1. Introduction

2. Materials and Methods

2.1. Materials

2.1.1. Soil material

This research was conducted on light textured soil materials characterized by different basic properties and diverse land uses. Soil 1 (S1) – classified as Brunic Arenosols according to FAO-World Reference Base [50] – located in central Poland (Spała Forest District), occurring in the fresh coniferous forest habitat (with Pinus sylvestris and Picea abies, with Quercus petraea in the undergrowth). Soil samples were taken from the sideric horison (Bvs). This material showed also the features of spodic horizon. S1 is a sand-textured material characterised by pedogenic accumulation of immobile humic complexes with sesquioxides (R₂O₃) and the presence of rusty coatings at mineral grains composed of Fe and Al (not associated with humus), which were formed by silicate weathering and low humic acid synthesis. The features related to the illuvial accumulation of Fe and Al sesquioxides as a result of the podzolic process in the soil horizon are also visible in S1. Soils 2 and 3 (S2; S3) – classified as Eutric Arenosols [50] – arable loamy sand textured soils located in north-eastern part of Germany (S2– Prohn; S3 – Poppendorf). Under natural conditions these soils have been Podzols, but the original litter layer and leached A-horizons have been incorporated into the tilled A-horizon. Beginning in the early 1990ies, deeper ploughing with stronger machinery incorporated the previous oxide-rich spodic horizon material into the surface Ap horizon, causing the problem of phosphate-fixation. This Ap-material has been sampled.

2.1.2. Experimental organic materials and soil incubation

The different types of organic matter derived from agricultural waste materials – compost (CPT), biogas digestate (BD), cattle manure (CM) and horse manure (HM) – have been acquired locally and introduced into the soil in the form of dissolved and colloidal organic matter (abbreviated as DOM). The DOM was extracted using cold tap water at a ratio of 1 : 5 (w : v) of waste to water. After 24h incubation the suspensions achieved passed through a 1-mm-sieve and were filtered through Macherey-Nagel, MN 616 G-filters. We have thus obtained an organic matter fraction from the waste materials, which can be assumed to be naturally the most active in soil, as it has the ability to be rapidly released into solution under natural conditions, such as during rainfall. Next, the air-dry soil samples were placed in beakers and slowly filled up with that dissolved organic matter until the saturation levels of soils were achieved. We have schematically presented the soil filling up procedure with DOM derived from organic waste in Figure 1.

Our intention was to fill the spaces between the soil particles with DOM to allow the introduced organic matter to occupy the active sorption sites at the soil particles, mainly derived from iron and aluminium oxides and hydroxides (Figure 2). Our aim was to block potential sorption sites for P by the introduced organic matter (DOM), which competes with P for the same sorption sites [29]. Thus, we hoped to lower the P sorption capacity of soil and at the same time to increase P availability to plants.

Figure 2. Expected effect of soil mineral particles saturated with dissolved organic matter.

Figure 2. Expected effect of soil mineral particles saturated with dissolved organic matter.

After saturation, the samples were incubated at 70°C and then the soil saturation procedure was repeated several times in the same way. The final amount of DOM added into each soil is shown in Table 1. These soil samples saturated with DOM derived from 4 types of agricultural waste materials were used in P sorption studies in comparison to unamended soils, which were the reference (control) materials.

3. Results and Discussion

3.2. Process of P sorption

3.2.1. P sorption in unsaturated soils (S1, S2, S3)

The results obtained for P sorption in S1-S3 – soils with different P sorption properties – are the reference for the soils saturated with DOM. The early stage of sorption (at the low P doses) in S1 (B horizon) indicates a very high affinity for P, which weakens considerably with the sorption of further P doses (Figure 4). The slope of the sorption isotherm decreases then meaningfully and reaches the highest P adsorption value (Q) of approximately 0.2 mg P g^-1 soil. The P sorption in arable soils (S2 and S3) at the early stages of isotherms is comparable for these both soils. The much lower slope of the curve – in comparison to S1 – after the sorption of the initial P doses indicates substantially smaller

Figure 4. Phosphorus sorption in soils under study (S1, S2, S3). *Explanation: Q – the amount of P adsorbed to soil at equilibrium concentration; Ce – the P concentration in the equilibrated solution.

