Effects of Betula platyphylla invasion in North China on soil aggregate stability, soil organic carbon and active carbon composition of larch plantation

In order to better understand the changes of soil carbon sequestration capacity in forest after forest mixing, the effects of broadleaf tree invasion on soil aggregate stability and carbon sequestration were studied. In northern China, the pure Larix principis-rupprechtii plantations and the Larix principis-rupprechtii plantations invaded by Betula platyphylla at various degrees with the same site conditions were selected (Betula platyphylla had mixed degrees of 0.2 and 0.4). The distribution and stability of soil aggregates were analyzed, and soil organic carbon and active carbon components were determined. The distribution of soil macroaggregates (> 0.25 mm) increased with the increase in the mixed degree of Betula platyphylla. The mixture of Betula platyphylla could effectively increased SOC, EOC, DOC and MBC of the original soil and soil aggregates of different diameter classes. The invasion of Betula platyphylla had a positive indirect impact on soil carbon sequestration by affecting the soil physical and chemical properties and the aggregate stability. The invasion of Betula platyphylla had significant positive effects on soil aggregate stability, erosion resistance and soil nutrient status in Larix principis-rupprechtii plantation. Maybe the selection of suitable broadleaf mixed species can improve the soil quality and soil organic carbon sequestration of the Larix principis-rupprechtii plantation in this area.


Introduction
Forests are the largest terrestrial carbon pool in the world, accounting for 56% of the global terrestrial carbon pool (Schuur et al. 2008). Changes in forest ecosystems will have a impact on the global carbon cycle. At present, only 10% of the world's forests have been effectively managed, and most forests (especially secondary forests and artificial forests) have not been fully realized their carbon sequestration potential (Lunstrum and Chen 2005). China's plantations have developed rapidly, and the area of plantations currently ranks first in the world (Chen et al. 2014). The management of plantations is mostly pure forest mode of single tree species. Under the long-term management of this mode, there are problems such as biodiversity reduction, poor ecological stability, productivity decline and fertility decline, which affect the sustainable development of plantation ecosystems (Andrew et al. 2020). Currently, the establishment of mixed forests is regarded as an opportunity to maintain forest services in the process of global climate change (Zhang and Chen 2007). Mixed forests can improve the biodiversity of the community, promote the growth of trees and improve the productivity of the forest, improve the soil nutrient cycle and soil fertility, enhance the adaptability to climate change, and maintain the stability of the ecosystem (Da et al. 2008;Vinicius Cianciaro et al. 2021).
The soil is the largest carbon pool on land, and soil organic carbon play an important role in soil fertility, forest ecosystem productivity and the global carbon cycle (Trumbore and Czimczik 2008;Iqbal et al. 2013;Spivak et al. 2019). To improve forest productivity, reduce erosion and mitigate climate change, enhancing soil organic carbon sequestration and soil structure stability are important objectives of forest ecosystems (Shrestha and Lal 2010;Prescott et al. 2016). Soil aggregates are the basic unit of soil structure, storing most of the soil carbon, regulate many physical, chemical and biological processes in the soil, especially the availability of nutrients and soil carbon dynamics, and play a crucial role in soil organic carbon sequestration (Blanco-Canqui et al. 2005;Noellemeyer et al. 2008). The sequestration mechanism of soil carbon by soil aggregates mainly includes reducing the contact between organic matter and extracellular enzymes, limiting the diffusion of oxygen, and limiting the contact between microorganisms and substrates (Bouajila and Gallali 2010). The organic carbon in the macroaggregates is mainly derived from plant residues and fresh organic matter with high activity, so it is easily decomposed and utilized by microorganisms. However, the organic carbon in the microaggregates mostly comes from more stable organic matter, such as microbial residues and humus, so it is not easily decomposed and utilized by microorganisms (González-Rosado et al. 2020).
According to the turnover rate and chemical characteristics of soil organic carbon in the ecosystem, soil organic carbon is often divided into active and inert organic carbon components, and these components have different responses to environmental factors (Liao et al. 2012). Active organic carbon is soil carbon with poor stability, easy oxidation, decomposition and mineralization, and certain solubility. It mainly includes organic carbon with high degrees of freedom, such as plant residues, microbial biomass, soluble carbohydrates, proteins, etc. Although the content in soil is low, but the turnover is fast, the activity is high, can directly participate in the chemical transformation process of soil organisms.
In contrast, the inert organic carbon content is relatively high, and the turnover rate is slow, which plays an important role in the long-term sequestration of soil carbon . The migration and transformation of soil active organic carbon has an important impact on soil nutrients, plant growth and the atmospheric environment, and is a leading indicator of soil organic carbon change (Bi et al. 2013). In recent years, many studies have focused on the distribution of soil aggregates and the response trend of total organic carbon content to changes in environmental factors. Research shows that soil active organic carbon is more susceptible to interference from environmental factors than soil total organic carbon, and its change will indicate the change in total soil organic carbon, thereby affecting the process of soil nutrients and the carbon cycle (Bin et al. 2006).
