Difference of Soil Aggregates Composition, Stability, and Organic Carbon Content between Eroded and Depositional Areas after Adding Exogenous Organic Materials

Abstract

Black soil in northeastern China has suffered widespread soil degradation due to long-term cultivation while causing eroded–depositional landscapes, leading to soil-associated carbon redistribution. In agricultural systems, adding exogenous organic material to degraded soil is a common measure to improve soil aggregate stability and soil quality. However, differences in soil properties may alter the decomposition and turnover of organic material in aggregates. Using a uniform method to restore the eroded (E) and depositional (D) soils is inefficient. Therefore, an indoor constant temperature and humidity incubation experiment with the addition of three organic materials, namely, straw (S), biochar (B), and swine manure (M), was designed with an equal amount of carbon. Soil aggregate composition, stability, and organic carbon from eroded and depositional soils were analyzed for evaluating the amendment efficiency of soil quality by exogenous organic material addition. The main results were as follows: adding straw and swine manure could effectively promote >2-mm aggregates formation (E: 7.1%, 8.8%; D: 17.3%, 8.6%) and significantly improved the mean weight diameter (MWD) (E: 0.45 mm, 0.52 mm; D: 0.96 mm, 0.54 mm), while the addition of biochar significantly increased the proportion of 0.25–2-mm aggregates (E: 7.9%; D: 10.9%), but the effect of improving MWD was less than straw and swine manure. All the three organic materials could significantly increase soil total organic carbon (TOC) (S, B and M: 1.95, 3.12 and 2.46 g·kg⁻¹) in the eroded area, and the effect of biochar was the best, whereas it was not significant for the soil in the depositional area. Specially, adding swine manure and adding straw is more beneficial to the restoration of eroded areas and depositional areas, respectively.

1. Introduction

Black soil in northeast China is characterized by a high organic matter content and is almost the most fertile soil across the world. Thus, the black soil region has been the top commodity grain production base for China, providing nearly 40% commodity grains for the domestic market [1]. However, tillage and water erosion are responsible for widespread soil degradation, causing an agricultural landscape with a predominate process of soil erosion at the top slope position and soil deposition at the foot slope position. Soil redistribution due to erosion and deposition processes consequently leads to soil organic carbon (SOC) redistribution in a sloping farmland [2,3,4]. The impact of SOC redistribution is generally considered negative for eroding sites (i.e., low productivity and SOC) and positive for depositional sites (i.e., input of nutrients and SOC) [5,6,7].

SOC is a critical factor for soil fertility. It plays a key role in the maintenance of soil productivity due to its significant contribution to the physical, chemical, and biological properties of soil [8]. SOC stores approximately more than three times the atmospheric carbon; it is the largest organic carbon pool in the terrestrial ecosystem [7,9]. Even small changes in SOC stocks can have a substantial influence on atmospheric CO₂ concentration [10]. Therefore, the importance of SOC in the terrestrial ecosystem has been increasingly acknowledged [8]. Soil aggregate is the fundamental unit of soil structure and its formation and stability are relevant to the C sequestration process in soil [11]. SOC encapsulated within soil aggregates can be physically protected against microbial attacks; therefore, the increase of SOC and the improvement of aggregate stability can play a crucial role in maintaining soil quality and soil function.

In agroecosystems, adding exogenous organic material to soil is a common measure to improve SOC sequestration and soil aggregate stability [12,13]. Previous studies showed that adding swine manure and straw can increase the SOC level, enhance soil aggregate stability, and accelerate the turnover of macro-aggregates [13,14]. In addition to applying organic fertilizer directly to the field, organic material can also be used to produce biochar through high-temperature pyrolysis. Moreover, biochar has been proven to be an optional organic amendment for improving soil quality [15,16]. For instance, Dai et al. [17] indicated that the SOC via biochar amendment was approximately 1.5-times higher than amendments by manure, straw, and biogas residue in barren sandy soil. Zhang et al. [12] also confirmed that biochar can increase the proportion of macro-aggregates and MWD of water-stable aggregates as well as maize straw does.

