Competition between Organic Matter and Phosphate for Binding Sites in Sandy Soils

Transcription

1 Competition between Organic Matter and Phosphate for Binding Sites in Sandy Soils Number of words: Marco Mng ong o Stamnummer: Promotor: Prof. dr. ir. Steven Sleutel Tutor: ir. Lisa Mabilde Master s Dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Science in Physical Land Resources - main subject Soil Science Academic Year:

2 This is an unpublished M.Sc. dissertation and is not prepared for further distribution. The author and the promoter give the permission to use this Master dissertation for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using results from this Master dissertation. Gent, The Promoter(s), The Author, Prof. Dr. ir. Steven Sleutel Marco Mng ong o i

3 ACKNOWLEDGMENTS First and foremost, I would like to thank Almighty God for His grace without which I would not have made my studies journey possible. I would like to offer my heartfelt gratitude to my Promoter, Prof. Dr. ir. Steven Sleutel, and my Tutor, ir. Lisa Mabilde for their valuable suggestions, patience, guidance and supervision they gave me during the entire duration of my M.Sc. research. I also extend my thanks and appreciation to Luc, Sophie and other lab technicians for their technical assistance and Administration of Department of Soil Management of Ghent University- Belgium for allowing me to use their facilities and expertise for the whole period of my studies. I express gratitude to Physical Land Resource students, staff members and friends for their encouragement and support to me during my studies. I thankfully acknowledge the European Union through Caribu Scholarship Project for their financial support during two-year period of my Master program at Ugent-Belgium. My deepest gratitude to my beloved mother, Amaria Mgona, my father, Exavery Mng ong o and all my brothers and sisters for their love, sacrifices, understanding, blessing and encouragement in my academic career. Gent, June 2017 Marco Mng ong o ii

4 ABSTRACT When P-fertilizer is applied, orthophosphate is quickly adsorbed on sesquioxides and becomes very immobile in the soil. Addition of organic matter (OM) in the soil can increase soil phosphorus (P) availability via soil mineralization or desorption of bound soil P. OM can also reduce phosphate adsorption, thereby increasing P in soil solution. The acid sandy soils, as in Flanders, have low clay% and are more vulnerable to P deficiency. Because of historical redundant P fertilization P leaching risk is high, exacerbated by sandy soils high hydraulic conductivity. The main adsorbents of P in these soils are sesquioxides, which are also binding sites for OM compounds. The main objective of this study was to investigate the competition of OM and P for binding sites in sandy soils, i.e. to compare phosphate sorption in soils with different soil organic carbon levels. The interactive effect of phosphate saturation degree (PSD) was taken into account by selecting soils covering three ranges in PSD (10-25, and 55-65%). No co-variation of carbon (C) content, PSD and phosphate sorption capacity (PSC) was assured. This allows us to investigate the isolated effect of C on P sorption, within each PSD-range. The competitive adsorption of OM and phosphate was analyzed by way of P sorption experiments. Fifty sandy cropland soils, varying in soil OM content and within three PSD-ranges were selected and equilibrated for 24 hours with a 0.01 M CaCl2 solution, containing 0, 0.1, 0.5, 1, 2, 5, 7.5 and 10 mg P L -1 applied as KH2PO4. Additionally, soil phkcl, NH4-oxalate-extracted Al, Fe and P, total and particulate C% were measured, as these are factors involved in the adsorption process. The Langmuir adsorption isotherms fitted well with the data, with correlation coefficients (R 2 ) ranging from 0.67 to 0.86 in all the soils series. The Langmuir fit (R²)was good for PSD-series 1 and 2, but less for PSD-series 3 (probably because the binding sites are almost saturated with P so new P adsorption was less likely Among the soil properties (organic matter, ph, Feox, Alox and non-particulate OM), organic matter and Alox was significantly positively correlated with the Langmuir adsorption maximum (Qmax) and negatively correlated with K, whilst the remaining soil properties studied were not correlated with the adsorption parameters. - lower Qmax in PSD were observed in -series 1: because of lower Alox (unintentionally), significant positive correlation Qmax and Al, Al could be more important than Fe as Al is the dominant adsorbent at ph 4-5.5) Qmax was much higher in PSD-series 2 (i.e. P sorption higher) compared to series 1 was unexpected. Probably due to higher Al contents compared to PSD-series 1. iii

5 -Ratio of C/Al and np-c/al negatively correlated with Qmax which means higher C and np-c content compared to amount of Alox will result in P desorption. In conclusion, However, no single soil property was managed to explain the P sorption alone, suggesting that P sorption in this study was controlled by a combination of number of physicalchemical soil properties. Al appears to be more important adsorbent for P than Fe, in the sandy soils we studied, C-content was not directly confirmed to stronger influence the P sorption in soils with higher PSD due to insufficient binding sites while the relative proportion of C to Alox does impact o-p sorption, confirming the hypothesis that higher carbon levels in soil reduce P adsorption. since the strength of C: Alox does not becomes greater at higher PSD, the hypothesis 2 that the competition of OM and P for binding will be pertain in higher PSD was rejected iv

6 Table of Contents ACKNOWLEDGMENTS II ABSTRACT...III LIST OF FIGURES... VII LIST OF TABLES.VIII CHAPTER 1 INTRODUCTION BACKGROUND RESEARCH OBJECTIVES... 3 CHAPTER 2 LITERATURE REVIEW BACKGROUND Soil P cycle Forms of phosphorus in soils MAJOR REACTIONS OF PHOSPHORUS IN SOIL Speciation of orthophosphate ions Precipitation - dissolution of phosphate minerals Adsorption desorption of o-p ions on soil minerals P sorbing surfaces Factors that affect P-sorption Soil P Saturation ORGANIC MATTER INTERACTIONS WITH MINERAL SURFACES Free POM Dissolved OM Mineral-bound OM ORGANIC MATTER STABILIZATION Stabilization of OM by interaction with mineral surfaces and metal ions Sorption of organic matter in soil MECHANISMS OF COMPETITION FOR MINERAL BINDING BETWEEN OM AND PHOSPHATE Direct competitive Mechanisms of OM on P sorption Indirect mechanism POSITIVE EFFECTS OF OM ON SORPTION OF O-P IN SANDY SOILS CHAPTER 3 MATERIALS AND METHODS STUDY AREA AND SOIL SAMPLE COLLECTION SOIL CHARACTERIZATION PARTICULATE OM (POM) v

