Background

Phosphorus (P) is one of the limited elements for life survival and growth on earth [1]. Excessive application of P fertilizer in arable lands to guarantee crop production may induce P loss from soil to waterbody, causing adverse effect such as eutrophication [2, 3]. Leaching is one of the major pathways of P transport from soils to groundwater. It has received much attention since a large amount of P was detected in leachate [4,5,6]. Evaluating the P leaching risk is the prerequisite for making pollution control management. However, traditional monitoring methods like soil column or lysimeter experiments are difficult to implement because of the time- and cost-consuming [6]. Soil P tests such as CaCl2-extractable P, water extractable P, ammonium oxalate extractable P have developed as reliable means to identify the risk of P leaching in the past studies [5].

Soil CaCl2-extractable P (CaCl2-P) is highly correlated with both dissolved reactive P concentrations in surface runoff and the leached P concentration in column trials so that it can work as an indicator for assessing the potential P leaching risk [5, 7, 9]. Soil available P (AP) is usually used as an agricultural proxy index which is the soil P that potentially can be used by plants. It is less strongly bound than soil residual P so that can be easily transported in soils resulting in leaching [10,11,12]. Based on the relationship between AP with CaCl2-P, the change point at which CaCl2-P concentrations dramatically increased has been defined as the maximum acceptable AP concentrations in soils to avoid P leaching [13, 14]. It means that the P leaching risk can be evaluated by comparing the soil AP with the change point value. However, AP is not a constant value, it changes over time. For example, after fertilizer application it increases suddenly and then declines over time. AP can be adsorbed, but adsorbed P can become plant-available over timescales ranging from months to years [15, 16]. On the other hand, organic P (PO) is not included in AP of change point and the relationship between available organic P (APO) and CaCl2 extractable organic P (CaCl2-PO) has not been explored in the past studies.

P is easily fixed in the soil through, e.g., adsorption, precipitation or biological immobilization, hence only 20% of P fertilizer is taken up by crops in arable lands [17]. The mobility and loss of inorganic P (PI) in soils have been explored by many studies [18,19,20]. Sorption is considered as the key process to controlling PI mobility in arable lands, which is affected by certain soil properties including pH, cation exchange capacity (CEC), clay content, soil organic matter (SOM) and Al, Fe, Mn oxides [18, 21, 22]. However, the role of PO in soil P leaching has received poor attention and is less understood [23]. In fact, PO has been found to account for a large part of P leachates because some PO fractions reveal smaller affinity to the soil solid phase than PI [19, 24]. Soil Po account for a large part of soil P pool and usually occurs as a complex mixture of monoesters (Mono-P) and diesters (Di-P), with smaller amounts of phosphonates (Phos-P) and organic polyphosphates [25, 26]. Research made by McDowell et al. [23] found that all dissolved PO fractions, with the exception of mono-inositol hexakisphosphate leached through aquifer gravels to depth at a higher speed than orthophosphate. Thus, PO fractions are easily transported to deep layers, and this has been proved both in montane ecosystem and agricultural lands [6, 27]. Besides, many species of PO in soils can become available as orthophosphate for plant via microbially mediated mineralization [28]. It means that the presence and quantity of soil PO fractions affects the overall total P leaching. Therefore, it is necessary to consider PO in P leaching risk assessment.

In this study, 63 samples were taken from agricultural lands with different soil types (i.e., paddy soil, red soil, chernozem, black soil and other soils) across China. The aims of the study were (1) to assess the Po leaching risk in agricultural soils across China and (2) to explore the relationship of soil Po leaching risk with soil basic properties, particularly focusing on the effect of soil available Po on P leaching.

Materials and methods

Soil collection

A total of 63 surface soils (0–20 cm) were point sampled by spade and pickaxes from agricultural lands across China (Additional file 1: Table S1). Soils were chosen to cover a range of soil types. All samples were air-dried at room temperature and then sieved (2, 1 and 0.15 mm) after removing the organic debris and impurities.