Figure 4. Phosphorus sorption in soils under study (S1, S2, S3). *Explanation: Q – the amount of P adsorbed to soil at equilibrium concentration; Ce – the P concentration in the equilibrated solution.

affinity for P, which is associated with the higher DPS value in both soils (Table 3), typical for arable – often overfertilized – soils [58]. The higher doses of P flatten isotherms in both soils and are sorbed better by S2 than S3, which is in agreement with the magnitude of the P sorption capacity in these soils (Table 3). The sorption capacity of S2 can be compared to S1 – it reaches also Q value of 0.2 mg P g^-1 of soil, while Q is lower in S3 and achieves 0.13 mg P g^-1. These results confirm a strong relation with the type of land use of these soils, which seems to be the main factor determining the course of P sorption here. Arable soils (S2 and S3), regularly fertilized with P, have a much higher DPS value, so their affinity for P is much weaker, in contrast to forest soil (S1) with a much lower DPS value (Table 3).

The P-sorption isotherms obtained in the studied soils are typical examples of so-called "L" isotherms, in which the concentration ratio of the compound remaining in solution and adsorbed on the solid decreases with increasing concentration of the solute, providing a convex curve [60,61]. The P adsorption by soil is regarded as being a multi-stage kinetic process involving an initial fast adsorption stage (chemical) and then a slower adsorption stage (physico-chemical, physical), and possibly further stages [26,62]. At relatively low P concentrations, the adsorption process is completed rapidly and ion exchange and ligand exchange are probably the dominant mechanisms contributing to the high adsorption rate [26,63]. This quick, chemical phase of adsorption process slows down promptly at high concentrations of added P because available adsorption sites quickly become saturated and P becomes physico-chemically and physically adsorbed from the liquid to the soil at a slower rate, and this process at this point represents the slow adsorption phase [26,62,63]. Then the "L" curve can reach a strict asymptotic plateau when the solid has a limited sorption capacity or the curve does not reach any plateau, when the solid does not show clearly a limited sorption capacity [60]. This is often observed in the case of sorption processes in natural soils, which are polydisperse and highly heterogeneous, and that is why sometimes it may be even difficult to clearly classify the isotherm subtype "L".

3.2.2. DOM effect on P sorption in soils

The saturation of soil samples (S1, S2, S3) with DOM derived from waste organic materials (CPT, BD, CM, HM) resulted in different effects on P sorption (Figure 5).

An increase of P adsorption (Q) at higher P doses was observed as DOM effect in all combinations except for soils saturated with DOM-HM. The highest Q reached about 0.25 mg P g^-1 and occurred as a result of DOM-CPT and/or DOM-BD saturation. The enhanced P sorption was also observed as an effect of the DOM-CM treatment in all studied soils. In S2, the DOM-BD and DOM-CM saturations slightly increased the sorption at the highest P dose, but the P sorption process occurred in a very similar way to the control soil in these both cases (S2). Comparable P sorption isotherms were also observed after saturation with DOM-CPT and DOM-CM in the S3 (Q was higher of about 0.05 mg P g^-1 than in S3). The soils saturated with DOM-HM behaved undoubtedly differently. In each soil we observed P desorption at the initial stages of sorption (low P doses) and the significant weakening of P sorption (Figure 5). A smaller angle of the inclination towards the OX axis indicated a much lower affinity for P in the DOM-HM soil samples than in the control soils, and the decrease of Q was also observed (Q was lower of about 0.05 mg P g^-1 than in control soils), unlike others DOM treatments: CPT, BD, CM.

The initial stage of the sorption is determined mainly by conditions related to the properties of reactants important in chemical sorption, i.e. the number of sorption sites on mineral surfaces (including the bonds related to Al and Fe oxides, clay minerals and other soil colloids), PSC or pH. So it can explain the differences in the soils affinity for P as indicated by the slope at the beginning of the sorption curves (Figure 4 and Figure 5). The affinity for P weakens in the order: S1 > S2 > S3 and is similar for all cases even after saturation with DOM materials, excluding DOM-HM, which significantly diminished the affinity for P in every soil. At a later stage, when the sorption mechanism goes beyond chemical binding, the type of adsorbent becomes important – so we clearly can observe the differences specific to the type of DOM introduced. The different behaviour of DOM–HM at an earlier stage may be linked to the introduction of increased P amount with this DOM type, which could change the equilibrium of the reaction and lead to previously sorbed P becoming desorbed.

3.3. The P adsorption indices

The P adsorption isotherms were quantitatively described by the two most popular models – Langmuir and Freundlich equations. Results for soils before and after treatments were fitted to these adsorption equations. The values of R-squared were calculated as a quality check (Table 4). The Freundlich and Langmuir equations are characterised by the very high values of R² in the soils under study. The weakest R² values were obtained after applications of DOM-HM in S3 (S3-HM).