7 3.4 P SORPTION IN SOIL TEST ISOTHERMS OF P SORPTION STATISTICAL ANALYSIS CHAPTER 4 RESULTS GENERAL SOIL PROPERTIES SORPTION EXPERIMENT AND AMOUNT OF P SORBED AT EQUILIBRIUM P SORPTION ISOTHERMS MAXIMUM ADSORBED P (Q MAX) AND BONDING ENERGY (K) CORRELATION OF SOIL PROPERTIES WITH P SORPTION CHARACTERISTICS Organic carbon and Non particulate C The amount of Fe ox and Al ox Relationship between Q max and K Effect of np-c: Al ox and C: Alox ratio to Q max and K CHAPTER 5 DISCUSSION GENERAL SOIL PROPERTIES AMOUNT OF P EXTRACTED AND ADSORBED P PARAMETERS OF LANGMUIR ADSORPTION ISOTHERM EFFECT OF OM ON P SORPTION AND DESORPTION SOIL C LEVEL AND O-P SORPTION CHAPTER 6 CONCLUSION AND RECOMMENDATION CONCLUSION RECOMMENDATIONS CHAPTER 7 REFERENCES CHAPTER 8 APPENDIXES vi

8 List of Figures Fig. 1. Phosphorus cycle in the natural environment... 6 Fig. 2. Influence of ph on the distribution of orthophosphate species in solution... 9 Fig. 3. The availability of phosphorus as affected by soil ph Fig. 4. Predicted PSD for agricultural soils in Flanders (interpolated map with a probability of 95%) Fig. 5. Direct effects of dissolved organic carbon (DOC) compounds on phosphorus (P) availability. M: metal (i.e. Fe or Al), LOA: low molecular weight organic acid Fig. 6. Direct and indirect effects of organic matter addition on phosphorus (P) phytoavailability. DOC, Dissolved organic carbon; LOA low molecular weight organic acids; Pi, inorganic P; Po, organic P; MBP, Microbial biomass P Fig. 7. Scatter plots of C-content and PSC Fig. 8. Location for all three soil series samples in Flanders-Belgium Fig. 9. The scatter plot showing covariation of Alox and C-content, boxplot showing Qmax of all three soil series Fig. 10. Example of P sorption isotherm curves (Langmuir model) fitted to the data Fig. 11. Boxplot showing Qmax for all PSD soil series Fig. 12. Scatter plot of Alox and Langmuir sorption parameters (Qmax and K) vii

9 List of Tables Table 1. Selected soil samples with their chemical characteristics (0-30cm depth) Table 2: Descriptive Statistics of Soil Properties of Series 1 (PSD %) Table 3: Descriptive Statistics of Soil Properties of Series 2 (PSD %) Table 4: Descriptive Statistics of Soil Properties of Series 3 (PSD %) Table 5: Table 5. Analysis of variance for CP and Q in 3 PSD soil series (ANOVA) Table 6: Average amounts of P remaining in solution and P adsorbed to the soil for added solutions with different initial P concentration (n = 15, 18, 17 for soil series 1, 2, 3 respectively) Table 7: Mean Qmax (µg P g -1 ) and K (dm 3 mmol -1) for the 3 soil PSD-series Table 8: ANOVA for Qmax and K for all three PSD soil series Table 9: Pearson Correlation coefficients of soil properties and Qmax and K across all three PSDseries Table 10: Pearson Correlation coefficients between soil properties and Qmax and K per PSDseries Table 11: Pearson correlation coefficients between soil C to Alox ratios and P sorption isotherm parameters viii

10 CHAPTER 1 INTRODUCTION 1.1 Background In the soil, phosphorous (P) is converted from soluble P to insoluble P through two important processes of P fixation or P retention i.e. adsorption and precipitation. Adsorption to clay minerals and iron(fe)- or Aluminium (Al)-oxides (or hydroxides) is the main process in acid soils, and often referred to as sorption instead of adsorption, because it consists of a fast reversible adsorption reaction at the surface of soil particles, and then followed by slow diffusion reaction into the soil particles. Precipitation of P with Al, Fe or Ca ions to form Fe-/Al-/Ca-phosphates occurs in more acidic or alkaline or calcareous soils. Generally, 70 90% of added P through fertilization or mineralization in the soil is fixed depending on soil characteristics, resulting in reduced plant available P (Liu et al., 2000a, McBeath et al., 2005), consequently increased P fertilizers application rates are adapted in agricultural fields (Guppy et al., 2005; Yu et al., 2013; Fink et al., 2016). Excess amounts of P added enhance eutrophication of aquatic ecosystems due to enhanced P leaching, also resulting in unprofitable farming, due to increased production cost, as high cost is invested in P fertilizer (Ohno et al., 2007). Sandy soils are more vulnerable to P deficiency, due few P binding sites and low level metal oxides (Alox and Feox) resulting in high P leaching, exacerbated by a high hydraulic conductivity. Sorption of phosphate in the soil depends on different factors and soil conditions i.e. soil texture, clay mineralogy, ph and sorption capacity as determined by oxide or hydroxide of Al and Fe, and soil organic matter (SOM) (Zhang et al., 2005; Yan et al., 2013; Fink et al., 2016). The main adsorbents of P in sandy soils are Fe- and Al- (hydr)oxides (Fink et al., 2016). Added organic matter (OM) to soil can increase or decrease P sorption depending on the type of OM, inert P concentration and the amount of P added by fertilization (Chintala et al., 2014; Bortoluzzi et al., 2015). Soil organic matter (SOM) can inhibit P sorption since SOM and phosphate anions both are negatively charged and they bind on the same binding sites in the soil sorption complex (Zhang et al., 2005; Yan et al., 2013). The negative charge of the SOM is a result 1