According to the hierarchical concept of soil aggregates (Six et al. 2004), the quality of plant litter returned to the soil determines the distribution of litter in aggregates of different particle sizes, and ultimately affects the dynamics of soil aggregates. Compared with pure pine forest, litters of mixed forest are more effective, which means that litters of these stand types are more likely to combine into macroaggregates during decomposition, thus promoting the formation of macroaggregates (Chen et al., 2020). Compared with pure pine forest, the increase of litter amount and coverage in mixed forest weakened rainfall leaching and further protected soil macroaggregates. Chen et al. (2017) studied Pinus tabulaeformis forest and mixed broad-leaved forest and found that the presence of deciduous broadleaved plants in Pinus tabulaeformis forest improved the stability of soil aggregates. Wang et al. (2021) studied three Cunninghamia lanceolata plantations and showed that the soil aggregate stability and nutrient storage of mixed forest were higher than those of pure Cunninghamia lanceolata forest. Changes in soil organic carbon content were related to tree species composition, vegetation coverage, litter accumulation and decomposition degree, soil microorganisms, and root distribution in soil (Yan et al. 2020). Coniferous forest litter contains a lot of organic matter which is difficult to decompose, which affects the return rate of soil nutrients. The litter layer of coniferous forest is thicker and the humus layer is thinner, which is not conducive to the decomposition of litter. The mixed forest has rich vegetation composition and thick humus layer. Broad-leaved tree species tend to have greater litter yield and decomposition rate, which increases the amount and rate of nutrient return, thus increasing the organic carbon content in soil (Hui et al. 2019). The study shows that suitable mixed species of broadleaved trees and coniferous pure forests could alleviate the decline of soil aggregate stability and soil nutrient loss, promote the sustainable use of soil resources, and protect soil quality and soil health.
Larix principis-rupprechtii is an important tree species widely planted in North China that plays an important role in water conservation and the formation and maintenance of forest ecosystems in North China. Through investigation, it was found that some pure Larix principis-rupprechtii plantations in the Taiyue Mountain area were invaded by Betula platyphylla, forming a typical coniferous broad-leaved mixed forest. Compared with pure forest alone, the forest with Betula platyphylla invasion had a more abundant community structure. Betula platyphylla invasion changes the storage and residence time of soil organic matter by affecting the content and quantity of litter in the soil and the soil properties, therefore affecting the function, structure and activity of the soil microbial community (Jiang et al. 2013;Scheibe and Gleixner 2014). In order to better understand the sequestration mechanism of soil carbon in larch plantation ecosystem, it is important to study the effect of Betula platyphylla invasion on soil carbon stability. The specific objectives of this study were (i) to study the changes in soil physical and chemical properties and soil aggregate stability of the Larix principis-rupprechtii plantation after Betula platyphylla invasion and (ii) to investigate the effect of Betula platyphylla invasion on soil organic carbon and active carbon sequestration. We hypothesized that (i) Betula platyphylla invasion increases the stability of soil aggregates and (ii) Betula platyphylla invasion is more beneficial to the stability and accumulation of soil organic carbon in pure Larix principis-rupprechtii plantation.

Study area and plot setting
This study was carried out in the eastern region of the middle section of Taiyue Mountain in central Shanxi Province, China, which belongs to the Taiyuan Basin in North China. The altitude of 1500-2500 m has a warm temperate semihumid continental monsoon climate. The annual average precipitation is 653 mm, and the annual average temperature is 8.6 ℃. The annual average sunshine time is 2612 h, and the frostfree period is 120-160 days (Wang et al. 2012). The main soil type is cinnamon soil of Eutric Luvisd. The forest types in this area mainly include coniferous forest and coniferous broad-leaved mixed forest, and coniferous forest is the main type of forest in this area. The dominant tree species are Larix principisrupprechtii and Pinus tabulaeformis. The common tree species include Quercus liaotungensis, Betula platyphylla, Populus davidiana, etc.
The Larix principis rupprechtii forest in the forest farm was planted uniformly in 1998 (3 years old Larix principis rupprechtii seedlings). Since then, the forest has been in a state of natural growth. In July 2020, on the basis of investigation and forestry data, pure Larix principis-rupprechtii plantation and Larix principis-rupprechtii and Betula platyphylla mixed forests (with the same parent materials, site conditions and afforestation measures) invaded by Betula platyphylla to different degrees were selected (marked as pure, mixed degree 0.2 and mixed degree 0.4). Pure forest was used as a control treatment. The initial soil carbon content/fractions and soil properties of the three sites were consistent. Three standard plots of 20 m × 30 m were set for pure and mixed forests, making a total of nine plots (Fig. 1). To eliminate the boundary effect, the spacing of each plot was more than 100 m. The stand characteristics and site conditions of the plots are shown in Table 1.

