Pelletized Straw Incorporation in Sandy Soil Increases Soil Aggregate Stability, Soil Carbon, and Nitrogen Stocks

Abstract

In China, increasing the quantity and quality of total carbon and nitrogen stocks in sandy soil used for crop production is an important research issue. Soil amendment with pelletized straw could improve both soil physical structure and fertility in sandy soils, but these aspects remain understudied. The present pot and field experiments examined the dynamic changes in sandy soil water holding capacity, soil bulk density, soil total carbon and nitrogen stocks, and the distribution of water-stable aggregates and soil total carbon stocks related to aggregates across the following treatments: no fertilization (i.e., study control (CK)), normal fertilizer rate (NM), soil amendment at 150 Mg ha⁻¹ (S150), manure amendment at 150 Mg ha⁻¹ (M150), pelletized straw amendment at 75 Mg ha⁻¹ (PS75), and pelletized straw amendment at 150 Mg ha⁻¹ (PS150). The results show that the pelletized straw incorporation significantly increased water holding capacity and decreased soil bulk density. PS150 notably increased the large macroaggregates (>2000 μm) proportion and decreased the ratio of <250 μm aggregate size fractions in comparison with CK, NM, S150, and M150 at 0–20 and 20–40 cm soil depths. Compared with the CK treatment, the bulk soil carbon and nitrogen stocks in the 0–20 cm layers under the PS150 treatment were significantly increased by 85.2% and 302.9%, and in the 20–40 cm layers those increased by 136.4% and 257.1%, respectively. The PS150 treatment resulted in higher soil organic carbon (SOC) and particulate organic carbon content than the CK and PS75 treatments, whereas the PS75 treatment achieved maximum soil inorganic carbon content. The pelletized straw treatment increased the large macroaggregate-associated soil total carbon content at 0–20 and 20–40 cm soil depths. The maximum soil total carbon stocks were in the small macroaggregates (250 < WSA < 2000 μm) rather than in the large macroaggregate and microaggregates under the PS75 and PS150 treatments. Additionally, the pelletized straw and manure amendments increased the yield of silage corn, which was dependent on the increase in soil total carbon and nitrogen content in the macroaggregates, whereas the soil and manure amendments did not facilitate sandy soil aggregation and soil total carbon stock increases. In conclusion, PS150 was found to be the optimal amendment for maintaining sandy soil profile physico-chemical properties through macroaggregate stabilization. These results will be beneficial for arid and semi-arid regions, thus contributing to soil carbon and nitrogen conservation.

1. Introduction

Soil plays a critical role in sequestrating atmospheric carbon dioxide (CO₂) and emitting greenhouse gases. The total carbon (C) stock of soil is at least three times the amount of carbon in the atmospheric and biotic pools [1,2]. Numerous studies indicated that soil organic carbon (SOC) is mainly affected by climatic conditions, tillage, fertilization, experimental duration, soil type, C inputs, and cropping systems [3,4,5]. SOC promotes soil aggregation and improves soil physical stability [6,7]. Increasing the stability of soil aggregates can promote SOC by protecting C from mineralization [8,9].

Some studies show that the application of animal manure to agricultural lands improves crop productivity and increases SOC [10]. However, in other studies, the effect on SOC is not significant after animal manure is applied [11]. Straw incorporation to soil can facilitate soil nutrition and enhance the capacity of SOC immobilization, thereby causing accumulation SOC [12,13]. For instance, it was reported that long-term straw incorporation was capable of increasing SOC content as compared to no straw addition [14]. However, existing research suggested that straw incorporation enhanced SOC content only in the first 3 years [15]. Possible reasons for these different results could be the decomposition rate of conventional crushed straw, straw incorporation amounts, the experimental duration, as well as the role of soil type [16].

Previous studies showed that straw incorporation is capable of increasing SOC in macroaggregates [17]. SOC in macroaggregates is considered an important cementitious material that can facilitate the formation of microaggregates into macroaggregates [7,8]. Macroaggregates are considered to be the foundation for maintaining soil structure stability, affecting soil C and nitrogen (N) retention and water holding capacity [18,19]. In addition, macroaggregates are mainly composed of instantaneous and temporary organic binders, which have a higher C to N ratio and are more conducive to SOC sequestration [20,21].