11 of ionization of functional groups which are present on it, i.e. carboxylic acid, phenol and hydroxide groups (Antelo et al., 2007). Despite the great role of OM to the soil for plant growth and microbial activities, its role to the availability of applied phosphatic fertilizers has been contradictory over different research studies; Some researchers reported decreased P sorption while others reported increased P sorption upon OM addition (Guppy et al., 2005; Chintala et al., 2014; Fink et al., 2016; Gerard et al., 2016). It is unclear to what extent SOM content is a relevant influencer of P-availability in acidic sandy soils, relative to other factors like pedogenic Fe and Al content and soil ph. In North-western Europe such soils usually have high P levels when under prolonged agricultural use as a result of historical P-fertilizer and animal manure loads. Frequently, in the topsoil high degrees of phosphate saturation exist and it is not known to what extent mediation of SOM content on P- sorption is effectuated at different degrees of phosphate saturation. 2

12 1.2 Research objectives The main research question for this study was to investigate the degree of competition between organic matter and phosphate for sorption sites in sandy agricultural topsoils. The objectives of the study were to test the difference in phosphate sorption in soils with different soil organic carbon levels, other factors influencing P sorption were kept constant. More specifically, the intention was to check the affinity of OM and P for Flemish sandy soils at varying degrees of phosphate saturation. We took advantage of the existing unique large soil set of sandy cropland soils sampled in the frame of the phosphate saturation degree mapping of the Flemish Sandy Region as a base for selecting soils with no covariation in OM, PSC and PSD. The objective was to use experimental P-sorption tests to verify following central hypotheses: Hypothesis 1: Higher organic carbon levels are hypothesized to reduce phosphate adsorption, thereby increasing the phosphate concentration in the soil solution, when other factors and soil conditions such as soil type and texture, ph and sorption capacity are the same. Hypothesis 2: It is expected that SOM content strongly affects P-sorption in soils with higher PSD. At low PSD, ample binding space may be still available for sorption of both native SOM and added phosphate, so the competition between both should be limited. At high PSD (e.g. 50%), available space for additional sorption of P is much more limited and OM could more strongly influence P-availability and new P sorption. 3

13 CHAPTER 2 LITERATURE REVIEW 2.1 Background Phosphorus (P) is an essential element for plant and animal growth and important during cell division and growth. It is a component of nucleic acids and it is important in biological energy transfer processes that are vital for life and growth. Complex soil process influence the availability of phosphorus applied to the soil and many soils have ability to tie up phosphorus, making it unavailable to plants. P in soils is often below that required for optimum crop growth, either due to precipitation of P with Ca, Fe, and Al, or to specific adsorption of P by metal oxides. To raise the fertility status of soils, large input of inorganic P fertilizer is required, which may result in the excessive buildup of soil-bound P and saturation of soil with P. Added P, through fertilization, will then not be able to bind to the soil anymore. Interaction of OM with P in the soil influences the amount of available P in solution (Guppy et al., 2005). Reduction in P sorption will have significant effects on plant P availability, but also P leaching and eventually eutrophication of ground and surfaces water resources. Multiple mechanisms explain how OM level could significantly control P availability, i.e. the subject of this MSc thesis research. These will be detailed in section and section 2.3 after a general introduction of P behavior in soil Soil P cycle Elemental P is extremely reactive and reacts with oxygen when exposed to air to form phosphate, a chemical form in which each P atom is surrounded by 4 oxygen (O) atoms, PO4 3-. The existence of phosphate in the soil depends on the soil ph (see 2.2.1) and prevailing forms are termed jointly ortho-phosphate (o-p) hereafter. o-p in soil is present in several pools, that are constantly subject to internal cycling. P from primary minerals is released into the soil solution, from which plants can absorb it (Fink et al., 2016). Apatite mineral (3[Ca3(PO4)2]CaX2) is usually the primary source of P from the parent material where X is anions (F -, Cl -, OH - or CO3 2- ). Deposits rich in apatite (e.g. rock phosphate) 4

14 which are in sediments deposited at bottom of ancient seas. Rock phosphate reserve are on order of 5x10 13 kg, making a total of 12x10 12 kg P basing on the average P content of 10% (Stevenson and Cole, 1999). P can also be added to the soil in form of inorganic fertilizers such as triple superphosphate (TSP) or organic fertilizers such as farmyard manure, pig slurry and composts. Rocks phosphates from various sources are treated with inorganic acids (e.g. Sulphuric and Phosphoric acid) to produce more soluble phosphate fertilizer. For example, Super phosphate fertilizer is produced by mixing approximately equal quantities of 60 to 70% H2SO4 and rock phosphate. 3[Ca3(PO4)2CaF2+7H2SO4+3H2O 3Ca(H2PO4)2H2O +7CaSO4 +2H2O+2HF (1) When gypsum is removed with excess H2SO4 concentrated Superphosphate, containing 20% P is produced. Total reserve of phosphate rock appears to be appreciable, but phosphate is a limited resource when considered in world demand. However, P-deficiency are likely to intensify in the near future due to shortage of mineable phosphate, and crop yield will be limited by P release from insoluble form in the soil. Due existing current Forecasts as when shortage will develop vary considerably, depending on basis on which the estimates are made for unknown reserves and the P content of the rock. At the present mining rate (about 7x10 10 kg of rock per annum), reserves will be depleted in about 700 years. However, if the usage of P fertilizer continues to grow at recent rates of billion tons per annum (Jasinski, 2017), the known reserves will be depleted in a much short time (about 100 years). Recycling of soil Fixed P and reuse of waste materials with high P content as fertilizer i.e. sewage sludge, manure, compost, crop residues and slurry should be introduced (Stevenson and Cole, 1999). Dissolved o-p from primary minerals and fertilizers can precipitate metal cations such as Ca 2+, Fe 3+ Al 3+, K + to form secondary P minerals. In acidic soils, mainly Al-P and Fe-P based minerals 5