Soil sampling and soil aggregate separation
Five sample points were randomly selected from each standard sample along the diagonal to remove litter from the topsoil. A standard cutting ring with a volume of 100 cm 3 was used to determine the bulk density at 0-20 cm, 20-40 cm and 40-60 cm soil by the ring knife method (Al-Shammary et al. 2018). A soil drill with a diameter of 8 cm was used to collect soil samples from the 0-20 cm, 20-40 cm and 40-60 cm soil layers. Five samples of the same soil layer in the same plot were combined into a mixed soil sample, and all visible roots, plant residues and stones were manually removed. The soil sample was immediately taken back to the laboratory for treatment. See Fig. 2 for the specific treatment process of the soil sample.
The method of Sithole et al. (2019) was used to separate soil aggregates of different sizes. One hundred grams air-dried soil was soaked in distilled water and allowed to stand for 2 min. Then the soil was quickly immersed in distilled water with 0.053, 0.25, 1, and 2 mm sieves and oscillated at a displacement of approximately 4 cm at a speed of 30 rpm for 10 min. The soil samples were divided into five aggregates with different particle sizes: > 2 mm, 1-2 mm, 0.25-1 mm, 0.25 − 0.053 mm and < 0.053 mm. All fractions were dried at 70 °C for 24 h and then weighed. The mean weight diameter (MWD) of aggregates, soil aggregate content > 0.25 mm (WSA) and soil erodibility K value (K) were calculated by calculating the proportion of each aggregate component. The calculation formula (Shrestha et al. 2007;Shi et al. 2010;Dou et al. 2020) is as follows: The calculation formula of the geometric mean diameter GMD (Zhou et al. 2007) is as follows: Fig. 1 The map of the sample location where X i (mm) is the mean diameter of aggregates in any particle size range, W i (%) is the percentage of the mass of aggregates in any particle size range in the mass of soil samples, M i > 0.25 (g) is the mass of aggregates > 0.25 mm, and M t (g) is the mass of total soil samples.

Laboratory analysis
The soil pH value was measured with a pH meter (the ratio of soil to water was 1:2.5). The particle size of the soil was measured by the pipetting method, and the texture of the soil was analyzed. The determination of soil total nitrogen (TN) was first digested with concentrated sulfuric acid and then measured with a continuous flow analyzer (SEAL AutoAnalyzer3 HR, Germany) (Li et al. 2009). The total phosphorus (TP) content of the soil was first digested with perchloric acid and then determined by molybdenum antimony ascorbic acid colorimetry (McDowell et al. 2001). The soil organic carbon (SOC) was determined by the potassium dichromate external heating method (Yu-Jie et al. 2014). We also calculated the contribution rate of soil aggregate organic carbon and active carbon components to soil total organic carbon and active carbon components. The formula ) is as follows: where P i (%) is the contribution rate of i-size aggregates to SOC, S i (g · kg − 1 ) is the organic carbon content of i-size aggregates, W i (%) is the percentage of i-size aggregates, and Q (g · kg − 1 ) is the total organic carbon content of soil. The soil easily oxidized organic carbon (EOC) was determined by 333 mmol L − 1 KMnO 4 oxidation colorimetry (Purakayastha et al. 2008). Soil soluble organic carbon (DOC) was extracted by potassium sulfate (K 2 SO 4 ) and determined by a TOC analyzer (Multi N/C 3100, Germany) (Smucker et al. 2007). Soil microbial biomass carbon (MBC) was extracted by chloroform fumigation and then determined by a TOC analyzer. The calculation formula of MBC (Vance et al. 1987) is as follows: where MBC (mg · kg − 1 ) is the microbial carbon content, Ec (mg · kg − 1 ) is the difference in organic carbon content in fumigated and unfumigated extracts, and 0.45 is the correction factor.

Statistical analysis
Microsoft Excel 2019 software was used to preprocess the data, and SPSS (version 23.0) software was used for statistical analysis. Before statistical analysis, the normality of the data was tested by the Kolmogorov-Smirnov test. The Levene test of SPSS 23.0 was used to test the homogeneity of variance. To meet the normality requirement of geostatistical analysis, logarithmic transformation of nonnormally distributed data was carried out. One-way analysis of variance (ANOVA) was used to analyze the effects of mixed Betula platyphylla on the soil physical and chemical properties, aggregate stability, soil organic carbon and active carbon components of the Larix principis-rupprechtii plantation in North China. Tukey's HSD test was used to determine the significance level (P < 0.05), and the value was the mean

Basic physical and chemical properties of soil after the invasion of Betula platyphylla
Soil pH increased with soil depth. Compared with the pure Larix principis-rupprechtii plantation in North China, the pH of the Betula platyphylla mixed forest increased, and the mixed degree 0.4 increased more than the mixed degree 0.2 (P < 0.05). RWC, BD, TN and TP decreased with the the soil layer increasing ( Table 2). The effect of Betula platyphylla mixed forest on the basic physical and chemical properties of the soil was more significant in the topsoil (0-20 cm) than in the deep soil (20-60 cm). In the topsoil, the mixed degree of 0.4 increased pH by 8.66%, RWC by 12.37%, TN by 16.02%, TP by 23.64% and sand content by 9.33% compared with the pure forest ( Table 2). The results showed that the Betula platyphylla mixture was beneficial for improving soil properties, and the positive effect of mixing degree of 0.4 on basic soil physical and chemical properties was more significant than that of a mixing degree of 0.2.
