organo-Al Complexes Research Articles

Protective role of reactive aluminum phases to stabilize soil organic matter against long-term cultivation in the humid tropics under volcanic influence

ABSTRACT Cultivation can cause rapid depletion of soil organic matter (SOM), particularly in the humid tropics. Understanding the factors controlling SOM storage is a key step toward effective soil management for sustainable crop production. While clay + silt content is known to be an important factor for soil organic carbon (SOC) storage, recent studies have shown greater importance of oxalate-extracted Al (Alo; roughly corresponds to short-range-order aluminosilicate minerals and organo-Al complexes) and Fe (Feo). However, the extent to which these mineralogical factors control the response of SOM pool to land-use changes remains unclear, especially in the tropics. This study aimed to clarify the factors regulating SOM content in croplands compared to secondary forests or home gardens in Negros Occidental, Philippines. Along two line transects having a wide range of Alo contents, soil samples were collected at a depth of 0 − 10 cm from sugarcane sites (n = 33; long-term continuous cultivation of >70 years) that were paired with either secondary forests (n = 10) or home gardens (n = 20). Sugarcane sites had significantly lower SOC and total nitrogen (N) contents than secondary forest and home garden sites, whereas there was no clear difference between the latter two land uses. Both SOC and total N contents showed significant positive correlations with Alo content and, to a much less extent, with Feo content, but not with clay + silt content. The slope of the regression line between SOC and Alo was indifferent between sugarcane and the other two land uses, while the intercept was significantly lower in the sugarcane sites. These results suggest the protective role of Alo phases in both agricultural and forest soils and the presence of unprotected SOM (e.g. plant litter) in the latter sites. Furthermore, δ13C analysis suggested that the proportion of forest (C3 plant) derived carbon remaining in the sugarcane soils ranged between 10% and 50% and increased with Alo content. Together, our study showed indirect evidence of Alo-induced stabilization of SOM against long-term cultivation under the humid tropical environment.

Organic–inorganic calcium lignosulfonate compounds for soil acidity amelioration

Soil acidification is a problem widely occurring worldwide, which severely threaten food security and agricultural sustainability. Calcium lignosulfonate (CaLS), a cheap and ecofriendly compound, is used for the first time to amend acid soil by utilizing its unique organic and inorganic functional moieties simultaneously. Both column leaching and incubation experiments were conducted to investigate the comparative effects of CaLS (four rates at 5, 10, 15, 20gkg-1) and compared with conventional amendments, including gypsum (5gkg-1), lignin (5gkg-1), L + G (each at 5gkg-1), and control. The soil pH, exchangeable acidity and base cations, organic carbon, and different Al fractions were determined to unravel the ameliorative performance and mechanism of the treatments. Regardless of application modes and dosages, the results demonstrated that CaLS incorporation significantly increased soil pH, exchangeable Ca2+, cation exchange capacity, and organic carbon and decreased the contents of exchangeable acidity, especially exchangeable Al3+. The ameliorative mechanism was that amendment material led to the displacement of H+ and Al3+ off soil colloids by Ca2+. These released H+ and Al3+ which complexed with lignosulfonate anions into soluble organo-Al were all quickly leached from the soil column. The CaLS addition enhanced the transformation of exchangeable Al3+ and low-to-medium organo-Al complexes into highly stable organically bound fractions and immobilized into the soil. The complexing of CaLS functional groups with Al3+ impeded Al3+ from undergoing hydrolysis to produce more H+. As an environmental-friendly material, CaLS can be a promising amendment for soil acidity and Al toxicity amelioration.

Distribution and chemical species of phosphorus across density fractions in Andisols of contrasting mineralogy