15 are dominant (e.g. strengite) while Ca-P predominantly in alkaline soils. These P removal processes are discussed in more detail in On the other hand, from the soil, o-p can be removed through crop uptake, soil erosion, leaching and water run-off (Vanden Nest, 2015). Dissolved o-p can also be transformed by plants and microorganism to organic P forms such as ATP and DNA (Bortoluzzi et al., 2015). o-p transformation between organic and inorganic forms is determined by different factors which determine mineralization and immobilization process i.e. microbial activity, soil moisture, chemical and mineralogical soil properties (Tiecher et al., 2012). The soil P cycle is dynamic, involving soils, plants and microorganisms. The P cycle does not include an atmospheric component, it includes only the aquatic and soil compartments. In natural systems, all utilized P by plants is returned to the soil through plant and animal residues. As the basic source and reservoir of phosphorus are the rocks phosphate or other rock deposits, these gradually release o-p into the ecosystems by dissolution into soil pore water as inorganic o-p (Fig. 2). Fig. 1. Phosphorus cycle in the natural environment (Pierzynski et al., 2000) 6

16 2.1.2 Forms of phosphorus in soils The existence of P in the soil is divided into three major pools, the solution P pool, the active P pool and a fixed P pool. P in solution pool usually occurs in the o-p form, but small amounts of organic P may exist as well. P in the solution pool is very important, because it is the pool from which plant P-uptake occurs and it has measurable mobility. Growing plants or crops can deplete the P in the soluble P pool (Bushman et al., 2009). The active P pool consist of inorganic o-p adsorbed to the soil, which is weakly bound in solid phase and is relatively easily released into the soil solution. As plants takes up o-p, the concentration of o-p in solution is decreases and o-p from the active P pool is released to restore the equilibrium, o-p in the solution pool is very low, so the active o-p pool is main source plant available P. soil is considered fertile with respect to P when the active o-p pool have ability to replenish the soil solution o-p pool. The fixed P pool comprises inorganic P compounds which are insoluble and organic P compounds that are resistant to soil microbial mineralization (Oelkers, 2008). 2.2 Major reactions of phosphorus in soil Speciation of orthophosphate ions In nature phosphate exist in three o-p forms, depending on the soil ph, i.e. H2PO3 -, HPO4 2- and PO4 3-. These chemical forms react with up to three single positive ions such as hydrogen (H + ), potassium (K + ), ammonium (NH4 + ) Ca 2+, Al 3+, Fe 3+. The amounts of various o-p ions in the soil solution are determined by soil ph, availability of metal cations i.e. Fe 3+, Al 3+, Ca 3+, and presence of other competing ligands (e.g. Citrate and oxalate) in the soil solution (Hinsinger, 2001; Sato, 2003; Fink et al., 2016). At ph (H2O) 7.2, there are approximately equal amounts of H2PO3 - and HPO4 2- forms in soil solution (Fig.2). at ph below 7.2, H2PO4 - is dominant while at ph above 7.2, HPO4 2- become a major specie. At soil ph the concentration of H + available for reactions is very low, the amount 7

17 of o-p in H2PO3 - and HPO4 2- forms are only a small and transient component of the total soil P reserve. That means phosphates will react with other positively charged ions in the soil such as Ca 2+, Al 3+ and Fe 3+ to form stable components in acidic (i.e. Al-P or Fe-P) and alkaline (i.e. Ca-P) soil condition (Oelkers, 2008; Barrow, 2016). The speciation of P ions in the soil solution, also is dependent on the availability of metal cations such as Al 3+, Fe 3+, Ca 2+ and Mg 2+ which has strong tendency to react and form complex species with them e.g. Fe 3+ +H2PO4 - FeH2PO42 +. Also the availability of these meatal cations depend on soil ph. In acidic soil (ph<6) the availability of Al 3+ and Fe 3+ increase while Ca 2+ and Mg 2+ is dominant in neutral and alkaline soils. The presence of other competing ligands in the soil solution control the P speciation. Presence of organic ligand such as citrate and oxalate compete with o-p ions to form complexes with metal cations which in turn determine the P species available in the soil solution (Hinsinger, 2001) Therefore, phosphorus can be found in different positively or negatively charged ions or uncharged species in addition to the o-p ions in the soil solutions. 8

18 Fig. 2. Influence of ph on the distribution of orthophosphate species in solution (Sato, 2003) Precipitation - dissolution of phosphate minerals The o-p ions precipitate readily with metal cations such as Al 3+, Fe 3+, Mn 2+, Ca 2+ and Mg 2+, forming a range of P-minerals. The type of mineral formed depends on the soil ph because it determines the occurrence and abundance of metallic cations in the soil solution that can precipitate with o-p. Hence, in neutral to alkaline soils, o-p ions precipitate as Ca phosphates i.e. dicalcium hydroxyapatite and soluble apatites (Sato, 2003; Fink et al., 2016). Ca 2+ +H2PO H2O CaHPO4.2H2O (brushite) + H + log k =0.63 (2) In contrast, under acidic conditions o-p ions will precipitate as Fe-P and Al-P such as strengite, vivianite, variscite. Al 3+ (aq) + H2PO4 - (aq) +2H2O Al(OH)2H2PO4(s) + 2H + log K= (3) 9