Long-term application of manure and soil amendments to soils can increase and sustain soil total C (TC) and soil total N (TN) levels, thus increasing soil aggregate stability [10,22]. However, many farmers do not implement such measures, as they are perceived as having longer degradation times and higher operation costs [23,24,25]. Before straw incorporation, it is crushed, extruded, and granulated to produce pelletized straw, which can then be used as a soil amendment in agriculture [26]. Compared with conventional crushed straw, pelletized straw can break the original organizational structure of the straw, destroy the cuticle on the outer surface of the straw, and thus increase the contact surface between the refractory substances and the soil [27]. Pelletized straw can shorten the length of straw, improve the amount of straw absorbed by the soil, accelerate the decomposition of straw by increasing microbial biomass [28], and also protect some fine straw by soil aggregates to reduce C loss [29]. Existing research suggested that use of pelletized straw is capable of increasing SOC as compared with the direct incorporation of conventional straw [26,27,28,29]. Previously, conventional straw incorporation showed that a higher rate of straw incorporation also consumes more soil-available nitrogen, which results in lower soil-available N used for crop growth and finally reduces crop yield [30]. Accordingly, the effects of pelletized straw and conventional organic fertilizer (manure and soil taken from another location termed “foreign” soil herein) on crop growth and yield are investigated in this study.

Existing research on pelletized straw achieved good results with respect to soil fertility in the North and Northeast Plains [31,32]. Sandy soils are widely considered to have poor soil structure, low water holding capacity, and poor soil nutrient status, which affect the quality and yield of crops [33,34], and the effect of soil amendment in these soils is understudied.

Therefore, the present one-year pot and field experiments examined how soil aggregate stability, soil TC stocks, and soil TN stocks in a sandy soil were affected after pelletized straw amendment and how this change occurred across different aggregate sizes.

2. Materials and Methods

2.1. Pot Experiment Design and Sample Collection

The soil required for the pot experiment was collected from 0 to 20 cm soil of Horqin newly reclaimed sandy land in September 2021 at the Horqin Sandy Land Reserve Project Area of Balaqirude Sumu, Alhorqin Banner, Chifeng City, Inner Mongolia (43°35′ N, 120°6′ E). The sandy soil had 6.5% clay, 2.4% silt, and 91.0% sand, with SOC of 1.7 g kg⁻¹. In the field, plant roots were removed from the soil and the soil was passed through a 5 mm sieve. In the laboratory, each soil sample was dried and passed through a 2 mm sieve before use.

The pelletized straw application amount was proportional to the weight of sandy soil as follows: 0, 2%, and 4%, respectively; 0%, 2%, and 4% pelletized straw corresponded to CK, PS75, and PS150, respectively, in the field experiment. Each treatment had three replicates.

The pot experiment was conducted in September 2021 in a greenhouse at 25 ± 2 °C. The pelletized corn straw used for the experiment was produced with a diameter of nearly 9 mm and a density of nearly 1.14 g cm⁻³. Pelletized straw was mixed evenly with the sandy soil and the mixed soil was put into plastic pots with holes at the bottom (45 cm height and 25 cm diameter). The bottoms of the plastic pots were covered with perforated plastic film. Subsequently, plastic gauze was laid on the bottom of the pots. Two days later, 3 kg of water was placed in all pots, the irrigation process simulated field drip irrigation by slowly dropping water into the pots; after that, 3 kg of water was poured every 30 days, and no additional watering treatment was conducted for the rest of the time. All pot surfaces were covered with plastic film with holes, which simulated plastic film covering in the field. The silage corn was planted on 6th June 2022, and the sowing depth was 3–4 cm. After 270 days of incubation, the soil samples were taken from the respective pot to determine SOC, TC, TN, and particulate organic carbon (POC); after soil incubation for 30, 60, and 360 days, water holding capacity (WC) and soil bulk density (BD) were determined after sampling; the silage corn was harvested on 11th October 2022. The silage corn variety of the pot experiment was consistent with that in the field. Soil aggregate composition was determined after the silage corn harvest of the day was completed.

