Increase soil aggregate stability can limit colloidal phosphorus loss potentials from agricultural systems

Background Colloid-facilitated phosphorus (P) transport is a recognized important pathway for soil P loss in agricultural systems, but limited information is available on the soil aggregate-associated colloidal P. To elucidate the effects of aggregate size on the loss potential of colloidal P (P coll ) in agricultural systems, soils (0-20 cm depth) from six land use types were sampled in Zhejiang province in the Yangtz river delta region, China. The aggregate size fractions (2–8 mm, 0.26–2 mm, 0.053–0.26 mm and <0.053 mm) separated by wet-sieving method were analyzed. Results Results showed that the 0.26–2 mm small macroaggregates had the highest total P (TP) content. For acidic soils, the highest P coll content was also found in the 0.26–2 mm aggregate size, while the lowest was found in the <0.053 mm (silt+clay)-sized particles, the opposite of that found in alkaline soils. Paddy soils contained less P coll than other land use types. The P coll in total dissolved P (TDP) was dominated by <0.053 mm (silt+clay)-sized particles. Aggregate size did strongly influence the loss potential of P coll in paddy soils, where P coll contributed up to 83% TDP in the silt+clay sized particles. The P coll content was positively correlated with TP, Al, Fe and mean weight diameter (MWD). Aggregate associated total carbon (TC), total nitrogen (TN), C/P, and C/N had significant, but negative effects on the contribution of P coll to potential soil P losses. The P coll content of the aggregates was controlled by aggregate associated TP and Al content as well as soil pH value, with P coll loss potential from aggregates being controlled by its organic matter content. management practices P coll from lower loss of P coll ; and use management with single rice has a higher loss potential of P coll


