in grain production within China’s black soil region—a core area safeguarding national food security. Against this global and national backdrop, addressing albic soil degradation is not merely a local soil improvement initiative, but a prerequisite for securing agricultural productivity and food security at both regional and global scales. These soils exhibit inherent limitations, including a fragile plow layer, limited nutrient reserves, and a mechanically restrictive albic horizon, which collectively impede root penetration [7,8]. In addition, the application of large amounts of nitrogen fertilizers exacerbates the acidification of albic soils, while traditional tillage has resulted in a thinning of the soil tillage layer and an upward shift of the subsoil layer, which leads to frequent episodes of surface droughts and flooding, forcing the crop root system to confine itself to a shallow level of development, which results in crop yields in white pulp soils that are 10 to 20 percent lower than those of the black soils [9]. Such multidimensional soil limitations necessitate systematic remediation strategies to address both physical and chemical constraints while enhancing productivity.

Traditional soil improvement methods, such as straw returning, forage grass planting, and progressive deep tillage, primarily enhance the physicochemical properties of the plow layer but fail to effectively disrupt the physical barrier of the albic layer. Numerous researchers have focused on enhancing soil structure and nutrient availability through the addition of external amendments, with straw returning emerging as a widely adopted and effective practice [10,11,12]. A meta-analysis of 41 long-term trials in Europe demonstrated that straw returning increased yields by 6.0% [13], while a 22-year field experiment in China revealed a significant 13.71% yield enhancement compared to conventional tillage [14].

The experimental study demonstrated that deep burial of crop residues achieved a 22.3% higher decomposition rate compared to surface straw mulching, while simultaneously increasing soil organic carbon and total nitrogen content by significant margins [15,16]. These practices also optimize microbial community structure, accelerating nutrient cycling and enhancing soil fertility sustainability [17,18,19]. However, such surface level interventions inadequately address the albic layer’s inherent physical constraints. Research shows that even with chemical amendments, albic layer hardness remains elevated at 14 kg/cm², while enzyme activities show negligible improvement relative to pre-treatment levels, underscoring persistent limitations in nutrient release and root uptake. As emphasized by Prof. He Wanyun [20], the albic layer is like the “cancer” of albic soil. Its mechanical discontinuity destroys the hydrological regulation and root–soil interactions, blocking the crop rooting, nutrient and water exchanges up and down, and is neither drought-resistant nor waterlogging-resistant, so the transformation of the albic layer is the core of the improvement of albic soil.

Mechanical deep tillage was initially considered a promising solution [21,22,23], but its ameliorative effects diminished within one year due to rapid re-compaction of silty albic soils following rainfall. This technical bottleneck prompted Zhao Delin’s team [24,25,26] to propose a two-layer restoration framework: 20 cm of ploughing in the upper layer versus 30–40 cm of mixed layer in the lower layer, which utilizes the clay content of the sediment layer to neutralize the sandiness of the albic layer. Based on this principle, Araya et al. [27] and Liu’s team [28] developed a three-stage subsoil mixing plough that achieved sustained physical property improvements over 5 years through soil integration. However, while such machinery enhances physical properties, it fails to ameliorate chemical characteristics. To date, no effective method has been established to simultaneously improve both physical and chemical properties of albic soils, underscoring the need for integrated soil management strategies.

Based on this, this study proposes a new mode of combining straw returning to the field with subsoil improvement machinery, and on the basis of the existing three-stage subsoil mixing plow, develops a straw subsoil mixing plow capable of mixing root stubble straw and soil modification materials into the subsoil, and puts forward a new method of deep straw returning to the field by means of the straw subsoil mixing plow. This system facilitates deep straw burial while addressing subsoil compaction through mechanical loosening. The proposed technology not only enhances soil organic matter and nutrient storage in the plow layer, thereby improving fertility, but also breaks up the albic layer to increase subsoil permeability. While preserving the black soil layer, this approach provides a viable solution to address nutrient depletion in the albic subsoil while simultaneously opening new avenues for managing surplus crop residues generated from agricultural restructuring.

Through a comprehensive analysis of this novel system’s effects on soil pore dynamics, vertical nutrient distribution, and plant physiological responses, this study establishes a mechanistic framework for sustainable management of albic soil. This study integrates physical improvement and soil fertility enhancement. Straw returning to the field is a conventional soil fertility enhancement method; mixing straw into the soil while physically eliminating the albic horizon barrier can not only boost soil fertility but also enhance the stability of improvement effects and extend the soil restoration period. The present study contributes to a better understanding of the physicochemical and biological mechanisms underlying soil improvement. Its findings provide both technical and theoretical support for overcoming the historical disconnect between physical rehabilitation and chemical amelioration in agricultural soils.

2. Materials and Methods

2.1. Site Description and Design

The experimental site was located at the Dryland Farming Experimental Station of Bawusan Farm, Heilongjiang Province, China (46°31′26.1″ N, 133°00′15.8″ E). Bawusan Farm is a state-owned agricultural reclamation unit located in Baoqing County, Heilongjiang Province. Founded on 17 September 1956, it covers a total area of 1228.6 square kilometers. This experimental station has maintained long-term positioning research on dryland soil fertility and crop cultivation. Before the formal implementation of this experiment, the previous crop of the plot was soybean, with conventional tillage management; the long-term basic soil fertility indicators of the plot were stably maintained at the typical level of hilly albic soil in this region, providing a standardized historical background for the comparative analysis of soil improvement effects in this study. The region experiences short spring, summer, and autumn seasons, with long, harsh winters typical of a subarctic continental monsoon climate. Annual precipitation averages 560.0 mm, annual evaporation is 1373.3 mm, and mean annual temperature is 3.7 °C. The soil is classified as upland albic soil, with a profile structured as follows: 0–20 cm for the black soil layer (cultivated layer), 20–40 cm for the albic layer, and 40–60 cm for the sedimentary layer. The particle composition includes 65.51% silt, 29.73% clay, and 4.76% sand. Initial physicochemical properties are presented in

Table 1. Initial physicochemical properties of albic soil at the Dryland Experimental Station of Bawusan Farm, China.