Soil characterization

Soil pH was measured in water at a soil/water ratio of 1:2.5 by a FE20-FiveEasy Plus pH meter (Mettler Toledo, Switzerland). Water-soluble salt (WSS) was determined by a conductivity meter (SX-650; Tester, China) at a soil/water ratio of 1:5. The cation exchange capacity (CEC) and CaCO3 content in soils were measured by the NaOAc compulsive exchange method [29] and gasometric method [30]. Dissolved organic carbon (DOC) was extracted by 2 M KCl with the soil/water of 1:10 and the DOC in soil extracts was analyzed by a TOC/TN analyzer (Multi N/C 2100, Analytik Jena, Germany). Soil organic matter (SOM) was analyzed by wet oxidation with H2SO4–K2CrO7 [31]. For total Fe and Al, soils were digested by with HF–H2O2–HNO3 at 180 °C using a MARS 5 Xpress microwave system (CEM, USA), water-soluble Fe and Al were extracted by deionized water, free Fe and Al oxides were extracted by the sodium dithionite–sodium citrate–bicarbonate (DCB), amorphous Fe and Al oxides were extracted by acid ammonium oxalate, and complexing Fe and Al oxides were extracted using sodium pyrophosphate. The Fe and Al content were measured by ICP-OES (Optima 8000, Perkin Elmer, USA).

P status

Soil samples were digested by HClO4 and H2SO4 at 420 °C to determine the TP [32]. Extractable water-soluble P was extracted by 0.01 M CaCl2 for 30 min at a soil/water ratio of 1:5. After filtering, a part of clear solution was taken to detect the CaCl2-extractable inorganic P (CaCl2-PI) by the ammonium molybdate–ascorbic acid method immediately [33]. The other supernatant was digested with 10% K2S2O8 at 120 °C and 1.5 MPa for 30 min and then the total P content in water extractable P (CaCl2-PT) by the ammonium molybdate–ascorbic acid method was determined [34]. The organic P content (CaCl2-PO) was determined by the difference between CaCl2-PT and CaCl2-PI.

Available P (AP) was extracted by 0.5 M NaHCO3 (pH 8.5) at a soil/water ratio of 1:20 [35]. After shaking and filtering, part of clear solution was taken to measure the available inorganic P (API) using the ammonium molybdate–ascorbic acid method [33]. The other solution was digested with 10% K2S2O8 at 120 °C and 1.5 MPa for 30 min, and then the solution was measured by the ammonium molybdate–ascorbic acid method to determine the total content of available P (APT). The difference between APT and API was considered as available organic P (APO).

Data analysis

A one-way analysis of variance (ANOVA) was used on SPSS v22.0 software (IBM, USA) to detect differences in soil properties and P status between different soil types. Pearson correlations were used to assess the relationships between CaCl2-extractable P with climate, basic soil properties, Fe/Al status and P status. Then, a heatmap was constructed by the Origin Pro v9.1 software (OriginLab, USA).

Results

Basic soil physicochemical properties

The soils collected across China had a wide range of physicochemical properties due to differences in soil type, climate characteristics, and agronomical variations (Table 1). The soil pH values ranged from 4.5 to 9.2 with an average of 7.2. Generally, paddy soils and red soils were neutral and chernozem soils were alkaline (Table 1). The soil CEC ranged from 6.2 to 49.3 cmol g−1, and WSS in soils were 0.01–0.95 mS cm−1 (Table 1). The soils contained 0.35–14.8% soil organic matter (SOM) in weight basis and the dissolved organic carbon (DOC) varied from 0.04 to 0.36 mg kg−1 (Table 1). The CaCO3 in the study soils was in a relative wide range with 0.81 to 73.7 mg kg−1. Compared with the other soil types, chernozem soils were found to be higher in CEC and in the content of SOM, DOC and CaCO3.

Table 1 Basic properties of soil collected across China
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P status

In the present study, total content of P (TP) of soil showed higher variation from 378 to 3387 mg kg−1. There were only three soils which contained less than 400 mg kg−1 TP (Table 2). TP was higher in paddy soils than in other soil types (Table 2). However, the potentially leachable P (CaCl2-P) and AP of paddy soils were relatively lower than that of other soil types. The AP values were higher on average in red soils than the other soils (Table 2). And the highest values of CaCl2-P appeared in the group of “other soils” with 3.03 mg kg−1. Among total soil types, the average percent of APO was 57 ± 25% of AP (Table 2).

