Distribution of organic carbon and nutrients in soil aggregates under different stand types of Cunninghamia lanceolata in southern Guangxi of China

ABSTRACT

Revealing the dynamics of soil organic carbon (C_org) and nutrients at aggregate scales is essential for improving our understanding of soil C_org mitigation and nutrient restitution in the Cunninghamia lanceolata plantations. In this study, soil C_org, total nitrogen (N_tot), available phosphorus (P_ava), and exchangeable cations (including calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and sodium (Na⁺)) received the analysis in aggregate fractions acquired from 0 to 10 cm and 10–20 cm depths in the three different stand types, namely mixed stands of Cunninghamia lanceolata and Michelia macclurei (stand A), Cunninghamia lanceolata and Mytilaria laosensis (stand B), and pure stand of Cunninghamia lanceolata (stand C). Soil aggregates were classified into macro-aggregates (>2 mm), meso-aggregates (2–0.25 mm), and micro-aggregates (<0.25 mm) by the dry-sieving process. The two mixed stands displayed higher level of soil aggregate stability than the pure stand. Moreover, micro-aggregates acted as the main fractions that carried soil C_org, N_tot, and P_ava, and both micro-aggregates and macro-aggregates referred to the main fractions that carried exchangeable cations. As for the soil nutrient stocks, only the exchangeable K⁺ stock of the pure stand dominated among the C_org and nutrient stocks, in addition, the rest of the other nutrient stocks of the mixed forests (stands A and B) took an advantage over the pure stand. Moreover, the C_org and nutrient stocks in stands A and B were reflected in the macro-aggregates, differently, those of stand C were mainly reflected in the micro-aggregates. Thus, selecting suitable broadleaf tree species mixed with Cunninghamia lanceolata can alleviate the reduction of soil aggregate stability and the loss of soil nutrients, thus promoting soil resources to be sustainably utilized and protecting soil quality and health in southern Guangxi of China.

KEY WORDS:

• Fir plantation
• soil aggregate
• organic carbon
• nutrient

1. Introduction

As the basic unit of soil structure, aggregate can coordinate the water, fertilizer, air, and heat in soil, it is the material basis for the formation of good soil structure and influences the changes of soil quality and health (Six, Bossuyt, and Degryze 2004). Aggregates of different sizes not only determine soil physical and chemical characteristics such as soil morphology, distribution, and pore quantity, but also impact the supply, conservation, and transformation of soil nutrients (Six and Paustian 2012; Egan, Crawley, and Fornara 2018). Aggregate stability is influenced by a number of factors, including plant residues, soil organic carbon (C_org), Fe/Al-oxide, and clay contents, which play important roles in the formation of soil aggregates (Almajmaie et al. 2017). In general, primary particles are combined by durable binders (that is, polyvalent metal cation complexes and humified organic matters), highly disordered aluminosilicates, and oxides to form micro-aggregates (<0.25 mm) (Voltolini et al. 2017). On the other hand, macro-aggregates (>0.25 mm) can be created by a micro-aggregate aggregating process with transient and temporary organic binders; they are majorly covered in plant roots, fungal hyphae, and polysaccharides of microbial and plant origin (Li et al. 2019). Macro-aggregates with a stable structure can store more organic matters and nutrients, thus reducing rainwater erosion and surface runoff as well as achieving water and fertilizer retention (Zhao et al. 2017). But physical disturbance will cause the stability of macro-aggregates to be reduced, and the micro-aggregates will receive the creation, serving as the basis of the following macro-aggregate formation cycle (Ma et al. 2015). Therefore, it is important to study the different sized aggregates because they can significantly maintain soil C_org and nutrients.

For insights into mechanism to sequester soil organic matters, we need to quantify their location at aggregate scales, because the distribution of soil aggregate-related C_org and nutrients will impact microbial degrading processes (Ge et al. 2019). Recently, there are various studies about soil aggregate-related C_org and nutrient distribution patterns. For example, soil C_org, total nitrogen (N_tot), and available phosphorus (P_ava) contents declined noticeably with the increase of aggregate sizes (Wu et al. 2018; Zou et al. 2018), while in other studies, the opposite trends were achieved (Ranatunga, Reddy, and Taylor 2013; Egan, Crawley, and Fornara 2018). Obviously, the soil aggregate-related C_org, N_tot, and P_ava were widely reported, yet the exchangeable cations (including calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and sodium (Na⁺)) are rarely explored, although they can form cationic bridges with clay particles and organic matters to promote the formation and stability of soil aggregates, and to protect soil C_org and nutrients from loss (Jiang et al. 2011).