Considering that the amendment of an eroded site usually takes place with a low SOC level relative to the depositional site, the depositional site is probably wasteful of the exogenous organic material, causing extra greenhouse effects if the same amount of exogenous organic material is applied to both the eroded site and the depositional site. In addition, the microbial community composition and metabolic activity correspond to the SOC level at different slope positions due to the fact that a difference of water conditions can significantly affect the exogenous organic material decomposition [18,19]. It seems inappropriate to use a unified method to amend erosion-deduced soils. Thus, we hypothesized that soil at eroded sites and deposition sites have different impacts on the quantity and quality of organic C and soil aggregate stability, responding to different organic matter amendments. Therefore, three types of commonly used exogenous organic materials (straw, biochar, and swine manure) were taken to explore the response differences between eroded sites and deposition sites.

The objectives of our research were to: (1) evaluate the effect of three organic amendments on the composition and stability of soil aggregates, and the content of organic C in both bulk soil and soil aggregate fractions; (2) compare their differences between eroded soils and deposited soils; (3) identify the effectiveness of organic amendments by extending the incubation period.

2. Materials and Methods

2.1. Study Sites

This study site is located on Keshan farm (48°12′~48°23′ N, 125°08′~125°37′ E) in Heilongjiang province, China, situated 240–340 m above sea level (MASL) and a with temperate continental monsoon climate. The average temperature is 0.9 °C, the average annual precipitation is 501.7 mm, and the average annual evaporation is 1329 mm. The soil types in this region are mainly black soil (Phaeozems in the WRB; Mollisols in the USST).

2.2. Experimental Materials

Surface soils (0–20 cm) at top slope position (eroded sites) and foot slope position (depositional sites) were taken from the sloping arable land with a reclamation period of about 60 years (silage maize was the previous crop) and was sieved through a 2-mm sieve for indoor incubation. The soil was air-dried and milled to measure chemical properties and the samples were performed in triplicate. The initial soil properties are listed in

Table 1. pH, organic carbon(C), total nitrogen(N), total phosphorus(P) and soil granular composition.

Soils	pH	C (g·kg−1)	N (g·kg−1)	P (g·kg−1)	Sand (%)	Silt (%)	Clay (%)
Eroded site	5.91	23.12	1.84	0.52	27.87	37.18	34.95
Depositional site	6.67	28.48	2.25	0.69	33.26	34.46	32.27

.

The following organic materials were used: maize straw, swine manure, and biochar. The carbon and C/N ratio for maize straw and swine manure was 398 g·kg⁻¹, 306 g·kg⁻¹ and 25.8, 11.0, respectively. Meanwhile, biochar was originated from maize straw at 500 °C for 10 h in anaerobic conditions (from Nanjing Zhironglian Technology Co., Nanjing, China). The carbon and C/N of this biochar was 518 g·kg⁻¹ and 36.1 g·kg⁻¹, respectively.

2.3. Experimental Design

Eight treatments were examined as follows: eroded soil without any exogenous organic material addition (ECK); eroded soil with straw addition (ES); eroded soil and biochar addition (EB); eroded soil and swine manure (EM); depositional soil without any exogenous organic material (DCK); depositional soil with straw addition (DS); depositional soil with biochar addition (DB), as well as depositional soil with swine manure addition (DM). Each treatment was performed in triplicate.

The straw addition amount was firstly determined according to the actual straw mulching amount (12,000 kg·ha⁻²) when returning to the soil depth of 0–20 cm and at an average soil bulk density of 1.1 g·cm⁻³ for the arable layer of the sloping farmland in the typical black soil area. The equivalent amount of C in biochar and swine manure was calculated according to the C amount in the straw addition. Consequently, 5.45 g of straw, 4.18 g of biochar, and 7.10 g of swine manure were added to 1 kg of air-dried, 2-mm-screened soil samples. After mixing evenly, water was proofed to 65% of the field water capacity and put it into a constant-temperature (25 °C) ventilation incubator for incubation. Water was regularly added to maintain constant humidity (65% of the field water capacity for both the eroded sites and depositional sites) and incubated for 180 days.