2.2. Field Experiment Design and Sample Collection

A field experiment was performed from November 2021 to October 2022 at the Horqin Sandy Land Reserve Project Area of Balaqirude Sumu, Alhorqin Banner, Chifeng City, Inner Mongolia (43°35′ N, 120°6′ E). According to USDA’s soil classification system, the soil was classified as entisols [31,35]. This experiment had a randomized block design with three replications. The area of each plot was 54 m² (6 × 9 m), and a total of 18 plots were used. The experiment was composed of six treatments: no fertilizer (i.e., control, CK), returning normal (i.e., chemical) fertilizer (NF), returning manure at a rate of 150 Mg ha⁻¹ (M150), and returning another soil at a rate of 150 Mg ha⁻¹ (S150), returning pelletized straw at a rate of 75 Mg ha⁻¹ (PS75), and returning pelletized straw at a rate of 150 Mg ha⁻¹ (PS150). Manure, soil, and pelletized straw were applied after the previous maize grain was harvested in November 2021. The TC and TN content of the soil, manure, and pelletized straw were 1.14%, 0.064%; 12.16%, 0.67%; and 28.00%, 0.58%, respectively. The reason for 150 Mg ha⁻¹ of manure and soil incorporation was that the fertility of the local sandy soil and corresponding crop yields were extremely low, and therefore, farmers and local governments usually applied a high dosage of manure and soil to quickly improve sandy soil fertility without causing yield reductions. In general, the local conventional straw incorporation rate is nearly 3.75 Mg ha⁻¹, whereas 75 Mg ha⁻¹ and 150 Mg ha⁻¹ of a high dosage of pelletized straw return were adopted in this study to rapidly improve soil fertility. Chemical fertilizers were applied as urea sulfate compound fertilizer (N:P₂O5:K₂O = 13:23:12), and the same fertilization rates (30 kg ha⁻¹ of urea sulfate compound fertilizer) were adopted for all treatments; except for the CK treatment in accordance with local agronomic management practices before maize sowing in June 2022, all treatments were manually ploughed into the 0–40 cm soil layer. Moreover, the silage corn was planted in July 2022 and ended on 11 October 2022 after silage corn harvesting was completed. The silage maize variety of Aoyu 3804 was adopted in the experiments, and the silage corn was sown at 83,300 seeds ha⁻¹. Furthermore, other field cultivation and management measures were consistent with local traditional management practices.

The yield of silage corn this year was lower than that in previous years under the effect of climate factors (e.g., low temperatures, waterlogging, and other factors), and soil samples were collected from the respective plot at depths of 0–20 cm and 20–40 cm. The samples were collected from three points per plot and composited. The soil sample was collected on 11 October 2022 and placed onto a 5 mm-aperture sieve in the field, with the passing soil sampled.

The basic soil properties of the experimental field (0–20 cm) comprised the TC content of 4.50 g kg⁻¹, the TN content of 0.17 g kg⁻¹, the soil total phosphorus content of 0.09 g kg⁻¹, the soil total potassium content of 1.43 g kg⁻¹, as well as the BD of 1.62 g/cm³. The initial soil texture included 93.8% sand, 2.4% silt, and 3.7% clay. The annual precipitation of the whole Horqin sandy region was 271.6 mm. The annual average temperature was about 6.8 °C (Figure 1), and annual total evaporation was 1950–2000 mm. The frost-free period was 95–140 d, the accumulated temperature of ≥10 °C was over 3100 °C, and the annual sunshine hours were 2854 h.

2.3. Soil Samples and Crop Analysis

The BD and WC were examined using an aluminum cutting ring (5 cm in height and 5 cm in diameter) [29]. Size grades in the aggregate samples were separated through wet sieving [36]. The samples were wet sieved using a series of three sieves to determine four aggregate size fractions. The above aggregates were classified as large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm), and silt and clay (<53 μm), respectively. Particulate organic carbon (POC) was determined using the method presented by Cambardella and Elliott (1992) [37]. SOC was examined using the dichromate oxidation method [38]. TC and TN content in the bulk soil and those in aggregates were examined using the CNHS-O analyzer (EA3000, Milan, Italy). All the aboveground silage maize in the respective plot of the field and pot experiments was cut after the silage corn was harvested. The silage corn obtained from the respective plot was blanched at 105 °C for 30 min in an oven, the dry matter straw and the silage corn yield were calculated using oven drying subsamples at 70 °C for 48 h, and the dry yield of silage corn was calculated after weighing.

2.4. Calculation

The mean weight diameter (MWD) of the aggregates was obtained by Equation (1) [39]:

$$\mathrm{MWD}={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{X}}_{\mathrm{i}}\times {\mathrm{W}}_{\mathrm{i}}$$

(1)

where MWD represents the mean weight diameter of the aggregates (mm), X_i denotes the weight fraction (proportion) of the respective aggregate size fraction with coarse particle correction; W_i denotes the mean diameter of each size fraction.