The area of plantation forests in China is about 79.0 million hectare in 2019, accounting for about 36.0% of the national forest area and 48.5% of the world’s total, which ranks first in the world. Cunninghamia lanceolata with the characteristics of fast growing, good quality, and high economic value, it is an important forestation species in China. Approximately 19.0% of the country’s plantation forest area is planted with Cunninghamia lanceolata (Zhou et al. 2020). Because of the unique geographical location and climate, Guangxi has become a major forestry province. However, low productivity of planted forests has been a major problem in China’s planted forests for a long time. For instance, the forest stock of Guangxi was only 64 m³ ha⁻¹ in 2019, which was significantly lower than the national (95 m³ ha⁻¹) and international (131 m³ ha⁻¹) averages. The multi-generational replanting of pure forests in pursuit of timber productivity has serious implications for the sustainable management of planted forests, which leads to homogeneous stand structure, poor ecological stability, and declining soil fertility. However, the second-generation Cunninghamia lanceolata plantation caused soil fertility and productive capability to be severely reduced (Guan et al. 2015). For solving the issue, the managing process of mixed forests is considered as a vital orientation to sustainably manage plantations. For example, degraded Cunninghamia lanceolata plantations are gradually rebuilt through intercropping with Michelia macclurei, thus reestablishing the autotrophic mechanisms of the forest ecosystem and increasing the productivity of forest stands.

Soil is strongly influenced by vegetation factors and revealing the dynamics of soil C_org and nutrients at aggregate scales is essential for improving our understanding of soil C_org mitigation and nutrient restitution in the Cunninghamia lanceolata plantations (Wang, Zhang, and Ye 2020). Therefore, the present study aimed at determining the influences of different stand types on the contents and stocks of C_org and nutrients in soil aggregates. It is hypothesized that (i) the micro-aggregates are the major carriers carrying soil C_org and nutrients since micro-aggregates exhibiting wider specific surfaces are able to offer appropriate sites for C_org and nutrients; (ii) the C_org and nutrient stocks of mixed forests are higher than those of pure forest, due to the findings of our previous research (Huang et al. 2020) that the mixed plantations exhibit the relatively high levels of tree litter quantity and litter surface covering protection.

2. Materials and methods

2.1. Experimental site

This work performed in June 2019 at the experiment center of tropical forestry (Chinese academy of forestry) in Pingxiang, Guangxi, China (106°41′-106°59′ E, 21°57′-22°16′ N) (Figure 1). The common climate refers to subtropical monsoon with the annual sunshine about 1220–1600 h, the annual temperature between 19.5°C and 21.5°C, and the annual rainfall about 1200–1550 mm. Low mountains and hills are the mainly landform in this area, with the gradient about 25–30 °. The native rock is mainly limestone gravel, argillaceous sandstone, granite, and limestone. The soil is mainly latosol and krasnozem with a loose texture and the soil layer thickness is over 80 cm. The soil belongs to acid soil with pH of 4.8–5.5. The primary vegetation is mainly monsoon forest and rainforest. Nevertheless, as impacted by long-term human interference activities, utilization, and logging, the primary monsoon forest vegetation can hardly be found. The secondary forest vegetation is only residual and replaced by a large number of man-made forests.

Figure 1. Location of the experimental site

2.2. Experimental design

In this study, forests mixing Cunninghamia lanceolata and Michelia macclurei (stand A), Cunninghamia lanceolata and Mytilaria laosensis (stand B), and pure forest of Cunninghamia lanceolata (stand C) in Qingshan experimental site were selected according to the geological conditions and site conditions. The three stand types were constructed in 1992, with a row spacing of 2 m × 3 m, and a 3: 1 mixed proportion of Cunninghamia lanceola with Michelia macclurei and Cunninghamia lanceolata with Mytilaria laosensis. The experiment site with a crown density about 0.85 has a slope of 27–35 ° and an altitude of 725–730 m.

For studying the influence exerted by Cunninghamia lanceolata stand types on the soil C_org and nutrients in aggregate fractions, every stand type was duplicated in quintuplicate, and then produced 15 tree plantations. Separation among these tree plantations was made with distances of ~1000 m between each other, thus reducing the spatial autocorrelation and prohibiting the pseudoreplication. For every tree plantation (S ≈ 1 × 10⁴ m²), one plot (S = 20 m × 20 m) was randomly constructed at a distance of >50 m away from the edge of plantation.

2.3. Soil and litter sampling

According to the respective plot, 5 litter samples received the collection from soil surface to the plastic bags by using the 5 randomized sub-plots with an area of 1 m² (1 m × 1 m). Subsequently, 5 litter samples received the composition process to one. All of the 15 composite litter samples received the oven-drying process at 80°C until a constant weight was achieved. Finally, the dried composited litter samples received the weighing process and litter C (Nelson and Sommers 1996) and N (Bremner 1996) contents were measured. The litter quantity was 723.66 g m⁻², 566.84 g m⁻², and 340.58 g m⁻², and the litter C/N ratio was 16.34, 17.61, and 24.36 in stand A, stand B, and stand C, respectively.

