Phosphorus from adsorbed OP compounds is available to plants
Our first hypothesis that P from adsorbed OP compounds is available to ryegrass and varies with soil mineral properties was confirmed in our experiment. The two main findings supporting the hypothesis are: the ryegrass was able to take up about 3-18% of OP adsorbed to soil minerals; OP compounds as well as their interactions with soil minerals significantly affected P uptake for ryegrass.
The amount of P recovered by ryegrass reached 5% from adsorbed IHP, 9% from G6P and 18% from adsorbed GLY. In general, P uptake decreased in the order GLY >> G6P > IHP. Taken together, these results indicate that, irrespective of soil mineral, adsorbed OP was at least partially available to plants. The magnitude of this availability depended on the OP compounds and the types of soil minerals. The extent to which adsorbed OP may be available for plant uptake remains poorly understood and controversial. Several authors have suggested that they are not available to plants (Barrow and Shaw, 1975; Guzman et al., 1994; Hingston et al., 1974), while others show experimental evidence of their availability and uptake by plants (Andrino et al., 2019; D’Amico et al., 2020; Parfitt, 1979). However, in all of these studies, the compound of P used was IP with only one compound of OP without considering other important OP pools such as GLY or G6P. Furthermore, the soil minerals used in most cases were Fe oxides, namely goethite, while Al oxides and clays have been ignored. To our knowledge, no study has evaluated and compared the availability of P to plants from the major OP compounds (IHP, GLY, and G6P) that were adsorbed onto the major soil minerals (Fe and Al oxides and clay minerals). Thus, our work after comparing several main soil mineral-OP complexes, argued that some amount of P can be available from the adsorbed OP compounds. Therefore, adsorbed OPs can act as a source of P in the soil depending on their availability of adsorbed species.
The availability of P from adsorbed OP differed between OP compounds. The P uptake from the adsorbed GLY compound was much higher than that of adsorbed G6P which was more available than adsorbed IHP. It appears that the difference in availability among the adsorbed phosphates was determined by the adsorption capacity more than their binding strength. The trend in availability (GLY >> G6P > IHP) would be mainly explained by the distinct sorption and desorption dynamics of the different minerals, leading to the lowest desorption for IHP-P, followed by G6P-P and then GLY-P. Basically, we expected G6P to be more available than GLY due to its higher desorption and mineralization rate and its likely instability on soil minerals due to its higher molecular weight than GLY ( Annaheim et al., 2010; Annaheim et al., 2013; Ruttenberg and Sulak, 2011; Yan et al., 2014). The opposite result we found may be the consequence of its more negatively charged surface than GLY (Giaveno et al., 2008; Ruttenberg and Sulak, 2011). These reasons would also explain why the binding energy of G6P predicted from the Langmuir model is often higher than that of GLY or IP(Goebel et al., 2017). On the other hand, it may also mean that G6P was more desorbed than GLY but after desorption, its mineralization would have been lower, resulting in its low absorption by plants. Finally, adsorbed GLY was the higher source of P for plant uptake compared to other compounds, consistently with Adams and Pate (1992). In conclusion, the availability of OPs for plant nutrition from organic inputs and soil would be determined by the stability of OP-mineral complexes or by their (de)sorption dynamics. Consequently, the IHP would have formed with the minerals, stable complexes with multiple rings and had a lower availability than other OP compounds.
To summarize, the different OP compounds showed different availability to ryegrass. Phosphorus uptake decreased in the order GLY >> G6P > IHP. This trend is mainly explained by the (de)sorption capacities and strength of the different minerals, leading to the least desorption for IHP-P, followed by G6P-P and then GLY. The high uptake of P from adsorbed GLY is thought to be due to its low binding to soil minerals, high desorption and ability to be hydrolyzed by most soil enzymes. In addition difference in P uptake between adsorbed GLY, G6P, and IHP might also be related to their different degrees of hydrolysis by enzymatic activity as shown by Annaheim et al., (2010) and Bünemann, (2008). Hydrolysis of OP by enzymatic activity is generally affected by the chemical nature of P and especially its solubilization from soil minerals (Amadou et al., 2021; Greiner et al., 2007; He et al., 2004; He and Honeycutt, 2001; Richardson et al., 2005). Thus, higher molecular weight and higher phosphorylated compounds such as IHP may be more resistant to enzymatic activity especially when adsorbed, limiting their degradation from mineral surfaces. Thus, in addition to the nature of the OP compounds and their (de)sorption mechanisms, their capacity for hydrolysis by enzymes is also an important factor affecting their uptake by the plant.