Abstract
Background Colloid-facilitated phosphorus (P) transport is a recognized important pathway for soil P loss in agricultural systems, but limited information is available on the soil aggregate-associated colloidal P. To elucidate the effects of aggregate size on the loss potential of colloidal P (P coll ) in agricultural systems, soils (0-20 cm depth) from six land use types were sampled in Zhejiang province in the Yangtz river delta region, China. The aggregate size fractions (2-8 mm, 0.26-2 mm, 0.053-0.26 mm and <0.053 mm) separated by wet-sieving method were analyzed.
Results Results showed that the 0.26-2 mm small macroaggregates had the highest total P (TP) content. For acidic soils, the highest P coll content was also found in the 0.26-2 mm aggregate size, while the lowest was found in the <0.053 mm (silt+clay)-sized particles, the opposite of that found in alkaline soils. Paddy soils contained less P coll than other land use types. The P coll in total dissolved P (TDP) was dominated by <0.053 mm (silt+clay)-sized particles. Aggregate size did strongly influence the loss potential of P coll in paddy soils, where P coll contributed up to 83% TDP in the silt+clay sized particles. The P coll content was positively correlated with TP, Al, Fe and mean weight diameter (MWD). Aggregate associated total carbon (TC), total nitrogen (TN), C/P, and C/N had significant, but negative effects on the contribution of P coll to potential soil P losses. The P coll content of the aggregates was controlled by aggregate associated TP and Al content as well as soil pH value, with P coll loss potential from aggregates being controlled by its organic matter content.
Conclusion Therefore, we conclude that management practices that increase soil aggregate stability or its organic carbon content will limit P coll loss from agricultural systems. Background Phosphorus (P) loss from agricultural soils has been identified as one of the main causes of the eutrophication of lakes in the lower reaches of the Yangze River in southern China [1]. Soil colloidal phosphorus (P coll ) is the P fraction bound to colloids. Colloidal particles are highly mobile and are effective adsorbents for organic and inorganic contaminants and nutrient elements, such as P, because of high specific surface area and charge density, resulting in a high adsorption capacity [2,3]. Colloid-facilitated P transport is an important pathway for the migration of P into water bodies [4][5][6]. Studies have shown that more than 75% of P in cultivated soil solution was combined with fine particles smaller than 240 nm [7]; similarly, 40-58% of molybdate-reactive P with size less than 450 nm in water extract of grassland soil was fine-grained P with size of 25-450 nm [5]. Other studies suggest that P coll can reach up to 50% of total P (TP) in surface runoff, rivers, and lakes [8], which may lead to eutrophication and environmental risks.
Water-dispersible colloids in the soil adhere to soil aggregates, forming a stable system [9]. Soil aggregate stability plays a key role in controlling erosion processes and soil nutrients losses [10][11][12].
Colloids either can bind to soil aggregates, or be physically strained from water flowing though pores between aggregates [13,14]. Soil organic carbon (SOC) promotes aggregate stability and thus reduces the degree of clay colloid dispersion [15][16][17]. Some scholars determined the colloid content in 1-2 mm aggregates in 39 soils, and found a significant positively correlation between water dispersible colloid (WDC) content and clay content in soil aggregates [15]. Furthermore, they reported that, WDC content is a function of total organic C (TOC) and total clay. Other scholars modeled the release characteristics of colloids from soil aggregates, the attachment and detachment processes at the air-water interface, and flocculation and straining from interstitial water [18]. The release of colloids from aggregates may result in the disintegration of aggregates [19]. However, soil aggregation mainly depends on the availability of active mineral surfaces and the dispersion/flocculation behaviors of the colloidal components [20]. Therefore, the stability of soil aggregates directly affects the migration of soil colloids.
The retention of P to soil aggregates depends on the particle size [21,22]. Phosphorus has a relatively closed cycle in which most of the mineralized and dissolved P from the microaggregates is adsorbed onto unaggregated clay-sized particles (< 53 µm) or is utilized by plants [23]. Some studies have shown that soil aggregate stability and size can affect soil P distribution [24][25][26]. Higher percentages of both water-extractable and Mehlich III-extractable P have been found in both the 0.50-0.25 and 0.25-0.125 mm aggregate fractions [27]. In contrast, some other researchers suggested that TP is highest in small soil aggregates [28], or the TP content was uniform in soil aggregates of all sizes, whereas the available P was higher in small soil aggregates [29]. Soil aggregation can significantly reduce the loss of organic P in aggregates and increase the adsorption of inorganic P by silt and clay particles [23]. Some scholars have studied different forms of P in soil aggregates. For example, a study showed that Al-P was mainly dominated by soil aggregates of < 1 mm, while those of 2-8 mm were mainly Fe-P and Ca-P [30]. Others claimed that labile P in macroaggregates was higher under native land use than other land uses, they further confirmed that soils under native use contained more Ca-bound P in macro-aggregates than the disturbed soils [31]. These studies provide a first basis for the better understanding the relationship between soil aggregates and P. To date, there is still limited information on P coll content and its loss potential from aggregates. It is unclear how aggregation affect the P coll content in soils. Moreover, the composition and structure of soil aggregates do vary under different land use managements [32]. There are less large-sized aggregates present in rice soil than dryland, which was due to the long-term flooding and anaerobic conditions that caused the macro-aggregates to be dispersed [32,33]. In addition, alternation between dry and wet conditions generally destroys macro-aggregates and enhances the decomposition of SOC in paddy soils [34]. Therefore, we suspect that the content of P coll in macroaggregates of paddy soils is less than dryland soils, but mainly exists in micro-aggregates and has a higher loss potential.
The main purposes of this study were to understand the effect of soil aggregate stability on soil P coll content and its loss potential, and to assess the main environmental factors affecting P coll in soil aggregates. To this end, we collected soil samples from 15 sites and 6 land use types in Zhejiang Province in the Yangtz river delta region for aggregates and P coll analysis. Firstly, we isolated the different sized aggregates in the soil samples. Secondly, we then determined the P coll , total carbon (TC), TP, total nitrogen (TN), Al, Fe and Ca content in each aggregate size fraction. We hypothesized that 1) Larger sized aggregates have higher TP and P coll content; 2) Aggregates with higher organic carbon content have lower loss potential of P coll ; and 3) Land use management with single rice has a higher loss potential of P coll .

Soil sampling and preparation
In total, soils with different land use types (Figure1) were collected from 15 sites, which were almost evenly distributed in Zhejiang Province (an area of 1,055,000 km 2 ). Information on specific sampling points is shown in Table 1. The 15 sampling points covered six land use types, generally established in the past 5 years.
Soil samples of 0-20 cm were collected from typical fields (Long-term farmland with conventional fertilization by local farmers) in May 2018 in the second season of the rotation systems. Two samples with three replicates were taken at an interval of 1000 m in each site with same land use type, the replicates were brought back to the laboratory and mixed. Then, the mixed soil samples from each site were divided into four equal parts by the diagonal quartering method. One part was retained for a follow-up test. All samples were air-dried and separated into two parts; one was finely milled and sieved through a 2-mm mesh to determine basic physical and chemical properties, and another was broken carefully into small pieces by hands to segregate aggregates and determine P coll .