Strong control of iron (Fe) and aluminum (Al) phases on soil phosphorus (P) dynamics is well established. How organic and inorganic P forms are associated with Fe and Al phases remains poorly understood, and sequential density fractionation thus allows one to assess the soil continuum from organic-rich particles (< 1.8 g cm−3) to organo-mineral particles (1.8–2.25 g cm−3), and finally to mineral rich particles (> 2.25 g cm−3). We combined the density fractionation approach with wet extraction techniques, 31P NMR spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy to investigate the distribution and chemical species of P in relation to Fe and Al phases for two Andisols: an allophanic Andisol rich in short-range-order aluminosilicates (silandic) and a non-allophanic Andisol rich in organic matter (OM) and organo-aluminum complexes (aluandic). We fractionated the soils to the density classes of 1.6–1.8, 1.8–2.0, 2.0–2.25, 2.25–2.5, and > 2.5 g cm−3 using the sodium polytungstate solutions. Across five density fractions, inorganic and total P were distributed in low to meso-density fractions (1.6–2.25 g cm−3) where major portions of extractable Fe and Al were found. The major P pool was found in the NaOH extraction step for low- to meso-density ranges (< 2.25 g cm−3), suggesting that Al and Fe (oxyhydr)oxides and aluminosilicates have significant roles in P sorption in allophanic and non-allophanic Andisols. In contrast, the residual P remaining after the sequential extraction was predominant in heavier-density fractions (> 2.25 g cm−3). Molybdate-reactive and total P extracted by NaOH in the density fractions were positively correlated with oxalate extractable Al (Alox) in the allophanic Andisol (r > 0.91) and with pyrophosphate extractable Al (Alp) in the non-allophanic Andisol (r > 0.97), suggesting stronger control of Al phases to P over Fe phases. The non-destructive XANES analysis confirmed that P in both Andisols was associated primarily with Al, not Fe, and suggested the presence of P bound to organo-Al complexes in the non-allophanic Andisol (up to 76% of P in the 1.6–1.8 g cm−3 fraction where Alp is enriched). Solution 31P NMR spectra of NaOH-EDTA soil extracts showed that the primary organic P group was phosphomonoesters that decreased with increasing density in allophanic and non-allophanic Andisols. Myo-inositol hexakisphosphate (IHP) and scyllo-IHP were identified in a density fraction between 1.8 and 2.25 g cm−3 of non-allophanic Andisol. Our study highlights the importance of low- to meso-density fractions in organic and inorganic P reservoirs in Andisols.

Mineralogical associations with soil carbon in managed wetland soils.

Carbon (C)-rich wetland soils are often drained for agriculture due to their capacity to support high net primary productivity. Increased drainage is expected this century to meet the agricultural demands of a growing population. Wetland drainage can result in large soil C losses and the concentration of residual soil minerals such as iron (Fe) and aluminum (Al). In upland soils, reactive Fe and Al minerals can contribute to soil C accumulation through sorption to poorly crystalline minerals and coprecipitation of organo-metal complexes, as well as C loss via anaerobic respiration by Fe-reducing bacteria. The role of these minerals in soil C dynamics is often overlooked in managed wetland soils and may be particularly important in both drained and reflooded systems with elevated mineral concentrations. Reflooding drained soils have been proposed as a means to sequester C for climate change mitigation, yet little is known about how reactive Fe and Al minerals affect C cycling in restored wetlands. We explored the interactions among soil C and reactive Fe and Al minerals in drained and reflooded wetland soils. In reflooded soils, soil C was negatively associated with reactive Fe and reduced Fe(II), a proxy for anaerobic conditions (reactive Fe: R2 =.54-.79; Fe(II): R2 =.59-.89). In drained soils, organo-Al complexes were positively associated with soil C and Fe(II) (Al R2 =.91; Fe(II): R2 =.54-.60). Soil moisture, organo-Al, and reactive Fe explained most of the variation observed in soil C concentrations across all sites (p<.01). Reactive Fe was negatively correlated to soil C concentrations across sites, suggesting these Fe pools may drive additional C losses in drained soils and limit C sequestration with reflooding. In contrast, reactive organo-Al in drained soils facilitates C storage via aggregation and/or formation of anaerobic (micro)sites that protect residual soil C from oxidation and may at least partially offset C losses.