19 The precipitation dissolution equilibria that govern the solubility of P-minerals depend on ph, concentration of o-p and metal cations. For example, considering the equation 4 below which applies for hydroxyapatite: Ca5(PO4)3OH + 7H3O + 3H2PO4 + 5Ca H2O (4) It is observed that in equation 4, the equilibrium can be shifted to the right, i.e., the dissolution of the hydroxyapatite can be enhanced if protons are supplied or if o-p or Ca 2+ ions are removed from the soil solution. Adsorption of o-p ions by other soil constituents may thus favor the dissolution of this Ca-phosphate Adsorption desorption of o-p ions on soil minerals P sorbing surfaces Sorption and desorption reactions in the soil equilibrate o-p in solids with o-p in the soil solution. o-p can adsorb to the surfaces and edges of hydrous oxides, clay minerals and carbonates by replacing OH - (De Bolle, 2013). Adsorption of o-p in the soil is usually enhanced by presence of metal oxides of Fe or Al, collectively named sesquioxides, organic complexes of Al and Fe, edges of silicate clays and calcite (De Bolle, 2013; Gérard et al., 2016). Added inorganic o-p can be adsorbed weakly (electrostatically) or strongly (covalent bond) onto these variable charged surfaces. As mostly o-p species in soil solution are negatively charged, o- P will be sorbed to soil constituents that bear positive charges such as hydroxyl, carboxyl, clays groups. Metal hydroxides have a variable charge, and their capacity to adsorb anions such as P ions will increase with decreasing ph, due to higher protonation at low ph (Barrow, 2016). Both Al and Fe oxides have a point of zero charge between 7 and 10 which makes their surfaces net positively charged over the whole ph range, usually encountered in the soil (i.e. 3.5 to 9.5). Weakly crystalline Al, usually determined as NH4-oxalate extractable Al (Alox), is effective at adsorbing P selectively in the presence of competing anions such as Cl, NO 3 and SO4 2. Alox it is also abundant in nature, and has a relatively higher point of zero charge (PZC) (Kumar et al., 2014). The o-p adsorbed by Al can also be stable over a wide ph range. o-p adsorption and desorption furthermore depends on concentration, crystallinity and SSA of hydroxyl groups on the surface of pedogenic oxides (Fink et al., 2016). It is observed that there is 10

20 preferentially adsorption of o-p by hydroxyl surface groups in Fe hydroxides as they are protonated below ph 7 9 (Hinsinger, 2001; Fink et al., 2016). The o-p may be adsorbed in monodentate or bidentate form depending on the number of OH groups in the phosphate that are bonded to Fe atoms, or in binuclear form when two OH phosphate groups are adsorbed by two Fe atoms (Fink et al., 2016). In clay minerals, o-p is adsorbed by functional groups at the edges of 1:1 and 2:1 mineral, and this affects the amount and energy of o-p adsorption and its availability to plants. o-p adsorption capacity of clay minerals may be similar to or higher than that of Fe and Al(hydr-)oxides, depending on the specific surface area (SSA) of the particular soil components (Gérard et al., 2016) Factors that affect P-sorption Organic matter: The application of OM can increase soil phosphorus (P) availability through decomposition and mineralization of included organic-p, or by abiotic processes such as ligandexchange. However, presence of mineral-bound OM may also reduce phosphate adsorption capacity, favoring release of o-p in soil solution (Yusran, 2010). Three abiotic mechanisms are proposed to explain how addition of OM can reduce P sorption in soils. Firstly, soluble organic molecules may specifically adsorb to soil minerals by ligand exchange in competition with o-p (Guppy et al., 2005; Yusran, 2010; Fink et al., 2016). Secondly, the soluble OM may react with bound Al 3+ or Fe 3+ at the surface of the soil mineral phase to form soluble complexes and release o-p which was previously sorbed or which was present as insoluble Al and Fe-phosphate (Guppy et al., 2005; Yusran, 2010). Third, OM can be sorbed to soil particles, resulting in higher negative charge of the particles, and hence increased repulsion force for o-p anion. These mechanisms are discussed in detail in section 2.3. Mineralogy of the soil: has a great effect on o-p-sorption. Volcanic soils tend to have the greatest o-p-sorption of all soils since volcanic soils contain large amounts of amorphous minerals like Allophane with high content of Al 3+, Ca 2+, Fe 3+. These minerals also have a large surface area for o-p sorption. 11

21 Amount of clay: As the amount of clay increases in the soil, the P-sorption capacity increases as well since clay particles have a tremendous large surface area onto which phosphate sorption can take place. At smaller clay contents, the role of pedogenic Fe and Al becomes more important (Chintala et al., 2014). That s why sandy soils are expected to have less P sorption capacity and the pedogenic Fe and Al then should be the primary binding sites for o-p. Soil ph: There is a general relationship between soil ph and phosphorus availability, which is based on the kinds of o-p compounds associated with the various phs. Reactions that reduce P availability occur in all ranges of soil ph but can be very pronounced in alkaline soils (ph > 7.3) and in acidic soils (ph < 5.5). At low ph, soils have greater amounts of Al and Fe in the soil solution, which form very strong bonds with o-p. In contrast at high ph, o-p precipitates with calcium as Ca-P (Sato, 2003; Oelkers et al., 2008). Maintaining soil ph between 6 and 7 will generally result in the most efficient use of phosphate with maximum P availability. (Busman et al., 2009). Fig. 3. The availability of phosphorus as affected by soil ph (Bushman et al., 2009). Presence of competing anions in soil solution. Ligands with higher affinity for the soil surface than phosphate will destabilize P minerals (Sato, 2003) and resulting desorption of o-p mostly 12