Table 2 Soil P status
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Fe/Al status

As shown in Table 3, the average of total Fe (TFe) in soils was 24.8 mg kg−1, which was higher than that of total Al (TAl, 3.73 mg kg−1). However, water-soluble Fe (WFe) with an average of 1.10 mg kg−1 was lower than water-soluble Al (WAl, 1.36 mg kg−1). Three types of soil Fe/Al oxides were distinguished: free Fe/Al oxides, amorphous Fe/Al oxides, and complexing Fe/Al oxides. Highest mean values of free Fe oxides (Fefr, 72%) and free Al oxides (Alfr, 73%) was found in total Fe oxides (Feox) and total Al oxides (Alox), respectively (Table 3). Compared with other soil types, the chernozem contained more TFe, TAl, WFe, WAl, Feox and Alox. The paddy soils contained highest TFe, WFe, WAl compared with other soil types while their Feox (14.2 mg kg−1) and Alox (3.30 mg kg−1) were less than those of red soils (18.8 mg kg−1 and 5.41 mg kg−1, respectively).

Table 3 Fe and Al oxides in soils collected across China
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Relationships of CaCl2-extractable P with climate, soil properties and Fe/Al status

There was a positive correlation in the red soils between the measured climate parameters, i.e., mean average temperature (MAT, r = 0.730) and mean average precipitation (MAP, r = 0.685), with potentially leachable inorganic P (CaCl2-PI) (Fig. 1). Soil properties showed limited effects on the CaCl2-P. WSS was positively correlated with CaCl2-PT, CaCl2-PI under “total soils” (r = 0.258, r = 0.263). SOM was positively correlated with the CaCl2-PT under paddy soils (r = 0.460), but was negatively correlated with CaCl2-PO under chernozem (r = − 0.596) (Fig. 1). DOC showed a significantly positive relationship both with CaCl2-PT (r = 0.589) and CaCl2-PI (r = 0.647) for the “other soils”. TFe was negatively correlated with both CaCl2-PT and CaCl2-PI under red soils (r = − 0.650, r = − 0.699), while TAl was negatively correlated with CaCl2-PI (r = − 0.436) and positively with CaCl2-PO (r = 0.507) in the paddy soils. However, no significant correlation was found between Fe/Al oxides with CaCl2-P under all the soil types except that Alar had a weakly positive correlation with CaCl2-PO under “total soils” and red soils (Fig. 1).

Fig. 1
figure 1

Heatmaps demonstrating the correlation between CaCl2-extractable P under different soil types and climate, basic soil properties, Fe/Al status and P status

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Relationships between CaCl2-extractable P and soil P content

TP had significantly positive relationship with both CaCl2-PT and CaCl2-PI under “total soils” and type “other soils” while it showed no special effect on CaCl2-extractable P under paddy, red and chernozem soils (Fig. 1). The relationships between soil CaCl2-P and AP were adequately described by linear equations (Figs. 2–4). CaCl2-PT was significantly affected by APT under all investigated soil types (total soils: p < 0.001; red soils: p < 0.001; chernozem: p = 0.001; other soils: p < 0.001) excerpt paddy soils (Fig. 2). Under paddy soils, CaCl2-PI also showed no relationship with API while there were strong positively relationships between CaCl2-PI with API in “total soils”, “red soils”, “chernozem” and “other soils” (Fig. 3). No relationship of CaCl2-PO and APO was found neither in paddy soils (Fig. 4). As shown in Fig. 4, the strongest relationship between CaCl2-PO and APO was found in red soils with the R value being 0.901 (p < 0.001) (Fig. 4c). In chernozem, the relationship was relatively weaker (Fig. 4d).

