Mixed Plantations Induce More Soil Macroaggregate Formation and Facilitate Soil Nitrogen Accumulation

Abstract

Nitrogen plays a crucial role in limiting plant growth and determining net primary productivity in forest ecosystems. However, variations and influencing factors of soil nitrogen distribution on the aggregate scale in pure and mixed Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations remain unclear. This study aimed to explore how soil aggregate composition, geometric mean diameter (GMD), mean weight diameter (MWD), total nitrogen (TN), total acidolyzable nitrogen (TAN), available nitrogen (AN), nonacidolyzable nitrogen (AIN), amino acid nitrogen (AAN), nitrate nitrogen (NO₃⁻-N), cidolyzable ammonia nitrogen (AMN), unknown-acidolyzable nitrogen (HUN), amino sugar nitrogen (ASN), and ammonium nitrogen (NH₄⁺-N) contents and stocks varied with aggregate size (>2, 1–2, 0.25–1, and <0.25 mm) and stand type [Chinese fir mixed with Michelia macclurei Dandy (CF + MM), Chinese fir mixed with Mytilaria laosensis Lecomte (CF + ML) and pure stands of Chinese fir (CF)] in 0–20 and 20–40 cm soil depth. Soil N content in different stand types of Chinese fir plantations decreased as the aggregate size increased, whereas the soil N stock exhibited the opposite trend. In contrast to CF soil, CF + MM and CF + ML soil displayed a significant increase in MWD, GMD, and aggregate-associated TN, AN, NO₃⁻-N, NH₄⁺-N, AIN, AAN, ASN, and AMN contents and stocks, especially CF + MM soil. Organic N was more sensitive to the response of aggregate size and stand type than mineral N. Redundancy analysis and Pearson’s correlation analysis indicated that the 0.25–1 mm aggregate proportion was the main controlling factor for the variations in soil N content and storage. Overall, this study contributed significantly to the promotion of the sustainable use of soil resources and reference information for the scientific management and sustainable development of Chinese fir forests.

1. Introduction

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) has the characteristics of fast growth, fine material, high yield, and economic benefits and is broadly planted around the world, especially in China [1]. The planting area of Chinese fir plantations (CFPs) occupied 10.96 million hectares in 2019 [2]. Guangxi became the most major Chinese fir-planting province in China due to its geographical location and climatic conditions. However, CFPs were subject to successive cropping, short rotation periods, clearcutting, and unreasonable fertilization management practices for a long time, resulting in the constant decline of CFPs’ productivity, soil fertility, and the loss of biodiversity. To solve these problems, the conversion of pure stands into mixed stands, especially the construction of mixed Chinese fir (CF) and broadleaf tree species [3], is considered to be an effective way to alleviate the soil quality and productivity declines of successive cultivated pure plantations and to enhance the sustainable development of plantations [4]. Compared with pure CFPs, mixed CFPs have a better stand structure, quantity of plant residues, and litter decomposition rate. This can improve stand productivity and reduce soil nutrient loss. Conversion from pure forest to mixed forest can improve soil structure and enhance soil fertility [1]. Selecting suitable broadleaf species (such as Michelia macclurei Dandy) to mix with Chinese fir can effectively improve the soil’s physico-chemical properties, thus promoting the sustainability of soil resources and protecting soil quality and health and providing new plantation management measures [5].

Nitrogen (N) is not only one of the most critical indexes for the assessment of soil fertility but also the limiting element in forest ecosystems, determining the primary productivity of forest ecosystems [6]. Soil N acts as a major nutrient source for plant growth due to its availability and high mobility [7]. In general, soil organic nitrogen (ON) accounts for over 90% of total nitrogen (TN) and is the main source and reservoir of mineral nitrogen (MN) [8,9]. Furthermore, ON must undergo transformations to MN (such as nitrate nitrogen, NO₃⁻-N; ammonium nitrogen, NH₄⁺-N) by microbial mineralization, which allows it to be absorbed by plants [10]. Although the MN content in soil is at a relatively low level, it serves as the main available N source for plant growth because it is characterized by high mobility, activity, and availability. As a result, MN is often used as an indicator of soil N supply capacity and N effectiveness. As discussed above, the contents and availability of various N fractions play different regulatory roles in soil N cycling and transformation processes and are pivotal for maintaining soil fertility and keeping the carbon and N balance [11,12]. Notably, N is widely considered a limiting element for Chinese fir growth and development in subtropical China [13]. Meanwhile, soil N content and its distribution are mostly influenced by soil parent material, species composition, aboveground biomass, and vegetation type [14]. Moreover, previous studies reported that soil N is sensitive to the response of stand age [15], stand type [1], and soil aggregate size [16]. Mixed plantations have a stronger capacity for soil N accumulation than pure plantations [17]. However, these studies mainly focused on bulk soil N. Therefore, the impacts of forest management on N variation characteristics in aggregates and their controlling factors should be paid more attention, which provides a vital theoretical and practical guide for improving the quality of CFPs and soil fertility as well as mitigating soil degradation in subtropical regions.

Soil aggregates are not only building blocks of soil structure but also indicators for evaluating soil quality and health; their quantity and quality reflect the nutrient supply and storage capacity of the soil [18,19]. Moreover, aggregates also provide a key storage site for soil N fixation and N availability; different-sized aggregates have various effects on soil N adsorption, retention, supply, and transformation, further affecting the distribution and composition of different N fractions [20]. Therefore, N distribution in aggregates could directly affect soil N fixation and cycling. However, a large number of studies have focused on soil aggregation and organic carbon [21,22], while the characteristic of aggregate-associated nitrogen remains unclear. To better understand soil N fixation, some studies reported that soil aggregate-associated N content was significantly influenced by the aggregate size, indicating that N content decreased significantly as aggregate size decreased [23]; however, the contrary tendency was also observed in other research [24,25]. In addition, the changes in aggregate-associated TN content perform close contact with the variation in the proportion of aggregates [26]. Recent studies [4,14] further indicated that the macro-aggregate properties appear to be core mechanisms and sensitive indicators by providing physical protection for soil TN content. Moreover, soil C input can maintain and improve soil aggregate-associated N content, promoting its cycling and accumulation processes [27]. Additionally, aggregate-associated N content is significantly influenced by forest management practices [1]; specifically, mixed forests significantly increase the TN content with different-sized aggregates compared to pure forests [4]. Notably, the influencing factors of the changes in the characteristics of soil N have received widespread attention, but they remain unclear. Furthermore, exploring the variations in N content and stabilities in soil aggregates is critical to the biogeochemical cycling of terrestrial ecosystems. Thus, on the aggregate scale, the effect of stand type on soil N fractions as well as their main driving factors should be further explored to better understand the N fixation mechanism of soil aggregates in CFPs.