Influence of Pinus pinaster age on aluminium fractions in acidic soils

The influence of plantation age on the chemical properties of acidic soils was studied in 16 plots in adult &lt;em&gt;Pinus pinaster&lt;/em&gt; stands established in Galicia (NW Spain). The Al fractions in the soil solid phase and the total Al in soil solution were determined in the upper soil layer (0-20 cm) and the lower soil layer (20-40 cm) in each plot. The pH, total C and N, exchangeable Ca, Mg, Na, K, and Al and Al saturation (% Al) were determined in the solid fraction. Aluminium was extracted from the solid phase with the following solutions: ammonium oxalate (Al&lt;sub&gt;o&lt;/sub&gt;), sodium pyrophosphate (Al&lt;sub&gt;p&lt;/sub&gt;), copper chloride (Al&lt;sub&gt;cu&lt;/sub&gt;) and ammonium chloride (Al&lt;sub&gt;NH4&lt;/sub&gt;). The total Al in the liquid phase was also determined. All soil chemical parameters, except total N, C/N ratio and % Al, were significantly influenced by soil depth. The mean pH was lower in the upper than in the lower layer (4.57 vs. 4.97), but the opposite was observed for the organic C (77.2 vs. 50.4 g kg&lt;sup&gt;-1&lt;/sup&gt;), the effective cation exchange capacity (eCEC) (9.43 vs. 6.25 cmol&lt;sub&gt;(+)&lt;/sub&gt; kg&lt;sup&gt;-1&lt;/sup&gt;), P (8.95 vs. 4.65 mg kg&lt;sup&gt;-1&lt;/sup&gt;) and the exchangeable cations. Organic matter, total N and eCEC were significantly and positively correlated with plantation age (r = 0.69 in the upper layer and r = 0.82 in the lower layer, p &amp;lt; 0.01; r = 0.62, p &amp;lt; 0.05 in the upper layer and r = 0.78, p &amp;lt; 0.01 in the lower layer; r = 0.77, p &amp;lt; 0.01 in the upper layer and r = 0.85, p &amp;lt; 0.0001 in the lower layer, respectively), and pH&lt;sub&gt;KCl&lt;/sub&gt; was negatively correlated with plantation age (r = -0.55 in the upper soil layer and r = -0.61 in the lower soil layer, p &amp;lt; 0.05). The concentrations of the different Al forms in all soils decreased in the order Al&lt;sub&gt;p &lt;/sub&gt;&amp;gt; Al&lt;sub&gt;o &lt;/sub&gt;&amp;gt; Al&lt;sub&gt;cu &lt;/sub&gt;&amp;gt; Al&lt;sub&gt;NH4&lt;/sub&gt;. Highly stable organo-aluminium complexes (Al&lt;sub&gt;p-cu&lt;/sub&gt;) predominated over moderate and low stability complexes (Al&lt;sub&gt;cu&lt;/sub&gt;) in all soil plots. The highly stable organo-Al complexes were significantly more abundant in the lower layer, whereas the opposite was observed for the exchangeable Al and the total Al in soil solution. The concentrations of all Al forms (except Al&lt;sub&gt;p-cu&lt;/sub&gt;) were significantly and positively correlated with plantation age (Al&lt;sub&gt;o&lt;/sub&gt; r = 0.50, p &amp;lt; 0.05 for the upper layer and r = 0.67, p &amp;lt; 0.01 for the lower layer; Al&lt;sub&gt;p&lt;/sub&gt; r = 0.64, p &amp;lt; 0.01 for the lower layer; Al&lt;sub&gt;cu &lt;/sub&gt;r = 0.84 for the upper layer and r = 0.83 for the lower layer, p &amp;lt; 0.0001; Al&lt;sub&gt;cu-NH4&lt;/sub&gt; r = 0.65 for the upper layer and r = 0.78 for the lower layer, p &amp;lt; 0.01; Al&lt;sub&gt;NH4&lt;/sub&gt; r = 0.76, p &amp;lt; 0.01 for the upper layer and r = 0.84, p &amp;lt; 0.0001 for the lower layer; total Al in soil solution r = 0.61 for the upper layer and r = 0.60 for the lower layer, p &amp;lt; 0.05). Stepwise linear regression analysis showed that plantation age, pH and total C explained between 67% and 93% of the variance in the Al forms. In all regression models, plantation age was a significant predictor variable for the different Al fractions, except total soluble Al, which is an important variable to consider in the study of chemical properties in forest soils.