Effect of mineral-OP interactions on P uptake by ryegrass.
The potential availability of phosphate adsorbed by different minerals was strongly affected by mineral-OP interactions properties. The P uptake increased in the following order: kaolinite-OP << gibbsite-OP ≤ goethite OP << montmorillonite-OP. Montmorillonite-OP complexes showed the highest P uptake, with a maximum of 18%. On the other hand, P uptake from kaolinite-OP was the lowest. Less than 4% of the initially adsorbed P was desorbed and mineralized for plant uptake. These results indicated that regardless of the type of mineral and its sorption strength and stability toward OP, plants are able to take up at least a small portion of it for their nutrition. This agree with laboratory and greenhouse studies indicating that specifically adsorbed P was potentially available to plants although it was very difficult to desorb (Bollyn et al., 2017; D’Amico et al., 2020; He et al., 1994; Martin et al., 2002; Montalvo et al., 2015; Parfitt, 1979; Shang et al., 1996). Thus, even under conditions where the activities of plant microbiomes and roots that can help mobilize P are limited, OP-mineral complexes still have potential for supplying P to plants.
Montmorillonite-OP complexes, in particular, montmorillonite-GLY or G6P were found to serve as sources of P in soils more than the other complexes. Organic P adsorbed on kaolinite was released the lowest amount of P to the plants. Overall, there was a considerable difference in P uptake between the tested minerals. Montmorillonite-OP complexes would provide more P due to the negative charges of montmorillonite i.e. its zero point of charge values (He et al., 1994) causing its weak binding to OP, resulting in the formation of unstable OP-montmorillonite complexes (He and Zhu, 1998; He et al., 1994; Hingston et al., 1974). The low P uptake from kaolinite-OP agrees with previous observations showing that kaolinite-P is an inner-sphere surface and bidentate complex below pH 6 (Hu et al., 2020; Ruyter-Hooley et al., 2015), and the adsorbed P on kaolinite is difficult to desorb (Kafkafi et al., 1988; Manning and Goldberg, 1996). Hence, our result emphasized that Fe and Al oxides were less responsible for limiting OP availability than kaolinite despite their general higher adsorption capacity in soil. Therefore, apart from the nature of the OP compounds, the characteristics of the minerals is an important factor responsible for the difference in P uptake between the tested OP-mineral complexes.
In sum, our results evidence that the type of minerals is a key driver of P availability for plant. Based on our results, all OP-mineral complexes can act as a sink or source of P in soil. Our results showed good utilization efficiency of all OP compounds adsorbed on montmorillonite, in particular, montmorillonite-GLY or G6P compared to other minerals. The kaolinite-OP complex released very low amounts of P, probably due to its high stability with OPs. The contrasted physicochemical properties (specific surface areas, net surface charges, interaction properties) of the minerals are likely the major mechanisms responsible for the chemical release of specifically adsorbed OP in the tested soils. Thus, efficiency of OP utilization by plants in agricultural soils following the addition of organic input will depend on the dominant soil type and its characteristics. Soils rich in 2:1 phyllosilicate would have a higher availability of OPs than soils rich in Fe and Al oxides, which in turn would have a higher availability of OPs than soils rich in 1:1 phyllosilicate. Obviously, more research is needed to understand and test this phenomenon under real field conditions.
No correlation was found between P uptake and resin-extracted P concentration (Fig 3). In addition, rhizosheath resin P concentration for most mineral-OP complexes was higher than that of the bulk soil. Hence, these results suggest that plants would have taken up additional pools of P beyond what was initially available, so the plant must have induced additional desorption of OP. According to Le Chatelier’s principle (Law of Mass Action), the sink effect of roots caused an OP depletion in the solution, which in turn induced further OP desorption dissolution to replenish the solution (Houben and Sonnet, 2012). In addition, mobilization of OP by ryegrass roots might also have contributed to OP depletion. As shown by Martin et al., (2002), high efficiency of ryegrass in displacing strongly bounded P may be due to the fact that the plant can react to P deficiency by extending root surface and utilizing more than one mechanism of extraction at the same time.