Equation (2) expresses the geometric mean diameter (GMD) of the aggregates [39]:

$$\mathrm{GWD}=\exp \frac{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}\left({\mathrm{X}}_{\mathrm{i}}\ln {\mathrm{W}}_{\mathrm{i}}\right)}{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{X}}_{\mathrm{i}}}$$

(2)

Equation (3) was used to calculate the TC (M_TC) or TN (M_TN) stocks in bulk soil (g m⁻²) [40]:

$${\mathrm{M}}_{\mathrm{TC}}\left({\mathrm{or}\mathrm{M}}_{\mathrm{TN}}\right)=\mathrm{TC}\left(\mathrm{or}\mathrm{TN}\right)\times \mathrm{BD}\times \mathrm{D}\times 10$$

(3)

where M_TC (or M_TN) is the bulk soil TC (or TN) stocks in g m⁻²; TC (or TN), BD, and D represents the content (g kg⁻¹) of TC (or TN), soil bulk density (g cm⁻³), and the thickness (cm) of the soil layer.

Equation (4) was used to calculate the TC (M_TC) or TN (M_TN) stocks in the respective aggregate size fraction (g m⁻²):

$${\mathrm{M}}_{\mathrm{TC}}\left({\mathrm{or}\mathrm{M}}_{\mathrm{TN}}\right)={\mathrm{TC}}_{\mathrm{i}}{\mathrm{or}\mathrm{TN}}_{\mathrm{i}}\times \mathrm{BD}\times \mathrm{Wi}\times \mathrm{D}\times 0.1$$

(4)

where M_TC (or M_TN) denotes the TC (or TN) stocks of the fraction with its size grade (g m⁻² aggregate); TC_i (or TN_i) denotes the content of TC (or TN) of the fraction with its size grade (g kg⁻¹ aggregate), and Wi refers to the weight proportion of the total soil in this fraction (%), BD and D represent soil bulk density (g cm⁻³) and the thickness (cm) of the soil layer.

2.5. Statistical Analyses

The Pearson correlation coefficient was performed using SPSS version 21.0 (IBM SPSS Software Inc., Armonk, NY, USA). Significant treatment effects were deemed to occur where the probability (p) was < 0.05 for a greater F-statistic. Analysis of variance (ANOVA) was tested with Tukey’s honest significant difference (HSD) test, which was conducted for separation of the means at the 95% confidence level. Linear regression analysis was performed using Excel 2019. All relevant output plots were generated using Origin 9.1. The data were analyzed using Excel 2019 and SPASS 21.0 software.

3. Results

3.1. Influence on Water Holding Capacity of Sandy Soil

In the pot experiment (Figure 1A), incorporating pelletized straw into soil significantly increased the WC of the sandy soil. At the early stage of cultivation (30th day), the WC of PS75 and PS150 treatments were significantly higher than that of the CK treatment, which were 105.8% and 181.2% higher, respectively, with significant difference (Figure 1A, p < 0.05); at the middle stage of cultivation (60th day), compared with CK, the WC of PS75 and PS150 treatments increased by 56.7% and 128.7%, respectively, with significant differences (Figure 1 A, p < 0.05); at the late stage of cultivation (360th day), compared with CK, the WC of PS75 and PS150 treatments increased by 104.9% and 111.70%, respectively, with significant differences (Figure 1A, p < 0.05). In a field experiment (Figure 1B), at the 0–20 cm soil layer, PS150 and M150 significantly increased the WC of sandy soil by 262.8% and 275.3% compared with CK; however, in the 20–40 cm soil layer, the differences between the treatments were not significant.

3.2. The Distribution of Water-Stable Aggregates in Sandy Soil

In the pot experiment, when compared with the study control, PS75 increased in large macroaggregate (>2000 μm) proportions and decreased the microaggregates’ rate (<250 μm) (

Table 1. Effect of pelletized straw incorporation on soil physico-chemical properties in the pot experiment. Different lowercase letters indicate significant differences between different treatments (p < 0.05). Error bars show the standard error of the means (n = 3).

