Influence of low-molecular-weight organic acids on the solubilization of phosphates

A range of low-molecular-weight organic acids were identified in rhizosphere soil, leaf litter, and poultry manure compost. Laboratory and greenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated with monocalcium phosphate and North Carolina phosphate rock. Acetic, formic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic), and citric (tricarboxylic) acids were used in the study. The addition of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid>dicarboxylic acid>monocarboxylic acid. The decreases in P adsorption with organic acid addition increased with an increase in the stability constant of the organic acid for Al (logK _Al). Organic acids extracted greater amounts of P from soils meubated with both monocalcium phosphate and phosphate rock than water did. Although more phosphate was extracted by the organic acids from monocalcium phosphate — than from phosphate rock — treated soils in absolute terms, when the results were expressed as a percentage of dissolved phosphate there was little difference between the two fertilizers. The amount of P extracted by the organic acids from both fertilizers increased with an increase in logK _Al values. The addition of oxalic and citric acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalcium phosphate. The results indicated that organic acids increase the availability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.     

茶多酚与低分子有机酸活化土壤矿质元素的差异研究

通过浸提试验比较了浓度均为5 mmol L-1的茶多酚(TPs)和低分子有机酸(LMWOAs)对3种酸性土壤矿质元素活化的差异。结果表明:对于Si、Al、Fe,其活化量在漂洗水稻土和黄壤上为LMWOAsTPs,在酸性紫色土上则为TPs苹果酸(Si的活化量为TPsLMWOAs),表明TPs在初育土脱硅过程中发挥着重要作用,预示植茶有利于初育土向地带性土壤演化;对于盐基离子,3种土壤Ca的活化量均为苹果酸TPs,Mg、K的活化量均为LMWOAsTPs,Na的活化量均为茶叶提取物柠檬酸苹果酸(在漂洗水稻土上为TPsLMWOAs);TPs对Ca、K、Na的活化量均以漂洗水稻土最大,表明植茶可显著促进漂洗水稻土盐基离子的活化与迁移,在提高其有效性的同时可能加速土壤酸化。     

低分子量有机酸对土壤磷活化影响的研究

研究两种低分子量有机酸（柠檬酸和苹果酸）对土壤磷活化影响,并用修正的Hedley法测定土壤磷活化前后磷组分的变化。结果表明,低分子量有机酸能持续活化土壤磷,活化强度随低分子量有机酸浓度的增大而增强,并且柠檬酸活化土壤磷的能力强于苹果酸。低分子量有机酸能促进作物有效态无机磷组分（H2O-P和NaHCO3-Pi）的释放;同时还促进有机磷组分（NaHCO3-Po和NaOH-Po）的矿化。在低分子量有机酸浓度达到0.5 mmol/L以上时,其对土壤磷组分的活化量的顺序为:NaOH-Pi HCl-P NaHCO3-Pi H2O-P,即铁铝结合态磷 钙结合态磷 作物有效态磷。低分子量有机酸活化土壤磷的过程中伴有大量铁、铝释放,且铁或铝的释放量与磷活化量之间显著正相关（P0.05）。说明铁、铝结合态磷是低分子量有机酸活化土壤磷的主要磷源,并且其活化机制可能与铁、铝结合态磷的螯合溶解有关。     

Impact of low-molecular-weight organic acids on the availability of phenanthrene and pyrene in soil

The impact of low-molecular-weight organic acids (LMWOAs) on the availability of phenanthrene and pyrene was investigated using laboratory batch assays. Experiments were conducted with two types of soil with different organic contents. The LMWOAs used were citric and oxalic acid. A mild solvent extraction procedure and a sorption-desorption experiment were used to predict the availability of phenanthrene and pyrene. Results showed that the extractable amounts of phenanthrene and pyrene in both soil types increased with increased citric or oxalic acid concentration. Citric acid addition promoted phenanthrene and pyrene extraction to a greater degree than oxalic acid. Compared with freshly spiked soils, the extractable amounts of phenanthrene and pyrene with the addition of LMWOAs decreased significantly after 60 days of cultivation. Soils with higher values of soil organic carbon content (f_oc) showed decreased phenanthrene and pyrene availability after the addition of LMWOAs. The sorption and desorption results also demonstrated the enhanced availability of PAHs with LMWOA addition. Phenanthrene sorption could be described using a linear model, regardless of the addition of LMWOAs. The simulated distribution constants (K_d) for phenanthrene sorption decreased significantly with the addition of LMWOAs. In contrast, phenanthrene desorption clearly increased with the addition of LMWOAs. These results suggest that the availabilities of phenanthrene and pyrene can be increased with the addition of suitable LMWOAs.     