2.4. Soil Sampling and Analysis

The turnover time of the macro-aggregates and the micro-aggregates was reported as 30 days and approximately 90 days (88 days) [20]. Soil samples were collected at the 30th, 90th, and 180th day, respectively. Additionally, we assumed that 180 days can fully capture the effect of the added organic materials on soil aggregate dynamics. After air drying, the soil samples were passed through a 10-mm sieve. Soil water-stable aggregates were isolated by soil aggregate analyzer (Dianjiang technology Co., LTD., model: WS1020, Shanghai, China). The specific operation was as follows: 50 g of air-dried soil placed on a 2-, 0.25-, and 0.053-mm sieve with an amplitude of 5 cm and a vibration of 3 min for a total of 50 times, consequently obtaining large macro-aggregates (>2 mm), small macro-aggregates (0.25–2 mm), micro-aggregates (0.053–0.25 mm), and silt and clay fractions (<0.053 mm) [21]. Soil aggregates were dried at 60 °C and weighed, and the mass and organic carbon content of the aggregates of each sized were determined. The C content of SOC and AOC was determined by an elemental analyzer (Costech Elemental Combustion System 4024, Firenze, Italy). All soil samples used in this study showed no carbonate reaction [22]; thus, the measured organic carbon content was the soil total carbon content. The pH was measured by a digital pH meter (PHSJ-5, China) with a soil–water ratio of 1 to 2.5. The hydrometer method was used to determine soil mechanical composition. The amount of N and P was determined by the same elemental analyzer (Costech Elemental Combustion System 4024, Firenze, Italy) and H₂SO₄–HClO₄ digestion, followed by P-molybdenum blue colorimetric analysis, respectively.

2.5. Calculation and Statistical Analysis

Soil aggregate ability was evaluated by the mean weight diameter (MWD, mm), which was calculated as follows:

$$MWD=\frac{{\displaystyle {\displaystyle \sum }_{i=1}^n}\left(xiwi\right)}{{\displaystyle {\displaystyle \sum }_{i=1}^n}wi}$$

where n is the number of aggregate fractions, x_i is the mean diameter of each fraction class, and w_i is the weight percentage of each fraction class.

Significant differences between treatments were calculated by one-way analysis of variance (ANOVA) with the LSD test at p < 0.05. The regression relationships between aggregate composition, aggregate-associated organic carbon, and the three influencing factors (sloping position, incubation time, and organic material) were analyzed by unitary linear regression, with a significance level of 0.05. Data (means ± SE, n = 3) were analyzed using the SPSS 25.0 (IBM, New York, NY, USA). Origin 2018 software (OriginLab, Northampton, MA, USA) was used to create figures.

3. Result

3.1. Effect of Slope Position, Incubation Duration, and Organic Material Type on Aggregate Composition and Aggregate-Associated Organic Carbon (AOC)

Slope position, incubation duration, and organic material type significantly influenced soil aggregate composition (

Table 2. Relationships between aggregate composition, aggregate-associated organic carbon (AOC), and the three influencing factors.

Index	>2 mm		0.25–2 mm		0.053–0.25 mm		<0.053 mm
	A	AOC	A	AOC	A	AOC	A	AOC
S	***	***	***	***	***	***	***	***
I	***	ns	***	*	***	***	***	***
O	***	***	***	***	***	***	***	ns
S * I	**	***	***	ns	*	**	***	***
S * O	***	***	***	***	ns	***	ns	ns
I * O	***	ns	***	ns	*	***	**	***
S * I * O	***	***	***	***	ns	**	*	**

* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significant. S, slope position; I, incubation time; O, organic material. A, aggregate; AOC, aggregate-associated organic carbon.

); the macro-aggregates (>0.25 mm) were more sensitive to their interactions. In addition, slope position had the greatest impact on AOC (>2 mm, R: 0.828; 0.25–2 mm, R: 0.928; 0.053–0.25 mm, R: 0.841; <0.053 mm, R: 0.692), and the interaction of the three factors had a significant impact on AOC (>2 mm, R: 0.872; 0.25–2 mm, R: 0.947; 0.053–0.25 mm, R: 0.865; <0.053 mm, R: 0.699). Therefore, both soil aggregates, especially macro-aggregates and AOC, could be influenced by slope position, incubation time, and organic material.