Characteristic	Soil Layer	Measurement	Characteristic	Soil Layer	Measurement
Soil organic matter SOM (g/kg)	Ap 1	37.9 ± 1.40	Total nitrogen TN (g/kg)	Ap	2.19 ± 0.08
	Aw 2	12.9 ± 1.72		Aw	0.45 ± 0.06
	B 3	13.7 ± 0.98		B	0.61 ± 0.05
Total phosphorus TP (g/kg)	Ap	1.50 ± 0.07	Total potassium TK (g/kg)	Ap	19.70 ± 0.63
	Aw	0.78 ± 0.06		Aw	21.71 ± 0.77
	B	0.83 ± 0.05		B	20.83 ± 0.54
Alkaline hydrolysis nitrogen AN (mg/kg)	Ap	169.5 ± 5.50	Available phosphorus AP (mg/kg)	Ap	36.7 ± 5.71
	Aw	49.4 ± 5.41		Aw	9.7 ± 3.27
	B	42.4		B	14.1 ± 3.10
Available potassium AK (mg/kg)	Ap	100.0 ± 15.22	pH	Ap	6.10 ± 0.1
	Aw	90.0 ± 11.13		Aw	6.20 ± 0.2
	B	128.0 ± 12.15		B	6.10
Solid phase (%)	Ap	45.7 ± 1.73	Liquid phase (%)	Ap	34.8 ± 1.30
	Aw	64.1 ± 1.65		Aw	34.5 ± 1.35
	B	59.3 ± 1.87		B	33.1 ± 1.75
Gaseous phase (%)	Ap	19.5 ± 0.64	Porosity (%)	Ap	54.3 ± 0.50
	Aw	1.4 ± 0.73		Aw	35.9 ± 0.32
	B	7.6 ± 1.00		B	40.7 ± 0.35
Soil bulk density (g/cm3)	Ap	2.52 ± 0.03	Saturated hydraulic conductivity (cm/s)	Ap	2.45 × 10−4
	Aw	2.51 ± 0.04		Aw	6.08 × 10−6
	B	2.51 ± 0.05		B	4.44 × 10−7

¹ Ap: The black soil layer. ² Aw: The albic layer. ³ B: the sedimentary layer.

. Experimental data were obtained from three replicate experiments. The experiment included four treatments: conventional tillage (CT), straw mulching (SM), straw deep subsoil incorporation (SD), and straw burning incorporation (SR), each replicated three times. A large-scale comparative design was implemented, with treatments applied in autumn post-harvest using combined manual and mechanical operations (

Table 2. Detailed operational procedures for each treatment in the experiment.

Treatment	Tillage and Residue Management
CT 1	Shallow tillage combined with subsoiling of the topsoil layer.
SM 2	After crop harvest, the straws are left scattered on the soil surface without management.
SD 3	Deep incorporation of straws into subsoil using a 1LH-1 root–stubble straw subsoil mixer.
SR 4	After crop harvest, surface straw and stubble residues are burned in situ.

¹ CT: Conventional tillage. ² SM: Straw mulching. ³ SD: Straw deep subsoil incorporation. ⁴ SR: Straw burning incorporation.

). The experiment was designed as a maize–soybean rotation system. In the first year, maize variety “38P05” was planted at a density of 70,000 plants/ha, with soybean straw return of 2000 kg/ha for each treatment. In the second year, soybean variety “Kenfeng 16” was sown at a density of 240,000 plants/ha, accompanied by a maize straw return of 6000 kg/ha. All straw return treatments were conducted with full-amount straw application in October annually: specifically, SR and SM treatments were retained on the soil surface, while SD treatment was uniformly mixed with soil in the 20–60 cm soil layer. Fertilization followed conventional practices: urea (N 46%) at 60 kg/ha, diammonium phosphate (P₂O₅46%, N 18%) at 180 kg/ha, and potassium sulfate (K₂O 50%) at 45 kg/ha. The experiment lasted for two years, from 2022.

The detailed operating procedures for each treatment in the experiment are shown in Table 2.

The root stubble straw subsoil mixing plow works in the following way (

Table 3. The working process of the plow.

Plow Components	Function	Working Depth (cm)
Spade topsoil plow	Remove the black soil layer.	20
Topsoil scraping plow	Scrape the surface straw stubble into the furrow opened by the previous plow.	3–5
Subsoil crushing plow	Plow up the albic horizon soil to achieve the mixing of the albic horizon soil and straw.	40
Subsoil crushing mixing plow	Along the previous plow’s path, plow up the sedimentary horizon to mix it with the albic horizon and straw.	50–55

).

The root stubble ploughed up by the second plough and the two layers of subsoil ploughed up by the third and fourth ploughs fall down from the end of the fourth plough’s bar as the plough advances, so as to realize the soil mixing and the random mixing of the straw and the subsoil. In the next round of operation, the first plow plows the next district soil, and the topsoil, about 15–17 cm thick, which has been scraped off the root stubble, is tipped on top of the heart soil that has been mixed. The specific structure and principle of soil reforming are shown in Figure 1.

The mechanical performance parameters were as follows. Overall dimensions: The main beam is 3820 mm long, the plow frame is 2480 mm wide, and 2065 mm high. Total machine weight: 0.85 t. Operating parameters: The working width is 46–60 cm, the working depth is 40–60 cm, the working stroke is 3.1 m, the draft force is 2.0–3.5 t, and the working efficiency is 0.2–0.4 hm²/h.