Soil samples were collected in the identical site of the litter sample collection. According to the respective plot, 5 soil samples (rhizosphere soil) were collected from 0 to 10 cm and 10–20 cm layer separately, because the response of soil C_org and nutrients to stand type in topsoil (0–20 cm depth) was more sensitive than that in the deeper soil layers. Next, they received the incorporation to be the composite one. The 30 composite soil samples collectively received the fragmenting process to nature aggregates rigorously. Then, the samples were sifted via a sieve (mesh size of 5 mm) to remove large roots, stones and macro-fauna. In the meantime, 5 soil samples within the respective plot received the random collection with the use of the cutting ring approach (V = 100⁻³) for measuring the properties of whole soil, such as bulk density, as well as the contents of C_org and nutrients.

2.4. Soil aggregate separation

Based on the dry-sieving process shown by Six and Paustian (2012), the soil aggregates received the isolation. In this process, the air-dried soil sample (250 g) was filtered using sieves with the successive diameter of 0.25 and 2 mm. Next, 20 min vertical oscillation (along 5 cm amplitude) was achieved at the 1 oscillation s⁻¹ rate. Then, 3 aggregates were acquired, they were macro-aggregates (>2 mm), meso-aggregates (2–0.25 mm), as well as micro-aggregates (<0.25 mm). According to all the fractions exhibited by the aggregates, the contents of soil C_org and nutrients received the assessment. Notably, in this study the fractions of <0.25 mm were considered as micro-aggregates, there is no clay and silt fractions (<0.053 mm) in the soil, due to the findings of John et al. (2005) indicating that clay and silt fractions were included in the macro-aggregate and micro-aggregate fractions.

2.5. Soil property analyses

Soil samples received the oven-drying process at 105°C to the constant weight for measuring the bulk density, and then their clay content was ascertained with a hydrometer (Lu 2000). In addition, soil samples (including aggregates and bulk soil) received the air-drying process under the indoor temperature before soil physical and chemical characteristics were analyzed. Soil C_org and N_tot received the measurement by acid dichromate wet oxidization (Nelson and Sommers 1996) and micro-Kjeldahl (Bremner 1996) methods, respectively. Soil P_ava received the measuring process based on molybdenum blue colorimetric procedure (Bray and Kurtz 1945). Soil exchangeable cations received the measurement by ammonium acetate replacement method (Thomas 1982). Soil amorphous Fe and Al oxides were carried out by acid ammonium oxalate extraction method (Kämpf and Schwertmann 1982).

2.6. Calculations and statistical analyses

The stability of soil aggregates was estimated by the mean weight diameter (MWD, mm), which was calculated based on the equation put forward by Six and Paustian (2012):

$MWD=\sum _{\mathrm{i}=1}^3(X_{\mathrm{i}}\times M_{\mathrm{i}})$

where X_i denotes the aggregate average diameter at the size of i^th (mm); M_i denotes the aggregate proportion at the size of i^th in the bulk soil (%).

Based on the method established by Eynard et al. (2005), C_org stock (CS, g m⁻²) in the bulk soil was calculated as follows:

$CS=\sum _{\mathrm{i}=1}^3\left(M_{\mathrm{i}}+{C_{org}}_{\mathrm{i}}\right)\times B_d\times H\times 10,$

where M_i denotes the aggregate proportion at the size of i^th in the bulk soil (%); C_{org i} is the content of C_org in aggregates at the size of i^th (g kg⁻¹); B_d represents the bulk density in the bulk soil (g cm⁻³); H refers to the soil thickness (cm). Similarly, nutrient stocks in the bulk soil were also obtained.

Based on SAS software, we carried out the statistical analyses. The results have the expression of the average of five replicates. We made the one-way variance analysis for assessing how stand type influenced soil properties. Furthermore, based on a generalized linear model, we performed the variance analysis with the split-plot for determining the effects of aggregate size and stand type on the C_org and nutrients in soil aggregates.

3. Results

3.1. The physical and chemical properties in bulk soil

Soil C_org and nutrient contents were decreased with the deepening of soil layer while the ratio (R_dm) of divalent cations (Ca²⁺ and Mg²⁺) to monovalent cations (K⁺ and Na⁺) and bulk density showed an opposite trend (Table 1). In both depths, soil C_org, N_tot, P_ava, and monovalent cation contents, bulk density, and the R_dm were significantly different among the different stand types. In 0–10 cm depth, however, there were no significant differences in soil C/N ratio, total exchangeable base (total EB), and divalent cation contents among the three stands. Besides, soil C_org and nutrient contents were significantly different among the three stands in 10–20 cm depth. In short, soil fertility characteristics in stands A and B had relatively high levels than that in stand C in both depths. Nevertheless, there were no significant different in soil Fe/Al-oxide and clay contents among the three stands.