Previous studies demonstrated that stand type in CFPs exerts a remarkable influence on OC and nutrients in aggregates [1]. In addition, revealing the response of N within soil aggregates to stand type is essential to improve our understanding of the dominant mechanisms influencing soil N content and soil N fixation and supply capacity in CFPs. Nevertheless, the variation pattern of N content in soil aggregates of CFPs with different stand types remains unclear and should receive more attention. Hence, the effects of stand type in CFPs on N content and stock within different aggregate sizes (>2, 1–2, 0.25–1, and <0.25 mm) at 0 to 40 cm soil depth as well as their driving factors were determined in the present research. The hypotheses were as follows: (i) mixed plantations improved soil aggregate stability and thus promoted N content and stock within aggregates; (ii) soil N stock is highest in the >2 mm aggregates; (iii) 0.25–1 mm aggregate content is the main control factor affecting the variation in soil N content and storage in pure and mixed CFPs. The results are important for enhancing productivity and promoting soil N fixation in CFPs and provide valuable guidance for the virtuous N cycle and the rational strategies for soil N fertilizer management as well as forest management practices in plantation ecosystems.

2. Materials and Methods

2.1. Experiment Site

This research was conducted in the Qingshan Experimental Field (22°08′–22°44′ N, 106°33′–107°12′ E; Figure 1), which was established in 1992 and became the major region for planting Chinese fir in Longzhou County. The experimental site has a subtropical monsoon climate, with annual sunshine, mean temperature, and precipitation of approximately 1260 h, 21 °C, and 1400 mm, respectively [28]. Low mountains and hills are the major types of terrain, with gradients and elevations of 25 to 30° and 500 to 900 m, respectively. Sedimentary rock is the main native rock. The primary type of soil is laterite and red soil according to the Chinese soil classification, which is classified as a ferralsol based on the IUSS Working Group [29], with a loamy clay texture. Soil depth is generally over 80 cm [30]. As a result of long-term human disturbance, the native vegetation in the area has been almost destroyed, and many CFPs have been planted. Chinese fir mixed with Michelia macclurei and Chinese fir mixed with Mytilaria laosensis Lecomte are the primary experimental forests in this study region [1]. The study was conducted near nature management in the CFPs to decrease human disturbance; thus, the understory vegetation was subjected to natural succession. The shrubs in the understory of pure and mixed CFPs are mainly Maesa japonica (Thunb.) Moritzi, Melastoma candidum D. Don, Clerodendrum cyrtophyllum Turcz., and Rubus alceaefolius Poir., and the herbs are mostly Miscanthus floridulus (Labill.) Warb. ex K. Schum. & Lauterb., Cyrtococcum patens (L.) A. Camus, Pteris semipinnata L. Sp., and Blechnopsis orientalis (L.) C. Presl.

2.2. Experimental Design

In the current research, the dominant changes and controlling factors of soil N on aggregate scales as affected by stand types of CFPs (pure plantations of CF and mixed plantations of CF + MM and CF + ML) were explored in November 2020. Generally, there were confounding factors in the soil spatial variations. Therefore, the three stand types in similar units with geomorphologic status were selected to mitigate the effects of these possible confounders. Specifically, the spacing of the three stand types was 2 m × 3 m, and stand types CF + MM and CF + ML had a 3:1 mixed proportion. Each stand type was duplicated in quintuplicate. As a result, fifteen fixed 100 m × 100 m stands were completely randomized (Figure 1). Separation among the stands was made with >800 m between each other to avoid pseudoreplication and reduce the spatial autocorrelation. For each stand, a plot (20 m × 20 m) was randomly selected at more than 50 m across the stand edge.

2.3. Litter and Soil Sampling

In each plot, the five litter samples were randomly acquired from the five subplots (1 m × 1 m) from the soil surface. The thickness of the litter layer was 5 to 10 cm. Afterward, a total of fifteen (3 stand types × 5 replicates) mixed litter specimens were obtained and desiccated at 80 °C using an oven. The quantity of composite litter specimens was measured. Then, the five soil samples were collected from the 0 to 20 and 20 to 40 cm soil layers via spades in the same locations as the litter specimens. Subsequently, the thirty soil specimens produced (3 stand types × 5 replicates × 2 soil depths) were separated into natural aggregates. The coarse roots, stones, and macrofauna in the soil specimens were removed manually using 5 mm sieves before soil aggregate separation. For each plot, the other 5 soil specimens were acquired in the 5 subplots by cutting rings from 0 to 20 cm and 20 to 40 cm soil layers, respectively. Then, these soil specimens were prepared to measure the pH, OC, total porosity (Pt), bulk density (BD), available phosphorus (AP), TN, NO₃⁻-N, NH₄⁺-N, available nitrogen (AN), total acidolyzable nitrogen (TAN), amino sugar nitrogen (ASN), amino acid nitrogen (AAN), nonacidolyzable nitrogen (AIN), cidolyzable ammonia nitrogen (AMN), and unknown-acidolyzable nitrogen (HUN) in bulk soil.

2.4. Soil Aggregate Separation

In this study, sieves of various sizes (2, 1, and 0.25 mm) were used to separate the soil specimens (250 g) successively via the method of wet sieving [31]. Subsequently, the separation of various-sized aggregates was achieved and generated macro-(>2 mm), medium-(1–2 mm), small-(0.25–1 mm), and micro-(<0.25 mm) aggregates. The N content within different-sized aggregates was determined.

2.5. Soil Property Analyses

Soil BD and Pt were obtained through the cutting ring (V = 100 cm⁻³, Ø = 50.46 mm, and depth = 50 mm). Soil samples were dried in an oven at 105 °C to the constant weight for detecting BD and Pt [1,32]. Soil pH was measured using a glassy electrode in a 1:2.5 soil/distilled water suspension [31]. OC was determined via the approach from Nelson and Sommers [33]. AP was estimated using the Olsen approach [34]. TN and TAN were obtained via the micro-Kjeldahl method [35]. The ON compounds were determined according to the methods of Bremner [35]. AN and AMN were found by steam distillation with MgO. The AAN and ASN were measured by the methods from Lin [11]. HUN was calculated using the following formula [11]: HUN = TAN-AMN-AAN-ASN. AIN was measured by subtracting TAN and TN. NO₃⁻-N was measured by the ultraviolet spectrophotometry indigo blue colorimetric [36]. The indigo blue colorimetric was used for detecting NH₄⁺-N [36].