低分子量有机酸对紫色母岩中钾释放的影响

采用4种浓度为0.05mol/L的低分子量有机酸溶液分别浸提4种紫色母岩风化碎屑和2种紫色土3000h研究其钾的释放规律。结果表明,与纯水浸提相比,0.05mol/L的草酸、酒石酸、柠檬酸以及苹果酸4种低分子量有机酸溶液都能明显促进6种供试样中钾的释放,释放量提高0.3～15.2倍。4种有机酸活化6种供试样中钾的能力以草酸柠檬酸酒石酸苹果酸。侏罗纪沙溪庙组紫色泥岩、沙岩和侏罗纪遂宁组紫色泥岩以及灰棕紫泥4种土壤(或岩样)中钾的释放过程在3000h内用Elovich、双常数、抛物线、一级扩散和双曲线方程等5种动力学方程拟合,结果除一级扩散方程较差外,其它4种方程拟合的相关系数r=0.5988～0.9966(r0.01=0.6055),以双常数方程拟合效果最佳。拟合方程中常数b值可表征钾的释放速度。     

Water-soluble low-molecular-weight organic acids in automorphic loamy soils of the tundra and taiga zones

The formation features of water-soluble low-molecular-weight organic acids (LMWOAs) in a zonal series of automorphic soils on loose silicate rocks from the middle taiga to the southern tundra (typical podzolic, gley-podzolic, and surface-gley tundra soils) were first revealed by gas chromatography mass spectrometry and gas-liquid chromatography. The content of LMWOAs varies within the range of 1–14 mg/dm³, which corresponds to 1–5% of the total carbon of the water-soluble soil organic matter. It has been shown that a subzonal feature of gley-podzolic soils in the northern taiga is the high content of LMWOAs, including primarily the strongest aliphatic hydroxyl acids. Possible mechanisms of their formation and accumulation in soils have been considered.     

Effects of organic manure and NPK fertilization on earthworm activity in an Oxisol

The effects of fertilization with N, P, K, and organic manure (alone or in combination) on earthworm populations, biomass, and casting activity were measured in a cultivated soil (organic C 1.5%, annual rainfall 2000–2300 mm). These applications of fertilizer caused significant increases in earthworm numbers, biomass, and casts. N alone or in combination with P and K also influenced these earthworm parameters significantly. The inorganic NPK fertilizer in combination with organic manure had a significantly greater effect on earthworm activities than NPK fertilizer alone, and therefore the addition of organic matter appears advisable in order to obtain maximum benefits from NPK fertilizer in this soil.     

The effect of low-molecular-weight organic acids and inorganic phosphorus concentration on the determination of soil phosphorus by the molybdenum blue reaction

Low-molecular-weight organic acids play an important role in the mobilization of soil phosphorus (P). The molybdate blue colorimetric method, based on the formation of P molybdenum blue compound, is commonly used for analysis of phosphate in soil and environmental samples. However, some organic acids can act as a ligand to bond with molybdenum acid, which subsequently interfere with the colorimetric reaction. The recoveries of P were inhibited by the addition of oxalic (>2 mM) and citric acids (>3 mM) both in standard P solutions and soil extracts, but formic and maleic acids did not interfere with the P determination. The inhibition of oxalic and citric acids on P recovery remained even at higher level of P (up to 100 μg P 100 ml⁻¹) though such interferences decreased by increasing P concentration. Comparison between oxalic and citric acids revealed the more pronounced interference by the addition of oxalic acid. The results suggested that the interference of organic acids with P determination is related to the types of organic acids and the ratio of organic acid ligands to P anions in the solutions. Thus, analysis of P using the molybdate blue colorimetric method should be undertaken cautiously in the presence of relative strong ligands like oxalic and citric acids.     