2.2. Soil Sampling and Analysis

Soil sampling was conducted after crop maturity, with three random replicates collected from three soil layers in each plot. Samples were split into two portions: one stored at 4 °C for immediate enzyme activity analysis, and the other air-dried at 25 °C for 7 days, crushed manually, and sieved through a 1 mm mesh for physicochemical property analysis. Soil biological activity was characterized by three key enzymes. Catalase (CAT) activity was determined via titration with 0.1 mol·L⁻¹ KMnO₄: 5 g fresh soil was mixed with 5 mL 3% H₂O₂ and 50 mL deionized water, shaken at 150 rpm for 20 min, filtered, and the filtrate titrated with KMnO₄ until a 30-s persistent pink color, expressed as mL KMnO₄ g⁻¹ soil h⁻¹. Urease (UR) activity was measured using indophenol blue colorimetry: 5 g fresh soil was incubated with 10 mL 10% urea and 20 mL citrate buffer (pH 6.7) at 37 °C for 24 h; after filtration, 1 mL filtrate was mixed with 4 mL sodium phenate and 3 mL sodium hypochlorite, with absorbance measured at 578 nm (UV-2600, Shimadzu Corporation, Kyoto, Japan), expressed as mg NH₃₋N g⁻¹ released per gram of soil per day. Sucrase (SUC) activity was assayed via 3,5-dinitrosalicylic acid (DNS) colorimetry: 5 g fresh soil was mixed with 10 mL 8% sucrose and 5 mL phosphate buffer (pH 5.5), incubated at 37 °C for 24 h; after filtration, 1 mL filtrate was boiled with 3 mL DNS reagent for 5 min, cooled, and absorbance measured at 540 nm, expressed as mg glucose g⁻¹ soil day⁻¹.

Soil physicochemical properties were analyzed via standard methods: total nitrogen by Kjeldahl digestion using a Kjeltec 8400 autoanalyzer(FOSS Analytical A/S, Hillerød, Denmark); alkaline hydrolysis nitrogen via alkaline diffusion with 1 mol·L⁻¹ NaOH; total phosphorus by H₂SO₄HClO₄ digestion and molybdenum blue colorimetry at 700 nm; available phosphorus extracted with 0.5 mol·L⁻¹ NaHCO₃ (pH 8.5) and measured via molybdenum blue colorimetry; total potassium digested with NaOH at 720 °C and determined by flame photometry; available potassium extracted with 1 mol·L⁻¹ NH₄OAc (pH 7.0) and measured via flame photometry; soil organic matter by dichromate oxidation with titration of excess Cr²⁺ using 0.2 mol·L⁻¹ FeSO₄; pH in a 1:2.5 (w/v) soil–water suspension using a pH meter; and soil three-phase ratio was determined using the cutting ring method (100 cm³ ring), with bulk density calculated as oven-dried soil mass/ring volume and porosity derived as 1—(bulk density/particle density), where particle density was assumed to be 2.65 g·cm⁻³. Soil hardness was measured using a Japanese Yamamoto-type meter (cone angle 25°20′) and a penetration-type meter (cone angle 30°, cross-sectional area 2 cm², range 0–25 kg/cm²), with their correlation: $\mathrm{Y}=0.6\mathrm{X}$ ($\mathrm{Y}$: Yamamoto hardness; $\mathrm{X}$: penetration hardness, both in kg/cm²). Soil water content was determined by oven-drying. Field hydraulic conductivity was measured via the double-ring method [6] (inner ring: 30 cm diameter × 30 cm height; outer ring: 45 cm × 35 cm), with rings driven 20 cm into leveled topsoil, water maintained at 10 cm depth, and infiltration volume recorded at intervals. Soil moisture dynamics were monitored via tensiometry, daily at 8:00 a.m. from maize emergence (June 4) to pre-harvest (September 10) in the second year post-improvement. Maize and soybean yields were determined by randomly selecting three 1 m² quadrats per treatment plot at maturity.

3. Results

Because the chemical properties of the soil require two years to stabilize, this study recorded the chemical properties in the second year of soil amendment. Control experiments verified that deep straw incorporation effectively improves the chemical properties of albic soil. Additionally, the yields of maize and soybean after two years of crop rotation were measured, both showing significant increases. Subsequently, soil physical properties were monitored over two years to evaluate whether the proposed novel soil improvement model enhances soil physical conditions and achieves long term amelioration effects. The mechanisms underlying the relationships between soil physicochemical properties and crop yield were analyzed, with soil influence principles illustrated in Figure 1.

3.1. Soil Enzyme Activity

Different straw returning methods had different effects on soil enzyme activity. The effects of various straw return methods on soil catalase (CAT) activity varied significantly across soil layers (Figure 2a). In the 0–20 cm plow layer (Ap), the SM treatment showed a more pronounced enhancement of CAT activity compared to CT, while no significant difference was observed between SD and CT. The SR treatment exhibited slightly higher CAT activity than CT. In the 20–40 cm albic layer, CT displayed the highest CAT activity; SM showed no significant difference from CT, but both SD and SR treatments reduced enzyme activity, with SD resulting in a 13.51% decrease compared to CT. In the 40–60 cm illuvial horizon, no significant differences in CAT activity were found between SM, SD, and CT, whereas SR led to a marginal reduction.

As shown in Figure 2b, urease activity exhibited distinct patterns across soil layers and treatments. In the Ap layer, the SD treatment showed no significant difference compared to CT, whereas SM and SR treatments significantly increased urease activity by 26.62% and 18.78%, respectively. In the Aw layer, both SW and SR treatments led to slight decreases in urease activity relative to CT, while the SD treatment caused a significant reduction. In the B layer, SW and SR treatments showed no significant changes compared to CT, but SD treatment increased urease activity. A comparison across all four treatments revealed that, except for SD, the other three treatments followed a consistent trend of decreasing urease activity with increasing soil depth. Notably, the SD treatment equalized urease activity between the Aw and B layers.