A farm-scale investigation of the organic matter composition and soil chemistry of Andisols as influenced by land use and management

Andisols are characterised by having abundant reactive Al in the form of short-range ordered (SRO) Al constituents and organo-Al complexes, which facilitates the accumulation of soil organic matter (OM). However, recent studies of New Zealand pastoral systems have reported loss of carbon (C) from Andisols when under intense management. This study compares the organic and inorganic chemistry of Andisols on two adjacent pasture sites under different pastoral management regimes (Paddock 2 being more intensively managed than Paddock 1), as well as under a nearby pine stand (Forest). Mean soil pH-H2O in Forest (5.3) was significantly lower (P < 0.05) than that in Paddock 1 (5.7), which itself was significantly lower (P < 0.05) than in Paddock 2 (6.1). Soil C concentrations were significantly higher (P < 0.05) in the soils under pasture than under pine (63.8 g C/kg), and C in Paddock 1 (98.1 g C/kg) was significantly higher (P < 0.05) than in Paddock 2 (84.1 g C/kg). The ratio of Al in organo-Al complexes (as estimated with sodium pyrophosphate) to the sum of Al in both SRO and organo-Al complexes (Alp/Alo) was significantly smaller (P < 0.05) as the alkalinity of the soils increased (0.38, 0.23, 0.16 for Forest, Paddock 1 and Paddock 2, respectively). At the molecular level, soils under Forest had a larger relative contribution of degraded products of plant polysaccharides than those under pasture, while these had a larger contribution of fresh (e.g. cellulose and cutan/suberan aliphatic structures) and N-rich OM (e.g., microbial fingerprints, denoting a high microbial activity). Dissolved organic C content in the rhizosphere of pasture species was similar between paddocks, but Paddock 2 had a significantly (P < 0.05) greater contribution of organic acids of MW < 500 Da and higher pH (6.8 vs. 6.2). The results (1) confirm the common enrichment in organic C of New Zealand top soils under pasture compared to those under pine, and (2) reveal that the changes in the soil chemistry associated with pasture management may weaken the ability of these soils to preserve OM.

Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.

Phosphorus status and its sorption-associated soil properties in a paddy soil as affected by organic amendments

Yet the transformation and contribution of amorphous aluminum and iron which are usually indicated as oxalate-extractable forms (Alox and Feox) to phosphorus (P) sorption in paddy soils, under long-term organic amendments combined with alternating flooding and draining, were not fully understood. The aim of this study was to investigate the effects of organic amendments on P status and P sorption-associated attributes including Alox and Feox in a paddy soil. We selected 26 study sites that varied in three fertilization regimes (i.e., MF, straw, and manure) and three management histories (i.e., 2, 8, and 13 years) in the experiment station. Soil samples were assayed for (1) P accumulation status including total P, Olsen-P, oxalate-extractable (Pox) and degree of soil saturation with P (DPS), and (2) soil properties associated with P sorption including pH, soil organic matter (SOM), Alox, and Feox. Manure application presented significantly greater values for DPS (16.3 %, P < 0.01), Olsen-P / TP ratio (4 %, P < 0.01), and Olsen-P / Pox ratio (6.8 %, P < 0.01). Moreover, it increased the contents of Alox and Feox in the paddy soil, especially the content of Alox. Phosphorus accumulation in the paddy soil was not significant in straw treatment vs. MF treatment. Both Alox (P < 0.01) and Feox (P < 0.05) exhibited significantly linear relationships with SOM, suggesting that the organo-Al(Fe) complexes might widely generate in the paddy soil, which have strong affinity to P. In comparison with Feox, Alox showed greater correlation coefficients with soil P forms (varying from 0.56 to 0.81) and higher predictability for DPS of the paddy soil (R 2 = 0.3679, P < 0.01). These results suggest that Alox is the primary soil property associated with P sorption in the paddy soil under long-term organic amendments and repeated redox circulations.

Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication: Is allophanic soil different?