Variable	Treatment
	CK	PS75	PS150
BD (g cm−3)	1.66 ± 0.02 a	1.61 ± 0.01 b	1.55 ± 0.01 c
MWD (mm)	0.61 ± 0.01 b	0.71 ± 0.01 a	0.63 ± 0.06 b
GMD (mm)	0.38 ± 0.01 b	0.44 ± 0.01 a	0.39 ± 0.03 b
TC (g kg−1)	3.35 ± 1.17 b	6.96 ± 0.83 a	6.66 ± 1.16 a
TN (g kg−1)	0.16 ± 0.04 b	0.38 ± 0.11 ab	0.25 ± 0.11 a
SOC (g kg−1)	2.47 ± 0.86 b	3.82 ± 1.31 ab	5.15 ± 0.63 a
SIC (g kg−1)	0.88 ± 0.38 b	3.14 ± 0.42 a	1.51 ± 0.27 a
POC (g kg−1)	0.52 ± 0.16 b	1.20 ± 0.21 ab	2.43 ± 1.41 a
Large aggregates proportion (%)	0.27 ± 0.24 b	2.09 ± 1.21 a	0.28 ± 0.07 b
Small aggregates proportion (%)	46.85 ± 4.98 a	50.90 ± 3.74 a	48.56 ± 5.45 a
Microaggregates proportion (%)	50.11 ± 0.39 a	44.44 ± 3.20 b	47.91 ± 1.31 ab
Silt clay size class proportion (%)	1.99 ± 0.31 a	1.83 ± 0.42 a	2.55 ± 0.74 a

). In the field experiment (Figure 2), the proportion of large aggregates in PS75 and PS150 in the 0–20 cm soil layer increased by 384.8% and 886.6% in comparison with CK, respectively. The proportion of microaggregates and silt clay-sized fractions in the PS75 and PS150 was lower than in the CK, NM, S150, and M150 treatments (Figure 2). In the 20–40 cm soil layer, the proportion of large macroaggregates and small macroaggregates in the PS150 was notably increased in comparison with CK, NM, S150, M150, and PS75, the proportion of microaggregates and silt clay size fractions in the PS75 and PS150 was notably lower than in the CK, NM, S150, and M150 (Figure 2). In contrast, no significant difference was identified in the proportion of large macroaggregates among CK, NM, S150, and M150 at either soil depth; M150 notably increased the proportion of silt clay-sized fractions at 0–20 cm, whereas it decreased the proportion of small macroaggregates at 20–40 cm.

3.3. Soil Bulk Density and Aggregate Stability

Incorporation of pelletized straw to soils decreased BD (Table 1, Figure 3, p < 0.05). In the pot experiment, the BD of PS75 and PS150 treatments were 3.0% and 7.0% lower than that of CK (p < 0.05). In the field experiment, at 0–20 cm, BD in the PS150 was decreased by 51.2% in comparison with CK (p < 0.05). In the 20–40 cm soil layer, BD in the PS150 was decreased by 20.2% in comparison with CK (Figure 3). In contrast, the BD of CK, NM, S150, and M150 at the depth of 0 to 20 cm and 20 to 40 cm had no significant difference at any soil depth.

In the field experiment, PS75 and PS150 notably increased the MWD and GMD more than CK, NM, S150, and M150 at both soil depths (Figure 3). In the pot experiment, compared with CK, the MWD and GMD of PS75 and PS150 increased by 16.3%, 3.2% and 15.7%, 2.6%, respectively (Table 1).

3.4. Bulk Soil TC and TN Stocks

The pelletized straw incorporation increased TC and TN content, in the pot experiment, SOC content in the PS150 was increased by 108.5% in comparison with CK, and 34.8% in comparison with PS75. POC content in the PS150 was increased by 367.3% in comparison with CK, 102.5% in comparison with PS75. PS75 notably increased the SIC content in comparison with CK and PS150 treatments (Table 1, p < 0.05).

In the field experiment, at 0–20 cm, bulk soil TC stocks in the PS75 and PS150 increased by 200.6% and 82.2% in comparison with CK, respectively (

Table 2. Effect of different treatments on the bulk soil TC and TN stocks in 0–20 cm depth and 20–40 cm depth of the field soil. Different lowercase letters indicate significant differences between different treatments within each depth (p < 0.05). Error bars show the standard error of the means (n = 3).