The content and stoichiometric ratio of soil organic carbon and active carbon components after the invasion of Betula platyphylla The soil organic carbon and active carbon components content decreased significantly with increasing soil depth, and the mixture of Betula platyphylla had significant effects on thesoil organic carbon and active carbon components content (P < 0.05 The average soil C/N ratio was 17.40. The C/N ratio in the topsoil (0-20 cm) of the Betula platyphylla mixed forest was significantly higher than that in the pure forest, but there was no significant difference between the pure forest and the mixed forest in the deep soil (Fig. 4). The average soil C/P ratio was 56.55. The C/P ratio in the topsoil (0-20 cm) of the Betula platyphylla mixed forest was significantly higher than that of the pure forest, while the C/P ratio in the deep soil was significantly lower than that of the pure forest. The C/P ratio and N/P ratio showed a decreasing trend with the soil depth increasing. The average soil N / P ratio was 3.24 (Fig. 4). There was no significant difference in the N/P ratio between pure  forest and mixed forest in the 0-40 cm soil layer, but the N/P ratio of pure forest in the 40-60 cm soil layer was significantly higher than that of mixed forest.

Distribution and stability of soil aggregates after the invasion of Betula platyphylla
The soil aggregates of pure forest and mixed forest were mainly macroaggregates (> 0.25 mm) (74.29 − 90.81%) ( Table 3). With the decrease in aggregate size, the proportion of the soil aggregate fraction decreases. The soil aggregates > 2 mm and macroaggregates (> 0.25 mm) content increased with increasing mixing degree of Betula platyphylla but decreased with increasing soil depth. The range of MWD was 1.73-2.01 mm. Betula platyphylla invasion was beneficial for enhancing the stability of soil aggregates, and a mixed degree of 0.4 enhanced the stability of soil aggregates more than a mixed degree of 0.2 (Fig. 5). The proportion of macroaggregates (> 0.25 mm) decreased with the soil depth increasing and increased with the increase in the mixed degree of Betula platyphylla. The invasion of Betula platyphylla was beneficial to the accumulation of more macroaggregates in the soil, and the accumulation of macroaggregates in the mixed degree of 0.4 was greater than that in the mixed degree of 0.2. The range of the K value of soil erosion resistance was 0.2424-0.3177. The invasion of Betula platyphylla enhanced the erosion resistance of the Larix principis-rupprechtii plantation, and the erosion resistance of the mixed degree 0.4 was stronger than that of the mixed degree 0.2.

Content and contribution rate of organic carbon and active carbon components in soil aggregates after the invasion of Betula platyphylla
There were significant differences in the contents of organic carbon and active carbon components in soil aggregates between pure and mixed forests (Fig. 6). The organic carbon and active carbon content in the soil aggregates decreased with the soil depth increasing. The SOC content was the highest in the aggregates with a particle size of > 2 mm, and significantly increased in the 0.4 mixed degree (P < 0.05). The EOC content in the aggregates first increased and then decreased according to the size of the aggregates.
The EOC content was the highest in the 0.25-1 mm aggregates and the lowest in the > 2 mm aggregates. The EOC content in aggregates in topsoil (0-20 cm) increased with increasing mixed degree. The DOC content was higher in the aggregates with particle sizes of 0.053-2 mm. In the topsoil (0-20 cm), there was no significant difference in the DOC of the soil aggregates between the pure forest and mixed forest (P < 0.05). In the deep soil (20-60 cm), the intrusion of Betula platyphylla increased the DOC of the soil aggregates. The MBC content in the aggregates increases first and then decreases according to the size of the aggregates. The MBC content was higher in macroaggregates (> 0.25 mm). With the invasion of Betula platyphylla, the MBC in soil aggregates showed an increasing trend, and the increasing range of mixed degree 0.4 was larger than that of 0.2 (Fig. 6).
Aggregates > 2 mm had the highest contribution rate to SOC, followed by aggregates 0.25-2 mm ( Table 4). The contribution of aggregates > 2 mm to SOC in the mixed degree 0.4 was significantly greater than that in the pure forest and mixed degree 0.2 (P < 0.05). The aggregates of 0.25-1 mm contributed the most to EOC. In DOC and MBC, the contribution rate of aggregates > 2 mm was the largest (Table 4), which was consistent with the proportion of aggregates. With the invasion of Betula platyphylla, the organic carbon and active carbon components content in aggregates showed an increasing trend, but the contribution rate of aggregates to soil organic carbon and active carbon components did not follow any specific trend.