3.2. Aggregate Distribution and MWD

The distributions of aggregate fractions were strongly affected by organic amendment (Figure 1). On the whole, organic amendment increased the proportion of macro-aggregates fraction (>0.25 mm) and decreased the proportion of micro-aggregates fraction (<0.25 mm). Correspondingly, the addition of straw and swine manure significantly increased the >2-mm aggregate proportion (ES: 2.6–7.1%, EM: 0.7–8.8%; DS: 10.7–17.3%, DM: 5.6–8.6%) during the whole incubation. Although biochar treatment did not promote the formation of >2-mm aggregate, 0.25–2-mm aggregate was significantly increased (EB: 7.9%; DB: 10.9%) on the 180th day (p < 0.05). Notably, the change trend of large macro-aggregates proportion after adding straw was increasing with the whole incubation duration, while the biochar and swine manure treatments were increasing first before decreasing and then increasing again on the 30th, 90th, and 180th days, respectively. On the 30th and 180th day, the proportion of 0.053–0.25-mm aggregate treated with organic materials was significantly lower than that of CK, and reached the minimum value on the 30th day. For silt and clay, DS, DB, and DM were significantly lower than DCK at the end of incubation (180 d), while only ES and ECK showed significant differences (p < 0.05).

The change trend of MWD after adding organic amendments was the same as the distribution of large macroaggregates (Figure 2). Specifically, compared to ECK treatment, ES and EM treatment significantly increased by 48.9% and 56.5%, respectively, and DS and DM treatments significantly increased by 93.2% and 54.3%, compared with DCK on the 180th day (p < 0.05), respectively. In addition, EB treatment significantly increased MWD, compared to ECK treatment (p < 0.05); however, DB had a not-significant effect on MWD, compared to DCK. In general, the addition of swine manure showed a better effect on eroded soils, and the addition straw had a better effect on depositional soils.

3.3. Soil Total Organic Carbon (SOC)

The soil organic carbon of eroded and depositional soils had different effects after adding different exogenous organic materials (

Table 3. The content of soil total organic carbon.

Treatment	Soil Total Organic Carbon (g·kg−1)
	30 day	DV	90 day	DV	180 day	DV
ECK	22.35 ± 0.78 Ac		22.83 ± 0.37 Ab		22.11 ± 0.42 Ab
ES	25.40 ± 0.62 Aab	3.04 ± 1.15 Aa*	23.81 ± 1.08 Bab	0.97 ± 0.75 Ba	24.06 ± 0.41 ABa	1.95 ± 0.02 ABa*
EB	25.79 ± 1.16 Aa	3.44 ± 1.32 Aa*	24.84 ± 0.29 Aa	2.00 ± 0.19 Aa	25.22 ± 1.39 Aa	3.12 ± 1.01 Aa
EM	24.10 ± 0.59 Ab	1.75 ± 1.31 Aa	24.17 ± 0.71 Aa	1.34 ± 1.04 Aa	24.56 ± 1.16 Aa	2.46 ± 1.55 Aa
DCK	29.48 ± 0.52 Aa		28.01 ± 1.10 Ab		29.36 ± 0.42 Aab
DS	30.03 ± 0.40 Aa	0.55 ± 0.82 Aa*	29.56 ± 0.73 Aab	1.54 ± 1.73 Aa	29.49 ± 0.10 Aab	0.13 ± 0.32 Ab*
DB	29.27 ± 0.56 Aa	−0.21 ± 0.64 Ba*	30.39 ± 1.01 Aa	2.38 ± 1.63 Aa	30.09 ± 0.29 Aa	0.73 ± 0.18 ABa
DM	29.10 ± 1.02 Aa	−0.38 ± 0.55 Ba	29.67 ± 0.93 Aab	1.66 ± 1.16 Aa	28.94 ± 0.57 Ab	−0.42 ± 0.28 Bc