Aggregate separation and determination
Aggregate size distribution was determined for each soil sample using a modified wet sieving method [35]. Briefly, the second part of unground soil was first passed through an 8-mm sieve, and 50 g soil was placed carefully on the top of a nest of three sieves (2 mm, 0.26 mm, and 0.053 mm). Then, the sieves were submerged for 20 min in 2.5 L deionized water at room temperature and oscillated under water 300 times for 10 min with a 30 mm amplitude to separate aggregate fractions. Thus, four aggregates fractions were obtained on each sieve: large macroaggregates (2-8 mm), small macroaggregates (0.26-2 mm), microaggregate (0.053-0.26 mm), and (silt+clay)-sized particle (<0.053 mm) [36]. Aggregates of each size were carefully moved from the sieve into a beaker. The water used for wet sieving was left to rest for 48 h, silt and clay particles were collected, and the supernatant was used to determine total dissolved P (TDP), truly soluble P (TSP), and P coll content. All aggregates were oven-dried at 65 °C for 48 h, weighed, and placed in a zip lock bag. To obtain water-stable aggregates, sediment concentration was subtracted from that obtained by wet sieving because sand was not considered a component of water-dispersible aggregates [35]. Sand content was determined by the following process: 5 g of the dry aggregates obtained above were weighed, dispersed into 30 mL 5 g L -1 hexametaphosphate solution, placed into an ultrasonic cleaner, and Colloidal P was determined as described by Ilg [39]. Briefly, 10 g of unground soil was placed into a 250 mL flask, 80 mL deionized water (DDW) was added, and the sample was shaken at 160 rpm and 25°C for 24 h. The supernatant was pre-centrifuged at 3000 g for 10 min to remove coarse particles.
After pre-centrifugation, the supernatant was filtered with a 1 μm microporous membrane, 5 mL of the primary filtrate was discarded, and the total filtrate was collected (sample I). This suspension included the colloidal component and the dissolved component. The filtrate was ultracentrifuged at 300,000 g for 2 h to remove colloids (Optima TL, Beckman, USA; Sample Ⅱ), and the residue at the bottom of the ultracentrifuge tube was the water-dispersible colloid. The TDP in sample I and TSP in sample II in the solution were determined after digestion with acidic potassium persulfate. The difference between TDP in Sample I and TSP in Sample Ⅱ was the concentration of P coll . Previous studies have shown that soil P through leaching and surface runoff is usually in soluble forms that can pass through the 0.45-1 μm filter [40,41]; therefore, in the present study, TDP including P coll and TSP in aggregates was defined as the potential loss P. The TDP in the supernatant after 10 min wet sieving was considered as easy loss P.

Calculation of water-stable aggregate (WSA) size fractions
The proportion of WSA in each size fraction was obtained from equations (1) and (2) (1) where, i is the ith size fraction (2-8, 0.25-2, and 0.053-0.25 mm); dry soil aggregate (DSA i ) is the oven-dried mass of total, non-dispersed aggregates collected on each sieve; Sand is the oven-dry mass of sand collected after dispersed in hexametaphosphate solution on the 0.053 mm sieve; Total Soil is the oven-dried mass of soil (50 g) for aggregate separation.

Calculation of MWD and GMD
The mean weight diameter (MWD) and geometric mean diameter (GMD) of the aggregates can be obtained by Equations (3) and (4)

Contribution of aggregate associated P coll to TDP
To illustrate the contribution of aggregate associated P coll to TDP (potential loss P.), soil aggregates and P coll concentrations were integrated and calculated. The contribution rate (CR) of aggregate associated P coll to TDP was calculated using Equation (5): where, Agg_CP is the concentration of aggregate associated P coll (mg kg -1 ), TDP is the concentration total dissolved P (mg kg -1 ), and i is the ith size fraction (2-8, 0.26-2, 0.053-0.26, and <0.053 mm).

Statistical analysis
Microsoft Excel 2016 and Origin 8.0 were used for data processing and cartography. Data were statistically analyzed using SPSS Statistics 22.0 (SPSS Inc. Chicago, USA) software. One-way ANOVA was conducted using two samples of each site to examine differences of different variables in Table   S1, S2, and 3 and Fig S1 and S2. Pearson correlation analyses were used to identify the relationship between aggregate associated P coll and other soil parameters. Stepwise linear regression was performed to evaluate the relationships between P indicators (Content and loss potential of P coll ) and soil variables (pH, TP, TC, TN, C/N, Fe, Al, Ca, MWD, and GMD).