Organic matter (OM) bound to soil mineral particles (higher-density particles) tends to be more stabilized, enriched in 13C and 15N, and has a lower C:N ratio. Yet how these variations in OM chemistry are linked to the nature of organo-mineral assemblage remains poorly understood, especially in allophanic soils where high amounts of OM are stabilized by interactions with reactive inorganic phases such as short-range-order (SRO) minerals. We thus assessed the extent to which the degree of aggregation and its disruption during fractionation control the distribution and chemistry of the soil organo-mineral particles across six density fractions using a volcanic soil (allophanic Andisol) based on selective dissolution, microscopy (SEM), solid-state 13C NMR spectroscopy and δ13C and δ15N analyses. Intermediate-density fractions (2.0–2.5gcm−3) accounted for 63–86% of organic C and N, 73–93% of pyrophosphate-extractable iron and aluminum (Fep, Alp), and 78–95% of oxalate-extractable metals (Feo, Alo) in the bulk soil sample. While air-drying pretreatment had little effect, sonication during fractionation led to (i) fragmentation of both plant detritus and some of the aggregates of 30–100mm sizes, (ii) release of occluded low-density fraction (<1.6gcm−3) which largely originated from the aggregates of 1.6–2.0gcm−3 density range, and (iii) redistribution of organo-mineral particles (15–16% of total OM and 7–19% of the extractable metals) within the intermediate density fractions. Positive correlation of Alp with C:N ratio and negative correlation of Alp with δ15N among the fractions suggest preferential binding of Alp phase (e.g., organo-Al complexes) to decaying plant detritus. Positive correlation of Alo and Feo with δ15N, together with theoretical density calculations of idealistic organo-mineral association modes, suggests that 15N enrichment may be coupled with OM binding to SRO minerals and with the formation of physically-stable aggregates of micron/submicron sizes in accord with our conceptual model (Asano and Wagai, 2014). The general pattern of 13C and 15N enrichment and C:N decline with increasing particle density remained largely unchanged despite the sonication effects detected, indicating that sonication-resistant organo-mineral assemblages largely control the observed patterns. The similarity in the density-dependent changes of OM chemistry between the studied Andisol and the soils with crystalline clay and metal oxide mineralogies in previous studies strongly suggests a common biogeochemical control which deserves further investigation.

1500 years of soil use reconstructed from the chemical properties of a terraced soil sequence

Colluvial soils can store signals of the environmental conditions occurred during their formation, including the anthropogenic activities they supported. Their study may provide important information for reconstructing and interpreting the evolution of cultural landscapes. We studied the chemical properties of a terraced soil system located in the town of Santiago de Compostela (NW Spain). Aluminum, Fe and Si fractionation was studied by selective dissolution techniques with high vertical resolution, combined with elemental composition and other soil properties such as phosphate retention (Pret) and NaF pH, aiming to identify modifications produced by land use changes and agricultural management practices since Antiquity. The buried epipedon of the paleosol, which is considered to exemplify the pre-terracing soil, showed strong andic character. We argue that its attenuation in the anthropogenic soil layers is, firstly, a consequence of the decreasing amounts of reactive components (organic matter, organo-Al complexes and low ordered aluminosilicates) due to a dilution by mixing superficial and sub-superficial horizons. Secondly, the introduction of agricultural techniques led to modifications in the chemical stability of organo-metal complexes influencing the accumulation of organic matter. Other signals, such as variations in soil acidity, and P and Ca contents, point to management practices as fertilization or liming. The increasing amounts of Fe inorganic compounds in the more recent layers indicate a strong weathering and degradation, probably as a consequence of the intensification of the agricultural use. Our results indicated continued and progressively more intensive agricultural use during the last 15 centuries, linked to the development of the town.

Aluminium fractionation and speciation in bulk and rhizosphere of a grass soil amended with mussel shells or lime

