Effects of Poplar Shelterbelt Plantations on Soil Aggregate Distribution and Organic Carbon in Northeastern China

: This study aimed to determine the distribution, stability, and soil organic carbon (SOC) of aggregates, and the contribution of soil aggregate proportion, stability index, and aggregate-associated SOC to the total SOC. Three hundred and sixty soil samples were gathered from shelterbelts and neighboring farmlands in ﬁve layers of 1 m proﬁles in Songnen Plain, northeastern China. The shelterbelt plantations were found to increase by 69.5% and 103.8% in >2 mm and 0.25–2 mm soil aggregates, respectively, and their R 0.25 , mean weight diameter (MWD), and geometric mean diameter (GMD) were enhanced by 96.3%, 33.2%, and 40.0%, respectively, compared to those of farmlands in soil layers at 0–20 cm depth ( p < 0.05). The total SOC content increased by 13.3% at 0–20 cm soil depth, and the SOC content and stock in >2 mm aggregates increased by 21.5% and 18.7% in the 20–40 cm layer ( p < 0.05), respectively. The SOC content and stock in total soil had a signiﬁcantly positive relationship with the proportion of >2 mm soil aggregates and a negative relationship with the value of fractal dimension (D). The enhancement in the SOC of the total soil was dependent on the increase in aggregate-associated SOC, with larger-particle aggregates having a greater contribution. Based on the study results, afforestation improved soil stability and the structure of soil aggregates, and SOC accumulation in the total soil was not only governed by SOC concentration and stock within the aggregate size class, but also the proportion of >2 mm soil aggregates and the value of the fractal dimension.


Introduction
The global soil carbon (C) pool has been calculated to be over three-fold higher than the atmospheric pool and approximately four-fold higher than the biotical pool [1]. Changes in the soil C pool may markedly affect the atmospheric CO 2 concentration [2]. Forest ecosystems contain 60% as much C as land ecosystems, 70% of which is reserved in the soil [3]. C is also an important element that maintains the balance in terrestrial ecosystems [4]. Afforestation in abandoned farmlands has become an important measure to protect soil from degradation and recover degraded ecosystems [5,6]. The "Three-North Shelterbelts" program, called "Green Great Wall", was launched in Northwest, North, and Northeast China in 1978 to protect farmlands from serious erosion and windstorms. The program covers a total area of 4.07 Mkm 2 , accounting for 42.4% of the country's land area [7]. To date, inconsistent results have been reported regarding the influence of afforestation in agricultural land on soil organic carbon (SOC). Some studies suggested that SOC accumulation occurred following afforestation [8,9], while other studies revealed that SOC depletion occurred after afforestation [10,11]. Some studies also reported an initial loss followed by an increase in SOC concentration [12,13]. These contradictions warrant further studies to evaluate the significance of SOC accumulation from afforestation in farmlands and determine the dominant factors affecting SOC sequestration. The existing literature is limited in the distribution of SOC in different-size aggregates and the contribution of SOC in aggregates to total SOC after afforestation. Further investigation in these areas would help explain the mechanism of SOC change following afforestation on abandoned farmlands.
As primary components in the composition of soil, soil aggregates play a critical role in the movement of air through soil, the water-holding capacity of soil, and the protection of soil organic matter [14], which are regarded as the cores of all mechanisms of SOC sequestration [15,16]. As important indexes, the distribution and stability of soil aggregates may be used to assess changes in soil structure owing to quick responses to conversions in soil use types [17]. At present, the main parameters used to evaluate the stability of soil aggregates are soil's large-aggregate content (R 0.25 ), mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (D) [18]. The larger the R 0.25 , MWD, and GMD, and the smaller the D, the better the soil structure stability and the stronger the soil erosion resistance [19,20]. Soil-aggregate-associated organic carbon is usually used as a parameter to measure the stability of organic carbon in total soils following conversions in land use to protect SOC from microbial decomposition and oxidation [21]. Most SOC was found to be occluded with macroaggregates (>0.25 mm) in forest soils [22], and the macroaggregates in forest soils generally afford higher SOC content and stocks than microaggregates (0.053-0.25 mm) in most temperate soils [23]. Deforestation can reduce the distribution of macroaggregates and result in the loss of SOC in aggregate and total soil [24,25]. However, other research revealed that SOC accumulation depends on microaggregates (0.053-0.25 mm) [16] and slit and clay fractions (<0.053 mm) [26,27] following afforestation. These inconsistent results suggest the need for in-depth studies on the influences of afforestation on SOC accumulation in different soil aggregates with different particle sizes [28].
Only a few studies have reported the effects of afforestation in farmland on the distribution and stability of soil aggregates and the accumulation of SOC in aggregates in northeastern China. However, such information is crucial for understanding how SOC responds to the change in farmland owing to afforestation and assessing the function of the sequestered SOC following afforestation. We hypothesized that: (1) afforestation on farmland will increase the proportion of >0.25 mm soil aggregates and the stability of soil structure; (2) total SOC and aggregate-associated SOC in >2 mm aggregates will increase in the 0-20 cm and 20-40 cm layers following afforestation; and (3) the total SOC is dominated by aggregate-associated SOC, with larger-particle aggregates contributing more to the total SOC.
To test our hypothesis, the distribution of soil aggregates, the stability of soil aggregates, and the SOC concentration and stock in total soil and aggregates of different particle sizes were measured in paired plots of poplar shelterbelts and adjacent farmland. The aims of this study were to determine the effects of afforestation on aggregate size distribution, aggregate stability, SOC content and stock in different size aggregates, and the contribution to SOC in total soils.