Table 1. Soil physical and chemical properties under the different stand types

Item	0–10 cm soil layer				10–20 cm soil layer
	Stand A	Stand B	Stand C		Stand A	Stand B	Stand C
Bulk density (g cm^−3)	1.24 ± 0.01 b	1.26 ± 0.02 b	1.31 ± 0.02 a		1.26 ± 0.01 b	1.27 ± 0.01 b	1.33 ± 0.02 a
Clay (%)	35.12 ± 2.47 a	33.18 ± 3.06 a	34.27 ± 1.28 a		32.29 ± 3.17 a	30.47 ± 2.94 a	31.09 ± 2.16 a
Active Fe2O3 (g kg^−1)	12.13 ± 1.02 a	14.08 ± 1.38 a	13.24 ± 0.97 a		13.21 ± 1.24 a	13.96 ± 0.91 a	13.76 ± 1.33 a
Active Al2O3 (g kg^−1)	1.26 ± 0.14 a	1.37 ± 0.36 a	1.29 ± 0.27 a		1.34 ± 0.41 a	1.29 ± 0.36 a	1.30 ± 0.29 a
Organic C (g kg^−1)	15.20 ± 0.24 a	13.80 ± 0.36 b	10.10 ± 0.26 c		13.07 ± 0.21 a	9.97 ± 0.12 b	9.44 ± 0.23 c
Total N (g kg^−1)	1.16 ± 0.05 a	1.02 ± 0.07 b	0.75 ± 0.01 c		0.92 ± 0.08 a	0.82 ± 0.02 b	0.91 ± 0.06 a
C/N ratio	13.20 ± 0.60 a	13.61 ± 0.66 a	13.60 ± 0.40 a		14.46 ± 1.50 a	12.29 ± 0.35 b	10.42 ± 0.70 c
Available P (mg kg^−1)	6.81 ± 0.38 a	5.81 ± 0.61 b	4.05 ± 0.35 c		2.67 ± 0.42 a	1.29 ± 0.20 c	2.27 ± 0.12 b
Total EB (cmol kg^−1)	6.13 ± 0.16 a	6.04 ± 0.16 a	6.16 ± 0.18 a		5.74 ± 0.10 a	5.37 ± 0.11 b	5.81 ± 0.26 a
Exchangeable Ca^2+ (cmol kg^−1)	5.38 ± 0.18 a	5.43 ± 0.15 a	5.38 ± 0.13 a		5.18 ± 0.09 a	4.97 ± 0.06 b	5.29 ± 0.20 a
Exchangeable Mg^2+ (cmol kg^−1)	0.46 ± 0.01 a	0.41 ± 0.01 a	0.46 ± 0.06 a		0.40 ± 0.02 a	0.24 ± 0.08 b	0.13 ± 0.01 c
Exchangeable K^+ (cmol kg^−1)	0.15 ± 0.01 b	0.08 ± 0.01 c	0.24 ± 0.03 a		0.12 ± 0.01 b	0.11 ± 0.01 b	0.33 ± 0.08 a
Exchangeable Na^+ (cmol kg^−1)	0.14 ± 0.01 a	0.12 ± 0.01 b	0.08 ± 0.01 c		0.04 ± 0.00 b	0.05 ± 0.00 a	0.06 ± 0.01 a
Rdm	20.12 ± 1.80 b	29.49 ± 2.09 a	17.99 ± 0.72 b		34.05 ± 1.81 a	32.19 ± 2.77 a	14.86 ± 2.88 b

Note: Data represent the average of five replicates ± standard deviations. Different lower case letters indicate significant differences (p < 0.05) among the different stand types.

3.2. Soil aggregate distribution and stability

In the three stands, macro-aggregates took up the main parts in the bulk soil, with the average proportion of 43.1% and 37.3% in 0–10 cm and 10–20 cm depths, respectively (Table 2). Among the other two aggregates, the proportion of meso-aggregates was significantly higher than that of micro-aggregates in 0–10 cm depth, but this trend was opposite in 10–20 cm depth. Soil aggregates of various sizes were significantly impacted by the stand type, except for meso-aggregates in 0–10 cm depth. To be specific, the proportion of macro-aggregates in stand A was remarkably higher than that in stands B and C, but an inverse trend was observed for micro-aggregates. Besides, the MWD showed a significant difference between each stand, which was ranking as stand A > stand B > stand C.

Table 2. Soil aggregate distribution and stability under the different stand types

Soil layer	Stand type	MWD (mm)	Composition of soil aggregates (%)				Effects
			>2 mm	2–0.25 mm	< 0.25 mm		S	A	S × A
0–10 cm	Stand A	2.21 ± 0.10 a	51.90 ± 3.00 Aa	33.30 ± 4.40 Ba	14.80 ± 4.50 Cc		ns	**	**
	Stand B	1.96 ± 0.13 b	45.00 ± 3.90 Ab	31.20 ± 3.40 Ba	23.80 ± 4.20 Cb
	Stand C	1.54 ± 0.12 c	32.40 ± 3.00 Bc	31.80 ± 1.20 Ca	35.80 ± 5.00 Aa
10–20 cm	Stand A	2.10 ± 0.07 a	49.20 ± 1.40 Aa	31.50 ± 3.40 Ba	19.30 ± 4.30 Cc		ns	**	**
	Stand B	1.52 ± 0.09 b	33.10 ± 2.70 Bb	27.90 ± 1.90 Cab	39.00 ± 2.80 Ab
	Stand C	1.38 ± 0.12 c	29.70 ± 3.50 Bb	24.80 ± 2.90 Bb	45.50 ± 4.30 Aa

Note: Data represent the average of five replicates ± standard deviations. Different lower case letters indicate significant differences (p < 0.05) among the different stand types. Different capital letters indicate significant differences (p < 0.05) among the different aggregate sizes. S: stand type; A: aggregate size. ** and ns indicate significant differences at p < 0.01 and p > 0.05, respectively.