Different uppercase letters represent significant differences between the same treatment with different incubation times; lowercase letters represent significant differences between different treatment with the same incubation time; * represents significant differences between the same treatment with different slope positions (p < 0.05). The increment of soil organic carbon content (DV, g·kg⁻¹) in added exogenous organic material treatment is compared with unprocessed soil. ECK, untreated eroded soil; ES, eroded soil with straw addition; EB, eroded soil with biochar addition; EM, eroded soil with swine manure addition; DCK, untreated depositional soil; DS, depositional soil with straw addition; DB, depositional soil with biochar addition; DM, depositional soil with swine manure addition.

). For eroded soil, ES treatment significantly increased SOC by 8.8%, compared with ECK treatment on the 180th day; in addition, EB and EM treatments significantly increased SOC compared with ECK for the whole incubation period (p < 0.05). However, for depositional soil, DB treatment only significantly increased SOC by 8.5%, compared with DCK on the 90th day, and no significant difference was found among DS and DM for the whole incubation period (p < 0.05). Overall, the effect of organic materials amendment in eroded soil was better than depositional soil; meanwhile, biochar is the best material for improving SOC.

3.4. Aggregate-Associated Carbon Content (AOC)

The AOC of eroded and depositional soils had different effects after adding exogenous organic materials (

Table 4. Aggregate-associated carbon content (AOC) under different organic amendments.

Treatment	Incubation Time	Soil Aggregate Organic Carbon (g·kg−1)
		>2 mm	2–0.25 mm	0.25–0.053 mm	<0.053 mm
ECK	30 d	21.96 ± 1.56 c	21.13 ± 1.35 c	22.11 ± 0.53 c	13.90 ± 1.39 c
ES		25.69 ± 0.37 b	23.20 ± 1.23 b	25.16 ± 1.39 b	15.66 ± 1.56 bc
EB		27.18 ± 1.47 ab	25.47 ± 0.46 a	28.23 ± 0.95 b	20.43 ± 1.55 a
EM		28.68 ± 1.10 a	25.38 ± 0.43 a	24.96 ± 1.77 a	17.03 ± 1.11 b
ECK	90 d	23.43 ± 1.98 b	23.09 ± 0.34 c	22.61 ± 1.18 c	16.11 ± 0.57 a
ES		25.45 ± 1.28 b	24.96 ± 1.05 ab	25.26 ± 0.83 a	11.69 ± 0.43 b
EB		24.30 ± 0.26 b	23.52 ± 1.20 bc	22.97 ± 0.73 bc	11.93 ± 2.36 b
EM		28.26 ± 0.17 a	25.31 ± 0.69 a	24.30 ± 0.59 ab	10.80 ± 3.48 b
ECK	180 d	21.86 ± 1.84 c	20.52 ± 0.62 c	22.01 ± 0.51 c	14.78 ± 1.30 b
ES		24.94 ± 1.00 ab	23.48 ± 0.93 b	25.90 ± 0.99 a	17.57 ± 2.36 b
EB		23.30 ± 0.32 bc	23.19 ± 0.47 b	24.55 ± 0.57 b	15.42 ± 1.48 b
EM		25.89 ± 0.75 a	25.87 ± 1.63 a	26.95 ± 0.59 a	21.42 ± 1.42 a
DCK	30 d	30.98 ± 0.04 ab	31.37 ± 0.41 a	29.88 ± 0.71 a	20.87 ± 1.16 a
DS		31.32 ± 0.69 a	31.81 ± 0.79 a	29.78 ± 0.63 a	21.64 ± 0.95 a
DB		30.02 ± 0.83 b	30.33 ± 1.29 a	30.13 ± 0.63 a	20.25 ± 0.52 a
DM		29.87 ± 0.70 b	30.21 ± 0.82 a	28.46 ± 0.27 b	19.97 ± 1.30 a
DCK	90 d	29.39 ± 0.85 a	31.64 ± 1.16 ab	28.60 ± 0.52 a	24.15 ± 1.76 a
DS		29.84 ± 1.62 a	30.09 ± 0.45 b	28.66 ± 0.79 a	18.15 ± 1.74 b
DB		31.53 ± 0.91 a	32.63 ± 0.97 a	29.41 ± 0.94 a	20.02 ± 1.64 b
DM		31.04 ± 1.03 a	33.00 ± 1.74 a	29.04 ± 1.12 a	19.13 ± 1.18 b
DCK	180 d	31.43 ± 0.70 b	31.23 ± 0.20 ab	29.94 ± 0.59 a	21.23 ± 1.43 a
DS		30.55 ± 0.08 b	30.81 ± 1.00 b	31.56 ± 1.30 a	20.56 ± 1.70 a
DB		35.07 ± 2.29 a	32.33 ± 0.42 a	31.70 ± 1.50 a	23.15 ± 1.56 a
DM		31.83 ± 0.32 b	32.16 ± 1.14 ab	31.50 ± 1.14 a	22.86 ± 1.48 a