低分子量有机酸类物质对红壤和黑土磷有效性的影响

在20℃恒温培养条件下,研究低分子量有机酸类的钠钙盐(LA)和低分子量有机酸类与混合氨基酸的钠钙盐(LAA)不同用量水平对红壤和黑土磷有效性的影响。研究表明,两种低分子量有机酸类混合物在各不同水平下均能显著提高红壤和黑土磷有效性;随着培养的进行,添加有LA、LAA的各处理对红壤和黑土中磷有效性均有明显提高。无论是LA或LAA,红壤中以半量处理对提高磷有效性作用效果更为明显,而黑土中则以LAA作用效果更好,且以常量LAA处理的效果最好,常量LA和LAA处理间差异显著。在添加磷肥的基础上,与不加有机酸物质处理(CKp)相比,添加常量LA、LAA和半量LA(0.5 LA)、LAA(0.5 LAA)处理,在红壤中磷固定率平均值分别降低10.5%、22.3%和11.3%、19.4%;在黑土中分别降低8.6%、10.6%和14.1%、11.8%。LA和LAA对降低红壤和黑土磷固定率效果显著,红壤以半量处理的效果较好,黑土则以常量LAA处理效果最好。     

Adsorption of trichlorphon on phyllosilicate minerals: Effect of low-molecular-weight organic acids

The use of trichlorphon in large quantities causes a large number of organic pollutants to enter water, sediments, and soils. Phyllosilicate minerals are considered effective adsorbents for organic pollutants. However, the adsorption behavior of organic pollutants on soil minerals affected by low-molecular-weight organic acids(LMWOAs) is not fully understood. In this study, the effect of LMWOAs on the adsorption behavior of trichlorphon on phyllosilicate minerals was investigated using a combina...     

Sulfate adsorption by variable charge soils: Effect of low-molecular-weight organic acids

Sulfate (SO₄ ^2–) movement and transport in soils has received considerable attention in recent years. In most soils, SO₄ ^2– coexists with a variety of natural organic compounds, especially organic acids. Studies were conducted to assess the effect of low-molecular-weight organic acids (eight aliphatic and five aromatic acids) on SO₄ ^2– adsorption by variable charge soils from Chile and Costa Rica. The effects of type of organic acid, pH, type of soil, and organic acid concentration were investigated. In one experiment, a 1.0 g soil sample was equilibrated with 25 ml 0, 0.5, 1.0, 2.0, 4.0, or 6.0 mM K₂SO₄ in 1 mM NaCl in the presence or absence of 5 mM citric acid. In the second set of experiments, the adsorption of 2 mM SO₄ ^2– in soils at pH 4 or pH 5 in the presence or absence of one of 13 organic acids at a concentration of 2 mM or 5 mM was studied. Results showed that citric acid significantly decreased SO₄ ^2– adsorption by the two soils. Sulfate adsorption decreased with increasing pH of the equilibrium solution. Aliphatic acids, with the exception of cis-aconitic acid, decreased the amount of SO₄ ^2– adsorbed by the two soils, with oxalic, tartaric, and citric acid showing the greatest effect. The differences in pH values of the equilibrium solutions in the presence and absence of organic acids were significantly, but negatively, correlated with the amount of SO₄ ^2– adsorbed, suggesting chemisorption of SO₄ ^2– and the release of hydroxide ions. The ionization fraction values of the organic acids at the equilibrium pH were correlated with the amounts of SO₄ ^2– adsorbed, suggesting that the protonation of surface hydroxyl groups of the mineral phase increased as the strength of the ionization of the acid increased, thus creating more positively charged surfaces. Received: 12 February 1997     

低分子量有机酸作用下土壤中菲和芘的残留及形态

采用微宇宙试验方法,以菲和芘为多环芳烃(PAHs)代表物,研究了几种低分子量有机酸作用下黄棕壤中菲和芘的残留和形态。结果表明,老化60 d后土壤中菲和芘的残留含量明显减少;不施加有机酸的对照土壤中,菲和芘的残留含量为10.13和29.18 mg kg-1,去除率为87.33%和63.50%。与对照相比,供试浓度(0～64 mg kg-1)范围内,柠檬酸、草酸、酒石酸等3种低分子量有机酸作用下土壤中PAHs残留含量提高,去除率减小,表明供试条件下有机酸抑制土壤中菲和芘的降解;进一步分析发现,少量(≤4 mg kg-1)的有机酸即可对PAHs降解产生高的抑制效果。微生物降解在PAHs的去除中起重要作用,且芘比菲更抗微生物降解。供试条件下,可脱附态和有机溶剂提取态是土壤中菲和芘存在的主要形态,而结合态残留占总残留的比例很小(<8.5%)。3种有机酸均提高了土壤中可脱附态和有机溶剂提取态菲和芘的残留含量,施加有机酸使土壤中菲和芘的可脱附态含量较对照分别提高了46.67%～749.1%和1.83%～80.20%,有机溶剂提取态则提高了8.73%～375.2%和22.63%～114.3%;低分子量有机酸作用下结合态的菲和芘含量仍很小。     