2.6. Statistical Analysis

The mean weight diameter (MWD, mm) and geometric mean diameter (GMD, mm) are the key indicators to evaluate the stability of soil aggregates, and the formulae of the MWD and GMD were as follows [37]:

$$MWD={\displaystyle \sum _{i=1}^4({\mathrm{X}}_i}\times {\mathrm{W}}_i)$$

(1)

$$GMD=\exp [({\displaystyle \sum _{i=1}^4{\mathrm{W}}_i}\ln {\mathrm{X}}_i)/({\displaystyle \sum _{i=1}^4{\mathrm{W}}_i})]$$

(2)

where X_i (mm) is the mean diameter of the ith-sized aggregates and W_i (% in weight) is the proportion of the ith-sized aggregates in bulk soil.

The formulae of soil TN stock (TNS, g m⁻²) was as follows [38]:

$$TNS=\sum _{i=1}^4({\mathrm{X}}_i\times {\mathrm{W}}_i)\times \mathrm{B}\times \mathrm{H}\times 10$$

(3)

where B (g cm⁻³), H (cm), and X_i (g kg⁻¹) denote the bulk density, soil depth, and TN content within ith size aggregates, respectively. Ten stands for the unit conversion factor. Similarly, soil AN, TAN, AIN, AAN, ASN, AMN, HUN, NO₃⁻-N, and NH₄⁺-N stocks (ANS, TANS, AINS, AANS, ASNS, AMNS, HUNS, NO₃⁻-NS, and NH₄⁺-NS, respectively) were also calculated.

Stand type and aggregate size were two major variables in the present study. Thus, SPSS 22.0 was used to conduct data statistics by soil layer (

Table 1. Litter and soil properties as influenced by aggregate size and stand type in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations.

Variable	Litter	0–20 cm Soil Depth			20–40 cm Soil Depth
	S	S	A	S × A	S	A	S × A
Litter quantity (g m−2)	√√
Bulk density (g cm−3)		√√			√√
Total porosity (%)		√√			√√
pH		√√			√√
Soil aggregate proportion		×	√√	√√	×	√√	√√
MWD		√√	√√	√√	√√	√√	√√
GMD		√√	√√	√√	√√	√√	√√
OC (g kg−1)		√√	√√	×	√√	√√	√√
AP (g kg−1)		√√	√√	√√	√	√	×
TN (g kg−1)		√√	√√	√√	√√	√√	×
AN (mg kg−1)		√√	√√	×	√√	√√	×
TAN (mg kg−1)		√√	√√	√√	√√	√√	√√
AIN (mg kg−1)		√√	√	√√	√√	√	√
AAN (mg kg−1)		√√	√√	√	√√	√√	×
ASN (mg kg−1)		√√	√√	√√	√√	×	√√
AMN (mg kg−1)		√√	√√	×	√√	√√	×
HUN (mg kg−1)		√√	√√	√√	√√	√√	√
NO3−-N (mg kg−1)		√√	√√	×	√√	√√	×
NH4+-N (mg kg−1)		√√	√√	√√	√√	√√	√√
TNS (g m−2)		√√	√√	√√	√√	√√	√√
ANS (g m−2)		√√	√√	×	√√	√√	×
TANS (g m−2)		√√	√√	√√	√√	√√	√√
AINS (g m−2)		√√	√√	√√	√√	√√	√√
AANS (g m−2)		√√	√√	√√	√√	√√	√√
ASNS (g m−2)		√√	√√	√√	×	√√	√√
AMNS (g m−2)		√√	√√	√√	√√	√√	√√
HUNS (g m−2)		√√	√√	√√	√√	√√	√√
NO3−-NS (g m−2)		√√	√√	√√	√√	√√	√√
NH4+-NS (g m−2)		×	√√	√√	√√	√√	√√

S: stand type. A: aggregate size. MWD and GMD stand for mean weight diameter and geometric mean diameter, respectively. OC, AP, TN, AN, TAN, AIN, AAN, ASN, AMN, HUN, NO₃⁻-N, and NH₄⁺-N represent the contents of organic carbon, available phosphorus, total nitrogen, available nitrogen, total acidolyzable nitrogen, nonacidolyzable nitrogen, amino acid nitrogen, amino sugar nitrogen, cidolyzable ammonia nitrogen, unknown-acidolyzable nitrogen, nitrate nitrogen, and ammonium nitrogen in bulk soil, respectively. TNS, ANS, TANS, AINS, AANS, ASNS, AMNS, HUNS, NO₃⁻-NS, and NH₄⁺-NS represent the stocks of organic carbon, available phosphorus, total nitrogen, available nitrogen, total acidolyzable nitrogen, nonacidolyzable nitrogen, amino acid nitrogen, amino sugar nitrogen, cidolyzable ammonia nitrogen, unknown-acidolyzable nitrogen, nitrate nitrogen, and ammonium nitrogen in bulk soil, respectively. ×: no significant differences. √: p < 0.05; √√: p < 0.01.

). All datasets were tested for homogeneity of variances and normal distribution of residuals prior to doing an analysis of variance (ANOVA) in order to ascertain that the supposition of statistical analysis was met. One-way and two-way ANOVAs were utilized to explore how stand type affected the quantity of litterfall and physico-chemical properties in the bulk soil and how aggregate size and stand type affected properties in aggregates. The post hoc Tukey HSD test was used to test the significance in various stand types and aggregate sizes, with statistical significance defined as p < 0.05. Redundancy analysis using CANOCO 5.0 helped test the influence of soil aggregate parameters on the content and stock of N in bulk soil. Moreover, the association of soil aggregate parameters with the nitrogen content and stock in bulk soil was assessed via Pearson’s correlation.

3. Results

3.1. Litter and Bulk Soil Properties

The litter quantity, soil Pt, and NH₄⁺-N in CF + MM and CF + ML had significantly higher levels than those in CF, whereas the soil BD and pH showed opposite trends (

Table 2. Litter and bulk soil properties.