As shown in Figure 2c, the SUC activity trends under the SD treatment differed from those of the CT, SM, and SR treatments. Under SD, SUC activity followed the order Ap layer > B layer > Aw layer, whereas the CT, SM, and SR treatments showed a gradual decrease in SUC activity with increasing soil depth. In the Ap layer, the SM and SR treatments significantly increased SUC activity by 26.83% and 18.37% compared to CT, while SD showed no significant difference. In the Aw layer, SM and SR reduced SUC activity by 19.82% relative to CT, but SD caused a severe decline. In the B layer, no significant differences were observed between SM, SR, and CT, but SD significantly increased SUC activity. Notably, SD equalized SUC activity between the Aw and B layers.

3.2. Trends in Soil Organic Matter and Nutrient Content

Changes in soil nitrogen and organic matter were as follows. Through the laboratory analysis of the nutrient content of different soil layers under different straw returning methods of albic soil, and the variance analysis and multiple comparison of the data, it can be seen in Figure 3a–c that the change trends of soil alkaline hydrolysis nitrogen, total nitrogen, and soil organic matter are relatively consistent. The content of soil alkaline hydrolysis nitrogen in all treatments shows a significant decreasing trend with the deepening of the soil layer. The vertical distribution characteristics of total nitrogen and organic matter contents are as follows: the content in the Ap layer is the highest, while the contents in the Aw layer and B layer are generally low. It is worth noting that under the shallow tillage with CT and SD treatments, the contents of total nitrogen and organic matter both show a decreasing trend with the increase in soil depth. However, in the SM and SR treatments, the contents of total nitrogen and organic matter in the B layer are slightly higher than those in the Aw layer, but the difference between the two is not significant. Compared with the shallow tillage with CT treatment, SM and SR have no significant effects on the contents of soil alkaline hydrolysis nitrogen, total nitrogen, and organic matter; while the SD treatment significantly increases the contents of these three nutrients, especially in the Aw layer. The contents of alkaline hydrolysis nitrogen, total nitrogen, and organic matter in the Aw layer are 29.98%, 58.70%, and 40.46% higher than those in the CT treatment, respectively, and the differences reach a significant level.

Changes in soil phosphorus were as follows. As shown in Figure 3e,f, the variation trends of available phosphorus and total phosphorus are basically consistent except for the SD treatment. Under the shallow tillage with CT treatment, SM and SR treatments, both total phosphorus and available phosphorus contents present a distribution trend of Ap layer > B layer > Aw layer. In the SD treatment, the distribution trend of available phosphorus is consistent with the above three treatments, while the total phosphorus content shows a decreasing trend with the deepening of soil layers, which is characterized by a significant decrease from the Ap layer to the Aw layer and then tends to be stable, there is no significant difference between the B layer and the Aw layer. Compared with the CT treatment, only the available phosphorus and total phosphorus contents in the Aw layer of the SD treatment show significant differences, with increases of 36.86% and 20.46%, respectively; there are no statistical differences in the total phosphorus and available phosphorus contents in each soil layer under other treatments.

Soil total potassium (TK) and available potassium (AK) concentrations exhibited distinct distribution patterns across tillage treatments and soil layers, with significant variations in response magnitude and layer-specific trends in Figure 3g,h. Statistical analysis revealed that tillage practices exerted a more pronounced influence on AK than TK, while both potassium fractions showed consistent response patterns to certain treatments. For TK, the SD treatment consistently outperformed other practices in the Aw layer, with a concentration 21.7% higher than the average of CT, SM, and SR treatments (p < 0.05). This specifical treatment’s enhancement was not observed in the Ap layer, without significant differences. In the B layer, SD maintained relatively higher TK levels compared to SM, while CT and SR showed intermediate values with no statistical distinction. A parallel trend was observed for AK, where the B layer consistently exhibited the highest concentrations across all treatments, with SD achieving the maximum value, representing a 1.2% increase compared to the overall mean of other treatments (p > 0.05). In the Aw layer, AK under SD was significantly elevated by 5.0% relative to the average of CT, SM, and SR, mirroring the TK response pattern in this layer. However, AK concentrations in the Ap layer showed negligible variations among treatments, consistent with the TK distribution in the same horizon.

Notably, both potassium fractions displayed similar hierarchical responses to tillage intensity: SD treatment induced the most significant enhancements in the Aw and B layers, while Ap layer concentrations remained relatively stable across all practices. The coefficient of variation (CV) for AK was substantially higher than that for TK, indicating greater sensitivity of AK to tillage perturbations. These consistent response patterns suggest that SD tillage may promote potassium retention in deeper soil layers through similar mechanisms affecting both total and available fractions.

Changes in soil pH were as follows. Laboratory analysis of soil pH across different straw return methods revealed distinct trends by soil layer. In the Ap layer, all three treatments increased pH compared to CT. However, in the Aw and B layers, pH decreased under all treatments relative to CT. Notably, no significant differences in pH were observed among treatments within any layer (Figure 3d).

3.3. Crop Yields

As shown in

Table 4. Changes in soybean yield.

Crop Types	The Name of the Process	Average (kg/ha)	Yield Increase (%)
Maize	CT 1	7528.0 Cd*	—
	SM 2	7511.1 Ba*	−0.22
	SD 3	8686.6 Dc*	15.39
	SR 4	7598.0 Ab*	0.93
Soybean	CT	1872.0 Cd*	—
	SM	1775.0 Abb*	−5.18
	SD	2245.3 Aa*	19.94
	SR	1895.6 Bc*	1.26

¹ CT: Conventional tillage. ² SM: Straw mulching. ³ SD: Straw deep subsoil incorporation. ⁴ SR: Straw burning incorporation. * Letters denote significant differences at the 5% level (lowercase) and 1% level (uppercase) among treatments, respectively.