Acid soils have a high proportion of Al in the exchange complex and are usually amended with lime to reduce the toxic effect of the free Al (Al3+). To this purpose, in some places like Galicia (NW Spain), there is a high production of wastes such as mussel shells that may substitute the lime. The aim of this work was to study the effect of different amending products derived from mussel shells on the Al chemistry in both bulk and rhizosphere, and on pasture production. The experiments were conducted on plots of an acid soil cultivated with grasses (Dactylis glomerata L., Trifolium repens L. and Lolium perenne L.). Each plot received its own treatment consisting of 1) commercial lime, 2) mussel shells dried and ground till 0–2mm (MDF), 3) mussel shells dried and ground till 2–4mm (MDC), 4) mussel shells heated (550°) and ground till <63μm (MHF), or 5) mussel shells heated (550°C) and ground till 0–2mm (MHC); one plot acted as control and was not treated. Each plot was replicated four times. The soil samples were collected after 16months and submitted to a general characterization. The Al was extracted from the soil by different solutions: ammonium oxalate (Alo=non-crystalline Al), sodium pyrophosphate (Alp=organically bound Al), copper chloride (Alcu=organo-Al complexes of low to medium stability) and ammonium chloride (exchangeable Al); the difference Alp-cu estimated the organo-Al complexes of high stability. From the soil water extracts, the Al was fractionated into acid-soluble (Al-polymers, colloids and organic complexes), non-labile Al (organic monomers) and labile Al (inorganic monomers, namely the species Al3+, Al–OH, Al–F and Al–SO4). For each plot, the production of the total dry matter and of that produced by the sown species, were also determined. With respect to the control, all the amending treatments increased the pH in the rhizosphere (5.11 in the control plots, 5.48–5.89 in the amended plots), which showed pH values higher than the bulk (5.13–5.60 in the amended bulk soil; 5.48–5.89 in the amended rhizosphere). Also the contents of C and N were higher in the rhizosphere than in the bulk, without difference among treatments. Concerning with Al chemistry, all the mussel shells derived products had effects similar to those of the commercial lime, with no difference among them. With respect to the control, in the soil solid phase all the treatments induced an increase in the formation of hydroxyl-aluminium complexes in the bulk (Alo: 3500mgkg−1 in the control, 3700–5500mgkg−1 in the amended plots) and organo-Al complexes of high stability in the rhizosphere (Alp-cu: 3563mgkg−1 in the control, 3550–5000mgkg−1 in the amended plots). The exchangeable Al diminished with all treatments, more in the rhizosphere (1.36cmol(+) kg−1 in the control, 0.24–0.47cmol(+) kg−1 in the amended plots) than in the bulk (2.83cmol(+) kg−1 in the control, 0.46–1.23cmol(+) kg−1 in the amended plots). Concerning the water extracts, the treatments induced an increase of organic complexes (polymers and monomers), more pronounced in the rhizosphere (non-labile Al: 0.17μmolL−1 in the control, 10.7–35.7μmolL−1 in the amended plots) than in the bulk (non-labile Al: 0.25μmolL−1 in the control, 0.25–3.57μmolL−1 in the amended plots). Further, in the treated plots the labile Al was dominated by Al–OH complexes, which are considered toxic but a lower level than the Al3+ and abounded in the control. The production of dry matter due to the sown species was about double in the plots treated with CL and MHF with respect to the control. We conclude that the products derived from mussel shells can substitute the commercial lime in the amending of acid soils, so increasing the value of the shells and limiting the environmental problems derived by their disposal.

Comparing NaOH-extractable organic matter of acid forest soils that differ in their pedogenic trends: a pyrolysis-GC/MS study

Soil organic matter (SOM) in Alu-andic Andosols and Alu-humic Umbrisols is believed to accumulate because of the protection caused by binding to aluminium (Al). We investigated soils that differed in the abundance of organo-Al complexes to determine the effect of such binding on SOM chemistry. For this, the surface horizons of three types of acid soils in the Basque Country (northern Spain) under forest stands were studied: (i) Alu-andic Andosols (AND soils) on basalts and trachytes, (ii) Umbrisols or so-called ‘aluminic’(ALU) soils also on basalts and trachytes and (iii) soils with a podzolizing trend (POD), on quartzites. Values of Al extractable with sodium pyrophosphate (Alp) in the surface horizons of these soils ranged between 8.5 and 13.1, 1.9 and 9.3, and 0.8 and 3.7 g kg−1 dry weight, for the AND, ALU and POD soils respectively. For POD and ALU soils, surface horizons were sampled at two depths, 0–5 and 5–20 cm, whereas the AND soils were sampled at different depths down to the B horizon. NaOH-extractable SOM from three AND soils, 12 ALU soils and 12 POD soils was studied by pyrolysis-gas chromatography/mass spectrometry. The POD soils had the largest loads of plant-derived markers (lignin, long-chain alkanes and alkenes, methyl ketones, fatty acids); SOM of the AND soils had the smallest amounts of plant-derived SOM and the largest amounts of microbial products (microbial sugars and N-compounds) of the soils studied. ALU soils had an intermediate pattern, as expected. The results indicate that the SOM of Alu-andic Andosols, developed from basalt and trachyte rocks, is essentially dissimilar to that of soils derived from quartz-rich parent material, under the same climate conditions and similar forest stands. The dominance of secondary (microbial-derived) SOM in Alu-andic Andosols, also observed in previous research on Sil-andic Andosols (these are dominated by short-range ordered Si compounds in contrast to the dominance of organo-Al complexes in Alu-andic Andosols), reveals the small contribution of primary (plant-derived) material to SOM in soils with andic properties.