	TC Stocks (g m−2)		TN Stocks (g m−2)
Treatment	0–20	20–40	0–20	20–40
CK	1988.00 ± 524.03 bc	897.32 ± 764.98 bc	58.01 ± 4.89 cd	49.70 ± 27.73 d
NM	811.67 ± 464.95 c	623.37 ± 171.49 c	31.10 ± 7.60 d	31.77 ± 14.22 d
S150	1390.38 ± 290.77 bc	1513.38 ± 366.70 ab	97.12 ± 31.70 c	104.00 ± 7.52 bc
M150	1682.86 ± 28.92 bc	358.09 ± 42.41 c	154.11 ± 25.15 b	62.40 ± 6.30 cd
PS75	5976.32 ± 3313.86 a	1897.52 ± 376.81 a	206.71 ± 53.64 a	114.16 ± 26.76 b
PS150	3682.83 ± 619.21 ab	2121.76 ± 231.60 a	233.77 ± 11.49 a	177.48 ± 38.83 a

). While at 20–40 cm, bulk soil TC stocks in the PS75 and PS150 increased by 111.4% and 136.4% in comparison with CK, respectively. At 0–20 cm, bulk soil TN stocks in the PS75 and PS150 increased by 256.3% and 302.9% in comparison with CK, respectively (Table 2). Furthermore, at 20–40 cm, bulk soil TN stocks in the PS75 and PS150 increased by 129.70% and 257.10% in comparison with CK, respectively (Table 2).

3.5. Aggregate C Content and Stocks

At 0–20 cm, PS75 and PS150 had higher aggregate-associated TC content than CK, NM, and S150. M150 had higher large macroaggregates, small macroaggregates associated TC content than CK, NM, and S150 (Figure 4, p < 0.05). At a depth of 20–40 cm, PS150 had higher all aggregate-associated TC content than CK, NM, and S150, and PS75 had higher large macroaggregates, microaggregate-associated TC content than CK, and NM (Figure 4, p < 0.05).

At a depth of 0–20 cm, PS75 and PS150 increased large macroaggregates, small macroaggregate-associated TC stocks, and M150 increased small macroaggregate-associated TC stocks (Figure 4). At 20–40 cm, PS75 increased small macroaggregates, microaggregate TC stocks and PS150 increased large macroaggregates, small macroaggregates, and microaggregates TC stocks (Figure 4). In general, PS75 and PS150 had the highest TC stocks in the bulk soil, primarily attributed to the increase in TC stocks related to large and small macroaggregates.

The following table presents the correlation coefficients between the bulk soil TC stocks or TN stocks and the TC stocks or TN stocks of different aggregate sizes at 0–20 and 20–40 cm soil depths (

Table 3. Relationships between TC or TN stocks in bulk soil and those found in the different aggregate size fractions in the field experiment (n = 18).

	Large Macroaggregates		Small Macroaggregates		Microaggregates		Silt Clay Particles
	TC Stocks	TN Stocks	TC Stocks	TN Stocks	TC Stocks	TN Stocks	TC Stocks	TN Stocks
TC stocks (0–20 cm)	0.704 *		0.518 *		0.422		−0.126
TN stocks (0–20 cm)		0.737 *		0.713 *		0.263		−0.113
TC stocks (20–40 cm)	0.589 *		0.698 *		0.712 *		−0.024
TN stocks (20–40 cm)		0.686 *		0.865 *		0.301		−0.083

Values in the table are correlation coefficients and values with (*) are significant (p < 0.05).

). TC stocks and TN stocks related to bulk soil were significantly positively correlated with TC stocks and TN stocks in >250 μm size aggregates (p < 0.05).

Linear regression analysis (Figure 5) showed that the content of TC in soil was significantly positively correlated with MWD, and the pelletized straw incorporation increased the content of SOC in sandy soil and enhanced the stability of soil aggregates.

3.6. Crop Yield

In the field experiment, the pelletized straw and manure incorporation promoted the growth of silage corn. Dry yield of silage corn in the PS75, PS150, and M150 were notably increased by 1158.8, 712.7%, and 1450.1% in comparison with CK, respectively. However, although NM and S150 treatments increased crop yield, there were no significant differences with CK (Figure 6). In the pot experiment, compared with CK, PS75 and PS150 notably increased the dry weight of silage corn (Figure 6).