Comprehensive effect of Betula platyphyllabirch invasion on soil properties
Pearson correlation analysis and principal component analysis were used to evaluate the relationship between soil properties. SOC, EOC and MBC were significantly positively correlated (P < 0.05) and were significantly positively correlated with TN, WSA and RWC (Table 5). EOC and MBC were significantly positively correlated with TP. DOC was only positively correlated with SOC and BD. MWD was significantly positively correlated with K. In the principal component analysis, the soil properties of pure forest and mixed forest with different degrees of mixing are obviously separated, which indicates that the soil properties changed after mixing (Fig. 7). The total effect of the two main components was significant (77.53%). The total variance defined by PC1 was 49.07%, among which TN, EOC, SOC, MBC and WSA were dominant. PC1 defined 28.46% of the total variance, in which MWD, K, pH, TP and BD dominated. SOC, EOC and TN showed obvious aggregation relationships, and WSA and MBC showed obvious aggregation relationships (Fig. 7).
According to the index classification, a structural equation model (SEM) was established between the mixed degree and the stability of soil aggregates, soil texture, and basic physical and chemical properties of soil and soil carbon (Fig. 8). SEM analysis showed that the mixed degree had a significant positive effect on the stability of soil aggregates and the basic physical and chemical properties of soil, and the path coefficients were 0.927 and 0.556, respectively (P < 0.05). The mixed degree had little direct effect on soil carbon, and it mainly indirectly affected the soil carbon content by affecting the basic physical and chemical properties of the soil. The basic physical and chemical properties of the soil had a significant positive impact on soil carbon, and the path coefficient was 1.071 (P < 0.05).

Effects of Betula platyphylla invasion on soil physical and chemical characteristics and the stability of soil aggregates
Soil physical and chemical properties and nutrient processes are directly affected by the amount and decomposition rate of litter, which is the main pathway for plants to return nutrients absorbed to the soil (Zechmeister-Boltenstern et al. 2015). Compared with pure forests, mixed forests affect the annual litter yield, the composition and quality of surface litter, and the decomposition efficiency of soil roots, leading to changes in nutrient return efficiency and thus affecting soil related properties (Chen et al. 2018). The invasion of Betula platyphylla increased the pH of the soil. The absorption of different forms of nitrogen by plants with different genotypes can cause changes in pH (Zhang and Bai 2003). When plants absorb ammonium nitrogen, the soil releases H + , and the soil pH value decreases. In contrast, if plants mainly absorb nitrate nitrogen, the soil releases OH − and HCO 3 − , and the pH value increases. Generally, conifers absorb ammonium nitrogen and broadleaf trees absorb nitrate nitrogen (Cui and Song 2007). Another reason for the increase in pH is that the ash content of coniferous pine needles is low, and the resin and tannin contents are high. The organic acid produced after decomposition is generally fulvic acid, which is highly acidic (McTiernan et al. 2003). The ash content of Betula platyphylla is high and neutral, and the acidity of the soil will be neutralized after the decomposition of the fallen leaves (Meng and Jian 2010).
The mixture of Betula platyphylla significantly increased soil TN and TP. Nutrients can be transferred between the roots of mixed tree species, and the nutrient status of soil can be significantly improved through soil nutrient transfer, utilization and complementary utilization (Khanna 1997). Yu et al. (2020) showed that mixed broad-leaved trees in fir forests can improve the soil physical and chemical properties of the original pure fir forest on the whole in China. The effect of Betula platyphylla mixed forest on the basic physical and chemical properties of the soil was more significant in the topsoil (0-20 cm) than in the deep soil (20-60 cm). The organic carbon and nutrient elements in the soil mainly come from the decomposition of litter in the forest, and then leach into the lower layer of the soil with rainwater. Therefore, the organic carbon and nutrient elements in the soil decrease with the the soil depth increasing after forest mixing (Makumba et al. 2006).
In this study, the soil aggregates of pure forest and mixed forest were mainly macroaggregates (> 0.25 mm). The soil macroaggregates (> 0.25 mm) content increased with the increase in the Betula platyphylla mixed degree. The increase in the root system and its secretion by Betula platyphylla was conducive to the formation of macroaggregates (Ola et al. 2015). The MWD is an important parameter to evaluate the stability of soil aggregates (Barthès and Roose 2002). This study showed that the invasion of Fig. 6 Distribution of organic carbon and active carbon components in soil aggregates after the invasion of Betula platyphylla. Data represent the average of nine replicates, and the error bars represent standard errors. Different capital letters indicate significant differences (P < 0.05) among forest types, and different lowercase letters indicate significant differences (P < 0.05) among soil layers ◂ Vol:. (1234567890) Betula platyphylla could increase the stability of soil aggregates in the Larix principis-rupprechtii plantation, which was consistent with the research results of Dou et al. (2020) on pure Robinia pseudoacacia forests and mixed Robinia pseudoacacia and Armeniaca sibirica forests. Table 4 Contribution (P i ) of organic carbon and active carbon components in the soil aggregate to soil total organic carbon and active carbon components Data represent the average of nine replicates ± the standard errors. Different