Variable	0–20 cm Soil Depth			20–40 cm Soil Depth
	CF + MM	CF + ML	CF	CF + MM	CF + ML	CF
Litter quantity (g cm−2)	504 ± 27 a	455 ± 23 b	324 ± 12 c
Soil texture	Clay	Clay	Clay	Clay	Clay	Clay
Bulk density (g cm−3)	1.24 ± 0.01 b	1.26 ± 0.01 b	1.31 ± 0.02 a	1.26 ± 0.01 b	1.27 ± 0.01 b	1.33 ± 0.01 a
Total porosity (%)	53.13 ± 0.22 a	52.38 ± 0.38 a	50.42 ± 0.28 b	52.60 ± 0.19 a	52.92 ± 0.19 a	49.96 ± 0.83 b
pH	4.31 ± 0.01 c	4.33 ± 0.01 b	4.37 ± 0.00 a	4.27 ± 0.01 b	4.30 ± 0.01 b	4.41 ± 0.02 a
OC (g kg−1)	33.34 ± 1.16 a	24.80 ± 0.84 b	20.96 ± 0.42 c	17.16 ± 0.82 a	10.17 ± 0.73 b	7.25 ± 0.47 c
AP (g kg−1)	3.93 ± 0.10 a	3.75 ± 0.19 ab	3.36 ± 0.13 b	2.54 ± 0.03 b	3.11 ± 0.22 a	2.64 ± 0.03 b
TN (g kg−1)	1.46 ± 0.01 a	1.25 ± 0.03 b	1.26 ± 0.01 b	1.21 ± 0.03 a	0.75 ± 0.01 b	0.68 ± 0.02 c
C/N	22.9 ± 0.88 a	19.92 ± 0.53 b	16.67 ± 0.33 c	14.14 ± 0.61 a	13.38 ± 0.86 a	10.67 ± 0.40 b
AN (mg kg−1)	259.51 ± 12.46 a	163.54 ± 6.40 b	160.06 ± 6.47 b	184.40 ± 12.33 a	136.98 ± 7.03 b	124.21 ± 8.64 b
TAN (mg kg−1)	763.57 ± 5.16 c	1059.65 ± 8.63 a	797.65 ± 7.10 b	635.07 ± 7.70 a	523.61 ± 5.23 c	607.06 ± 12.33 b
AIN (mg kg−1)	401.83 ± 10.78 b	479.33 ± 16.02 a	454.74 ± 6.86 a	573.08 ± 31.36 a	231.07 ± 10.02 b	66.14 ± 3.47 c
AAN (mg kg−1)	285.26 ± 11.30 a	241.94 ± 3.80 b	200.63 ± 4.85 c	233.98 ± 6.04 a	175.06 ± 6.02 c	201.90 ± 4.81 b
ASN (mg kg−1)	81.48 ± 3.03 a	70.95 ± 3.86 b	59.15 ± 2.28 c	71.84 ± 5.08 a	71.48 ± 4.75 a	57.46 ± 3.60 a
AMN (mg kg−1)	277.40 ± 2.64 a	217.89 ± 4.17 b	213.38 ± 1.76 b	220.09 ± 4.56 a	160.40 ± 3.52 b	129.62 ± 5.55 c
HUN (mg kg−1)	415.51 ± 11.44 a	232.80 ± 4.42 c	324.5 ± 5.97 b	109.17 ± 5.80 b	116.66 ± 8.30 b	218.09 ± 13.50 a
NO3−-N (mg kg−1)	1.75 ± 0.07 b	2.18 ± 0.05 a	2.09 ± 0.10 a	0.95 ± 0.10 c	1.95 ± 0.03 a	1.67 ± 0.07 b
NH4+-N (mg kg−1)	7.54 ± 0.25 a	7.34 ± 0.08 a	6.93 ± 0.11 b	6.83 ± 0.26 a	6.62 ± 0.22 a	5.23 ± 0.12 b

Data represent the mean of 5 replicates ± standard deviations. CF + MM, CF + ML, and CF represent mixed plantations of (Cunninghamia lanceolata (Lamb.) Hook) and Michelia macclurei Dandy, Chinese fir and Mytilaria laosensis Lecomte, and pure plantations of Chinese fir. Different lowercase letters represent significant differences (p < 0.05) among different stand types.

). Soil OC, TN, AN, AAN, AMN, and NH₄⁺-N in CF + MM had significantly higher levels than those in CF + ML and CF among 0–40 cm (Table 2). Soil OC, AP, TN, AN, TAN, AAN, AMN, HUN, NO₃⁻-N, and NH₄⁺-N in the three stands decreased with increasing soil depth, whereas soil BD presented the opposite trend.

3.2. Characteristics of Soil Aggregates

The content of soil aggregates among the three stands exhibited a significant increase with increasing aggregate size at the 0–20 and 20–40 cm depths (

Table 3. Distribution and stability of soil aggregates as influenced by stand type in Chinese fir plantations.

Soil Depth	Stand Type	Aggregate Composition (%)				MWD mm	GMD mm
		>2 mm	1–2 mm	0.25–1 mm	<0.25 mm
0–20 cm	CF + MM	47.93 ± 0.12 Aa	21.84 ± 0.20 Bc	20.90 ± 0.13 Cc	9.34 ± 0.05 Da	3.35 ± 0.01 a	1.93 ± 0.01 a
	CF + ML	44.43 ± 0.15 Ab	23.56 ± 0.13 Bb	23.51 ± 0.05 Bb	8.49 ± 0.05 Cb	3.18 ± 0.02 b	1.83 ± 0.01 b
	CF	37.65 ± 0.06 Ac	28.60 ± 0.06 Ba	26.57 ± 0.04 Ca	7.18 ± 0.03 Dc	2.86 ± 0.01 c	1.68 ± 0.01 c
	Mean	43.34	24.67	23.66	8.34	3.13	1.81
20–40 cm	CF + MM	45.34 ± 0.10 Aa	23.78 ± 0.13 Bb	18.92 ± 0.09 Cc	11.96 ± 0.12 Da	3.21 ± 0.01 a	1.77 ± 0.02 a
	CF + ML	37.21 ± 0.22 Ab	27.21 ± 0.17 Ba	24.78 ± 0.11 Cb	10.80 ± 0.07 Db	2.81 ± 0.02 b	1.55 ± 0.01 b
	CF	35.22 ± 0.10 Ac	27.05 ± 0.07 Ba	27.96 ± 0.08 Ca	9.77 ± 0.07 Dc	2.71 ± 0.01 c	1.50 ± 0.01 c
	Mean	39.26	26.01	23.89	10.84	2.91	1.61

Data stand for the mean of 5 replicates ± standard deviations. Different capital letters stand for significant differences (p < 0.05) among different aggregate sizes in the same forest types. Different lowercase letters represent significant differences (p < 0.05) among different stand types with the same fraction.