Pearson coefficients showed significant correlation between soil physical and chemical properties and yield traits (Figure 7). In the pot experiment, the dry weight of silage corn had a significant positive relationship with TC, SOC, SIC, and WC, and negatively correlated with BD (two-tailed) (A, n = 9, p < 0.05). Likewise, in the field experiment, dry yield of silage corn had a significant positive relationship with TC related to large macroaggregates, TC related to small macroaggregates, TN, TN related to large macroaggregates, TN related to small macroaggregates, and WC at p < 0.05 (two-tailed) (B, n = 18).

4. Discussion

4.1. Soil Physical Structure and Composition of Soil Water Stable Aggregates

In degraded soil, good soil structure plays a significant role in improving soil physical and chemical properties and coordinating the contradiction between soil nutrient consumption and accumulation [39]. In this study, pelletized straw incorporation altered BD (Figure 3 and Table 1); this was due to the formation of more SOC after the decomposition of a high dosage of the pelletized straw [41]. The results of this study show that the proportion of small macroaggregates in PS75 and PS150 treatments was higher compared with other treatments (Figure 2), this indicated that the addition of the pelletized straw generated more organic cementing substances and facilitated the formation of microaggregates into macroaggregates [42,43]. In the field experiment, it was found that PS150 notably increased the large macroaggregates (>2000 μm) proportion and decreased the ratio of <250 μm aggregate size fractions in comparison with CK, NM, S150, and M150 at 0–20 and 20–40 cm soil depths (Figure 2), the MWD and GMD were significantly improved under PS75 and PS150 treatments (Figure 3). However, in the pot experiment, there was no significant difference in the large macroaggregate (>2000 μm) proportion between PS150 and CK treatments (Table 1), which was due to the inhibition of microbial reproduction caused by the absence of exogenous N at the later stage of cultivation, slowing down the conversion of lignin to stable humus, and affecting the formation of large macroaggregates [44]. In addition, the application of inorganic fertilizer slightly affected MWD and GMD of soil (Figure 3), and this is consistent with other research results [45]. Existing research suggested that the application of animal manure increases soil aggregate stability [46], whereas the application of animal manure in this study did not increase soil aggregate stability (Figure 3). The reason for this result is that additions of manure reduced the content of soil macroaggregates, which is not conducive to improving stability [47]. Furthermore, existing research suggested that returning soil to farmland is capable of increasing SOC content and improving soil texture in degraded soils [22,48]. However, in this study, the application of soil slightly increased soil aggregate stability (Figure 3). The reason for the above result was that the addition of SOC was not enough to promote the formation of macroaggregates [11,49].

4.2. Bulk Soil TC and TN Stocks

Numerous studies suggested that long-term straw returning is capable of notably increasing SOC content [12,13,14,15]. The pelletized straw incorporation notably increased SOC content and continuously improved the soil water holding capacity (Table 1), and SOC and WC notably increased with more pelletized straw returning in this study. This is consistent with previous research results [26,27,28,29]. Many studies showed that the increase in SOC content can greatly improve soil water capacity and avoid water loss [50,51]. In the field experiment, the TN stocks of PS75 treatment increased by 136.46% compared with CK (Table 2). This indicates that pelletized straw sequestrated more soil and additional N simultaneously with straw decomposition [52,53], and a high dosage of the pelletized straw reduced N runoff loss in a short time. This study found that manure returning notably increased TN stocks. This result is probably due to a lower C:N ratio being conducive to the rapid release of N from manure into soils [54]. However, in this study, manure returning did not have notably higher TC stocks than CK and NM at both depths (Table 2); the above results were probably due to relatively shorter experiment duration in this study. Furthermore, this study found that soil applications did not notably increase TC stocks more than CK at both soil layers; this suggested that the influence on soil carbon of incorporating soil is influenced by the thickness of the cover soil and experiment duration [55].

In the pot experiment, it was also found that TC content was increased notably at first and then decreased with the increase in pelletized straw returning (Table 1). The possible reason for this result is that the sandy soil pH in arid areas is higher than 7.5 and the available Ca²⁺ and Mg²⁺ are higher [56]. As a result, HCO₃²⁻ and CO₃²⁻ remained in the soil layer, thus facilitating the formation of carbonate and increased SIC content. To be specific, the amount of the pelletized straw increased continuously and soil moisture was increased, thus accelerating the dissolution of carbonate [57].