Table 4. The P isotherm parameters.

Table 4. The P isotherm parameters.

Soil sample / amendment	Freundlich isotherm					Langmuir isotherm					MBC mg g-1
	Kf	nf	sum SE	Variance	R2	KL	Qm	sum SE	variance	R2
S1 (CONTROL)	0.1	0.27	0.001	0.0053	0.99	3.68	0.18	0.003	0.0053	0.95	0.662
S1-CPT	0.12	0.34	0.003	0.0089	0.97	2.78	0.24	0.003	0.0089	0.97	0.667
S1-BD	0.1	0.43	0.002	0.0085	0.98	1.05	0.26	0.003	0.0085	0.96	0.273
S1-CM	0.09	0.40	0.002	0.0069	0.97	1.25	0.23	0.002	0.0069	0.97	0.288
S1-HM	0.03	0.67	0.001	0.0035	0.98	0.12	0.26	0	0.0035	0.99	0.031
S2 (CONTROL)	0.05	0.56	0.001	0.0053	0.98	0.23	0.27	0	0.0053	0.99	0.062
S2-CPT	0.06	0.63	0.001	0.0083	0.98	0.22	0.37	0.002	0.0082	0.98	0.081
S2-BD	0.05	0.70	0.002	0.0068	0.98	0.13	0.39	0.001	0.0068	0.99	0.051
S2-CM	0.05	0.66	0.002	0.0064	0.97	0.16	0.34	0.001	0.0064	0.99	0.054
S2-HM	0.008	1.12	0.005	0.004	0.88	0.01	1.34	0.005	0.004	0.87	0.012
S3 (CONTROL)	0.03	0.49	0	0.002	0.99	0.27	0.15	0	0.002	0.99	0.041
S3-CPT	0.04	0.55	0.001	0.0037	0.98	0.24	0.22	0	0.0037	0.99	0.053
S3-BD	0.04	0.73	0.001	0.0064	0.98	0.09	0.45	0.002	0.0064	0.97	0.041
S3-CM	0.04	0.64	0.001	0.004	0.98	0.15	0.26	0.001	0.004	0.98	0.039
S3-HM	0.002	1.51	0.004	0.003	0.85	0.07	0.2	0.009	0.003	0.67	0.010

3.3.1. Freundlich equation parameters

The capacity factor (K_f). The K_f value, which indicates the number of sorption sites [64], ranges in control soils from 0.03 (S3) to 0.1 (S1). The almost ten times higher value of K_f in S1 than in arable soils reflects a significantly higher P binding capacity of this soil, as we observed on the P sorption curves for the control soils (Figure 4). The S1 neither has more Al and Fe nor more Alox, Feox and PSC than the other two soils but its DPS is less than half of soils S2 and S3 (Table 2 and Table 3), indicating that K_f is determined by available sorption sites in soil, which had been confirmed in the literature [65].

The saturation with DOM revealed divers effects on the K_f parameter (Figure 6a). In each soil, a significant increase (by 20-35%) of the K_f parameter was observed after DOM-CPT addition, while a substantial reduction of K_f values (up to 90%) was caused by DOM-HM addition in all soils compared to the control soils. The saturation with DOM-CM and DOM-BD increased the K_f value only in S3 (of about 30%) which indicates the importance of soil properties influencing the value of K_f. No other effects on K_f were observed except for a decrease of about 10% in S1 after DOM-CM saturation (Figure 6a). The K_f parameter shows significant negative correlations with the contents of C, N and with pH (Table 5). This confirms the important role of organic matter content and pH in building the P sorption capacity in soils and its binding strength and had been observed by other researchers [40,64,65,66]. This may explain such strong decrease of K_f value after DOM-HM application and the increase of this value in all soils as an effect of the saturation with DOM-CPT. In addition to the basic soil properties, also the total P content has a very strong effect on the course of P sorption which is related to the number of available sorption sites for P in a given soil, as reflected in the DPS values. This was supported by statistically significant negative correlation coefficients between P content and the K_f parameter (Table 5), which may contribute to the explanation of the behavior of soils after the saturation with DOM-HM, which resulted in the largest P contents (Table 2). The missing significance of correlations of sorption isotherm variable with total contents of Al and Fe are plausible, because the total contents include atoms in the crystals that are not involved in any interaction with phosphate, e.g. internal parts of mineral particles.