, the SD treatment significantly increased crop yields. Compared with the CT treatment, maize yield increased by 15.39%, and soybean yield increased by 19.94%. In contrast, the other two treatments had little effect on plant yields. However, compared with the CT treatment, plant yields under the SM treatment decreased by 0.22% for maize and 5.18% for soybean.

3.4. Physical Properties of Soil Under the Action of Deep Burial and Returning of Straw Subsoil

The data of soil bulk density, macroporosity, microporosity, and total porosity are as follows.

Table 5. Changes in the physical properties of each soil layer of albic soil.

Dispose	Soil Horizon (cm)	Soil Bulk Density (g/cm3)	Macroporosity (%)	Microporosity (%)	Total Porosity (%)
CT	Ap	1.23 ± 0.03	20.44 ± 0.66	35.85 ± 1.31	56.29 ± 1.97
	Aw	1.47 ± 0.04	1.45 ± 0.73	36.54 ± 1.34	37.99 ± 2.07
	B	1.51 ± 0.04	5.57 ± 1.01	35.12 ± 1.76	40.69 ± 2.77
SM	Ap	1.31 ± 0.05	17.50 ± 1.04	34.88 ± 1.15	52.38 ± 2.19
	Aw	1.52 ± 0.05	1.46 ± 0.59	34.55 ± 1.16	36.01 ± 1.75
	B	1.51 ± 0.04	4.56 ± 0.88	36.11 ± 1.34	40.67 ± 2.22
SD	Ap	1.24 ± 0.02	21.05 ± 0.94	35.66 ± 1.56	56.71 ± 2.50
	Aw	1.38 ± 0.04	4.13 ± 0.70	40.93 ± 0.88	45.06 ± 1.58
	B	1.50 ± 0.05	6.21 ± 0.60	34.07 ± 1.31	40.28 ± 1.91
SR	Ap	1.28 ± 0.04	19.49 ± 0.93	34.86 ± 1.56	54.35 ± 2.49
	Aw	1.51 ± 0.05	1.45 ± 0.59	35.54 ± 1.68	36.99 ± 2.27
	B	1.51 ± 0.03	5.56 ± 0.40	35.13 ± 1.01	40.69 ± 1.41

CT: Conventional tillage. SM: Straw mulching. SD: Straw deep subsoil incorporation. SR: Straw burning incorporation. Ap: The black soil layer. Aw: The albic layer. B: the sedimentary layer.

presents the changes in bulk density and soil porosity of each soil layer under different treatments. Compared with the CT treatment, the total porosity ratio of the Ap and Aw soil layers after SM and SR treatments decreased slightly, which led to an increase in the bulk density of both soil layers. There was no significant difference in the B layer soil compared with the CT treatment. However, the SD treatment showed a different changing trend from the other two treatments. The Ap layer soil after SD treatment had little difference from the CT treatment, while the total porosity ratio of the Aw and B layer soils increased, resulting in a decrease in soil bulk density. Among them, the change in the Aw layer was the most obvious: its total porosity ratio increased by 3.71%, the microporosity ratio and macroporosity ratio increased by 4.39% and 2.68%, respectively, and these changes led to a 6.12% decrease in bulk density.

After two years of soil improvement treatments, the results of soil hardness measurement are shown in Figure 4a, while Figure 4b presents the profile characteristics and topsoil conditions of albic soil before treatment. As shown in Figure 4a, under the four treatments, the soil hardness in the Ap horizon all ranged between 1 and 2, with hardness in the order of SD < CT < SR < SM. With the increase in soil depth, soil hardness gradually increased, reaching the maximum in the Aw horizon. Compared with the CT treatment, the soil hardness of SR and SM treatments showed no significant difference, and their maximum hardness both exceeded three, with the soil hardness of SM treatment remaining the largest. After that, with the further increase in depth, the soil hardness of these three treatments gradually decreased, with hardness in the order of SR < CT < SM. In contrast, the soil hardness of the SD treatment decreased significantly, with a maximum hardness of approximately 2 MPa, representing a 43.56% reduction compared to the CT treatment. The soil hardness of the Aw horizon was close to that of the B horizon. Figure 4c illustrates soybean root distribution in the CT and SD treatments. The CT treated roots (left panel) were predominantly concentrated in the Ap layer, with sparse root penetration below 25 cm, where only perennial weed roots were observed. In contrast, SD treatment (right panel) promoted deeper root growth, with soybean roots extending beyond 30 cm and showing significantly increased root density in subsoil layers.

To evaluate soil permeability, the soil permeability coefficient (K) was measured after two years of soil improvement, with the results presented in

Table 6. The soil permeability coefficient after two years of soil improvement.

Dispose	The Soil Permeability Coefficient (×10−5)
	Ap	Aw
CT	8.5	0.02
SM	0.8	0.01
SD	8.9	0.23
SR	7.6	0.06

CT: Conventional tillage. SM: Straw mulching. SD: Straw deep subsoil incorporation. SR: Straw burning incorporation. Ap: The black soil layer. Aw: The albic layer.

. A comparison of soil permeability between the Ap and Aw horizons revealed that the permeability of the Aw horizon in albic soil was far inferior to that of the Ap horizon. A further analysis of the effects of different treatments showed that in the Ap horizon, the soil permeability coefficient under the SD treatment was the highest, slightly higher than those under the CT and SR treatments; while the permeability coefficient under the SM treatment was the lowest, at only 0.8 × 10⁻⁵. In the Aw horizon, the soil permeability coefficient under the SD treatment remained the highest, much higher than those under other treatments; the permeability coefficient under the SM treatment was the lowest, consistent with its performance in the Ap horizon.