3.3. Soil aggregate-related C_org, N_tot, and P_ava contents

Soil aggregate-related C_org, N_tot, and P_ava contents exhibited an obvious variation depending on the interaction of stand type and aggregate size in both depths (Table 3). As shown in Figure 2, soil aggregate-related C_org and nutrient contents were decreased with the deepening of soil layer. Regardless of the stand type, the content of soil C_org was significantly increased as the aggregate size decreased. And the content of soil P_ava in meso-aggregates and micro-aggregates was significantly higher than that in macro-aggregates in 0–10 cm depth. In both depths, the content of soil C_org in stand A was remarkably higher than that in stands B and C, while the contents of soil N_tot and P_ava in stands A and B were significantly greater than those in stand C.

Table 3. Effects of stand type, aggregate size, and their interactions on the soil aggregate-related C_org and nutrient contents

Item	Effects
	0–10 cm soil layer				10–20 cm soil layer
	S	A	S × A		S	A	S × A
Organic C (g kg^−1)	**	**	**		**	**	**
Total N (g kg^−1)	**	ns	**		**	**	**
C/N ratio	ns	**	**		**	ns	**
Available P (mg kg^−1)	**	**	**		**	**	**
Total EB (cmol kg^−1)	ns	ns	ns		**	ns	ns
Exchangeable Ca^2+ (cmol kg^−1)	ns	ns	ns		**	ns	ns
Exchangeable Mg^2+ (cmol kg^−1)	*	ns	ns		**	ns	ns
Exchangeable K^+ (cmol kg^−1)	**	**	**		**	ns	ns
Exchangeable Na^+ (cmol kg^−1)	**	**	ns		**	**	ns
Rdm	**	**	ns		**	**	ns

Note: S: stand type; A: aggregate size. **, *, and ns indicate significant differences at p < 0.01, p < 0.05, and p > 0.05, respectively.

Figure 2. Soil aggregate-related C_org, N_tot, and P_ava contents under the different stand types. Data represent the average of five replicates and error bars represent standard deviations. Different lower case letters indicate significant differences (p < 0.05) among the different stand types. Different capital letters indicate significant differences (p < 0.05) among the different aggregate sizes

Figure 3. a. Soil aggregate-related exchangeable base contents under the different stand types. Data represent the average of five replicates and error bars represent standard deviations. b. Different lower case letters indicate significant differences (p < 0.05) among the different stand types. Different capital letters indicate significant differences (p < 0.05) among the different aggregate sizes

Figure 3. Continued

3.4. Soil aggregate-related exchangeable cation contents

Soil aggregate-related exchangeable cation contents exhibited an obvious variation depending on the stand type and aggregate size, especially in 0–10 cm depth (Table 3). As shown in Figure 3a and 3b, the contents of soil total EB and various exchangeable cations (that is K⁺, Na⁺, Ca²⁺, and Mg²⁺) were decreased with the deepening of soil layer. Among them, K⁺ and Mg²⁺ were mainly concentrated in micro-aggregates. Na⁺ was mainly concentrated in macro-aggregates in 0–10 cm depth but it was concentrated in micro-aggregates in 10–20 cm depth. Meanwhile, the contents of soil exchangeable cations were fallowed the order of Ca²⁺ > Mg²⁺ > K⁺ > Na⁺. Soil Mg²⁺ content in 0–10 cm depth, total EB, Na⁺, Ca²⁺ contents in 10–20 cm depth, and K⁺ content in both depths in stand C showed relatively high levels than those in stand B. Besides, the lowest R_dm appeared in micro-aggregates regardless of the stand type.

3.5. Soil aggregate-related C_org and nutrient stocks

Soil aggregate-related C_org and nutrient stocks exhibited an obvious variation depending on the aggregate size and stand type, especially the aggregate size (Table 4). Soil C_org and nutrient stocks in macro-aggregates and meso-aggregates in stands A and B were significantly greater than those in micro-aggregates, but it was opposite in stand C (Figure 4). For instance, soil C_org stock in macro-aggregates in stands A, B, and C was 918.26 g m⁻², 717.98 g m⁻², and 347.07 g m⁻², contributing to 17.5%, 36.2%, and 46.3% of C_org stock in bulk soil, respectively. And the stocks of soil N_tot, P_ava, and exchangeable cations displayed the similar trends. The stocks of soil C_org, N_tot, P_ava, and exchangeable Na⁺ in 0–10 cm depth and the stock of soil exchangeable Mg²⁺ in 10–20 cm depth in stand A showed higher levels than those in other stands. Besides, the stock of soil exchangeable Mg²⁺ in 0–10 cm depth and the stocks of soil exchangeable Na⁺ and Ca²⁺ in 10–20 cm depth in stands A and C were higher than those in stand B.