低分子量有机酸对促进可变电荷土壤中铝溶解的影响

Low-molecular-weight （LMW） organic acids exist widely in soils and play an important role in soil processes such as mineral weathering, nutrient mobilization and A1 detoxification. In this research, a batch experiment was conducted to examine the effects of LMW organic acids on dissolution of aluminum in two variably charged soils, an Ultisol and an Oxisol. The results showed that the LMW organic acids enhanced the dissolution of A1 in the two investigated soils in the following order： citric 〉 oxalic 〉 malonic 〉 malic 〉 tartaric 〉 salicylic 〉 lactic 〉 maleic. This was generally in agreement with the magnitude of the stability constants for the Al-organic complexes. The effects of LMW organic acids on Al dissolution were greater in the Ultisol than in the Oxisol as compared to their controls. Also, the accelerating effects of citric and oxalic acids on dissolution of A1 increased with an increase in pH, while the effects of lactic and salicylic acids decreased. Additionally, when the organic acid concentration was less than 0.2 mmol L-I, the dissolution of A1 changed Iittle with increase in acid concentration. However, when the organic acid concentration was greater than 0.2 mmol L^-1,the dissolution of A1 increased with increase in acid concentration. In addition to the acid first dissociation constant and stability constant of Al-organic complexes, the promoting effects of LMW organic acids on dissolution of A1 were also related to their sorption-desorption equilibrium in the soils.     

Comparison of the influence of tannic acid and selected low-molecular-weight organic acids on precipitation products of aluminum

The crystallization and surface properties of hydrolytic reaction products of Al precipitated in the presence of 1.0 · 10^?6M and 1.0 · 10^?4M tannic acid from systems at the initial Al concentrations of 1.1 · 10^?4M and 1.1 · 10^?3M and OH/Al molar ratios of 2.0 or 3.0 were examined.X-ray diffraction analyses show that the structural distortion within the hydrolytic precipitation products of Al increases with the molar ratio of tannic acid to Al. The non-crystalline to poorly ordered hydrolytic precipitation products of Al, including pseudoboehmite, are formed in the presence of tannic acid and are shown by transmission electron micrographs to be fine, shapeless and hollow colloids which are deformed and aggregated. In contrast to low molecular weight organic acids (p-hydroxybenzoic, aspartic, malic, and citric acids), the influence of tannic acid on the surface reactivities of the precipitation products of Al cannot be solely explained on the basis of its chemical affinity towards Al. In addition to complexation, the physical adsorption of the tannic acid initially present at 1.0 · 10^?6M apparently promotes structural distortion in the precipitation products and thereby increases the specific surface and the formation of active sites for surface charge development and for phosphate and Ca retention to an extent even greater than that observed with the more strongly complexing malic acid. On the other hand, when the concentration of tannic acid is increased to 1.0 · 10^?4M, the physical adsorption of the tannic acid, while enhancing Ca retention through its ability to ionize and provide negative charges, hampers phosphate retention by masking some of the active sites.     

Effect of low-molecular-weight organic acids on Cl- adsorption by variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes.The objective of the present paper was to evaluate effect of two LMW organic acids, citric acid and oxalic acid, on Cl^- adsorption by three variable charge soils, a latosol, a lateritic red soil and a red soil, using a batch method. The results showed that the presence of citric acid and oxalic acid led to a decrease in Cl- adsorption with larger decreases for citric acid. Among the different soils Cl- adsorption in the lateritic red soil and the red soil was more affected by both the LMW organic acids than that in the latosol.     