To elucidate spatial moisture distribution patterns, soil water content was monitored before and after a 36 mm rainfall event (Figure 5a).

Before rainfall, the soil was in a dry state. The soil moisture content at the 10 cm depth under the four treatments followed the order of SD < CT < SR < SM. The moisture content in the Aw and B horizons were both higher than that in the Ap horizon, among which the soil moisture content under the SD treatment was the highest, while the differences among the other three treatments were insignificant. Compared with the CT treatment, the soil moisture content in the Aw and B horizons under the SD treatment increased by 3.31% and 2.59%, respectively. Observations at 48 h after rainfall showed significant differences in soil moisture distribution. The soil moisture content in the topsoil at the 2.5 cm depth followed the order of SD < SR < CT < SM. The soil saturation degree under the SM treatment was as high as 50.15%, and those under the CT and SR treatments were slightly lower than that under the SM treatment but both exceeded 45%. However, the moisture contents in the Aw horizon all dropped below 30%, indicating that obvious surface waterlogging occurred in these three treatments after rainfall. The surface saturation degree under the SD treatment decreased to 33%, and it promoted water infiltration into the deep layer, reaching a peak at the 35 cm depth in the Aw horizon. In addition, the investigation on soil submergence depth showed that the temporary submergence depth in the SD treatment area reached 70 cm, while that in the CT control area only appeared at 20 cm underground.

Soil moisture dynamics monitoring results showed that during the second growing season, daily monitoring of soil moisture dynamics under four treatments was conducted using tensiometers (Figure 5b), revealing significant differences in hydrological responses of underground drainage patterns. After rainfall, the soil pore water potential (mbar) of each treatment showed obvious differences: the SM treatment had higher pore water potential than the CT treatment at 10 cm and 20 cm depths, which was the highest among the four treatments, while the pore water potential at 30 cm depth was lower than that of the CT treatment; the SD treatment consistently had lower pore water potential than the CT treatment at 10 cm, 20 cm, and 30 cm depths; the difference in pore water potential between the SR treatment and the CT treatment was small, showing an overall fluctuating trend. Quantitative analysis indicated that compared with the CT treatment, the SD treatment reduced the number of days with pore water potential exceeding 1000 mbar by 3.5 days at 10 cm depth, 8.5 days at 20 cm depth, and 9.5 days at 30 cm depth. It is noteworthy that the surface soil of the CT treatment showed a rapid drainage response, with pore water potential dropping below 10 mbar after rainfall, indicating an oversaturated state. In contrast, the SD treatment had a moderate drainage rate, and the pore water potential at 10 cm depth remained at a relatively high level, which not only improved soil water retention capacity but also avoided waterlogging by enhancing underground drainage channels.

4. Discussion

By virtue of the principle of treating sand with clay, the new model of returning straw to the field by burying it deep in the soil breaks the soil barrier of the albic layer, effectively relieves the physical obstacle of albic soil, which is “sticky, board, thin”, and provides a loose and permeable micro environment for the root system, and helps the water to seep down. The enhancement of air permeability makes the growth of beneficial bacteria in the soil and the reduction in anaerobic harmful bacteria, which further improves the soil environment and facilitates the development of soybean plants. In addition, the process of straw decomposition inputs a large amount of organic carbon and nitrogen, phosphorus, and potassium nutrients into the soil, which can simultaneously improve the soil carbon and nitrogen cycle and nutrient supply capacity. The optimization of soil physical structure and the enhancement of nutrient effectiveness significantly promoted the morphology and physiological function of the soybean root system. The depth of rooting of the plant increased, and the ability to capture water and nutrients from the deeper layers was enhanced, which strengthened the mutual interaction between the root system and the soil. Ultimately, the synergistic optimization of soil and root system translates into significant yield gains. The following section provides a specific discussion of the effects of the new straw heart soil return model on albic soils.

4.1. Effects of Different Straw Returning Methods on Enzyme Activity

Soil enzyme activity is a critical indicator of soil quality and fertility [29,30], playing a central role in soil ecosystem processes such as nutrient cycling and energy flow. This enzyme activity also serves as a sensitive early-warning indicator for soil quality changes [31,32]. Catalase, a critical component of soil enzymatic systems, facilitates the decomposition of harmful peroxides that pose toxicity to organisms, effectively preventing oxidative damage during biological metabolic processes [33,34]. Comparative analysis revealed that catalase activity peaked in the Aw horizon under CT treatment, likely attributable to the multiple stress factors inherent in albic soils’ Aw horizon that necessitate elevated enzymatic detoxification. The SD treatment induced the most pronounced impact on Aw horizon catalase activity, reducing it by 13.51%. This reduction may stem from SD’s mechanism of alleviating peroxide toxicity through deep layer straw amendment, thereby diminishing the demand for catalytic decomposition. Conversely, both SM and SR treatments enhanced catalase activity in the Ap horizon compared to CT. This phenomenon potentially arises from exogenous organic matter inputs generating additional reactive oxygen species, thereby increasing the enzymatic requirement for peroxide decomposition in surface soil layers.

Soil urease activity serves as a vital indicator of soil biological activity and fertility status. This enzyme catalyzes the hydrolysis of amide organic nitrogen compounds into inorganic nitrogen forms directly assimilable by plants, thereby reflecting soil nitrogen supply capacity [35]. Except for the SD treatment, all other treatments exhibited progressive urease activity decline with soil depth in albic soils. This vertical trend may relate to long-term fertilization practices that maintain higher nitrogen levels in surface horizons, sustaining elevated enzymatic activity, while deeper layers show reduced nitrogen availability and correspondingly lower urease activity. Notably, SM and SR treatments significantly enhanced urease activity in the Ap horizon. This stimulation likely stems from increased nitrogen supply through surface organic amendments, requiring greater enzymatic hydrolysis capacity. In contrast, the SD treatment homogenized urease activity between Aw and B horizons. This phenomenon may result from mechanical mixing of contrasting soil layers during deep tillage, effectively equalizing enzymatic activities across previously distinct layers through integrated organic matter redistribution.