Study Area
The three study sites (Dumeng, Zhaodong, and Lanling) are located in central Songnen Plain, China ( Figure 1). The study region has a typical continental monsoon climate, with an annual average temperature of 3-4 • C. The annual average precipitation ranges from 350 to 500 mm, and the average frost-free period is 145 days. The soils in this region are typical black soils and degraded soils, including Chernozem (Lanling), Cambosols (Dumeng), and Solonetz (Zhaodong), according to the Chinese Soil Classification System [29]. The soil properties of the sampling sites are shown in Table 1. region are typical black soils and degraded soils, including Chernozem (Lanlin bosols (Dumeng), and Solonetz (Zhaodong), according to the Chinese Soil Class System [29]. The soil properties of the sampling sites are shown in Table 1.  The Songnen Plain is an area included in the "Three-North Shelterbelts" p 1978, which comprises plantation forests surrounding farmlands in northeaster western, and northern China [30]. Forest and farmland are the main land use typ study area. The major forest type is poplar (Populus alba × Populus berolinensis), dominant crop is maize (Zea mays L.). Four to six rows of poplar forest belts (in w usually cultivated to protect a farmland with an area of 500 m × 500 m. This ar proximately 10 m from the shelterbelt to the neighborhood farmland. The same co were secured between the control plots and the experimental plots. Related info regarding this study area has been presented previously [31,32].

Experiment Design, Soil Sample Collection, and Measurements for the Parameters o gates
Twelve paired plots of poplar plantation and neighboring farmland were sel soil sampling at each study site. We measured the tree height, diameter at breas and tree density in the research plot. In each paired plot, two soil profiles (1 m × m) were excavated to collect undisturbed soil samples at depths of 1 m (0-20 c cm, 40-60 cm, 60-80 cm, and 80-100 cm) in poplar and neighborhood farmland removal of the herbaceous and litter layer. Soil samples were collected using four cutting rings at each layer, and four samples obtained from the same soil lay mixed. Thus, a total of 360 samples were used (3 sites × 12 pairs × 2 profiles × 5 The collected samples were air-dried in the laboratory. The dried soil samples were mixed according to the same plots, the same la  The Songnen Plain is an area included in the "Three-North Shelterbelts" project of 1978, which comprises plantation forests surrounding farmlands in northeastern, northwestern, and northern China [30]. Forest and farmland are the main land use types in the study area. The major forest type is poplar (Populus alba × Populus berolinensis), and the dominant crop is maize (Zea mays L.). Four to six rows of poplar forest belts (in width) are usually cultivated to protect a farmland with an area of 500 m × 500 m. This area is approximately 10 m from the shelterbelt to the neighborhood farmland. The same conditions were secured between the control plots and the experimental plots. Related information regarding this study area has been presented previously [31,32].