). Specifically, macro-aggregates presented the highest proportion with mean values of 43.34% and 39.26%, respectively, and the lowest proportions were within micro-aggregates with only 8.34% and 10.84% (Table 3). The macro-aggregate and micro-aggregate proportions displayed markedly greater levels among CF + MM and CF + ML than that of the CF. However, there were inverse trends in small-aggregates, medium-aggregates, and macro-aggregates. At both depths, the soil MWD and GMD values of CF + MM and CF + ML were significantly higher than those of CF, and the highest levels were observed in CF + MM (Table 3).

3.3. Aggregate-Associated Nitrogen Content

On the aggregate scales, TN (Figure 2A,B), AN (Figure 2C,D), NH4+-N (Figure 2G,H), TAN (Figure 3A,B), AIN (Figure 3C,D), AAN (Figure 3E,F), AMN (Figure 3I,J), and HUN (Figure 3K,L) contents in the 0 to 40 cm depths of the three stands showed a significant rise with reducing aggregate size and were highest in micro-aggregates. However, the NO₃⁻-N content (Figure 2E,F) was mostly distributed in small-aggregates. TN, AN, NH₄⁺-N, TAN, AAN, AIN, ASN (Figure 3G,H), and AMN contents in different-sized aggregates of CF + MM and CF + ML exhibited significantly higher levels than CF at both depths. However, NO₃⁻-N content within aggregates was in the following order: CF + ML > CF > CF + MM. On the aggregate scales, the N content in the three stands decreased with increasing soil depth.

3.4. Aggregate-Associated Nitrogen Stock

At the aggregate scales, the soil TN and its fraction stocks (Figure 4A–J) with their contributions (Figure 5A–J) in the three stands were mostly distributed in macro-aggregates at both depths. For instance, the TNS of macro-aggregates in the three stands varied from 12.31 to 16.15 g m⁻² (0–20 cm) and 5.86 to 13.62 g m⁻² (20–40 cm) (Figure 4A), accounting for 37.39% to 44.61% (0–20 cm) and 32.84% to 44.78% (20–40 cm) (Figure 5A) of the TNS in the bulk soil, respectively. In the three stands, aggregate-associated TNS, ANS (Figure 4B), TANS (Figure 4E), ASNS (Figure 4F), AMNS (Figure 4H), AANS (Figure 4I), and HUNS AANS (Figure 4J) were significantly greater in CF + MM than in CF + ML and CF at 0–20 cm. However, aggregate-associated NO₃⁻-NS (Figure 4C) and AINS displayed opposite trends. Notably, there was no significant variation in aggregate-associated NH₄⁺-NS (Figure 4D) among the three stands. Regardless of the stand type, aggregate-associated TNS, ANS, NH₄⁺-NS, AINS (Figure 4G), AANS, and AMNS were significantly higher in CF + MM than in CF + ML and CF. Conversely, there were opposite tendencies in aggregate-associated NO₃⁻-NS and HUNS. Irrespective of the soil layer, no significant variation was found in aggregate-associated ASNS among the three stands. Regardless of the stand type, the aggregate-associated N stock decreased with increasing soil depth.

3.5. Relationship between Soil Aggregates and Nitrogen Content

The relative influences of soil aggregate parameters (including proportions of the micro-, small-, medium-, and macro-aggregates, MWD, and GMD) on the N content in bulk soil (including TN, AN, NO₃⁻-N, NH₄⁺-N, TAN, AIN, AAN, ASN, AMN, and HUN) in pure and mixed CFPs were determined via RDA (Figure 6). In addition, the variation in the soil N content of the three stands explained 87.73% (0–20 cm) (Figure 6A) and 91.62% (20–40 cm) (Figure 6B) of the total of the six soil aggregate parameters. In both soil layers, the effect order was as follows: small-aggregates proportions > medium-aggregates proportions > GMD > MWD > micro-aggregates proportions > macro-aggregates proportions. The results of the Pearson’s correlation analysis (

Table 4. Pearson’s correlation of soil N content and soil aggregate parameters.

Soil Depth	Response Variable	Soil Aggregate Parameters
		>2 mm	1–2 mm	0.25–1 mm	<0.25 mm	MWD mm	GMD mm
0–20 cm	TN (g kg−1)	0.696 **	−0.632 *	−0.771 **	0.718 **	0.702 **	0.724 **
	AN (mg kg−1)	0.732 **	−0.679 **	−0.797 **	0.763 **	0.737 **	0.752 **
	TAN (mg kg−1)	0.682 **	−0.61 *	−0.776 **	0.725 **	0.689 **	0.71 **
	AIN (mg kg−1)	−0.423	0.362	0.509	−0.467	−0.429	−0.448
	AAN (mg kg−1)	0.9 **	−0.881 **	−0.914 **	0.916 **	0.901 **	0.9 **
	ASN (mg kg−1)	0.821 **	−0.818 **	−0.817 **	0.836 **	0.82 **	0.814 **
	AMN (mg kg−1)	0.777 **	−0.719 **	−0.851 **	0.818 **	0.783 **	0.797 **
	HUN (mg kg−1)	0.323	−0.23	−0.452	0.372	0.333	0.366
	NO3−-N (mg kg−1)	−0.452	0.395	0.553 *	−0.551 *	−0.457	−0.459
	NH4+-N (mg kg−1)	0.596 *	−0.577 *	−0.623 *	0.634 *	0.597 *	0.588 *
20–40 cm	TN (g kg−1)	0.983 **	−0.965 **	−0.956 **	0.896 **	0.982 **	0.981 **
	AN (mg kg−1)	0.812 **	−0.8 **	−0.803 **	0.799 **	0.81 **	0.797 **
	TAN (mg kg−1)	0.507	−0.664 **	−0.369	0.244	0.501	0.527 *
	AIN (mg kg−1)	0.976 **	−0.916 **	−0.98 **	0.944 **	0.976 **	0.969 **
	AAN (mg kg−1)	0.707 **	−0.797 **	−0.605 *	0.485	0.704 **	0.726 **
	ASN (mg kg−1)	0.411	−0.258	−0.471	0.41	0.419	0.427
	AMN (mg kg−1)	0.952 **	−0.884 **	−0.972 **	0.966 **	0.952 **	0.939 **
	HUN (mg kg−1)	−0.648 **	0.463	0.755 **	−0.797 **	−0.655 **	−0.632 *
	NO3−-N (mg kg−1)	−0.853 **	0.913 **	0.769 **	−0.658 **	−0.851 **	−0.867 **
	NH4+-N (mg kg−1)	0.639 *	−0.473	−0.717 **	0.701 **	0.646 **	0.639 *

** and * represent significant correlations at p < 0.01 and p < 0.05, respectively.

) were relatively consistent with the RDA results. At 0 to 20 cm depth, small-aggregates and medium-aggregates proportions had significant negative associations with TN, AN, TAN, AAN, ASN, AMN, and NH₄⁺-N content, whereas the opposite pattern was observed within macro-aggregate and micro-aggregate proportions with these N contents, GMD, as well as MWD. At 20 to 40 cm depth, small-aggregates and medium-aggregates proportions had significant negative associations with TN, AN, AAN, AIN, AMN, and NH₄⁺-N content, but there were opposite trends within macro-aggregates and micro-aggregates proportions, GMD, and MWD with these N contents.