Sucrase, ubiquitously distributed in soils, plays a critical role in carbohydrate transformation and generation of assimilable decomposition products for plants and microorganisms, serving as a key indicator of soil carbon cycling and biochemical activity [36]. Except for the SD treatment, all other treatments exhibited progressive sucrase activity decline with soil depth, consistent with Zhang’s findings [37]. The reason for this is that the top layer of the soil aggregates more secretions of plants, animals, microorganisms, etc., which release enzymes and their organic residues, and the species and number of soil animals, plants, and microorganisms increase, and the physiological activity increases resulting in the release of more enzymes [38], leading to higher enzyme activity in the top layer of the soil than in the lower layers. Notably, SM and SR treatments significantly elevated sucrase activity in the Ap horizon compared to CT. This stimulation likely stems from increased carbon substrate availability through surface organic amendments, stimulating microbial enzymatic production [39]. In contrast, SD treatment homogenized sucrase activity between Aw and B horizons. This phenomenon may result from mechanical mixing of contrasting soil layers during deep tillage, effectively redistributing organic substrates and equalizing enzymatic activities across previously distinct layers.

4.2. Effects of Different Straw Returning Methods on Soil Organic Matter and Nutrients

Results analysis shows that conventional straw returning methods have little impact on alkaline hydrolysis nitrogen, total nitrogen, and organic matter in albic soil, while only the combined treatment of deep straw incorporation and subsoil mixing can effectively improve albic soil. Under the SD treatment, the contents of alkaline hydrolysis nitrogen, total nitrogen, and organic matter in the Aw horizon are 29.98%, 58.70%, and 40.46% higher than those under the CT treatment, respectively. The reason may be that deep straw incorporation can better promote straw degradation [40], thereby facilitating the proliferation of ammonifying bacteria and nitrifying bacteria, which convert urea and amino acids into ammonium and nitrate substances through microbial metabolic pathways [41]. In addition, straw amendment can increase the soil C:N ratio, alleviate microbial carbon limitation, and activate urease and protease activities to accelerate nitrogen release [39].

In the present study, under CT, SM, and SR treatments, the contents of total phosphorus (TP) and available phosphorus (AP) showed a distribution trend of Ap horizon > B horizon > Aw horizon. This characteristic can be attributed to the fact that these treatments only disturb the surface soil, and phosphorus input from SM or SR treatments is mainly retained in the surface layer. In contrast, the deep soil has significantly lower phosphorus contents due to the lack of exogenous phosphorus input and physical disturbance. This is consistent with the research conclusion on the “surface aggregation effect” of phosphorus in most agricultural ecosystems [42]. The results showed that the contents of AP and TP in the Aw horizon under the SD treatment changed significantly, being 36.86% and 20.46% higher than those in the CT treatment, respectively. However, no significant changes were observed in other horizons compared with the CT treatment. This result suggests that the new subsoil improvement model mentioned in this study can better regulate phosphorus distribution. On the one hand, after deep straw incorporation, the effective decomposition of straw enables phosphorus-containing organic matter to release inorganic phosphorus through microbial mineralization [43], directly increasing the total phosphorus pool in this horizon. Additionally, deep straw incorporation promotes microbial-mediated phosphorus cycling, thereby increasing the proportion of available phosphorus [44]. On the other hand, subsoil mixing may break the dense structure of the Aw horizon in albic soil, improve soil porosity, and reduce the risk of adsorption and fixation during phosphorus migration, allowing part of the phosphorus to migrate to the Aw horizon while maintaining its availability [45]. Since the SD treatment primarily targets the Aw horizon with limited effects on other horizons, it did not alter the phosphorus distribution pattern in other horizons. In contrast, the other three treatments failed to effectively break the physical barrier of deep soil to provide sufficient phosphorus input to the deep layer, thus showing poor improvement effects on deep soil.

Analysis of potassium revealed that the contents of total potassium and available potassium showed little difference between the Ap horizon and B horizon across all four treatments. This could be attributed to the fact that the Ap horizon, as the primary zone of crop root activity and residue input, undergoes rapid potassium turnover, resulting in negligible differences in potassium pools. Notably, the B horizon exhibited the highest AK content across all treatments, which may be related to its soil properties. The B horizon is rich in clay particles, which possess high cation exchange capacity and a strong ability to retain available potassium, thereby maintaining high AK levels [46]. The slightly increased potassium content in the B horizon under the SD treatment might be explained by the improved structure of albic soil induced by subsoil mixing tillage. This improvement enhanced soil connectivity, facilitating the downward migration of K. The SD treatment showed advantages in increasing TK and AK in the Aw horizon. One underlying reason is that straw, which is rich in soluble and exchangeable K, undergoes effective decomposition after deep burial and return to the soil, providing a continuous source of K. Additionally, straw-derived organic acids can accelerate the weathering of potassium-containing minerals, thereby promoting the long-term accumulation of TK in the Aw horizon. Furthermore, subsoil mixing tillage breaks the physical barrier of albic soil, increases soil porosity, enhances potassium diffusion, reduces potassium fixation, and maintains higher potassium availability [47].

The comparative analysis revealed that all three straw incorporation treatments elevated Ap horizon soil pH relative to conventional tillage, while depressing pH values in Aw and B horizons. This differential pH modulation demonstrates straw amendment’s capacity to regulate soil acidity through depth specific mechanisms. Surface treatments likely increased Ap horizon pH through organic matter accumulation and microbial nitrification processes, whereas deep tillage induced subsoil acidification via organic acid exudation during straw decomposition in anaerobic environments. These findings align with soil acid base equilibrium principles, where surface organic amendments enhance pH buffering capacity while subsurface biomass decomposition mobilizes aluminum/calcium cations that depress pH in deeper layers.