Experiment Design, Soil Sample Collection, and Measurements for the Parameters of Aggregates
Twelve paired plots of poplar plantation and neighboring farmland were selected for soil sampling at each study site. We measured the tree height, diameter at breast height, and tree density in the research plot. In each paired plot, two soil profiles (1 m × 1 m × 1 m) were excavated to collect undisturbed soil samples at depths of 1 m (0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, and 80-100 cm) in poplar and neighborhood farmland after the removal of the herbaceous and litter layer. Soil samples were collected using four 100 cm 3 cutting rings at each layer, and four samples obtained from the same soil layers were mixed. Thus, a total of 360 samples were used (3 sites × 12 pairs × 2 profiles × 5 layers). The collected samples were air-dried in the laboratory.
The dried soil samples were mixed according to the same plots, the same land type, the same soil depth, and the same DBH range (5-20 cm, 20-35 cm, and 35-50 cm). Thus, 90 mixed samples were used for further experiments on soil aggregates. Each mixed sample was divided into two parts: one part was passed through a 0.25 mm mesh to measure the SOC concentration. The oil bath-K 2 Cr 2 O 7 titration method was used to determine the SOC concentration, and the cutting ring (100 cm 3 volume) method was used to determine soil bulk density [33]. The remaining part was used to analyze the proportion of soil aggregates with different particles via the wet-sieving method. The soils were transferred to a nest of sieves, including 2 mm, 0.25 mm, and 0.053 mm sieves, to separate the >2, 0.25-2, 0.053-0.25, and <0.053 mm soil aggregates, and 360 samples (3 sites × 3 DBH ranges × 2 profiles × 5 layers × 4 aggregate sizes) were obtained through this method. The aggregates of all sizes obtained were dried at 60 • C for 24 h in an oven and then weighed. Details of the procedure have been described by Cambardella and Elliott [34].

Data Analysis
R 0.25 (the proportion of >0.25 mm soil aggregate), MWD, GMD, D, and SOC stock in total and different soil aggregates were calculated using the following formulas: where M X < 0.25 represents the weight of <0.25 mm aggregates, and M T represents the total weight of the soil aggregate.
where n represents the number of separated aggregate, w i represents the proportion (%) of aggregates of the ith size in the total sample, and x i represents the mean diameter of each aggregate fraction (mm) [35].
where D represents the fractal dimension, δ represents yard measure, d i is the average value of soil particle diameter between d i and d i+1 (i = 1, 2, . . . ), W (δ < d i ) represents the cumulative mass of aggregate particles with sizes δ < d i , and W 0 is the total mass of different sizes of soil aggregate particles [20].
where 0.2 represents the thickness of the soil layer (0.2 m), α represents the SOC concentration (g/kg) in total soil or soil aggregates with different particle sizes, BD represents the soil bulk density (Mg/m 3 ) of the farmland or poplar plantation in different soil layers, and V gravel represents the proportion of gravel. In this study, the relative change in all variables was calculated using farmland as the control. A paired-samples T test was used to determine the significance of the differences in soil-aggregate-associated indexes between poplar shelterbelt and adjoining farmland in five layers. Pearson correlation analysis was conducted to confirm the effect of the distribution and stability of aggregate on SOC content and stock of total soils. The relationship between total SOC content (stock) and aggregate-associated SOC content (stock) was also examined using this method. The level of significant differences was p < 0.05. SPSS 22.0 was used for the statistical analysis of the data.