3.6. Relationship between Soil Aggregates and Nitrogen Stock

The relative influences of soil aggregate parameters on the N stock in bulk soil (including TNS, ANS, NO₃⁻-NS, NH₄⁺-NS, TANS, AINS, AANS, ASNS, AMNS, HUNS) among the three stand types were determined via RDA (Figure 7). Furthermore, a total of the six soil aggregate parameters together explained 85.68% (0–20 cm) (Figure 7A) and 91.27% (20–40 cm) (Figure 7B) of the variation in the soil N stock, respectively. In both soil layers, the effect order was as follows: small-aggregates proportions > macro-aggregates proportions > medium-aggregates proportions > GMD > MWD > micro-aggregates proportions. According to

Table 5. Pearson’s correlation of soil N stock and soil aggregate parameters.

Soil Depth	Response Variable	Soil Aggregate Parameters
		>2 mm	1–2 mm	0.25–1 mm	<0.25 mm	MWD mm	GMD mm
0–20 cm	TNS (g kg−1)	0.696 **	−0.632 *	−0.771 **	0.718 **	0.702 **	0.724 **
	ANS (mg kg−1)	0.732 **	−0.679 **	−0.797 **	0.763 **	0.737 **	0.752 **
	TANS (mg kg−1)	0.682 **	−0.61 *	−0.776 **	0.725 **	0.689 **	0.71 **
	AINS (mg kg−1)	−0.423	0.362	0.509	−0.467	−0.429	−0.448
	AANS (mg kg−1)	0.9 **	−0.881 **	−0.914 **	0.916 **	0.901 **	0.9 **
	ASNS (mg kg−1)	0.821 **	−0.818 **	−0.817 **	0.836 **	0.82 **	0.814 **
	AMNS (mg kg−1)	0.777 **	−0.719 **	−0.851 **	0.818 **	0.783 **	0.797 **
	HUNS (mg kg−1)	0.323	−0.23	−0.452	0.372	0.333	0.366
	NO3−-NS (mg kg−1)	−0.452	0.395	0.553 *	−0.551 *	−0.457	−0.459
	NH4+-NS (mg kg−1)	0.596 *	−0.577 *	−0.623 *	0.634 *	0.597 *	0.588 *
20–40 cm	TNS (g kg−1)	0.983 **	−0.965 **	−0.956 **	0.896 **	0.982 **	0.981 **
	ANS (mg kg−1)	0.812 **	−0.8 **	−0.803 **	0.799 **	0.81 **	0.797 **
	TANS (mg kg−1)	0.507	−0.664 **	−0.369	0.244	0.501	0.527 *
	AINS (mg kg−1)	0.976 **	−0.916 **	−0.98 **	0.944 **	0.976 **	0.969 **
	AANS (mg kg−1)	0.707 **	−0.797 **	−0.605 *	0.485	0.704 **	0.726 **
	ASNS (mg kg−1)	0.411	−0.258	−0.471	0.41	0.419	0.427
	AMNS (mg kg−1)	0.952 **	−0.884 **	−0.972 **	0.966 **	0.952 **	0.939 **
	HUNS (mg kg−1)	−0.648 **	0.463	0.755 **	−0.797 **	−0.655 **	−0.632 *
	NO3−-NS (mg kg−1)	−0.853 **	0.913 **	0.769 **	−0.658 **	−0.851 **	−0.867 **
	NH4+-NS (mg kg−1)	0.639 *	−0.473	−0.717 **	0.701 **	0.646 **	0.639 *

** and * represent significant correlations at p < 0.01 and p < 0.05, respectively.

, Pearson’s correlation analysis results were relatively consistent with the RDA results. At 0 to 20 cm depth, ANS, AANS, ASNS, and AMNS had significant negative associations with small-aggregates and medium-aggregates contents and had significant positive associations with macro-aggregates and micro-aggregates proportions, GMD, and MWD. At 20 to 40 cm depth, TNS, ANS, AINS, and AMNS had significant negative associations with small-aggregates and medium-aggregates proportions and had significant positive associations with macro-aggregates and micro-aggregates proportions, GMD, and MWD.

4. Discussion

4.1. Aggregate-Associated Nitrogen Content

Understanding the distribution of aggregate-associated N content is vital for mitigating soil degradation, regulating soil fertility, and increasing soil nutrient storage. It was found that micro-aggregates had the highest distribution of TN, AN, NH₄⁺-N, TAN, AAN, AIN, AMN, and HUN contents, indicating that micro-aggregates improved adsorption to soil N compared with other sized aggregates. This may be because the smaller the aggregate size is, the larger the specific surface area [39], thus increasing the elevated adsorption to soil organic matter and facilitating the enrichment in the contents of TN and its fractions. The loamy clay soil has a larger specific surface area, stronger cation exchange capacity, and higher total porosity and organic matter content but less macroporosity than that of the sandy soil, which has a pronounced effect on the supply, conservation, and transformation of soil nutrients [40,41]. On the other hand, clay fraction plays an important role in the formation of soil aggregates, which can provide protection through physical entrapment for most organic materials to retard the decomposition process of soil organic matter [42,43]. Moreover, compared to macro-aggregates, micro-aggregates have a higher stability and availability of N due to the greater specific surface area and the lower N loss in micro-aggregates [44]. Similarly, micro-aggregates have lower air permeability and physical protection than macro-aggregates, which is not conducive to N transformation but rather to N accumulation. In addition, coupling effects exist in forest soil C and N; TN is positively correlated with OC at the aggregate scale [25]. OC in micro-aggregates decomposes slowly and contributes to long-term storage, with a greater ability to sequester OC and a low microbial decomposition rate [45], thus resulting in improving the ability of micro-aggregates to absorb and accumulate N derived from litter residues and root exudates.