Table 4. Effects of stand type, aggregate size, and their interactions on the soil aggregate-related C_org and nutrient stocks

Storage	Effects
	0–10 cm soil layer				10–20 cm soil layer
	S	A	S × A		S	A	S × A
Organic C (g m^−2)	**	**	**		**	**	**
Total N (g m^−2)	**	**	**		**	**	**
Available P (g m^−2)	**	**	**		**	*	**
Total EB (cmol m^−2)	ns	**	**		*	**	**
Exchangeable Ca^2+ (cmol m^−2)	ns	**	**		*	**	**
Exchangeable Mg^2+ (cmol m^−2)	ns	**	**		**	**	**
Exchangeable K^+ (cmol m^−2)	**	**	**		**	*	**
Exchangeable Na^+ (cmol m^−2)	**	**	**		**	**	**

Note: S: stand type; A: aggregate size. **, *, and ns indicate significant differences at p < 0.01, p < 0.05, and p > 0.05, respectively.

Figure 4. Soil aggregate-related C_org and nutrient stocks under the different stand types. Data represent the average of five replicates and error bars represent standard deviations

4. Discussion

4.1. Soil aggregate distribution and stability

The distribution of soil aggregates influences the material circulation at aggregate scales, and determines the soil pore-size proportion (Ferro et al. 2012). Stable aggregates can reduce soil erosion and surface runoff, protect soil C_org, and improve soil fertility (Fattet et al. 2011; Wiesmeier et al. 2012). Thus, it is necessary to analyze the distribution and stability of soil aggregates to evaluate the soil structure. In this study, stands A and B (in 0–10 cm depth) benefited the formation of macro-aggregates, and stands B and C (in 10–20 cm depth) produced a significant increase in micro-aggregates, while no evident differences were found in meso-aggregates among the three stands. These results confirmed that the distribution of soil aggregates was effected by the stand type (Wang et al. 2019). The difference in soil aggregate stability among different stand types was indicated by the MWD among the three stands. The decline of MWD was caused by the variation in soil aggregate distribution, in particular for macro-aggregates were decomposed into micro-aggregates (Wang, Zhang, and Ye 2020), indicating that the soil aggregate stability of stand A was greater than that of other stand types, and stand C had the most unstable soil structure. Meanwhile, the stability of soil aggregates in upper soil layer (0–10 cm) was higher than that in lower layer (10–20 cm).

As we known that soil C_org was one of the most important aggregate binders, and the content of C_org was highly correlated with soil aggregate stability (Egan, Crawley, and Fornara 2018). The content of C_org in stands A and B was significantly higher than that in stand C, and decreased with the deepening of soil layer due to the reduction of soil macro-aggregates, it is also explained from another aspect that the soil structure of the mixed forests is better than that of pure forest. As a number of researches have presented (Ayoubi et al. 2012; Egan, Crawley, and Fornara 2018), there was a significantly positive correlation (p < 0.05) between C_org content and MWD in bulk soil across the three stand types in each soil layer (Figure 5). The peak of soil aggregate stability in 0–10 cm depth appeared in stand A, which might be due to the C_org content. Besides, the peak of soil aggregate stability in 10–20 cm depth also appeared in stand A, it might be related to the ratio (R_dm) of divalent cations to monovalent cations and C_org content, which have significant positive effects on the aggregate stability. Soil monovalent cations have dispersing effect on aggregates, especially Na⁺, while soil divalent cations have polymerization effect on aggregates. So the larger R_dm is good for the formation of macro-aggregates. In this study, the lowest R_dm in both depths was found in stand C (Table 1), which implied that the proportion of monovalent cations in stand C was higher than that in other stand types. This might explain the accumulation of micro-aggregates and the lower level of aggregate stability in stand C. Notably, although the positive relationships of soil aggregate stability with Fe/Al-oxide and clay contents have been reported, the present study indicated that the highest level of aggregate stability in the stand A was accompanied by no significant differences in the Fe/Al-oxide and clay contents among the different stand types (Table 1). Similarly, Araujo, Zinn, and Lal (2017) and Gartzia-Bengoetxea et al. (2020) also found that the change of aggregate stability was not affected by the Fe/Al-oxide and clay contents in forest ecosystems, mainly because soil structure primarily depends on the soil organic matters and tree litter residues, but soil Fe/Al-oxide and clay contents are mainly controlled by its parent material (Gelaw, Singh, and Lal 2015).