22 occurs through ligand exchange reactions. An increase in the concentration of competing anions will shift the adsorption desorption equilibrium towards enhanced desorption (Hinsinger, 2001). Anions such as silicates, carbonates, sulfates molybdate, and carboxyl-containing OM constituents compete with o-p for a position on the anion exchange site. Higher concentrations of competing ligands such as sulphate are, however, needed for effective ligand exchange as o-p ions have a strong affinity to be adsorbed on the surface of positively charged minerals Soil P Saturation Phosphate saturation degree (PSD) is an index of the actual phosphate accumulation in the soil relative to the maximum phosphate sorption capacity (PSC) of the soil to a reference depth (De Smet et al., 1996). PSC is the maximum amount of o-p that can be sorbed. The build-up of sorbed P relative to the PSC increases equilibrium solution P concentrations to the extent where P can be readily removed through runoff and leachate (Kleinman et al., 1999). In acidic soils, PSC is controlled by non-crystalline Al and Fe minerals. In the Netherlands and Flanders many phosphate saturation parameters (i.e. PSD and PSC) were developed to estimate susceptibility of soils to leaching and the risk of causing eutrophication (Van der Zee et al.,1990). The measurement of PSD is based on acid ammonium oxalate extraction of non-crystalline Al and Fe compounds in acidic soils. PSC= α (Feox + Alox) (5) PSD = [Pox/α (Feox + Alox)] x100 (6), where Pox = sorbed P fraction extracted with ammonium oxalate-oxalic acid (mmol P kg -1 ), PSC = P sorption capacity (in mmol kg -1 ), Feox = ammonium-oxalate-oxalic acid extractable Fe (in mmol Fe kg -1 ), Alox = ammonium-oxalate-oxalic acid extractable Al (in mmol Al kg -1 ). The factor alpha (α) = 0.5 represents that only 50 % of the ammonium-oxalate-extractable oxides is capable of binding P, though in reality this proportion varies between 40 to 60 % (Van Der Zee et al., 1990). 13

23 The general PSD for the soil profile is determined by calculating the average Pox and the average PSC, Alox and Feox, to a depth of 90 cm or to the depth of groundwater if it is shallower than 90cm. A concentration of 0.1 mg L -1 from the base of the soil profile is considered as the lower limit for eutrophication and the capacity to retain phosphorus in the soil (P saturation). The PSD should be less than 24% of the whole soil profile, this is the critical PSD, when exceeded, the threshold of 0.1 mg L -1 o-p will likely also be exceeded. In Flanders, the protocol for phosphate saturation was set to a total profile depth of 90 cm and agricultural soils with a PSD of 35% or more are considered to be P-saturated (De Smet et al., 1996). Soils with a PSD between 25 and 35% was classified as P critical, whereas soils with a PSD < 25% were categorized to be P unsaturated (Van Meirvenne et al., 2008; De Bolle, 2013). 14

24 Fig. 4. Predicted PSD for agricultural soils in Flanders (interpolated map with a probability of 95%) (Source: Van Meirvenne et al., 2008). 15

25 2.3 Organic matter interactions with mineral surfaces Soil organic matter (SOM) is the organic component of soil, consisting of plant and animal residues at various stages of decomposition. SOM exerts numerous positive effects on soil physical and chemical properties, as well as the soil s capacity to provide regulatory ecosystem services. The presence of SOM is regarded as being critical for soil function and soil quality (Beare, et al., 1994). The positive impacts of SOM result from a number of complex, interactive edaphic factors, some of the SOM's effects on soil functioning include improvements of soil structure, aggregation, soil biodiversity, absorption and retention of pollutants, and the cycling and storage of plant nutrients. SOM increases soil fertility by providing cation exchange sites and acting as reserve of plant nutrients, such as nitrogen (N), phosphorus (P), and sulfur (S), together with micronutrients. SOM is typically estimated to contain 50% C, and the terms soil organic carbon (SOC) and SOM are often used interchangeably, with measured SOC content often serving as a proxy for SOM (Troeh et al., 2005). The concentration of SOM in soils generally ranges from 1% to 6% of the total topsoil mass for most upland soils (Troeh et al., 2005). SOM fractions; Different techniques have been proposed to separate soil samples into different SOM fractions with distinct physical and chemical features, based on various stabilizing mechanisms and soil functions (Lützow et al., 2007). The most used technique is physical fractionation, since the physical conformation of OM and its location in the soil matrix explains its protection of SOM against mineralization. Also, as compared to chemical methods, physical fractionation may create less artifacts. Physical fractionation has consequently been very frequently used in soil studies to evaluate SOM accessibility. Splitting OM fractions by physical methods can result in separating SOM fractions which differ in turnover times, especially incompletely decayed plant litter in the non-aggregate related light fraction from mineral connected SOM and intra-aggregate particulate organic matter (ipom) (Whalen et al., 2000). Physical fractionation usually entails various degrees of soil dispersion, size fractionation (Torn et al., 2009), followed by fractionation according to density or aggregates to separate SOM based on their size and difference in density (Sollins et al., 1999). 16

26 2.3.1 Free POM SOM can be classified as mineral-free or mineral-associated OM (Theodorou, 1990). The former is composed of either free or occluded (fpom). fpom is a labile fraction constituted by partially decomposed plant material, fungal hyphae, spores, and pollen grains, with a particle size ranging between 53 and 2000 μm and often covered with soil particles (Plante & McGill, 2002; Wander, 2004). It is a primary source of food and energy for microorganisms (Christensen, 2001) and nutrients for plant growth since fpom is usually easily decomposable. fpom, as does bulk SOM, enhances aggregate stability, water infiltration and soil aeration, increases cation exchange capacity and buffers soil ph, binds environmental pollutants such as heavy metals and pesticides, though to a smaller degree than bulk SOM because of fpom s relatively small surface area due to its larger size. fpom is very sensitive to the changes in soil management and, therefore, its variation throughout time is more indicative of the effect of management practices than total organic matter (Fabrizzi et al., 2003) Dissolved OM Dissolved organic matter (DOM) consists of all organic compounds present in soil solution and is the most active and mobile form of SOM (Kalbitz et al., 2003). DOM is defined as SOM with size limit of 0.45μm (Christensen, 2001). According to Lützow et al. (2007), DOM can be obtained by using a number of extracts ranging from cold water to aqueous solutions of different ionic strength to simulate the soil solution. Kalbitz (2003) found that extracts from agricultural fields and forests differ in DOC content and quality. Agricultural fields DOC was faster degradable as indicated by high experimental OC mineralization values compared to DOM extracted from forest floors. They concluded that possibly this reflects a quicker turnover of DOM in agricultural soils than in forest soils Mineral-bound OM Mineral-bound OM can be defined as all OM that is adsorbed to minerals or entrapped in small micro aggregates (Chenu et al., 2006). Mineral binding of OM is a medium of OC stabilization and is a main aspect in soil functioning (Mikutta et al., 2010). Generally, organic molecules that form strong chemical bonds with mineral surfaces become less prone to desorption or 17