The conversion of forests affects the soil’s physical, chemical, and biological properties [46]. Plant litter is the most important source of soil N content and can directly increase soil N content [47]. Meanwhile, the input of readily degradable components and effective nutrients derived from fresh plant litter can promote the mineralization of existing N in the soil by promoting the reproduction and enzymatic activity of soil microorganisms [48,49]. Therefore, compared to CF, CF + MM and CF + ML were beneficial in increasing TN, AN, NH₄⁺-N, TAN, AAN, AIN, ASN, and AMN content in soil aggregates. This can be explained by the significantly higher litter quality and OC content in CF + MM (litter quality: 504 g cm⁻²; OC: 33.34 g kg⁻¹) and CF + ML (455 g cm⁻²; 24.80 g kg⁻¹) than in CF (324 g cm⁻²; 20.96 g kg⁻¹). There is a collaboration between TN and OC in aggregates [25]. Additionally, the high litter quality in the mixed forest not only increased N source input but also significantly promoted organic matter decomposition. C and N are important components of organic matter, providing more organic cementing material to promote macroaggregate formation [50]. Macroaggregates can enhance the physical protection for organic matter and reduce the rate of microbial decomposition of plant litter to increase OC content [18], thus increasing the soil N content. In addition, there is a stronger ability to supply and reserve soil N content in Michelia macclurei and Mytilaria laosensis than in Chinese fir [51]. The soil aggregates’ stability and ability to absorb and maintain N varies among different-sized aggregates [52], further impacting the distribution and composition of TN and its fractions. Likewise, previous studies have demonstrated that there is a significantly positive correlation between soil N composition and soil aggregate stability [53]. However, soil aggregate stability has a negative correlation with inorganic nitrogen (NO₃⁻-N and NH₄⁺-N) [54]. In this study, the MWD and GMD values were significantly higher in CF + MM and CF + ML than in CF, indicating that mixed stands were beneficial for improving soil aggregate stability, promoting the conversion of small aggregates to large aggregates, and protecting the soil structure from rainfall erosion to reduce N loss, thereby facilitating N content in mixed CFPs. Previous studies also confirmed that the content of N is affected by soil aggregation and structure, which mitigate soil erodibility and nutrient loss and improve soil fertility [18]. The increase in soil pH significantly reduced net nitrogen mineralization, net ammonification, and net nitrification rates [14]. We found that soil pH was significantly higher in pure stands of Chinese fir than in mixed stands of Chinese fir in this research. Moreover, soil acidification reduced the number and enzymic activity of microorganisms, resulting in slower rates of soil organic matter decomposition and ammonium ion conversion, thus further affecting the distribution pattern of soil N content [55]. In addition, the results showed that the regular changes in MN content in aggregates were not more significant than those in ON content in aggregates due to the low MN content with high activity. In particular, aggregate-associated NO₃⁻-N is easily absorbed and used by plants and microorganisms and lost from the soil through different pathways. However, the highest MN content in aggregates was also found in mixed CFPs. The above findings can support our first hypothesis.

In this study, the aggregate-associated N content in pure and mixed CFPs declined as soil depth deepened. A previous study has shown that soil depth plays a key role in changing soil TN content [56]. Soil N content in aggregates decreases as soil depth increases during Chinese fir planting [26]. As the soil layer decreased, soil BD decreased and Pt increased (Table 2), resulting in an increase in soil permeability and looseness and an improvement in soil nutrient absorption capacity. To be specific, the macro-aggregates proportion, MWD, and GMD values were higher in 0–20 cm soil depth than in 20–40 cm soil depth (Table 3), suggesting that the soil aggregate stability in 0–20 cm was higher than that in 20–40 cm, thus accumulating the highest organic materials from root exudates and litterfall in the upper soil layer. In addition, the large litter quality on the soil surface increased the organic matter input [57]. Likewise, the litter decomposition rate on the soil surface accelerated and increased microorganism activities to improve nutrient cycling and utilization rates [1], thereby promoting soil N accumulation.

4.2. Aggregate-Associated Nitrogen Stock

Aggregates play crucial roles in protecting soil N loss since aggregates could mediate the effect of soil structure on N accumulation. The distribution of soil N stocks in pure and mixed CFPs was preoccupied with macro-aggregates in 0–40 cm (Figure 3), indicating that macro-aggregates act as the primary carriers of N stock. This was because macro-aggregates are formed gradually through the action of micro-aggregates with transient cementing substances [18]. Meanwhile, macro-aggregates provide physical protection to organic matter, thus reducing the mineralization and decomposition of organic matter by microorganisms. Notably, the soil N content was higher in micro-aggregates. In general, the soil N storage was closely related to N content and aggregate size [39]. However, the macro-aggregate proportion exhibited the highest level within different-sized aggregates, therefore, the distribution and contribution of N storage were the highest in macro-aggregates under the three stand types. This was because aggregate size contribution to the N stock was jointly made up of the composition and N content of various sized aggregates [58]. Macro-aggregates-associated TN stocks play a critical role in enhancing soil N storage [59]. Thus, the present findings showed that aggregate-associated N storage was primarily determined by aggregate composition. Increasing the percentage of macro-aggregates could improve the stability of soil structure and affect the distribution of soil nutrients by providing physical protection for soil nutrients, thus contributing to the enhancement of soil N stock. In addition, this study confirmed the second hypothesis.

N storage in soil aggregates was significantly higher in mixed stands than in pure CFPs (Figure 4), indicating that mixed stands promoted the accumulation of soil N and improved soil N stability. Past studies confirm that mixed plantations show higher soil N sequestration capacity than monocultures by enhancing species richness [60]. Mixed stands significantly improved soil physicochemical properties (Table 2) and macroaggregate proportions and the aggregate stability index (Table 3). Thus, mixed stands achieved greater soil nitrogen sequestration capacity due to their soil structure stability to reserve more organic matter, effectively reduce rainfall erosion and surface runoff, and achieve nutrient retention. In addition, the interaction between the soil and plants can greatly change soil characteristics and affect nutrient storage [61]. Chinese fir exhibits characteristics of rapid growth, and its growth requires a large amount of nutrient absorption from the soil, which can improve organic matter decomposition. However, the amount of plant litter is low, resulting in much higher expenditures from soil nutrients than income. In contrast, Michelia macclurei and Mytilaria laosensis are shallow nutrient space utilization species that can improve the soil nutrient status and accelerate the rate of soil nutrient cycling. Compared with coniferous forests, mixed stands of Chinese fir with broadleaf tree species have better litter quality to promote the decomposition rate of Chinese fir litter and facilitate more organic matter input to soil nutrient return and accumulation [1]. However, Chinese fir litter in pure forests is difficult to decompose, and therefore, the nutrient return is low. This may be the main reason for the significantly higher N storage in CF + MM and CF + ML than in CF, particularly CF + MM. Moreover, litter quality and root distribution in Michelia macclurei were better than in Mytilaria laosensis and Chinese fir [62]; thereby Michelia macclurei has a stronger capacity for nutrient return and availability for plant residue decomposition and stimulating organic matter production for soil aggregation. Meanwhile, in successive cropping of Chinese fir, chemosensitive substance accumulation in the soil leads to the occurrence of self-toxicity and reduces soil nutrient availability, which inhibits plant growth and reduces soil N return [63]. In contrast, Chinese fir mixed with broadleaf species can effectively enhance the soil’s physicochemical properties by increasing the litter decomposition rate and nutrient effectiveness, reducing the release of autotoxic compounds, and increasing degradation to avoid self-toxicity, thus facilitating soil N sequestration. The soil BD and pH in pure stands had significantly higher levels than those in mixed stands among both layers. Soil TNS is negatively correlated with soil BD and pH [60]. Therefore, these findings supported our first hypothesis.