The linearity parameter (n_f). The value of n_f exponent is a measure of the linearity of the function, but sometimes is also considered as an indicator of sorption sites energy heterogeneity [67,68]. In control soils, the n_f value is lower than unity, in range of 0.27 to 0.56 (Table 4). The values obtained are in line with the results of other authors [69]. After the saturation with each type of DOM the exponent n_f was significantly increased, which indicates a substantial increase in the heterogeneity of the energy of P sorbing sites in soils. The effects on n_f value follow the order: HM >> BD > CM > CPT. The highest increase of n_f value after the DOM-HM saturation (Figure 6b), if indicating heterogeneity of P binding sites, eventually points to additional processes such as surface complexation and precipitation [70] that occurred preferably in the DOM-HM samples.

The n_f exponent is positively correlated with C, N, P and pH (Table 5). This implies that saturation with DOM and increase in C content (including elements related to the presence of organic matter: N and P content) and the pH conditions have a dominant effect on increasing the heterogeneity of binding energy in the soils. This may be related to the quantity, but also to the quality of the organic matter introduced into the soil, since it is the structure of the organic molecules that will determine the characteristics of the functional groups active in P sorption. This may also be attempted to explain the major changes in n_f values observed under the influence of DOM-HM, with which, in addition to a significant increase in C content, we also observe the introduction of a large amount of phosphate ions. All these components (DOM, P, Al and Fe), important for P binding in soils, could probably have interacted with each other in different ways depending on the pH and other reaction conditions such as the contribution of these components. These issues are unfortunately beyond the scope of our study and require further analysis with emphasis on the qualitative characterisation of the organic matter introduced.

3.3.2. Langmuir equation parameters

The maximum adsorption capacity for P (Q_m). The Q_m reflects the number of P adsorption sites per unit weight of soil and is therefore commonly used to estimate P adsorption capacity of soils [26,47,71]. The value of Q_m ranged from 0.15 mg P g^-1 (S3) to 0.27 mg P g^-1 (S2) in control soils. This value is comparable for S1 and S3 and lower than Q_m in S2. After the saturation with DOM the Q_m value increased in all cases (Figure 7a). All types of DOM amendments (CPT, BD, CM, HM) had a comparable effect on Q_m in S1 giving about 30-40% higher values than in control soil (Figure 7a). The DOM materials gave a similar effects in S2, excluding the DOM-HM amendment, which greatly increased the calculated Q_m value (by as much as 400% compared to the control sample). This indicates that DOM-HM initiates binding processes that are different from the other soils. A stronger impact on Q_m parameter was observed after the saturation with DOM-BD, DOM-CM and DOM-CPT in S3. The magnitude of this effect followed the order: BD > CM > CPT. In contrast, the DOM-HM impact was negligible compared to control soil (S3).

Such a comparable effect of the applied DOM types in S1 and S2 suggests that the origin of DOM is unlikely to have a significant effect on the value of the Q_m parameter. Also, the different properties of S1 and S2 did not affect the significant differences in Q_m between the tested variants of the experiment. The increase of Q_m values (Table 4) is attributed to an increase of organic matter (OM) content (Table 2), which is confirmed by a statistically significant (p < 0.05) correlation coefficient (r = 0.54) between Q_m and total organic C content (Table 5) and remains in line with the results of others [26,47,71,72]. It is likely that DOM has occupied sorption sites potentially available for P – mainly connections to Al and Fe, but providing some new sorption sites for P on its own organic particles, hence the increase in Q_m was observed. There are also results indicating that OM does not have a direct effect on Q_m and the influence of other soil properties is also important [71]. The relationship between Q_m and OM was also shown to be dependent on soil pH [26] after Zhao et al. [73], who found that at pH > 6.0 increasing the OM content increased the amount of P adsorbed by soils but decreased the amount of P adsorbed by soils at pH < 6.0. In our study we observed this relation as an increase in Q_m value with an increase of C content at pH > 6.