4.3. Effects of Different Straw Returning Methods on Yield

Yield comparisons across treatments showed that the SD treatment significantly increased yields, with maize and soybean yields increasing by 15.39% and 19.94%, respectively. This yield advantage may stem from multiple mechanisms: deep straw incorporation improves soil water-holding capacity, while subsoil mixing plowing enhances the physical state of albic soil by increasing porosity, thereby promoting crop root absorption of water and oxygen. In contrast, the SM treatment exhibited yield decreases, likely due to transient carbon and nitrogen imbalances caused by decomposing straw residues, which temporarily restricted plant nutrient uptake and delayed seedling emergence. During soybean planting, the returned straw was the previous year’s maize straw; its larger quantity more significantly impacted soybean yield. In contrast, the soybean straw returned during maize planting was lower in quantity, resulting in minimal yield difference for maize compared to the CT treatment. Results from the SR treatment indicated that straw burning and returning had limited effects on improving albic soil properties. The above analysis shows that the SD treatment has the best improvement effect on albic soil and can promote the increase in soil fertility [15]. The significant difference between the SD treatment and the other treatments can indicate that the new model is more suitable for the improvement of albic soil fertility.

4.4. Effect of the New Mode of Deep Burial and Returning of Straw Subsoil to the Field on Soil Physical Properties

Albic soil exhibits degraded structural properties characterized by poor physical condition, dual layer mechanical composition, elevated chalk content, restricted expansion of the Aw horizon, and insufficient resilience. These deficiencies manifest as soil crusting, compaction, reduced permeability, and restricted root penetration. Furthermore, impaired capillary rise during drought and poor drainage during heavy rainfall exacerbate surface hydrological extremes, alternating drought stress and waterlogging, which degrade soil fertility and crop productivity. The SD treatment addresses these limitations through subsurface integration of the Aw and B horizons while preserving the Ap horizon. This mechanical restructuring transforms the native two-layered profile into three functional layers: a plow layer, mixing layer, and precipitation layer. Notably, the mixing layer demonstrates reduced bulk density and hardness, facilitating root system development. Field trials reveal sustained improvements over two years, confirming persistent Aw horizon transformation and long-term soil rehabilitation. Mechanistically, SD treatment breaks up the impermeable Aw horizon barrier, enhancing pore space distribution by elevating the microporosity. This modification improves the total porosity, significantly boosting saturated hydraulic conductivity and moisture retention capacity. Such structural optimization mitigates both drought susceptibility and waterlogging risks in albic soils.

4.5. The Influencing Mechanism of the New Model on Crop Yield

The reduction in soil hardness significantly enhances plant root penetration and nutrient uptake [48]. By promoting deeper root growth and improving nutrient accessibility, softer soils facilitate efficient nutrient absorption while accelerating soil chemical reactions, thereby increasing nutrient bioavailability and optimizing soil pH. Enhanced water retention capacity due to improved soil structure effectively preserves moisture and water-soluble nutrients, elevating plant nutrient uptake efficiency. Furthermore, decreased bulk density reflects increased porosity, which amplifies ion exchange capacity and strengthens the soil’s nutrient retaining ability. Elevated porosity optimizes water storage dynamics, promotes nutrient dissolution, and elevates N, P, and K concentrations in the soil, thereby stabilizing soil chemistry [49]. Improved physical properties also foster organic matter accumulation and microbial activation, further boosting organic carbon sequestration. The interplay between enhanced physical properties and organic matter turnover strengthens nutrient cycling, significantly increasing N, P, and K availability. Nitrogen, a critical limiting nutrient for protein and chlorophyll synthesis [50], and phosphorus, essential for cell division and root development, synergize with potassium, which enhances drought resistance and water-use efficiency, to collectively improve soybean yield quality and quantity.

5. Conclusions

This study developed an integrated technology combining straw deep burial and soil mixing ploughing, which synergistically ameliorates both physical barriers and chemical infertility in albic soils.

Key outcomes show that this technology regulates soil properties effectively: in the Aw layer, it reduces catalase activity by 13.51% (alleviating oxidative stress for root growth) and increases soil organic matter and nutrients (e.g., alkaline hydrolysis nitrogen by 29.98%, organic matter by 40.46%). It also improves physical structure long-term, lowering soil hardness by 43.56%, enhancing permeability and water holding capacity, and forming stable, anti-subsidence pore structures (avoiding rain-induced compaction seen in traditional methods). Agronomically, it boosts maize yield by 15.39% and soybean yield by 19.94%, with root penetration depth extending from 20 cm to 45 cm. Notably, this single-operation technology cuts costs by 60% compared to traditional subsoiling, preserves the black soil layer, and maintains effectiveness for over five years.

In summary, this integrated approach, coupling physical restructuring with chemical activation, offers a practical solution to long-standing challenges in albic soil management. It provides empirical and theoretical support for sustainable agriculture, with scalable potential for soil rehabilitation across diverse agroecosystems.

The current study focuses on poor upland albic soils. Going forward, we will expand the research scope to systematically test the soil improvement capacity of this technology across various subclasses of albic soils, thereby avoiding the limitations of a single experimental condition. Meanwhile, more advanced sampling and measurement methods will be adopted to enhance the accuracy of results. Additionally, we plan to extend the monitoring period and add more supporting experiments (such as the determination of straw decomposition curves). By accumulating long-term data, we aim to deeply reveal the regular effects of this technology on soil, providing a more solid scientific basis for its widespread application.