capital letters indicate significant differences (P < 0.05) among different forest types, and different lowercase letters indicate significant differences (P < 0.05) among different soil layers of the same forest. The calculation of P i is shown in the "Materials and methods" section Soil erosion is an ecological force that affects the development of vegetation and is affected by vegetation. Its long-term effect will change the landform and soil characteristics and determine the development of vegetation to a certain extent (Jiao et al. 2009). Vegetation mainly regulates rainfall and reduces the kinetic energy of raindrops through vertical structures such as the canopy layer, shrub grass layer and litter layer, thus affecting the hydrological process and soil erosion process in forests (Cerdà et al. 2021;Zeng-Wen et al. 2011) showed that replanting broad-leaved trees can effectively reduce soil loss in coniferous pure forests, which is consistent with the results of our study. After the invasion of Betula platyphylla, the canopy density of the forest increased, the litter under the forest became thicker, and the interception was enhanced. Additionally, the plant root system also has a strong effect on enhancing the soil erosion resistance (Xiong et al. 2007), which leads to the enhancement of the soil erosion resistance of the pure forest of Larix principis-rupprechtii after the invasion of Betula platyphylla.  Huang et al. 2004;Zhang and Chen 2007;Blonska et al. 2018). Our results showed that the mixed Betula platyphylla in the pure Larix principisrupprechtii plantation could significantly increase the soil organic carbon and active carbon components content. After the invasion of Betula platyphylla, the increase in litter biomass and fine root matter increased the input of organic carbon in the soil, and the active carbon components also increased. Betula platyphylla with a mixed degree of 0.4 could accumulate more soil organic carbon and active carbon components than Betula platyphylla with a mixed degree of 0.2. Some studies have shown that soil nutrients can be significantly improved when the conifer broadleaf mixed ratio is 0.1-0.8 (Yan et al. 2015;Peng et al. 2014) studied the effects of different mixed ratios on the soil of Masson pine broadleaved mixed forest. The study pointed out that with the increase in the proportion of broad-leaved trees in Masson's pine broad-leaved mixed forest, the soil organic carbon increased and the functional diversity of the soil microbial community increased. When the mixed ratio reached 81-100%, the soil organic carbon and microbial community functional diversity began to decline. Our results showed that the soil organic carbon and active carbon components increased with the increase in the mixed ratio of Betula platyphylla when the mixed ratio of Betula platyphylla was 0-40%. The mixing of Betula platyphylla in more than 40% of Larix principis-rupprechtii plantations needs further study. Soil EOC provides energy for microbial activities, and its stability is relatively poor. Laik et al. (2009) found that the decomposition of litter can supplement soil EOC, and the impact on the soil layer is from top to bottom. Soil microorganisms are the living components in the soil, which are extremely sensitive to various changes in the soil environment and can fully reflect the ecological functions of microbial communities. Microbial biomass is not only an important source of soil nutrients, but also an important carrier for soil nutrient fixation . In this study, it was found that MBC showed the greatest increase after the invasion of Betula platyphylla. After the invasion of Betula platyphylla, a large amount of Betula platyphylla litter on the ground returned to the soil, providing a large number of carbon sources for soil microorganisms. The coverage of Betula platyphylla litter reduced the water loss on The R2 value represents the amount of variance of the dependent variable explained by the internal model. *** P < 0.001, ** P < 0.01, * P < 0.05 the topsoil. After the invasion of Betula platyphylla, the shelter of the canopy of the forest was enhanced, which avoided certain direct sunlight to the soil, and was more conducive to the growth of soil microorganisms. These conditions provide a good living environment for soil microorganisms, improve soil microbial activity, and lead to a significant increase in soil microbial biomass. Soil DOC is leached by surface vegetation and mulch and is the product of soil root exudates and microbial metabolism, which have an important impact on the soil organic carbon concentration and soil physicochemical cycle process (Huang et al. 2012). Soil DOC accounts for a small proportion of SOC, but it can be directly utilized by soil microorganisms (Scaglia and Adani 2009). Our study found that the DOC of the Betula platyphylla invasion had a significant increasing trend in the topsoil, but there was no significant difference in other soil layers. Soil water-soluble organic carbon mainly comes from soil organic matter, litter and root exudates, and water-soluble organic carbon and soil total organic carbon are often in dynamic balance (Nakanishi et al. 2012).
The topsoil contains a large amount of litter and organic matter, and the soil microbial activity is high. Litter and organic matter are less distributed in the deep layer of soil after decomposition, resulting in an obvious vertical distribution of soil organic carbon and active carbon components. The results showed that the soil active organic carbon components were largely determined by the SOC content. The forest type determines the input of organic matter, thus affecting the SOC content in the soil. Differences in plant litter, soil humus and the soil root system lead to differences in soil nutrients and soil organic carbon, which indirectly affect the soil active organic carbon components content (Wang et al. 2005).