27 mineralization and become relatively resistant to microbial mineralization (Watanabe et al., 2005). OM-mineral bonds occur through hydrogen bonding, anion and cation exchange, ligand exchange and van der Waals bonding (Feng et al., 2005). The relative amount of SOC associated to the mineral-bound OM fraction is highly variable and ranges from less than 1 to more than 100 g OC kg 1 soil. This wide distribution can partly be due to the (1) techniques used for dispersing aggregates, and (2) differences in density cut-offs ( g cm 3 ) used for isolating clay sized fraction (Kögel-Knabner et al., 2008). Apart from these methodological issues, variations in percentage of OC in the mineral bound OM fraction can be mainly due to variations in amount and type of minerals. For instance, soil richer in short-range ordered Fe-oxyhydroxides and Al-silicates show larger concentrations in mineralbound OC fractions than those soils richer in poorly reactive clay minerals. Short-range ordered Al and Fe sesquioxides contents appear to be strongly correlated with OC stabilization in many soil systems (Carter et al., 2003; Rasmussen et al., 2005). In addition, Fe(III) and Al form strong coordination complexes with humic substances and are probably the essential cations for bridging the negative charge of mineral and organic surfaces in well drained, neutral to acidic soils. 2.4 Organic matter stabilization Stabilization of OM is a processes or mechanisms that lead to prolonged turnover times of OM in soil as a result of protection OM from mineralization. Generally, stabilized OM is older than unstabilized OM. Turnover times of stabilized OM is not absolute values because they are dependent on environmental conditions and soil properties of the area (e.g. climate, soil mineralogy) (Lützow et al., 2006). Selective enrichment occurs when specific organic molecules resist degradation by micro organisms. Chemical stabilization occurs when the interactions of organic substances and inorganic substances lead to a decrease in availability of the organic substrate to microorganism due to surface condensation. Physical stabilization occurred when accessibility of the organic substrates to microorganism is reduced due to occlusion within aggregates, commonly observed in clayey soil for OM which are within the soil aggregate. 18

28 2.4.1 Stabilization of OM by interaction with mineral surfaces and metal ions OM can attain stability when bound to mineral surfaces. Kalbitz et al. (2005) reported that soil OM in fine silt and clay fractions were observed to older and have mineralization rate by having long turnover time than OM in other soil OM fractions. Sorption of soluble OM to subsoil material observed to reduce OM mineralization by 20 30% compared to OM in soil solution (Chenu, 2002). Ligand exchange: Anion exchange with OH groups on mineral surfaces, carboxyl groups or phenolic OH groups of the OM is one important mechanism for formation of strong organomineral complexes such as Fe O C bonds. It is observed that the complexation of OM on mineral surfaces due to ligand exchange is prominent at lower ph, with a peak at ph range of , which is a pka values ranges most carboxylic acids and organic carboxyl (Lützow et al., 2006). DOM with higher content of carboxylic-c and aromatic-c form strong complexes with sesquioxides via ligand exchange in acidic soils (Lützow et al., 2006). Polyvalent cation bridges: metal cations such as Ca 2+, Fe 3+ and Al 3+ in the soil bind with anions such as COO - and maintain the neutrality at the surface by neutralizing the charge both on the negatively charged soil colloids and the acidic OM functional group, acting as a bridge. The cations involved in cation bridges are determined by soil ph. In neutral to alkaline soil Ca 2+ and Mg 2+ are dominant while Fe 3+ and Al 3+ are dominant in acidic soils (Lützow et al.,2006) Sorption of organic matter in soil Sorption of DOM compounds such as low molecular weight organic acids (LOAs) to soils and metal oxides follows principles of inorganic anion sorption, and occurs primarily via ligand exchange and is closely linked with sesquioxides (Guppy et al., 2005). The preferred sorption sites for OM are mouths of micropores which result in very strong bonding and multiple ligand attachment, edges of minerals, rough surfaces and micropores and the first molecules of OM to sorb are best stabilized (Kaiser et al., 2003). There is a preferential sorption of hydrophobic OM (OM with aromatic rings and aliphatic C chains), compared to the hydrophilic OM (OM rich in carbohydrates) (Kaiser et al., 1996). Sometime competition exists between Hydrophilic and hydrophobic OM especially in the soil with few binding sites (Kaiser et al., 1996). The sorption 19

29 of DOM onto Fe 3+ and Al 3+ is determined by ph and other competing inorganic anions (i.e. SO 2-, o-p) (Gu et al., 1994). 2.5 Mechanisms of competition for mineral binding between OM and Phosphate There are many potential direct and indirect mechanisms which affect the sorption of P and OM on soil surfaces. Added OM decomposes and its products can adsorb to the binding sites of the mineral surface, resulting in reduced P sorption and hence increased P concentration in soil solution and available P for plant uptake (Kaiser et al., 2000; Guppy et al., 2005; De Bolle, 2013). As SOM and o-p in the soil both are negatively charged anions they bind on the same binding sites in the soil surfaces. Addition of organic supplements to the soil has been observed to increase P availability in P-fixing soils and reduce the o-p sorption (Bhatti et al., 1998; Guppy et al., 2005). The adsorption of organic functional groups onto iron oxides or other metal oxide can (i) promote anion adsorption via cation bridges (Al 3+ and Fe 3+ ), (ii) increase Specific Surface Area (SSA) by inhibiting mineral crystallization of Fe 3+ and Al 3+, (iii) alter surface charges, (iv) increase competition with other anions for adsorption sites and (v) increase desorption of adsorbed P (Guppy et al., 2005; Fink et al., 2016). 20