Different lowercase letters indicate significant differences between the same soil with different treatments in the same incubation time. ECK, untreated eroded soil; ES, eroded soil with straw addition; EB, eroded soil with biochar addition; EM, eroded soil with swine manure addition; DCK, untreated depositional soil; DS, depositional soil with straw addition; DB, depositional soil with biochar addition; DM, depositional soil with swine manure addition.

). For eroded soil, large macro-aggregates-associated carbon (>2 mm) declined along with incubation, which, for ES and EM treatment, were significantly increased by 14.1% and 18.4%, compared with ECK on the 180th day, respectively. In terms of small macro-aggregate (0.25–2-mm), all ES, EB, and EM treatments had a significantly positive effect compared with ECK for the whole incubation, except for EB treatment on the 90th day (p < 0.05), but DS, DB, and DM treatments had no significant increase compared with DCK on the 180th day. Additionally, the amendment effect on the micro-aggregates (0.053–0.25-mm) was less than that on the small macro-aggregates (0.25–2-mm). C in the silt and clay fraction (<0.053 mm) first declined and then increased along with the incubation process. Specially, all ES, EB, EM, and DS, DB, DM treatments significantly decreased the AOC (<0.053 mm), compared with ECK and DCK on the 90th day, respectively (p < 0.05). However, for eroded soil, AOC (<0.053 mm) in all three treatments was higher than in the ECK on the 180th day; in particular, EM treatment significantly increased AOC (<0.053 mm) by 44.9%, compared with ECK. However, the effect of organic materials amendment for depositional soil generally had no significant changes. To sum up, the effects of organic materials on eroded soil were greater than those on depositional soil.

The difference of aggregates proportion and AOC between treated and untreated soils were affected by both organic material type and slope position (

Table 5. The change of aggregates proportion (P, %) and aggregate-associated organic carbon content (AOC, g·kg⁻¹ aggregate) of treated soil with organic materials compared with untreated soil.

Treatment	Incubation Time	>2 mm		2–0.25 mm		0.25–0.053 mm		<0.053 mm
		P	AOC	P	AOC	P	AOC	P	AOC
ES	30d	2.55	3.73	6.22	2.07	−6.03	3.05	−2.75	1.76
EB		0.39	5.22	4.81	4.34	−5.98	6.12	0.77	6.53
EM		0.98	6.72	6.42	4.25	−6.08	2.85	−1.33	3.13
DS		7.89	0.34	−4.36	0.44	−2.93	−0.10	−0.60	0.77
DB		2.31	−0.96	1.63	−1.04	−3.42	0.25	−0.53	−0.62
DM		7.33	−1.11	−0.59	−1.16	−4.99	−1.42	−1.75	−0.90
ES	90d	5.48	2.02	1.62	1.87	−3.94	2.65	−3.17	−4.42
EB		−0.01	0.87	5.68	0.43	−4.39	0.36	−1.28	−4.18
EM		0.66	4.83	5.27	2.22	−2.88	1.69	−3.06	−5.31
DS		10.74	0.45	0.59	−1.55	−5.25	0.06	−6.08	−6.00
DB		0.65	2.14	1.80	0.99	−1.76	0.81	−0.69	−4.13
DM		5.62	1.65	2.39	1.36	−3.71	0.44	−4.29	−5.02
ES	180d	7.14	3.08	3.47	2.96	−7.44	3.89	−3.17	2.79
EB		1.59	1.44	7.87	2.67	−6.82	2.54	−2.64	0.64
EM		8.80	4.03	0.73	5.35	−7.83	4.94	−1.69	6.64
DS		17.27	−0.88	−6.17	−0.42	−5.52	1.62	−5.57	−0.67
DB		−1.41	3.64	10.91	1.10	−6.10	1.76	−3.20	1.92
DM		8.62	0.40	3.28	0.93	−6.38	1.56	−5.52	1.63