Numerous studies have reported the effect of soil organic matter on inhibiting P adsorption in soil by blocking adsorption sites on metal oxides, such as Al and Fe oxides, or by forming metal-SOM-P complexes with flexible structures [26,27,29,36,39,68]. Nevertheless, our results conducted on different types of DOM do not confirm this based on the Q_m value analysis. The Q_m value higher than in control soils indicates that new sorption sites for P are likely to be generated after soil saturation with DOM. This is clearly demonstrated in S2 after DOM-HM saturation, where significantly higher number of sorption sites can be related to the higher amount of introduced DOM and the higher content of Al and Fe (especially amorphous Al and Fe – Al_ox, Fe_ox; Table 2, Table 3). Possibly, in this case, the sorption sites for P were not blocked, but a different sorption mechanism/s prevailed. It is not known clearly what the role of the high concentration of P introduced together with DOM-HM might have been. Possibly in the presence of high concentration of P, the behaviour of organic molecules was different than in other cases. Competition and mutual blocking of these components may have occurred. Although it is also possible that the DOM saturation may have caused in this case that, on the one hand, DOM interacted with the surface functional groups of Al and Fe, but, on the other hand, generated additional sorption sites for P associated with the organic molecules' sorption active functional groups. It is possible that the high P concentration may also have favoured the formation of monodentate, rather than bidentate, bonds with P on active surfaces, which tended to bind P more weakly in the soil, thereby increasing the P pool that can more readily pass into solution. The probability of multilayer P sorption, formation of monodentate bonds on sorptive surfaces or increased P leaching under high P loading conditions has been repeatedly observed in the literature [74,75,76,77]. In the presence of much P bi- or more-layer bonding and co-precipitation may occur as well. Another possibility is that DOM-HM could support the disintegration of Al-/Fe-oxide microaggregates making more sorption sites available. All these mechanisms [70] may have resulted in such a large increase in the maximum P sorption capacity (Q_m) calculated from our results and correlations (Q_m : C) we obtained (Table 5). The question remains in this area as to the effect of high P concentration in the presence of DOM on the activity of the introduced organic matter, and what is the organic matter activity in blocking/enhancing P sorption depending on its qualitative nature [28,78].

The P bonding energy constant (K_L). The K_L parameter reflects the P bonding energy and is one of the most important indicators describing the soil affinity for P. A higher K_L value indicates stronger P sorption, and spontaneous sorption may occur more readily as the P supply becomes less intense [62]. The K_L parameter values calculated for control measurements can be ordered in the following way: S2 < S3 << S1. The comparable K_L values lower than 0.3 were calculated for arable soils (S2, S3), while the forest soil had the K_L value at least ten times higher (Table 4). These differences reflect various soil properties, first of all the mineralogy that governs P-binding sites and the land use/management history, e.g. the former P fertilization at the arable soils S2 and S3. This view is supported the apparent much higher DPS values in arable soils compared to forest soil S1.

The K_L parameter decreased considerably as a general effect of the saturation with DOM materials (Figure 7b). This indicates that the introduced organic materials weakened P binding energy in all 3 soils. The new sorption sites likely generated under the DOM influence are energetically weaker in holding P than the Al and Fe bonds [31,70], thus the marked reduction in the K_L parameter, which was greatest in S1 compared to other soils. A comparable effect of soil organic matter on K_L values was observed by Yang et al. [26] in studies on phaeozem soils that received addition of humic substances. The largest negative effect on K_L was observed after the saturation with DOM-HM, followed by DOM-BD, DOM-CM, and the weakest effect was observed after the introduction of DOM-CPT (Figure 7b). The strongest impact of DOM-HM is probably related to the large amount of P incorporated into the soil (Table 2), so P fixation is much weaker in such material and probably forming single bonds with P that are definitely more easily desorbable from soil.

The Maximum Buffering Capacity (MBC). The MBC is an joined parameter that combines the effects of Q_m and K_L values [26,62]. The higher MBC means that more P can be sorbed by soil. We observed an increase of MBC value (Figure 7c) solely as an influence of DOM-CPT in S2 and S3 (S1-CPT had no effect). The other DOM types predominantly caused a decrease in MBC value (Figure 7c), which was the strongest after DOM-HM saturation in all soils. No influence was observed in S3-BD and S3-CM.

The different response to the DOM-CPT introduction may be related to the specific nature of the raw organic waste composted and, above all, to the transformation of OM during the mineralisation and humification processes [79,80]. This probably allowed the production of a material that more strongly retains P in the soil, e.g. DOM with a more aromatic character than the others, because aromatic moieties exert the largest binding energies for phosphate among various organic functionalities [70]. Although the effect of CPT on Q_m is similar for other types of DOM (Figure 7a), it stands out as having the weakest effect on the reduction of the K_L parameter (Figure 7b) compared to other DOM materials, thus the effect on MBC finally is positive or not present after the introduction of DOM-CPT. However, confirmation of this is possible in further studies of the nature and composition of CPT organic matter.

4. Conclusions

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