低分子量有机酸对可变电荷土壤铝活化动力学的影响

从动力学角度研究了几种低分子量有机酸对2种酸性土壤中铝的活化和活化铝在土壤固/液相之间分配的影响。结果表明:对于络合能力弱的醋酸和乳酸,主要通过质子作用活化铝,且活化作用明显小于盐酸。而络合能力较强的苹果酸、草酸和柠檬酸,主要通过络合作用促进铝的释放,且这种作用随有机酸根阴离子络合能力的增强而增加。在氧化铁含量较高的砖红壤中,苹果酸、草酸和柠檬酸通过专性吸附增加土壤表面负电荷,从而增加土壤交换态铝;但在氧化铁含量较低的红壤中,草酸和柠檬酸主要通过形成可溶性铝络合物降低交换态铝。活化铝在土壤固/液相间的分配主要决定于溶液中有机阴离子与土壤固相表面对铝离子的竞争。醋酸和乳酸活化的铝主要以交换态铝存在;而草酸和柠檬酸活化的铝主要以有机酸-铝络合物存在于溶液中,特别是在氧化铁低的红壤中,这将促进铝在土壤-水体中的迁移。     

Subsoil retention of organic and inorganic nitrogen in a Brazilian savanna Oxisol

Abstract. Nitrogen (N) loss by leaching poses great challenges for N availability to crops as well as nitrate pollution of groundwater. Few studies address this issue with respect to the role of the subsoil in the deep and highly weathered savanna soils of the tropics, which exhibit different adsorption and drainage patterns to soils in temperate environments. In an Anionic Acrustox of the Brazilian savanna, the Cerrado, dynamics and budgets of applied N were studied in organic and inorganic soil pools of two maize (Zea mays L.) – soybean (Glycine max (L.) Merr.) rotations using ¹⁵N tracing. Labelled ammonium sulphate was applied at 10 kg N ha^?1 (with 10 atom%¹⁵N excess) to both maize and soybean at the beginning of the cropping season. Amounts and isotopic composition of N were determined in above‐ground biomass, soil, adsorbed mineral N, and in soil solution at 0.15, 0.3, 0.8, 1.2 and 2 m depths using suction lysimeters throughout one cropping season. The applied ammonium was rapidly nitrified or immobilized in soil organic matter, and recovery of applied ammonium in soil 2 weeks after application was negligible. Large amounts of nitrate were adsorbed in the subsoil (150–300 kg NO₃^?‐N ha^?1 per 2 m) matching total N uptake by the crops (130–400 kg N ha^?1). Throughout one cropping season, more applied N (49–77%; determined by ¹⁵N tracers) was immobilized in soil organic matter than was present as adsorbed nitrate (2–3%). Most of the applied N (71–96% of ¹⁵N recovery) was found in the subsoil at 0.15–2 m depth. This coincided with an increase with depth of dissolved organic N as a proportion of total dissolved N (39–63%). Hydrophilic organic N was the dominant fraction of dissolved organic N and was, together with nitrate, the most important carrier for applied N. Most of this N (>80%) was leached from the topsoil (0–0.15 m) during the first 30 days after application. Subsoil N retention as both adsorbed inorganic N, and especially soil organic N, was found to be of great importance in determining N losses, soil N depletion and the potential of nitrate contamination of groundwater.     

Relative importance of protons and solution calcium concentration in phosphate rock dissolution by organic acids

A series of experiments was conducted to quantify the relative contribution of protons and other mechanisms to the dissolution of phosphate rocks (PRs) from six countries in solutions of low-molecular-weight-aliphatic organic acids. The amounts of P and Ca released after 3 d of incubation at 28°C were determined in all the experiments. In the first experiment the solubility of the PRs < 500 μm particle size) in 100 M (25 mL g^-1 PR) oxalic, tartaric, and citric acids was compared with that in three mineral acids and four chelating compounds. There were no differences in the amount of P released by the mineral acids, but the organic acids released more P than could be accounted for by protonation. The chelating compounds were the least effective. In the second experiment, 1 g each of Sri Lanka and Togo PRs was incubated with 2.5 mmol of the organic acids using three acid concentration (mM) : acid volume ratios (250:10, 100:25, and 25:100). The amount of P dissolved from the PRs and the amount of acid remaining in the filtrate were about the same for all three treatments. A mixture of the organic acids and the respective Na-salts was prepared at six different acid: salt ratios in the third experiment. Total acid concentration in all the solutions was 100 mM. Higher proportion of the Na-salt in the mixture decreased PR dissolution, while the amount of the acid remaining in the filtrate increased. The Ca concentration in the filtrate showed an opposite trend. The results indicated that PR dissolution by the organic acids was initially dependent on protonation, which accounted for only 13–38% of the amount of P dissolved. Most of the amount of P dissolved could therefore be attributed to other mechanisms. Removal of dissolved Ca from the solution appears to be the major factor controlling the subsequent dissolution of the PRs. The ability of tartaric and oxalic acids to dissolve effectively the PRs was attributed to the formation of an insoluble calcium compound which was precipitated from the solution. For the use of organic acids as a potential amendment for improving the P availability of PRs, the Ca binding power of the organic acids should also be considered.     