Soil and aggregate characteristics
Within the collected 15 soils, ten were acidic soils, two were neutral soils, and three were alkaline soils (Table 1). There were six land use types: orchard, single cropping rice, rice-rape rotation, vegetable, double cropping rice, and rice-wheat rotation. The TC of the soils ranged from 3.32 to 20.19 g kg -1 ; the TN was between 0.53 and 2.17 g kg -1 . The TP varied from 0.23 to 1.64 g kg -1 . Soil pH values ranged from 3.95 to 7.83 ( Table 1).
The DSA and WSA of larger macroaggregates (2-8 mm) generally increased with increasing of pH values ( Figure S1 and Figure S2, The MWDs of acidic soils (pH <5.5) were significantly smaller than those of alkaline soils (P < 0.05).
The average MWD of acidic soils was 0.78 mm ( Figure S2), while that of neutral and alkaline soils was 1.36 mm. However, little difference in GMDs was found between acidic (0.85 mm), and alkaline soils (0.91 mm).

Total and colloidal phosphorus content
Generally, the 0.26-2 mm aggregate fraction had the highest TP content, which accounted for 29.6% of the soil TP (Fig.1a), and soil aggregates of 2-8 mm had the second highest TP content. Whilst that of (silt+clay) sized particles was significantly lower than that of other fractions (P < 0.05, Table S1), which only accounted for 19.7% of the soil TP (Fig.1a). Moreover, TDP, TSP, and P coll contents were related to soil pH, and the highest TDP content was found in 0.26-2 mm aggregates in acidic soils and in (silt+clay) sized particles in alkaline soils (P < 0.05, Table S1). In all soils, no significant difference was observed in the P coll fractions between different aggregate sizes (P > 0.05; Fig. 1b). However, the content of aggregate associated P coll was highest in the 0.26-2mm aggregates, and the lowest in (silt+clay)-sized particles in acidic soils; while in neutral and alkaline soils, (silt+clay)-sized particles possessed the highest TDP and P coll contents, followed by the 0.26-2 mm aggregates (Table 3).

Loss potential of colloidal phosphorus
After wet sieving, about 0.16-1.87% of the soil TP was lost into the supernatant as TDP, and the P coll accounts for 8.5%-84.1% of the TDP ( Table 2). The proportion of easy loss P content in the various soils was also different due to variations in soil physicochemical properties.
To make the P coll in different soils comparable, the indicator of P coll in TDP was used for standardization. The proportion of aggregate-associated P coll in TDP can reflect the loss potential of P coll in the TDP. In general, the P coll loss potentia gradually decreases as the size of soil aggregates increases. The P coll loss potential was the lowest in 2-8 mm and 0.26-2mm aggregates, and P coll accounted for 52.6% and 60.6% of TDP, respectively. However, the P coll loss potential of (silt+clay)sized particles was the highest, and P coll accounted for 75.3% of TDP (Fig. 1a). The CR value of (silt+clay)-sized particles were mostly larger than that of the other aggregate sizes except for S7, S11 and S15, while the CR of larger macroaggregate was lowest in most soils (Table 3).
Considering different land use types, the P coll content in the rice-dry land rotation and vegetable (VE) soils was significantly higher than that in paddy (RICE) and orchard (OR) soils regardless of different aggregate sizes (Figure 4  of TDP were as high as 83.0% and all significantly higher than that in orchard and rice-dryland rotation systems (Figure 4b). This indicates that the loss potential of P coll was dominated by fine grained and (silt+clay) sized particles in paddy soils. However, the loss potential of P coll carried by all sized aggregates in dryland and rice-dryland rotation systems (orchard, rice-rape rotation, vegetable, and rice-wheat rotation) exceeded 50% of TDP.