Fig. 2
figure 2

Correlation between (a) CaCl2-extractable total P with available total P under total soils, (b) CaCl2-extractable total P with available total P under paddy soils, (c) CaCl2-extractable total P with available total P under red soils, (d) CaCl2-extractable inorganic P with available inorganic P under chernozem, (d) CaCl2-extractable total P with available total P other soils

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Fig. 3
figure 3

Correlation between (a) CaCl2-extractable inorganic P with available inorganic P under total soils, (b) CaCl2-extractable inorganic P with available inorganic P under paddy soils, (c) CaCl2-extractable inorganic P with available inorganic P under red soils, (d) CaCl2-extractable inorganic P with available inorganic P under chernozem, (d) CaCl2-extractable inorganic P with available inorganic P other soils

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Fig. 4
figure 4

Correlation between (a) CaCl2-extractable organic P with available organic P under total soils, (b) CaCl2-extractable organic P with available organic P under paddy soils, (c) CaCl2-extractable organic P with available organic P under red soils, (d) CaCl2-extractable organic P with available organic P under chernozem, (d) CaCl2-extractable organic P with available organic P other soils

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Discussion

Role of soil physicochemical properties and Fe/Al oxides on P leaching

Many soil properties including soil pH, cation exchange capacity (CEC), organic matter, moisture, soil constituents and clay contents were considered as the factors for the P leaching by influencing P adsorption/desorption in soils [36, 37]. In our study, there was no clear correlation between soil physicochemical properties and CaCl2-P (Fig. 1). In paddy soils which were slightly acidic, SOM had a significantly positive correlation with CaCl2-PT. For paddy soil samples taken after drying conditions like our samples, some studies pointed out that organic onions increased P sorption through the formation of organic matter–Al complexes [38, 39]. In fact, many studies in other soil types showed that there was a competition of sorption sides on minerals between organic matters and phosphate [40]. A study about the colloidal phosphorus mitigation from paddy soils to waterbody also verified that the addition of organic matter increased both dissolved P and colloidal P in leachate because negatively charged organic acid bonded to particles and generated negatively hydroxyl groups, which result in both site competition and electrostatic force [41]. Other studies in acidic soils also showed that low molecular weight organic acids increased the mobility of inorganic phosphorus (P) through the chelation of Al and high-molecular-weight organic acids showed potential to inhibit phosphate adsorption [42,43,44]. However, there was a negative relationship between SOM and CaCl2-PO in group of “Chernozem” with alkaline conditions in the study. Yang et al. [45] reported on the influence of SOM on P adsorption/desorption under black chernozem and concluded that OM could enhance P availability by reducing the P bonding energy and increasing the P desorption to some degree. In fact, their results also showed the adsorption capacity of soil for P increased with SOM content increased and the change of P bonding energy along with SOM content first went down and then up. It means that beyond a certain SOM value, both P bond capacity and intensity were increased so that can be a reason for negative relation between SOM and CaCl2-PO in the alkaline chernozem soils studied. Besides, some studies made in alkaline soils suggested that SOM was beneficial to P adsorption because of anionic character of SOM which can bind electrostatically cations such as Ca2+, Fe3+, and Al3+ and these cations promote P retention [46, 47]. Overall, the influence of SOM on P adsorption depends on the type and concentration of organic material and the soil types, respectively, which is in accordance with the idea made by Debicka et al. [48].

Limited effect of Fe/Al oxides on CaCl2-P was detected under all soil types in our study maybe due to the relatively limited number of soil samples. In fact, the process of phosphate sorption was affected by multiple soil constituents such as, carbonate content, organo-mineral complexes, metal oxides and clay minerals, because they all worked as adsorbents [18, 49, 50]. Abundance of metal oxides and their crystallinity were considered as the key factor for P adsorption [51, 52]. Higher free Al/Fe oxides usually means higher P retention and amorphous Al oxides with higher specific area is better to adsorb phosphate [51,52,53,54].

Effects of soil P status on P leaching

More P stored in soils resulted in the higher concentration of CaCl2-P and thus higher risk of P loss (Fig. 1). Such a significant relationship of TP with CaCl2-P could have been expected, when all studied soil samples (n = 63) were taken in consideration. P is easily accumulated in soils when P inputs exceed crop requirement because of the discrepancy between high P fixation capacity and low P use efficiency by plants. The unbalance between application of fertilizer and crop P removal also decreases the availability of P sorption sites in soils [55]. Therefore, the P concentration in surface soils was demonstrated as a key factor for P leaching in some studies [56, 57]. It works as one of the major continuous sources for P pollution in waterbodies [17, 58]. In the study, we tested arable soils from all over the China and found the CaCl2-PI were 0.16–8.57 mg kg−1, which was higher than that reported in the study by Zhao et al. [59]. They investigated 23 agricultural soil samples across China and found 0–2.5 mg kg−1 CaCl2-PI. The mean available inorganic P (API) value that we analyzed (58.2 mg kg−1) was also higher than that reported (20.7 mg kg−1) in a study in 2006 [60]. Only 9.3% of the arable land in 2006 was over 40 mg API kg−1, while that proportion is currently 38%. The increased application of P fertilizer resulted in the accumulation of P in arable land [11]. In comparison, 549 mg API kg−1 was found in Danchi catchment [6] and 106 mg API kg−1 was detected in Beijing suburbs [61], higher than that detected in an earlier study referred previously by Li et al. [60].