4.3. Aggregate C and Content and Stocks

In general, macroaggregate fraction is likely to serve as a favorable predictor to respond to management changes since it takes on a great significance in storing more labile soil carbon [58]. In this study, application of pelletized straw increased TC content, especially in large macroaggregates (Figure 4). This finding supports that the accumulation of straw C is preferential within macroaggregates [59]. However, some studies showed that considerable straw return treatments are capable of restraining the decomposition of crop straw, thus reducing the nutrient release rate of straw [60]; this showed that the pelletized straw can decompose faster, and at the pelletized maize straw’s addition level over 75 Mg ha⁻¹, the TC content in the silt clay fraction was increased (Figure 4). The possible reason for the above difference is that the decomposed straw first converted to the organic carbon of physical protection [61], and finally converted to the MOC with a high degree of humification [62]. It is likely to facilitate long-term TC sequestration since the silt clay fractions are characterized by higher stability and a longer time of turnover in comparison with macroaggregates [63,64,65].

In this study, the maximum TC stocks were in the small macroaggregates (250 < WSA < 2000 μm) rather than large macroaggregates and microaggregates under PS75 and PS150 treatment (Figure 4); this showed that TC stocks of soil aggregates depended on the quantity of soil aggregates rather than the TC content of soil aggregates [66]. Furthermore, due to the lower stability of microaggregates [61], lower TC stocks in microaggregates were observed under PS75 and PS150 treatments (Figure 4).

4.4. Crop Yield

In general, soil amendment with conventional straw may reduce crop yield, primarily because excess incorporated straw cannot be rapidly decomposed and mineralized [67]. In the present study, amendment of soil with pelletized straw and manure increased crop yield (Figure 6), and crop yield was notably positively correlated with the large macroaggregate TC and TN content (Figure 7). A possible reason for this is that compared with the pelletized straw, the release rate of nutrients (e.g., C and N from conventional straw) is slower. On the other hand, the initial SOC level of sandy soil was low [68]. It was reported that SOC content is significantly positively correlated with crop yield [69]. Furthermore, crop yield was significantly positively correlated with water capacity and negatively correlated with soil BD (Figure 7), and these showed that the pelletized straw incorporation can prevent soil wind and water erosion, effectively restraining land degradation and sandstorm [70].

4.5. Prospect and Supplement

The Horqin sandy land is characterized by poor soil nutrients and low land productivity. Local farmers primarily employed soil incorporation to improve the quality of sandy soil quickly. In general, conventional straw decomposes slowly, and the effect of improving fertility is slow; therefore, local farmers are reluctant to use conventional straw incorporation. The method of incorporating high rates of pelletized straw was employed in this study, with an aim to obtain higher yield in the current season to replace conventional fertilization, soil, and manure. In addition, other research confirmed that the application of Bacillus in the sand-blown area with a great amount of straw incorporation in the same desertification soil can significantly improve the content of SOC [71]. Some studies adopted the method of burying and covering tree branches, biochar, and functional fungal agents, utilizing combined application to improve the sandy soil [72,73]; however, the above amendments should be mixed with microbial agents or be applied for a longer time to achieve expected results. In this study, only the application of a high dosage of the pelletized straw could achieve the goal of rapidly improving soil fertility of sandy soil. However, whether the continuously increased amount of the pelletized straw can still rapidly decompose and whether the co-application of microbial agents will increase its fertility effect remains unclear and should be studied further.

5. Conclusions

Throughout the field experiment, pelletized straw incorporation in sandy soil increased bulk soil TC and TN stocks compared with other treatments. A higher dosage of pelletized straw increased the large macroaggregate (>2000 μm) proportion and decreased the ratio of <250 μm aggregate size fractions while decreasing the soil BD. The results of the pot experiment and field experiment are similar, and the increase in SOC content increased the water holding capacity and TN content of sandy soil and was beneficial for both water and fertilizer conservation. Moreover, the pelletized straw incorporation increased TC stocks related to large and small macroaggregates compared with other treatments, and this was attributed to the significant increase in the macroaggregate content and macroaggregate-associated carbon content. The pelletized straw and manure incorporation increased the yield of silage corn by increasing the content of TN and TC in macroaggregates, whereas soil and manure incorporation did not facilitate sandy soil aggregation and increase TC stocks. The results of this study indicate that PS150 treatment might be the optimal measure to improve sandy soil structure, enhance SOC and TN sequestration, and increase crop yield in sandy soil. This study lays a theoretical basis for Horqin sandy soil structure improvement and soil quality maintenance and provides a technical reference for making the sandy soil in this area serve as the reserved arable land.