It is generally accepted that P extraction with KCl is negatively correlated with the binding energy of the P compounds to soils. Thus, the more P can be extracted with KCl to become available to plants, the lower its binding energy (Martin et al., 2004; Yan et al., 2014). However, our results showed no correlation between P uptake and P extracted with KCl. The lack of correlation in our case may imply that the P taken up by plants did not depend on the strength or energy with which OP compounds are bounded to soil minerals. Nevertheless, this lack of correlation between P taken up by plants and P extracted by KCl could also mean that OP compounds may have been released from the soil minerals but remained in organic form without being mineralized for uptake by plants. Therefore, we hypothesize that although OP was desorbed from some minerals, it may not have been absorbed because it was not readily hydrolyzable by enzymes. This supports our earlier explanation of the resistance of some of OP compounds to enzymatic hydrolysis because of their chemical properties.
Finally, P uptake by ryegrass was positively correlated with ΔP resin (R = 0.73, p<0.01), showing that the variation of available P in the rhizosphere determined the P uptake by plant (Fig 3). The ΔP resin values of the majority of the mineral-OP complexes was generally negative, showing that in addition to the spontaneously released P, there would have been a subsequent release of P from soil minerals that would occur in response to plant-induced depletion of available P in the rhizosphere. This that plant roots mobilized additional P from minerals. However, this finding did not apply to all OP-minerals complexes. For instance, GLY-mineral complexes (Fig 4) showed almost no variation in ΔP resin, i.e. no variation between P resin in the bulk soil and P resin in the rhizosphere, suggesting that plant absorbed only the spontaneously available P without any other mobilization. This confirms that GLY was sufficiently available from soil minerals. Thus, when the plant was in the presence of soil minerals involving GLY, there was little or no additional P mobilization.
Here, we showed that the binding strength of OPs to the mineral surface did not necessarily explain P availability and uptake by plant. Furthermore, plant uptake, by inducing depletion of the available P pool, would have resulted in additional mobilization of OP. However, for some complexes, no additional mobilization was required due to sufficient P availability. Thus, plant P uptake from OP-mineral complexes may depend on the variation of available P in the rhizosphere, which results from (de)sorption dynamics and the ability of the plant species to mobilize specific OP compounds via its uptake effect or via physiological processes.
Phosphorus uptake from adsorbed OP relative to adsorbed IP
Our third hypothesis was that P from adsorbed OP would be less available than P from adsorbed IP because it desorbs less and requires also enzymatic cleavage before being taken up by ryegrass. Our results support only partially this hypothesis as they showed that the different adsorbed OP compounds had contrasted P uptake compared to the adsorbed IP compound. Phosphorus uptake from adsorbed IHP and G6P was significantly lower than that of adsorbed IP, whereas P uptake from adsorbed GLY, depending on the mineral, was slightly equal or higher than that of IP. Several studies have shown lower plant availability of adsorbed OP compounds compared to IP compounds (Andrino et al., 2019; D’Amico et al., 2020; Klotzbücher et al., 2019), probably due to the specific binding mechanisms of OP and its low desorption from soil (Bollyn et al., 2017; Ruyter-Hooley et al., 2015). However, in all of these studies, "organic P" refers only to IHP. In addition to IHP, GLY and G6P are also commonly found in soils, soils treated with organic inputs, sediments, and wetlands (Missong et al., 2016; Vincent et al., 2013) but these OP forms have not been considered potential sources of P for plants, except in a few studies (Adams and Pate, 1992). Here, our results highlight the relative availability of these major OP forms compared to that of IP. In agreement with previous studies(Andrino et al., 2019; D’Amico et al., 2020; Ruttenberg and Sulak, 2011), IHP was less available than IP. Similar to IHP, we also found that adsorbed G6P was less available than IP. This might be explained by the greater adsorption and binding strength of G6P to external surface sites than IP (Goebel et al., 2017; Ruttenberg and Sulak, 2011). Finally, we found that the adsorbed GLY compound, depending on the minerals, was almost equivalent or more available than the IP compound. This is attributed to its low affinity for soil minerals and especially to its high desorption in soil. Thus, it can be inferred that soil amendment with organic inputs with high concentration of GLY might be a sustainable alternative to mineral P fertilizer.
In the present studies, GLY regardless of soil type was generally as available as IP. More importantly, we showed that on Al oxides, GLY was significantly more available than IP. Thus, based on comparisons of different P compounds, our results showed that in soil, unlike IHP, another major compound of OP such as GLY can be equal or superior to IP in terms of plant availability.