Factors affecting colloidal P content and loss potential
Correlation analysis revealed that no significant correlations were found between P coll and TC or TN (Table 4). The P coll content was significantly positively correlated with TP in all aggregate sizes except for (silt+clay)-sized particles (Fig 3a). Soil pH was positively correlated with the aggregate-associated P coll content, but was only significantly correlated with the P coll content of (silt+clay)-sized particles (P < 0.01) (Fig 3b). Al and Fe were also found significantly positively correlated with P coll in total sizes of soil aggregate (P < 0.05) (Fig 3c and d), however, only P coll in 2-8 mm aggregate were found significantly positively correlated with Al and Fe (P < 0.05). Moreover, P coll was negatively correlated with C/P (Fig 3e) and positively correlated with C/N (Fig 3f). The forward results of the stepwise regression showed that P coll can be described by Al, TP, TN and MWD as: , (R 2 =0.605, P < 0.001, Table 4). Table 5 shows that the TC, TN, pH, Ca, C/P, C/N ratios of aggregates were negatively correlated with the proportion of P coll in TDP in total sizes of soil aggregate (Table 4, P < 0.05). Considering different sizes, TC and TN were only found significantly negatively correlated with P coll in 0.26-2 mm sized aggregates. Except for (silt+clay)-sized particles, Al was significantly positively correlation with P coll in TDP (P < 0.05). While Fe was significantly positively correlation with P coll in TDP in 2-8 mm sized aggregates (P < 0.05). However, Ca was negatively correlated with P coll in TDP in 0.053-2 mm sized aggregates (P < 0.05). The results of the stepwise regression showed that P coll /TDP can be described by TC, Fe, MWD, and GMD as: , (R 2 =0.539, P < 0.001, Table 5).

Total P in aggregates
In our study, we found that the TP content was highest in small macroaggregates of the 15 soils, while the TP content in the silt+clay particles was the lowest. These results indicated that soil P was mainly carried by larger aggregates, which confirmed our hypothesis and were in line with those of previous studies [25, 44,45]. For examples, some scholars found that P tended to concentrate in large WSAs in long-term fertilization experiments in a reddish paddy soil [44], and others claimed that aggregateassociated TOC, TN, and TP were preferentially enriched in large WSAs (4.76-2.0 mm) [45], and found that the proportion of TP increased with increasing aggregate size for native lands [31]. Higher P levels may be associated with higher levels of TC and TN in large aggregates [45,46]. Studies have shown that the content of organic and inorganic P in 2-4 mm aggregates was higher [47].

Macroaggregates ([(Cl-P-OM)x]y) are usually formed by organic matter (OM), clay (Cl) and multivalent
ions of P and other substances [48,49]. Organic matter (and associated P) are protected within stable aggregates against microbial degradation [50]. Moreover, it has been shown that the organic P forms that accumulate in soils are less available to enzymatic hydrolysis when bound to mineral surfaces [51,52]. On the other hand, the aggregation promoted by the organic matter counteracts the dispersion of the small mineral particles (mostly Fe and Al (hydr) oxides) where P is retained [53,54]. This is also evidenced by the positive correlation between aggregate-associated TP and TC, and the significant correlation between aggregate-associated TP and TN observed in this study (Tables S2).

Colloidal P content in aggregates
In this study, we found that TDP, TSP, and P coll contents were related to soil pH. The TDP and P coll contents were high in large aggregates of acidic soils, and low in micro-aggregates and silt-and claysized particles; however, contrasting results were found in alkaline soil aggregates. This is not consistent with our hypothesis. First this may be due to the acidity of the soil leading to the dissolution of organic matter and inorganic cement (Al 3+ , Fe 2+ , and Ca 2+ ) in the aggregates, causing the macroaggregates to be extremely unstable [55]. Moreover, soil clays are mostly negatively charged, but the aggregation of another soil colloids strongly depends on their surface charge, being favored when approaching their point of zero charge [56]. Protonation of Fe-, Al-oxides and organic matter in colloids under acidic conditions results in positive charges, leading to their association with soil particles. However, the dissociation of Fe-and Al-hydroxyl and humic functional groups (R-COOH, R-CH 2 -OH, R-OH) under alkaline conditions results in a negative charge of the colloid [56], which promote the release of fine particulate P and colloidal substances, thus increasing the TDP and P coll contents in small sized aggregate and particles.
In addition, we found that the P coll content in soil aggregates was positively correlated with the aggregate-associated Al and Fe content. This is because the contents of Fe and Al in acidic soil are greater than that in alkaline soil and the presence of Al and Fe oxides may have enhanced the adsorption of P and stabilization of soil aggregates [9,28,57,58]. Al and Fe oxides also had been recognized as important carriers of P coll [59][60][61][62].
We also found that the CR value of (silt+clay) sized particles was larger than that of aggregates of other sizes, while the CR of large macroaggregates was the lowest in most soils, which also indicates that the (silt+clay) sized particles contribute more for the P Coll loss potential, while macroaggregates immobilized the soil P coll . Colloidal P is highly bonded to Fe and Al on the surface of the macroaggregates [9,63], making it easy to form a stable composite structure that can resist the shear force of pore flow [59,60]. Similarly, soil macroaggregates also can increase the adsorption of P coll on the surface and reduce its mobility [23]. The ratio of P coll to TP in soil aggregates was not related to soil TP content (Table S2), indicating that the aggregate associated TP content had no significant effect on the proportion of P coll in TP.