30 2.5.1 Direct competitive Mechanisms of OM on P sorption OM directly affects P sorption in the soil through different mechanisms, as summarized in Fig. 5. Metal complexation by Fe and Al oxides: OM reacts with metal oxides of Fe 3+ and Al 3+, and forms complexes with surface metals and releases these metals into soil solution. The resulting in a decrease in number of sorption sites due to removal of metal oxides in soil surfaces (Fig.5). The reduction of P binding sites results in reduced P sorption (Hunt et al., 2007; Fink et al., 2016). Reduction of Specific Surface Area (SSA): OM in the soil act as a binding agent, it binds the small soil particles to a large aggregate to produce good soil structure. Kaiser et al., (2003) reported that the SSA of the mineral soils was positively related to the content of Fe oxyhydroxides and negatively related to OM content, meaning that an increase of OM in the soil resulted in a decrease of SSA and consequently a decrease in number of potential P sorption sites on the mineral soil surfaces. This effect of OM on reduction of SSA is greater at larger OM loadings like in grassland (Kaiser et al., 2003). Competition for adsorption sites: Dissolved organic carbon (DOC) compounds such as humic (HA) and fulvic acids (FA) which released upon decomposition of OM, they compete with added P for binding sites, since both are negatively charged (Hunt et al., 2007). According to Yu et al. (2013) on his research of effects of OM application on o-p adsorption reported that decomposition products from manure such as humic acids and citrate were observed to have greater affinity for Al oxides which is important sorbent for o-p. Hence, OM addition will induce competitive effect to binding sites between decomposition products and o-p on the soil surfaces, resulting in reduced P sorption. But its effect is small in soil with higher content of Al and Fe such as Ferralsols. Since the DOC bind in the same binding sites of the soil complexes, the added o-p find the binding sites are already occupied, or if both are added at the same time the DOC is likely to bind than o- P (Yu et al., 2013). 21

31 Alteration of surface charge: Sorption of OM compounds to the soil mineral surface increases the negative charge or decreases the point of zero charge (PZC), thus inhibiting P sorption due to increased repulsion for incoming negatively charged anions such as phosphate (Guppy et al., 2005). It is observed that sorption of OM can alter the surface charge of iron oxides and to cause phosphates to be electrostatically repelled (Antelo et al., 2007). Fig. 5. Direct effects of dissolved organic carbon (DOC) compounds on phosphorus (P) availability. M: metal (i.e. Fe or Al), LOA: low molecular weight organic acid (Guppy et al., 2005). 22

32 2.5.2 Indirect mechanism OM indirect can influence the P sorption in different ways as follows; Added OM can improve physical soil properties such as water holding capacity, soil ph, aeration and other soil properties which can promote plant growth and increased utilization of soil P reserves. Resulting in shifting of P equilibrium between P pools in the soil encouraging more P desorption from soil surfaces (Fink et al., 2016). Added OM may promote microbial activity in the soil, resulting in more microbial P immobilization, resulting in increased organic P and reduced adsorbed inorganic P concentrations (Guppy et al., 2005). OM addition can increase in soil ph in short-term, thereby decreasing the availability of metal cations such as Fe 3+ and Al 3+ by affecting their solubility, resulting in decreased P sorption (Borggaard et al., 2005). Fig. 6. Direct and indirect effects of organic matter addition on phosphorus (P) phytoavailability. DOC, Dissolved organic carbon; LOA low molecular weight organic acids; Pi, inorganic P; Po, organic P; MBP, Microbial biomass P (Guppy et al., 2005). 23

33 2.6 Positive effects of OM on sorption of o-p in sandy soils The interactions between OM and pedogenic Fe and Al- can inhibit the crystallization of Aluminum and iron oxides resulting in increased surface area, thereby increased o-p adsorption capacity (Borggaard et al., 2005). Metal bridging; OM can adsorb to positively charged sites on the soil surface. Metal cations such as Al 3+, Fe 3+, Ca 2+ bind themselves to the adsorbed OM creating additional positive sites, which allow further o-p adsorption (Fig. 4) (Sato, 2003; Yaghi et al., 2013). 24

34 CHAPTER 3 MATERIALS AND METHODS 3.1 Study area and Soil sample collection Soil samples covering ranges in OC level and PSD were selected from the soil archive of Department of Soil Management of Ghent University. These soils were sampled previously in the period across the sandy region of Flanders, as part of a large soil survey to map P saturation in sandy arable land in Flanders. (Baert et al., 1997). Fifty soil 0-30cm depth samples were selected covering three distinct ranges PSD, i.e. PSD 10-25%, 30-40% and 55-65%. All further analyses were completed with three replications. Since each series had a restricted range of PSD, the P already sorbed to the soil is comparable for all samples within a PSD-series. All samples originate from acid sandy soils, thus ph and clay content will also not be of major influence on the ability for P sorption between samples. Additionally, an attempt was made to keep the PSC (capacity of soil to bind P) relatively constant within one series. We specifically assured that there was no co-variation of C-content, PSD and PSC (Fig.7) This was to assure that within each PSD-range, the soil C-content was a main variable factor and so comparison of sorption characteristics of the soils within a single series allows us to detect a hypothesized mediation of soil C level on P sorption (hypothesis 1). The three different PSD-series were included to compare the sorption characteristics of soils with similar C-content but different PSD%. This is required to investigate the hypothesized increase in competition between P and OM at higher PSD (hypothesis 2). 25