Figure 5. Relationships of soil aggregate stability (MWD) with the soil C_org content and R_dm across the different stand types

According to soil aggregates’ hierarchy concept (Six, Bossuyt, and Degryze 2004), quality of plant litter returning to soil determines litter distribution in the various sized aggregates, finally affecting the dynamics of soil aggregates. Compared to the pure Cunninghamia lanceolata stand, tree litter in the mixed forests (Cunninghamia lanceolata and Michelia macclurei, Mytilaria laosensis) showed higher availability (based on the smaller C/N ratio of litter), implying that the litter was easily combined into macro-aggregates during its decomposition process at these stand types, thus facilitating macro-aggregate formation (Wang, Wang, and Huang 2008; Niu, Wang, and Ouyang 2009). In additional, the quantity of tree litter in stand A, stand B, and stand C was 723.66 g m⁻², 566.84 g m⁻², and 340.58 g m⁻², respectively, which was ranking as stand A > stand B > stand C. The lower litter quantity and coverage in the pure stand boosted the rainfall eluviation, which further led to the destruction of soil macro-aggregates.

4.2. Soil aggregate-related C_org, N_tot, and P_ava contents

More C_org and N_tot contents in soil received the observation in the micro-aggregates in all the stands, because the adsorption capacity of soil aggregates for nutrients was directly proportional to the specific surface area. Under the same quality, the smaller the particles size of soil aggregates, the larger the specific surface area. And the turnover time of C_org in micro-aggregates was longer than that in macro-aggregates (Mangalassery et al. 2013). Thus, the C_org content was increased with the decrease of aggregate size. Soil C/N ratio is an important parameter affecting soil organic matter degradation (Ostrowska and Porebska 2015). Meanwhile, the soil C/N ratio was increased with the decrease of aggregate size, indicating that soil organic matters in micro-aggregates were more older compared to macro-aggregates. Besides, soil P_ava content was more distributed in meso-aggregates and micro-aggregates in 0–10 cm depth, but distributed evenly in different sized aggregates in 10–20 cm depth. Moreover, the content of soil P_ava in the three stands was lower than 7 mg kg⁻¹, which was a common phenomenon in Southern China. Because the soil in this area is generally acidic and the soil P_ava content was relatively low. With the deepening of soil layer, the C_org, N_tot, and P_ava contents in soil aggregates were decreased, which was consistent with the previous researches (Zhang et al. 2019; Lu et al. 2020).

Among the three stand types, mixed stands (stands A and B) could positively impacted the accumulation of soil C_org and N_tot in the aggregate fractions, which was mainly due to the following mechanisms. Firstly, the dead branches and leaves of Cunninghamia lanceolata have the characteristics of remaining in the crown before falling off, that was one of the reasons why the litter quantity of Cunninghamia lanceolata pure stand was the lowest, and litter residue was one of the sources of soil organic matters (Castellano et al. 2015). Secondly, in the mixed forests, the fine root biomass of Michelia macclurei and Mytilaria laosensis was larger in the surface soil, and the fine roots of vegetation in the topsoil had a significant positive impact on the decomposition of litter leaves (Jiang et al. 2011). Meanwhile, the quantity of dead fine roots rapidly decomposed under the action of microorganisms, returning a large amount of nutrients to the soil (Yin, Wheeler, and Phillips 2014). Thirdly, compared to Cunninghamia lanceolata, Michelia macclurei and Mytilaria laosensis have stronger nitrogen retention capacity (Wang, Wang, and Huang 2008; Niu, Wang, and Ouyang 2009), which can provide more nitrogen for soil, and the soil microbial activity was stronger in the nitrogen rich space. Moreover, soil C/N ratio did not remarkably change among the different stand types, because soil C and N are structural elements and their consumption and accumulation are relatively fixed (Zhou, Boutton, and Wu 2018).

4.3. Soil aggregate-related exchangeable cation contents

According to Adesodun, Adeyemi, and Oyegoke (2007), soil exchangeable cations were primarily focused on the 4.76–2 and <0.25 mm aggregate fractions in the non-cultivated soil. Nonetheless, soil exchangeable cation contents were significantly decreased in the 4.76–2 mm aggregate fractions and then were significantly increased in the <0.25 mm aggregate fractions during the cultivation process. These results indicated that soil exchangeable cation redistribution in aggregates of various sizes could be caused by the cultivation. By contrast, our results showed similar distribution patterns of these exchangeable cations in soil aggregates under different stand types, indicating that stand type made no remarkable difference to the distribution patterns of the exchangeable cation in soil aggregates. In this study, soil exchangeable cation contents were the highest in the micro-aggregates, which might be due to the wider specific surface exhibited by micro-aggregates. Soil aggregates with larger specific surface area could increase adsorption of soil exchangeable cations derived from litter residues (Adesodun, Adeyemi, and Oyegoke 2007). The trend with exchangeable cations indicated that the divalent cations (Ca²⁺ and Mg²⁺) were obviously higher than the monovalent cations (K⁺ and Na⁺), since divalent cations might be held at the exchange complexes in a tighter manner as compared with the monovalent cations (Jiang et al. 2011).