The dynamic change in soil organic carbon is not only dependent on the input of organic matter, but also closely related to the structure of soil aggregates (Zhu et al. 2021). The Betula platyphylla mixture and the soil aggregate particle size had significant effects on the soil organic carbon and active carbon components. This study found that in pure and mixed forests, macroaggregates contributed more to the organic carbon pool, while microaggregates contributed less to the organic carbon pool (Table 4). Cheng et al. (2020) showed that the contribution of aggregates > 0.25 mm to the organic carbon pool was significantly greater than that of aggregates < 0.25 mm. We found that the organic carbon and active carbon content of surface soil aggregates was slightly higher than that in deep soil, which was consistent with the research results of Zhu et al. (2021). After mixing, especially in the forest with a mixed degree of 0.4, the contribution rate of aggregates with a diameter of > 2 mm to SOC increased significantly. After mixing, the newly imported organic matter first enters the macroaggregates, which physically protects the original and newly added organic carbon from microbial utilization and mineralization . Macroaggregates are formed by the aggregation of microaggregates with plant roots through hyphae (Bach and Hofmackel 2016). After mixing, the soil roots increased, and a large number of microaggregates of soil were combined to form macroaggregates through the action of cementing materials, which improved the protection effect of organic carbon. Therefore, the stability of soil aggregates increased, and the organic carbon content stored in macroaggregates increased.
The EOC content in the aggregates first increased and then decreased according to the particle size of the aggregates. The particle size of the aggregates with the largest contribution to EOC was 0.25-1 mm. Soil EOC is greatly affected by the soil microenvironment and is very sensitive to changes in the external environment . The change in EOC reflects the availability and timeliness of soil organic carbon to different degrees (Liu and Tang 2021). The release of DOC was related to the soil organic carbon content. Soil with a high organic carbon content has a better soil structure, stable soil aggregates, gradual release of organic carbon wrapped by soil aggregates, and increased DOC content (Liu et al. 2019). DOC is both a product of microbial metabolism and a substrate that can be used by microorganisms. The presence of DOC provides a carbon source for microbial activities (Huang et al. 2016). DOC released by SOC was quickly utilized by microorganisms, so the DOC content in the soil did not change significantly. The change in DOC content in the soil is caused by the decomposition of dead microorganisms in the soil, the new release of DOC from the soil, and the DOC decomposed by mineralization (Liang et al. 2011). The Betula platyphylla mixed significantly increased the MBC content in soil macroaggregates (> 0.25 mm). With the decrease in aggregate size, the contribution rate of aggregates to soil MBC decreased. This is consistent with the research results of Xiao et al. (2021). The MBC content was significantly positively correlated with the water-stable aggregates content, and the MBC of macroaggregates is higher than that of microaggregates. Zhao et al. (2006) and Carter et al. (2003) showed that there was a "threshold" of soil organic matter bound to soil particles. At first, the increase in SOC would increase the SOC in fine-grained soil, but when the SOC content of soil is close to saturation, SOC will be stored in the larger aggregates, which will increase the SOC enrichment factor of the larger size aggregates. Our results also support this view. In general, the Betula platyphylla mixture not only improved the stand structure, soil fertility and productivity but also effectively increased the organic carbon and active carbon components content in the original soil and soil aggregates of different diameter classes.
Soil C/N and C/P are important indicators of soil organic matter quality, which determine the strength of soil organic matter decomposition. Soil N/P can reflect the limiting process of ecosystem elements (Hui et al. 2021). High soil C/N is conducive to the accumulation of organic matter, while low soil C/N is conducive to the decomposition and release of organic matter (Alberti et al. 2015). The soil C/N in this study was approximately 17.40, which was lower than the average value of 18.20 in China (Zhang et al. 2011). The C/N of this study showed that the invasion of Betula platyphylla increased the soil organic carbon content, but the accumulation of organic carbon was still insufficient, and most of the organic carbon was decomposed and released. The soil C/N ratio of topsoil (0-20 cm) of mixed forest was significantly higher than that of pure forest, with an average value of 18.08. This indicated that Betula platyphylla invasion can accumulate part of the organic carbon in the topsoil and increase the retention of soil organic carbon and nutrients in the short term. Soil C/P can reflect the availability of soil phosphorus. When the C/P value was less than 200, soil nutrients were mainly mineralized, which is conducive to microbial activities (Bandyopadhyay et al. 2010). The average value of the soil C/P ratio in this study was 56.55, which was lower than the average value of 61.0 in China (Tian et al. 2010). The C/P in the topsoil (0-20 cm) of the Betula platyphylla mixed forest was significantly higher than that in the pure forest, which again confirmed the previous view that the accumulation of organic carbon caused by the invasion of Betula platyphylla mainly occurred in the topsoil (0-20 cm). Soil N/P is an important indicator to measure the balance of soil nutrients, and can also predict the nitrogen and phosphorus limitations of plants in the study area (Tian et al. 2010). When N/P is less than 14, plant growth is limited by nitrogen, and when N/P is greater than 16, plant growth is mainly limited by phosphorus. When N/P is between 14 and 16, plant growth is limited by nitrogen and phosphorus (Bui and Henderson 2013). The average value of the soil N/P ratio in this study was 3.24, which is far less than 14, so the stand growth in this region was limited by nitrogen.