Relationship between lead uptake by lettuce and water-soluble low-molecular-weight organic acids in rhizosphere as influenced by transpiration

The relationship between Pb uptake by leaf lettuce ( Lactuca sativa L.) and water-soluble low-molecular-weight organic acids (LMWOAs) in rhizosphere, as influenced by transpiration (high and low), has been studied. Studies were carried out by culturing lettuce plants grown for 2 weeks in pots filled with quartz sand mixed with anion-exchange resin and then for 30 days in a greenhouse. The potted lettuce plants were subjected to stress by the addition of Pb(NO 3) 2 solutions (100, 200, and 300 mg of Pb L (-1)) and by high and low transpiration treatments for another 10-day period. Blank experiments (without addition of Pb(NO 3) 2 solutions to the pots) were also run. There were no significant differences in the growth of the plants with the addition of Pb(NO 3) 2 solutions in either of the transpirations studies. Uptake of Pb by the shoots and roots of the plants was found to be proportional to the concentration of Pb solutions added, and more accumulation was observed in the roots than in the shoots at the end of days 3 and 10. High transpiration caused more Pb uptake than did low transpiration. One volatile acid (propionic acid) and nine nonvolatile acids (lactic, glycolic, oxalic, succinic, fumaric, oxalacetic, d-tartaric, trans-aconitic, and citric acids) in rhizosphere quartz sand or anion-exchange resin were identified and quantified by gas chromatography analysis with a flame ionization detector. The amount of LMWOAs in rhizosphere quartz sand or anion-exchange resin increased with higher amounts of Pb in quartz sand solution and also with longer duration of the study. The total quantities of the LMWOAs in the rhizosphere quartz sand or anion-exchange resin were significantly higher under high and low transpiration with a 300 mg of Pb L (-1) solution addition at the end of day 10. Compared with our previous related studies (published work), the present study shows that the presence of LMWOAs in rhizosphere does not significantly affect Pb uptake by lettuce plants under high and low transpiration. A physiological mechanism of the roots of lettuce plants governing the relationship between Pb contamination level and quantity of water-soluble LMWOAs in rhizosphere quartz sand and resin, as influenced by transpiration, was proposed.     

Assessing the organic phosphorus status of an Oxisol under tropical pastures following native savanna using 31P NMR spectroscopy

³¹P nuclear magnetic resonance (NMR) spectroscopy, P fractionation, and a P sorption experiment were used to follow the changes in P in the A horizons (0–10 cm) of acid savanna soils, Colombia, after little P fertilization and 15 years' continuous growth of a grass (Brachiaria decumbens) and a grass/legume (B. decumbens+Pueraria phaseoloides) pasture. Ready P supply as analyzed by Bray P was low under native savanna (1.3 mg kg^-1 soil) and responded moderately on pasture establishment. Concurrently, the affinity of the soil for inorganic P declined slightly after pasture establishment. ³¹P NMR spectroscopy revealed that P associated with humic acids was dominated by monoester P followed by diester P. Smaller proportions were observed for phosphonates, teichoic acid P, orthophosphate, and pyrophosphate. P associated with fulvic acids had lower proportions of diester P and higher contents of orthophosphate. Under native savanna the reserves of labile organic P species (phosphonates and diester P including teichoic acid P) associated with humic and fulvic acids were 12.4 and 1.1 kg ha^-1, respectively, and increased to 18.1 and 1.8 kg ha^-1 under grass pasture, and to 19.5 and 2.3 kg ha^-1 under grass/legume pasture. These data emphasize the importance of labile organic P species in the P supply for plants in improved tropical pastures, and further indicate that humic acid P in particular responds to land-use changes within a relatively short time-scale. Earthworm casts were highly abundant in the B. decumbens+P. phaseoloides plot and were enriched in labile organic P species. We conclude that earthworm activity improves the P supply in soil under tropical pastures by creating an easily available organic P pool.Dedicated to Professor J.C.G. Ottow on the occasion of his 60th birthday