Proportion and Stability of the Soil Aggregates
Shelterbelt construction increased the proportion of aggregates at the five depths, except for the >2 mm size class at 20-40 cm and <0.053 mm class at 0-20 cm. The changes in the proportion of >2 mm aggregates increased by 3.67%-71.69% at 0-20 cm, 46-60 cm, 60-80 cm, and 80-100 cm, with a significant difference only found at 0-20 cm (p < 0.05). The changes in the proportion of >0.25-2 mm and 0.053-0.25 mm aggregates increased by 8.99%-103.81% and 0.96%-14.60% at the five depths, respectively, with a significant difference only found in the >0.25-2 mm size class at 0-20 cm (p < 0.001). The proportion of <0.053 mm aggregates increased by 1.25%-20.14% at the 20-40 cm, 46-60 cm 60-80 cm, and 80-100 cm depths and decreased by 9.27% at 0-20 cm (Figure 2). The shelterbelt-induced changes in R0.25, MWD, and GMD significantly increased by 96.3%, 33.2%, and 40.0%, respectively, in the 0-20 cm soil depth following afforestation (p < 0.001) ( Figure 3); however, the differences were not significant in the other four soil layers between the poplar shelterbelts and farmland. The change in the value of D was weak, and no significant differences were found in the five depths following poplar shelterbelt establishment (Figure 3). The shelterbelt-induced changes in R 0.25 , MWD, and GMD significantly increased by 96.3%, 33.2%, and 40.0%, respectively, in the 0-20 cm soil depth following afforestation (p < 0.001) ( Figure 3); however, the differences were not significant in the other four soil layers between the poplar shelterbelts and farmland. The change in the value of D was weak, and no significant differences were found in the five depths following poplar shelterbelt establishment (Figure 3).

SOC in Total Soil
The SOC concentration and stock of total soil increased by 7.12%-19.86% and 1.11%-14.48% at five depths, respectively, with a significant difference only observed in shelterbeltinduced SOC concentration at the 0-20 cm depth (p < 0.05) (Figure 4).

SOC in Total Soil
The SOC concentration and stock of total soil increased by 7.12%-19.86% and 1.11%-14.48% at five depths, respectively, with a significant difference only observed in shelterbelt-induced SOC concentration at the 0-20 cm depth (p < 0.05) (Figure 4).

SOC in Aggregates
The SOC concentration in >2 mm aggregates increased by 9.80%-60.56% at five depths, and a significant increase was found in the poplar shelterbelt compared to the

SOC in Total Soil
The SOC concentration and stock of total soil increased by 7.12%-19.86% and 1.11%-14.48% at five depths, respectively, with a significant difference only observed in shelterbelt-induced SOC concentration at the 0-20 cm depth (p < 0.05) (Figure 4).

SOC in Aggregates
The SOC concentration in >2 mm aggregates increased by 9.80%-60.56% at five depths, and a significant increase was found in the poplar shelterbelt compared to the

Relationships between SOC in Total Soils and Aggregates
A significantly positive correlation was found between the SOC content and stock in total soils and the proportion of >2 mm soil aggregates (p < 0.001), while a significantly negative correlation was found between the SOC concentration and stock in total soils and the D value (p < 0.01) ( Table 2). No significant correlations were found between the SOC content (stock) in total soils and the proportion of 0.25-2 mm, 0.053-0.25 mm, <0.053 mm soil aggregates, and R 0.25 , MWD, and GMD ( Table 2).
The SOC content in total soils was positively correlated with the aggregate-associated SOC content in the different size classes (Figure 7). The SOC content in the total soil was more dependent on the SOC contents in the >2 mm and 0.25-2 mm size classes than that in 0.25-0.053 mm and <0.053 mm soil aggregates. For example, the R 2 values of the linear equations between total SOC content and SOC content in >2 mm or 0.25-2 mm aggregates were 0.89 and 0.81, while those between the total SOC content and SOC content in the 0.25-0.053 mm and <0.053 mm soil aggregates were 0.71 and 0.79 (Figure 7). Similarly, a significantly positive correlation was found between the SOC stock in total soils and the SOC stocks in aggregates (Figure 7). The SOC stock in total soils was more dependent on the SOC stocks in the >2 mm and 0.25-2 mm (R 2 = 0.87, R 2 = 0.81) aggregates than on those in the 0.25-0.053 mm and <0.053 mm (R 2 = 0.77, R 2 = 0.68) aggregates (Figure 7).