The total EB in bulk soil plays an important role in buffering soil acidification, because the higher total EB represents more H⁺ exchange sites. The more H⁺ exchange sites it has, the ability of soil acid resistance is stronger. In this study, the total EB contents in stands A and C were significantly higher than those in stand B, which indicated that the ability of soil acid resistance in stands A and C was better than stand B. Soil base cations are mainly derived from primary minerals in the soil, and the contents of base cations vary with edaphic properties, such as soil pH, organic matter fractions, and soil particle sizes (Wang et al. 2017). In this study, the contents of exchangeable cations in soil aggregates showed a pattern that Ca²⁺ > Mg²⁺ > K⁺ > Na⁺, which was the same as the findings of Wang, Zhang, and Ye (2020), because the exchangeable cations received the easy losing from soil aggregates based on leaching, especially the exchangeable K⁺ and Na⁺. Moreover, the exchangeable Na⁺ content in macro-aggregates was the highest in 0–10 cm depth but was the lowest in 10–20 cm depth, which might because the exchangeable Na⁺ in macro-aggregates was accumulated in the upper soil layer. In short, the monovalent cation contents in stand C were obviously higher than those in stands A and B, because the soil monovalent cations in pure forest have a strong biological cycle effect and the deep roots of pure forest can transform the deep non exchangeable bases into exchangeable bases, so as to improve the contents of soil monovalent cations in pure forest. Notably, it was evidently that the soil exchangeable K⁺ content was the highest in pure stand, because the soil exchangeable K⁺ was mainly affected by soil P_ava (Wen, Wang, and Wang 2019), and negatively correlated with P_ava. In acidic soil (Table 1), P_ava is adsorbed preferentially onto the clay mineral and Fe/Al oxide surface through forming different complexes, which can strongly compete with K⁺ for the sites of adsorption (Shen and Zhang 2011).

4.4. Soil aggregate-related C_org and nutrient stocks

In order to sustainably use soil resources, it is necessary to consider how to maintaining the soil C_org and nutrient stocks, because it noticeably impacts soil structure and fertility level (Sarker et al. 2018). In this study, the stocks of soil C_org and nutrients in macro-aggregates had more noticeably contributing effect in stands A and B. And the stocks of soil C_org and nutrients in micro-aggregates had higher contributing effect in stand C, which implied that the both macro-aggregates and micro-aggregates acted as the primary fractions that carried soil C_org and nutrients. The mentioned results indicated that the effect of stand type on the stocks of C_org and nutrients in soil aggregates largely depend on the composition of soil aggregates with different sizes. As the decrease of soil aggregate size, the soil C_org and nutrient stocks were decreased in stands A and B, and were increased in stand C. Besides, the results showed that the stocks of C_org, N_tot, P_ava, and exchangeable bases (including Na⁺, Ca²⁺, and Mg²⁺ cations) in stands A and B noticeably reached over those in stand C, and the reason was that the good soil structure of mixed stands had stronger ability to hold C_org and nutrients. Moreover, only the exchangeable K⁺ stock in stand C was more dominant than that in other two stands (stands A and B). That is because Cunninghamia lanceolata has the characteristics of fast-growing, needs to absorb a lot of nutrients and water from the soil for its own growth, so soil organic matter decomposition can be improved, but it is not conducive to the stocks of C_org and nutrients in the soil. In the pure forest, due to a small amount of tree litter, soil nutrient income is far less than the expenditure, which is not conducive to the accumulation of soil nutrients. In the mixed forests, however, the spatial structure, root distribution, and litter quality were better than those in pure stand, which increased the activity of soil microorganisms and improved nutrient cycling and utilization rate (Niu, Wang, and Ouyang 2009), thus promoting the forming process of soil macro-aggregates. Moreover, the pattern mixing coniferous and broad-leaved species increased the stocks of soil C_org and nutrients.

5. Conclusions

Compared to the Cunninghamia lanceolata pure forest (stand C), the soil structure of mixed forests of Cunninghamia lanceolata with Michelia macclurei (stand A) and Mytilaria laosensis (stand B) was more stable. Due to the larger amount of tree litter, the soil structure in stand A was better and stable than stand B. In all stand types, soil C_org, N_tot, and P_ava contents in aggregate fractions were significantly increased with decreasing aggregate size, while the exchange base cation (including Ca²⁺, Mg²⁺, K⁺, and Na⁺) contents mostly were decreased first and then increased. The two mixed stands displayed higher level of soil aggregate stability than the pure stand. Soil micro-aggregates acted as the main fractions that carried soil C_org, N_tot, and P_ava, and both micro-aggregates and macro-aggregates referred to the main fractions that carried exchangeable cations. In addition, soil C_org and nutrient stocks of the mixed forests (stands A and B) took an advantage over the pure forest (stand C). Thus, selecting suitable broadleaf tree species mixed with Cunninghamia lanceolata can alleviate the reduction of soil aggregate stability and the loss of soil nutrients, thus promoting soil resources to be sustainably utilized and protecting soil quality and health in southern Guangxi of China.

Disclosure of potential conflicts of interest

No potential conflict of interest was reported by the author(s).

Acknowledgments

This work was funded by the National Natural Science Foundation of China (No. 31460196). We thank editors and anonymous referees for their constructive comments on the early version of this manuscript, which greatly improved the quality of our article.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [31460196].