Loss potential of colloidal P in aggregates
The ratio of P coll to TDP reflects the release potential of P coll in soil aggregates to soil solution. The high value indicates the high loss potential of P coll . We observed a negative correlation between TC and TN content and P coll /TDP in aggregates indicating that the higher the TC and TN content, the less likely release of the P coll from soil aggregates, which is also confirmed by the negative correlation between TC and P coll /TDP in the regression model (Table 4). Studies had showed that C and N are important carriers of P coll (Most organics act as organic colloidal complexes) [25, 64,65], and the organic matter would stabilize Al/Fe colloids [66]. Therefore, increasing the carbon content in soil aggregates may be an important strategy to reduce the migration of P coll in soil. The ratio of P coll to TDP for all macroaggregates in paddy soils was lower, which may be related to the long-term flooding of rice fields. Notably, the lower loss potential of P coll in macroaggregates of paddy soils does not mean it is unlikely to be lost to water bodies, however, it may indicate that the P coll has been lost into water and discharged into the water body through the channels during rice seasons when under longterm flooding [67]. Moreover, flooding resulted in an anaerobic state in soil aggregates, and Fe bound to colloids and aggregates was reduced, resulting in excessive release and loss of P coll [9]. We found that aggregates of all sizes in dryland and rice-dry rotation systems carried higher P coll loss potential, and the loss potential of P coll carried by large-grained soil aggregates in paddy soil was lower than 50% of TDP. However, P coll in (silt+clay)-sized particles (<0.053 mm) accounted for the proportion of TDP was as high as 83% and significantly higher than that in in orchard and rice-dryland rotation systems. Therefore, we believe that P coll in paddy soil is mainly carried by small silt and clay particles, which may indicate that the loss of P coll is remarkable severe in paddy soils.

Conclusions
We found that the aggregate size distribution of soil P coll was positively affected by pH values, and the highest P coll content of small macroaggregates occurred in acidic soils. Small macroaggregates contributed the most to the immobilization of P coll . The soil P coll content was affected by multiple factors including soil Al, Fe, and TP. However, soil P coll loss was primarily associated with aggregate stability and negatively associated with OM for any given soil type. Paddy soils, because of their high P coll /TDP ratio, are particularly at risk of high P coll loss by smaller particles. Therefore, management practices that increasing soil aggregate stability or its organic carbon content will help to control and limit P coll and thus also overall P loss from agricultural systems.

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Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare no competing interests.   Table 2 The total dissolved P (TDP), and truly soluble P (TSP), colloidal P, colloidal P/TDP, and ratio of TDP to soil total P (TP) in the supernatant of different soils after wet sieving. Data represent the average of three replicates ± standard deviations. Table 3 The colloidal P content (mg kg -1 ) of different-sized soil aggregates and the contribution rate of aggregates to colloidal P (CR) in the total 15 soils.
Size of aggregate s S1 S2 S3 S4 S5 S6 S 7 S8 S9 S1 0 S 1 1 S1 2 S1 3 S1 4 S1   Letters in different sized aggregates indicated significant difference at P < 0.05 level. Table 4 Results from correlation analyses and stepwise linear regressions of colloidal P and colloidal P/TDP in different sized aggregate with soil aggregate associated mean weight diameter (MWD), geometric mean diameter (GMD), pH, total carbon (TC), total nitrogen (TN), C/P, C/N, Al, Fe, and Ca. * P < 0.05.  Figure 1 Location of sampling sites. Location sites of S1-S15 correspond to Kaihua, Kecheng, Longquan, Zhuji, Lingxi, Changshan, Qiandaohu, Liandu, Tonglu, Zhoushan, Wuxing, Tiantai, Shengzhou, Mazhan, and Luqiao in Zhejiang province, China, respectively. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

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