ECK, untreated eroded soil; ES, eroded soil with straw addition; EB, eroded soil with biochar addition; EM, eroded soil with swine manure addition; DCK, untreated depositional soil; DS, depositional soil with straw addition; DB, depositional soil with biochar addition; DM, depositional soil with swine manure addition.

). The variation range of the large macro-aggregate (>2-mm) proportion after adding straw and swine manure was higher than that of biochar, indicating the greater impact of straw and swine manure on >2-mm aggregate aggregation, compared with biochar; DB treatment even showed a negative effect on the large macro-aggregate aggregation (>2-mm) after 180 days. However, adding biochar had the greatest effect on promoting small macro-aggregate (0.25–2-mm) formation. In general, the amendment effect of organic material on AOC in the eroded area was better than that in the depositional area, and the improvement effect of organic material on soil aggregation in the depositional area was better than that in the eroded area.

4. Discussion

4.1. Effect of Organic Amendment on Soil Aggregate Distribution and MWD

As the basic unit of soil structure, soil aggregates are an important indicator for the diagnosis of soil quality, and their formation and stability are crucial to soil carbon sequestration [11]. In this study, the addition of straw and swine manure can effectively promote the formation of large macroaggregates (>2-mm). This is because the straw contains rich lignin, which is not easy to decompose, and can be directly used as the core of large aggregates to adsorb fine particles to form large aggregates [23]. Secondly, in the decomposition process of fresh organic materials, a variety of organic cements (such as carbohydrates, proteins, etc.) are produced, which enhance the activity of soil microorganisms and increase the metabolites secreted by microorganisms, thus promoting the formation of large macroaggregate [24]. However, biochar produced by high-temperature pyrolysis has a stable aromatization structure, which is difficult to be further degraded and decomposed by microorganisms [25]. On the other hand, the addition of biochar will increase the total porosity of soil and hinder the contact between soil particles [26], so it is not conducive to the formation of large macroaggregate. Therefore, MWD based on aggregate distribution also showed a consistent change trend; that is, adding straw and swine manure significantly improved soil aggregate stability, and the improvement effect was better than that of biochar. Due to a high level of organic C in the depositional site, it was also favorable for the formation of macroaggregates compared to the low level of organic C in the erosion site [19]. In addition, fine mineral particles preferring migration accumulate in the depositional area, and their huge surface area and strong adsorption capacity combine with SOC to form organic mineral complexes, which promote the aggregation of soil particles [27]. As a result, the effect of soil aggregation in the depositional area is better than that of the eroded area on the 180th day.