Relationships between SOC in Total Soils and Aggregates
A significantly positive correlation was found between the SOC content and stock in total soils and the proportion of >2 mm soil aggregates (p < 0.001), while a significantly negative correlation was found between the SOC concentration and stock in total soils and the D value (p < 0.01) ( Table 2). No significant correlations were found between the SOC content (stock) in total soils and the proportion of 0.25-2 mm, 0.053-0.25 mm, <0.053 mm soil aggregates, and R0.25, MWD, and GMD ( Table 2).
The SOC content in total soils was positively correlated with the aggregate-associated SOC content in the different size classes (Figure 7). The SOC content in the total soil was more dependent on the SOC contents in the >2 mm and 0.25-2 mm size classes than that in 0.25-0.053 mm and <0.053 mm soil aggregates. For example, the R 2 values of the linear equations between total SOC content and SOC content in >2 mm or 0.25-2 mm aggregates were 0.89 and 0.81, while those between the total SOC content and SOC content in the 0.25-0.053 mm and <0.053 mm soil aggregates were 0.71 and 0.79 (Figure 7). Similarly, a significantly positive correlation was found between the SOC stock in total soils and the SOC stocks in aggregates (Figure 7). The SOC stock in total soils was more dependent on the SOC stocks in the >2 mm and 0.25-2 mm (R 2 = 0.87, R 2 = 0.81) aggregates than on those in the 0.25-0.053 mm and <0.053 mm (R 2 = 0.77, R 2 = 0.68) aggregates (Figure 7).

Improvement of Soil Structure and Stability following Afforestation
In the present study, significant increases in the proportions of >2 mm and 0.25-2 mm

Improvement of Soil Structure and Stability following Afforestation
In the present study, significant increases in the proportions of >2 mm and 0.25-2 mm soil aggregates and the values of R 0.25 , MWD, and MGD were observed at a soil depth of 0-20 cm following afforestation on abandoned farmland (Figures 2 and 3), ultimately supporting the hypothesis (1) that afforestation on farmland will increase the proportion of >0.25 mm soil aggregates and the stability of soil structure. Various studies revealed that soil structural stability improved upon the enhancement of the proportion of soil aggregates > 0.25 mm in diameter [36]. Further, the higher numbers of MWD and GMD represented less fragmentation and increased stability of the soil aggregates [37,38]. This improvement might be attributed to the increase in root biomass and litter inputs, which contribute to the aggregation of soil particles, as these soils are not tilled compared to those of farmland [39,40]. Vegetation cover plays a key role in the formation of soil organic matter and enhances soil stability [41]. The increased biomass from plants increases the input of soil organic matter following afforestation [42]. In this study, a poplar shelterbelt increased the SOC concentration in 0-20 cm (Figure 4), and there were remarkable correlations between SOC and >2 mm soil aggregates, as outlined in Table 2. Similar relationships were found in other studies [43,44]. In contrast, the breaking up of the bigger aggregates and decreases in the stability of soil aggregates have been widely reported due to deforestation [45,46].
Five soil depths were considered in the 1 m profile for this study. However, significant increases in >0.25 mm aggregate distribution and stability indexes, for example, R 0.25 , MWD, and MGW, were increased by 96.3%, 33.2%, and 40.0% at the 0-20 cm depth (Figures 2 and 3). A heterogeneous improvement in the soil aggregate index was discovered in different layers following afforestation, and more notable differences in soil aggregate particle distribution, MWD, and GMD were found in the topsoil than the deeper layers [9]. Our results showed that the soil aggregate structure and the stability of topsoil (0-20 cm) were sensitive following the conversion of farmland into shelterbelts; this is because afforestation mainly affects the processes of soil aggregation in the topsoil [38,47], and the input of organic matter from litter and the products of microbial decomposition are directly received at the uppermost layer [48,49]. Some organic by-products play a critical role in the formation of soil aggregates and the improvement of soil structure stability [38,50].