Regardless of the Chinese fir plantation stand type, aggregate-associated N stocks decreased as soil depth increased. This difference in vertical distribution may be due to the higher proportion of macro-aggregates in 0 to 20 cm than in 20 to 40 cm. In addition, plant residues in the soil surface can directly input soil organic matter, and the hydrothermal aeration in the soil surface was better, thus promoting the accumulation of soil N stock mainly in the soil surface.

4.3. Relationship between Soil Aggregates and Nitrogen Content

Soil N is a key element in regulating the production, structure, and function of forest ecosystems and is often an important nutrient-limiting factor for plant growth [64]. Soil aggregates are the key sites for the transformation of N between organic and inorganic states [65]. The TN and OC contents in different-sized aggregates are microscopic representations of organic matter formation and decomposition [18], which has a dual significance for effects on soil C and N sequestration together with soil fertility. The variation pattern of soil N content is significantly influenced by soil aggregate properties [14,23]. Consequently, investigating the relation between N content in bulk soil and soil aggregate parameters is crucial for alleviating soil fertility decline and the sustainable utilization of soil resources. The N content of soil aggregates in pure fir forests was significantly lower than that in mixed fir forests (Figure 2 and Figure 3), which may be related to the higher proportion of small-aggregates and medium-aggregates in pure fir forests. The results of RDA (Figure 6) and Pearson’s correlation (Table 4) analyses in this study also confirmed that the properties of small-aggregates and medium-aggregates were the main controlling factors affecting the soil N content in the three stands, especially the small-aggregates, which explained 52.1% (0–20 cm) and 82.5% (20–40 cm) of the variation in the N content, respectively. Similarly, aggregates in the small-aggregates and medium-aggregates composition had marked and negative correlation with TN, AN, AAN, AMN, and NH₄⁺-N content in bulk soil and a marked positive correlation with NO₃⁻-N. Therefore, the results of this study confirmed our third hypothesis. Furthermore, small-aggregates are detected to have the highest mineral N content [16]. Some studies confirmed that the greater aggregates of macro-aggregates, the steadier the aggregates are and the greater the soil N content, which is inconsistent with the findings of this study. As observed, micro-aggregates were the main carriers of the soil N content in this research. TN content in aggregates increases with the decrease in aggregate sizes [24]. Moreover, the proportion of >0.25 mm aggregates was significantly higher than that of <0.25 mm aggregates (Table 3). Therefore, the proportion of >0.25 mm aggregates had a greater effect on the variation in soil N content than micro-aggregate proportions, especially the proportion of the small-aggregates, therefore supporting the third hypothesis.

4.4. Relationship between Soil Aggregates and Nitrogen Stock

The proportion of soil aggregates and their stability are important drivers of soil nutrient storage [19]. Moreover, the structure of soil aggregate could significantly affect soil N transport and distribution [66]. Regardless of stand type in Chinese fir plantations, soil aggregate parameters were more consistent with soil N stock and their contribution in soil aggregates, not the soil N content, indicating that the influence of the aggregate structure on soil N stock was more significant. Meanwhile, macro-aggregates had the highest proportion (Table 3). Additionally, macro-aggregates were the primary carrier of N storage (Figure 4). Therefore, the increase and maintenance of macro-aggregates is the key to promoting the enhancement of soil N storage in this study area. Similarly, based on the results of RDA (Figure 7) and Pearson’s correlation (Table 5), the proportion of macro-aggregates exhibited a significant influence on the variation in soil N storage and was significantly and positively correlated with soil TN and most ON fraction storage. Additionally, the ON fraction stock accounted for 99.12% to 99.30% of the TN stock. As a consequence, increasing the proportion of macro-aggregates was beneficial to soil N storage, which was consistent with the finding that it contributes the most to N storage, and is the same as the results of preliminary research [67]. Moreover, it has been studied that TN is the most essential element impacting the distribution of macro-aggregates [43]. However, such effects of the small-aggregate proportion on N stock changes were more significant and explained more. Furthermore, small-aggregates exhibited a significantly negative relation with TN and most ON fraction stocks, since persistent cementing agents benefited the formation of aggregates in the <0.25 mm size and then formed >0.25 mm aggregates gradually under the action of transient and temporary cementing agents. The aggregates break up in a manner and with an intensity that disturbs the soil structure and thus reduces N storage capacity [18]. The formation of soil aggregate content rose with increasing aggregate size (Table 3), showing a shift from the aggregate of <0.25 to >0.25 mm size, which can easily lead to nitrogen loss during this transformation process [68]. Therefore, N stock associated with small-aggregates was lower than that related to other sized aggregates, but it laid the cornerstone of the N stock in macro-aggregates; this finding also supported our third hypothesis.

5. Conclusions

Stand type significantly influenced the aggregate-associated N. Furthermore, the influence of aggregate size and stand type on organic N was more obvious than mineral N. Compared to pure stands, mixed stands showed a significant improvement in the proportions of macro-aggregates and micro-aggregates, aggregate stability, N content, and stock, particularly in mixed stands CF + MM. On the aggregate scale, N content in the three stands decreased with increasing aggregate size. However, N stock displayed the opposite trend. Regardless of the stand type, the small-aggregate proportion was the dominant factor affecting N content and stock in bulk soil. Therefore, selecting Michelia macclurei mixed with Chinese fir and protecting macro-aggregates can contribute to aggregate stability, soil N retention, and supply capacity of CFPs. Overall, this study advanced the understanding of the soil aggregate-associated N variation in different stand types of CFPs and provided supplementary information for maintaining soil quality and health, and the sustainable development of plantations.