API was identified as important parameter for the P transport [12]. Our results also confirmed the significant correlation between API and CaCl2-PI under “total soils” (r = 0.752, Fig. 3). In the past research, a split-line model was usually used to describe the relationship between API and CaCl2-PI. There was a specific point (change point) that can be used to determine the optimal API concentrations in soils without causing PI leaching [13, 14]. However, less study focused on the organic forms both in AP and CaCl2-P. In fact, organic P transport with subsurface flow plays a vital role in P transport and organic P was identified as the main form in leachate [19, 24, 62, 63]. In our study, the CaCl2-P included both inorganic P and organic P, with the average 34% of organic P under all soils (Table 2). However, the AP in all soils consisted of more organic P (57 ± 25%). A relatively high correlation between AP and CaCl2-P under “total soils” was found when considered both the organic and inorganic forms (R = 0.822, Fig. 2). Furthermore, a significant positive relationship of APO with CaCl2-PO was found among all soil types except paddy soils (Fig. 4). The r value was up to 0.901 in red soils, which revealed that APO was one of the key factors for P leaching in that soil type. A study conducted in red soils under arable crops found that API was positively correlated with deposition capacity and negatively with the maximum P adsorption capacity, which may result in more P leaching [64]. Fewer studies did research about the role of organic part in AP on P leaching and our results showed the mean APO was more than mean API in red soils (Table 2). Change point is a useful tool when assessing the need for implementing P leaching control management. Different with the change point can be found from the relationship between API and CaCl2-PI. No critical level of APO was identified where CaCl2-PO increased dramatically. It means that the leaching risk of organic P is harder to predict and thus control than that of inorganic P.

Paddy soils showed different results from other soil types (Figs. 2–4). AP had no correlation with CaCl2-P neither with organic or inorganic form. In fact, some studies believed that soil P test measured in the topsoil should not be used alone for P leaching risk assessment [56]. But fewer studies suggested that AP should not be used in P leaching prediction. Paddy soil is a special type of agricultural land which experiences waterlogging during rice growing period for over 6 months in the double-cropping rice system, resulting in the alternating redox cycles [65]. The shift of redox condition may alter the P adsorption and desorption by changing organic fractions and anions related to P bonded [66]. The alteration may affect AP behavior without affecting P leaching.

Conclusions

CaCl2 extractable total P (CaCl2-PT) in arable soils sampled from all over the China was in a large range of 0.97–9.33 mg kg−1 with 35% of them in organic forms on average. The mean percentage of organic P was over half of available total P (APT) (57 ± 25%) and the maximum even reached 88.5%. Therefore, the potential risk of organic P (PO) leaching from arable land is higher and Po should be included in P leaching proxy measurements. Positive correlation was found between CaCl2-P (organic forms, inorganic forms and total forms) with AP (organic forms, inorganic forms and total forms) under all investigated soil types except paddy soils. CaCl2 extractable inorganic P (CaCl2-PI) and available inorganic P (API) was highly correlated in the type of chernozem (r = 0.968). And the relationship of CaCl2 extractable organic P (CaCl2-PO) with available organic P (APO) was strong in the type of red soils (r = 0.901) which both CaCl2-PO and APO of it were significantly higher than those of other soil types. It means that the control of APO accumulation in arable land may be the key point to limit PO leaching and the mitigation measures should be constructed based on the soil type. However, rare effects of soil basic properties and Fe/Al oxides on PO leaching was found in our study because of the limited number of samples, thus increasing the sample size is necessary for further study.