Accumulation of SOC in Total Soil and Aggregates
The importance of SOC has been recognized by many scientists [2,4], and afforestation can accumulate SOC in the total and aggregates soil, as shown in this study. The SOC concentration in total soil significantly increased by 13.3% at the 0-20 cm depth following shelterbelt establishment (p < 0.05) (Figure 4). SOC in the topsoil is considered to be more vulnerable to land use type than deep soil [24,51]. Wang et al. [8] observed that larch plantations can accumulate SOC in the top 20 cm soil layer at a rate ranging from 57.9 to 139.4 gm −2 yr −1 ; however, deep layers are very little affected by larch trees in northeastern China.
The physical preservation of SOC via soil aggregates is a key mechanism in SOC sequestration [23]. According to our results, shelterbelts enhanced SOC concentration and stock by 21.5% and 18.7%, respectively, in >2 mm and 0.25-2 mm soil aggregates in the 20-40 cm depth (p < 0.05) (Figures 5 and 6). The accumulation of SOC in >0.25 mm aggregates by afforestation was observed in other studies [19,52]. The accumulation was mainly due to the input of new OC from litterfalls, root biomass, and dead microorganisms [53], which reduced the loss of SOC in aggregates owing to mineralization [46]. Among them, the relatively shallow root system of poplars is an important factor that should be considered [8]. According to our survey (data not shown), a 28 mg cm −3 poplar root system was located at a 1 m depth, and 95% of the root was distributed in the 0-60 cm soil layer, especially the 20-40 cm layer (57%). Based on the aggregate hierarchy theory [54], the adsorption of small particles into the organic skeleton contributes to the formation of large aggregates [55,56], which can physically protect SOC from mineralization and microbial decomposition [57].

Relationship of SOC in Total Soil and Aggregate-Associated SOC
The enhancement in SOC in total soils was dependent on the increase in aggregateassociated SOC (Figure 7). The contributions of aggregate-associated SOC to the SOC accumulation in total soil following afforestation on farmland have been reported by many scientists [19,52]. Some studies revealed that SOC accumulation depended on the >2 mm soil aggregates [58] and 0.25-2 mm soil aggregates [15,59], while according to other studies, SOC accumulation was mainly determined by the 0.053-0.25 mm soil aggregates [16]. Based on our findings, the SOC stock in total soils was more dependent on the SOC stock in the larger-size aggregates (>0.25 mm) than those in the smaller-size aggregates (<0.25 mm) ( Figure 6). These observations are supported the hypothesis that the total SOC is dominated by aggregate-associated SOC, with larger aggregates having a greater contribution. Some similar conclusions were supported by the findings that total SOC was controlled by the SOC in bigger aggregates in the forest [21,60], and the decrease in SOC in total soils was mainly due to the decrease in SOC in macroaggregate following the change from natural forest to farmland [52]. These contributions of the larger soil aggregates to SOC accumulation are related to the influences of plant roots and fungal hyphae-rendered cementing [61], which stimulate the root exudation rate [62].
An increase in total SOC content and stock exhibited a notable positive correlation with the proportion of >2 mm aggregates and a notable negative correlation with the D value. By integrating the contribution of SOC in the aggregate and proportions of the aggregate to the enhancement of total SOC, SOC accumulation in total soils was found to be controlled by not only the proportion of aggregates in each size class and the D value, but also the SOC content and stock in the soil aggregates [63,64]. In brief, our results emphasize the important role of increased >2 mm aggregate proportion and the larger aggregate-associated SOC content in total SOC accumulation.

Conclusions
Our results revealed that poplar shelterbelts significantly enhanced the distribution of >2 mm and 0.25-2 mm soil aggregates, R 0.25 , MWD, and GMD in the 0-20 cm depth; shelterbelt establishment on farmland improved soil stability and structure, especially those of the topsoil. The total SOC concentration in the 0-20 cm layers and the aggregateassociated SOC concentration and stock in >2 mm aggregates at 20-40 cm significantly increased following afforestation. In the 1 m soil layer, the increase in total SOC was mainly dependent on the aggregate-associated SOC and the proportion of >2 mm aggregates.