Comprehensive impact of Betula platyphylla invasion on Larix principis-rupprechtii plantations
The analysis found that SOC, EOC and MBC were significantly positively correlated (P < 0.05), which was consistent with the results of Yuan et al. (2021) and Cwabc et al. (2020). The contents of EOC and MBC in soil generally depend on soil organic carbon storage. After the invasion of Betula platyphylla, SOC, EOC and MBC increased to varying degrees. The invasion of Betula platyphylla caused new litter species in the soil of the Larix principis-rupprechtii plantation. This would stimulate the growth and reproduction of soil microorganisms, promote the activation and decomposition of soil insoluble substances, improve soil quality, promote forest growth, and increase the return of soil organic matter. The SOC, EOC and MBC were significantly positively correlated with TN, but not with TP, which was consistent with the research results of Yuan et al. (2021). Organic carbon and its active carbon components had a good correlation with total nitrogen, but a poor correlation with total phosphorus. Organic matter with a high nitrogen content is easily decomposed and utilized by soil microorganisms, and the rate of organic matter migration and transformation is fast, which affects the soil organic carbon and its active carbon components content. The SOC, EOC and MBC were significantly positively correlated with WSA.
The invasion of Betula platyphylla can promote the formation of macroaggregates, increase the organic carbon components content, and enhance the enrichment ability of organic carbon components in aggregates. Soil organic carbon is one of the main cementing agent in the formation of soil aggregates, and the distribution and stability of aggregates affect the decomposition and transformation of soil organic carbon (Steffens et al. 2010). In this study, the Betula platyphylla mixture promoted the formation of macroaggregates from microaggregates. After entering the soil, organic matter (1) can directly become the core in the formation of aggregates, adsorbing fine particles in the soil; (2) as a cementitious material, it can effectively promote the formation of macroaggregates; and (3) it can enhance the microbial activity in the soil and the growth of fungal hyphae so that small particles are entangled to form macroaggregates. Similar to the research results of Guang-Ping et al. (2019), SOC, EOC, MBC and RWC were significantly positively correlated in this study. Soil moisture can change the soil SOC content by affecting plant community productivity, soil physical and chemical properties, and microbial activity. (Deng et al. 2016;Humphrey et al. 2021). The MWD was significantly positively correlated with K, both of which were significantly positively correlated with pH and negatively correlated with BD. Soil erosion resistance refers to the ability of soil to resist water dispersion and suspension, which is closely related to the stability of soil aggregates. The size of the soil erosion resistance is mainly determined by the cementation between soil particles and the affinity between soil particles and water (Bouajila and Gallali 2010). Soil organic carbon helps to improve the cohesion between soil particles, which is conducive to the formation of soil macroaggregates, and then improves the structure of soil aggregates, thus indirectly affecting soil erosion resistance (Zhang et al. 2022).
This study found that mixed transformation (Betula platyphylla invasion) had a significant positive impact on the total nitrogen, total phosphorus, organic carbon and active carbon components of coniferous pure forest soil, which was consistent with the research results of Xie et al. (2013) and Lu and Scheu (2021). The species of undergrowth plants in coniferous broad-leaved mixed forests are richer than those in coniferous pure forests, which is conducive to litter decomposition and soil nutrient regression (Lixiong et al. 2018).
Organic carbon components are preferentially lost in macroaggregates and preferentially enriched in microaggregates, which conforms to the hierarchical theory that organic carbon input preferentially accumulates in microaggregates (Six et al. 2004). When the accumulation is saturated, the organic carbon is mainly enriched in the macroaggregates. The organic carbon and active carbon components in macroaggregates are more sensitive, unstable, and easier decompose and utilize. Studies have shown that in the early stage of forest mixing, the carbon and nutrient contents of macroaggregates are generally higher than those of microaggregates (Qiu et al. 2015;Zhan 2018). With the increase in the age of the mixed forest, a large amount of root gap damage may promote the disintegration of macroaggregates and organic matter, leading to the formation of microaggregates with high carbon and nutrients (Six et al. 2002;Kumar et al. 2017). Therefore, it is of great significance to protect the components of macroaggregates in the early growth stage of mixed forests. After mixing, the change in aggregate stability with forest age requires longterm study. At longer time scales, mixed forests may have a greater impact on the amount and form of soil carbon (Kumar et al. 2017).

Conclusion
The effects of Betula platyphylla invasion on soil quality, aggregate stability, and organic carbon and active carbon sequestration of the Larix rupprechtii plantation were studied. The results showed that the invasion of Betula platyphylla had a positive effect on species diversity and soil physical and chemical properties and promoted the stability of aggregates and soil erosion resistance. The Betula platyphylla mixture could effectively improve the sequestration of soil organic carbon and active carbon components. Soil macroaggregates contribute to soil accumulation of more organic carbon and active carbon components. Betula platyphylla invasion enhanced soil carbon sequestration capacity by affecting soil physical and chemical properties and aggregate stability. The positive effect of Betula platyphylla mixed degree 0.4 was more significant than that of mixed degree 0.2. The study showed that management measures of mixed coniferous and broadleaved pure forests of single forest stands should be encouraged in the area to improve soil structure, enhance soil carbon sequestration and prevent soil organic carbon loss.