4.2. Effect of Organic Amendment on Soil Total Organic Carbon (SOC) and Aggregate-Associated Organic Carbon (AOC)

Since biochar contains a large amount of inert carbon, it can directly increase soil organic carbon when being combined with soil aggregates [25]. Meanwhile, its special pore structure and large specific surface area enable carbon to be adsorbed on the surface or in the pores, which plays a certain isolation role and reduces the decomposition of soil organic carbon [28], resulting in the highest soil total organic carbon content on the 180th day. However, the addition of biochar did not significantly increase aggregate organic C content on the 90th day; this might be because the incubation period is shorter and biochar, as a direct and self-stable C source, is not easy to decompose to produce cemented materials; thus, partial biochar of unstable combinations exhibits losses during the acquisition of water-stable aggregates. In this research, C in the silt and clay fraction (<0.053 mm) decreased significantly after the addition of three organic materials, which was because new exogenous C accelerated the turnover of organic C and improved the mineralization and decomposition of organic C [29]. On the other hand, the <0.053-mm particle size aggregates might be aggregated to larger particle sizes, leading to the physical migration of C [30], and new exogenous C is not transferred to the silt and clay fraction. Previous studies have shown that the residue-C was transferred from large macroaggregates to the smaller aggregates over time, which might be associated with the decline in microbial metabolic activity [31,32]. In addition, the C in fresh organic materials mainly accumulates in the form of particulate organic carbon (POC) in the form of larger and small macroaggregates and microaggregates, and finally, in the form of mineral-associated organic carbon stored in the silt and clay fraction [33]. In this study, with the increase of incubation duration, >2-mm aggregate organic C content was in a downward trend and exogenous carbon preferred to being stored in 0.25–2-mm and 0.053–0.25-mm aggregates. This might be because the POC in macroaggregates is more likely to diffuse into the small pore in the microaggregates during the migration process and be adsorbed on the surface of minerals, making its use by microorganisms easier. This may also be one of main mechanisms for organic materials to increase the organic C in soil aggregates in the short term. Furthermore, it was explained that the silt and clay-associated organic C was replenished after 180 days. Because of the biochemical protection of the silt and clay fraction, this part of C was relatively stable and not easy to mineralize and decompose, which is the internal mechanism of adding organic materials to improve soil C sequestration for a long time [11]. Xu et al. [34] have also shown that the higher the C/N ratio of organic materials, the less easily it is decomposed. Compared with straw, swine manure in this study had a lower C/N and a faster decomposition rate, which promotes the absorption and transformation of new exogenous C in aggregates. Therefore, under the condition of an equal amount of C addition, swine manure is better than straw for improving soil C sequestration.

Lower SOC sequestration in the depositional area was possibly due to several reasons: (1) although adding organic material can stimulate the activity of microorganisms and accelerate the mineralization rate of SOC, the soil clay content of the eroded area is higher, and it can effectively protect SOC from microbial decomposition, while the soil in the depositional area has a higher sand content and pH, which increases soil respiration [35] (Table 1). (2) In the process of long-term severe soil erosion, the eroded site is far away from the carbon saturation state, while the depositional site has large amount of organic accumulation, which obviously promotes the carbon saturation level [36] and is less sensitive to the input of new exogenous C than the eroded area. (3) Don et al. [37] proposed a dilution effect and suggested that the lower the SOC, the less mineralization. In general, easily decomposed organic carbon in an eroded area is consumed, the relatively stable organic carbon is enriched, and the SOC deficit is larger; thus, the amendment for organic carbon is more urgent. (4) Generally, soil water content in the eroded area is lower than that in the depositional area. This leads to a lower level of microbial activity and mineralization rate in the eroded area compared with the depositional area [38]. To sum up, the site-specific soil properties at different slope positions (e.g., water content) may play a critical role in controlling SOC sequestration capacities after organic material addition.

5. Conclusions

The study confirms the effect of exogenous organic materials amendments on soil aggregation, stability, and C sequestration between treatments. Adding fresh organic materials (straw, swine manure) and biochar enhanced soil aggregate stability by promoting large macroaggregate and small macroaggregate formation, respectively. Specially, the amendment effect on the soil aggregate stability of straw and swine manure was higher than the biochar enhancement, while no significant differences were found in soil total organic carbon among straw, biochar, and swine manure. Overall, the amendment effect of adding organic materials on SOC was better in the eroded area than that in the depositional area, and the improvement effect of soil aggregate stability in the depositional area was better than that in the eroded area; in particular, swine manure is more beneficial to the restoration of eroded areas, and straw is more beneficial to the restoration of depositional areas. This finding highlighted the key role of site-specificity for controlling the capacities of SOC sequestration after organic material addition. Field experiments should be carried out in the future, and the effectiveness of the three exogenous organic materials for improving the soil quality should be further studied.