Site-to-site variability and temporal trends of DOC concentrations and fluxes in temperate forest soils

Here, we report site-to-site variability and 12–14 year trends of dissolved organic carbon (DOC) from organic layers and mineral soils of 22 forests in Bavaria, Germany. DOC concentrations in the organic layer were negatively correlated with mean annual precipitation and elevation whereas air temperature had a positive effect on DOC concentrations. DOC fluxes in subsoils increased by 3 kg ha−1 yr−1 per 100 mm precipitation or per 100 m elevation. The highest DOC concentrations were found under pine stands with mor humus. Average DOC concentrations in organic layer leachates followed the order: pine>oak>spruce>beech. However, the order was different for mean DOC fluxes (spruce>pine>oak>beech) because of varying precipitation regimes among the forest types. In 12 of 22 sites, DOC concentrations of organic layer leachates significantly increased by 0.5 to 3.1 mg C L−1 yr−1 during the sampling period. The increase in DOC concentration coincided with decreasing sulfate concentration, indicating that sulfate concentration is an important driver of DOC solubility in the organic layer of these forest sites. In contrast to the organic layer, DOC concentrations below 60 cm mineral soil depth decreased by <0.1–0.4 mg C L−1 yr−1 at eight sites. The negative DOC trends were attributed to (i) increasing adsorption of DOC by mineral surfaces resulting from desorption of sulfate and (ii) increasing decay of DOC resulting from decreasing stabilization of DOC by organo-Al complexes. Trends of DOC fluxes from organic layers were consistent with those of DOC concentrations although trends were only significant at seven sites. DOC fluxes in the subsoil were with few exceptions small and trends were generally not significant. Our results suggest that enhanced mobilization of DOC in forest floors contributed to the increase of DOC in surface waters while mineral horizons did not contribute to increasing DOC export of forest soils.

Influence of Agricultural Practices on the Stability of Organo-Al Complexes in an Alu-Andic Andosol

The influence of agricultural practices on the stability of organo-Al complexes in alu-andic andosols was investigated in a 16-week experiment. Soil samples from an A horizon of a Typic Fulvudand were placed in filtration funnels and subjected to (i) liming (L), (ii) fertilization ([F] 700 kg ha1 N, 422 kg ha1 P2O5), and (iii) a combination of L + F. The amendments were added at time 0 and at the start of Week 8. All soils were subjected to simulated tilling (T) once a week. Controls with and without tillage were also established (T and Ctrl, respectively). A subset of samples from each treatment was also subjected weekly to heat treatment at 40°C (H), with the subsequent drying effect. All soils were watered 3 days each week, and leachates were collected After 16 weeks, a significant (P < 0.05) degree of acidification was observed both in the soils without chemical amendment (Ctrl, T, and T + H) and in the fertilized soils (pH <4.6). Acidification was accompanied by a flush of Al into solution. Heated soils had a large decrease in LaCl3-and CuCl2-extractable Al, especially in the soils that underwent all treatments (T + F + L + H). This was paralleled by an increase in dissolved organic C and Al released into solution and an increase in soil pH. The wetting and drying cycling induced by the heat treatment followed by irrigation may have promoted the effects observed, although other factors cannot be discarded. The findings showed how the loss of andic soil properties after land-use change occurs as a result of the combination of the different agricultural practices investigated, with heat and moisture fluctuations being the most important factors. Copyright © 2010 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

Consideration on the occurrence of the Al 13 polycation in natural soil solutions and surface waters

Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al 13 that could be reached in waters in the absence of complexing ligands other than OH −. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al 13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al 13 concentration in mildly acidic natural waters may not exceed a few μmol l −1 at Al T on the order of 10 −4 mol l −1 and should be less than 1 μmol l −1 at Al T=10 −5 mol l −1. Monomeric Al–Si species may not significantly interfere with the formation of Al 13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al 13 concentration. The maximum concentration of Al 13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al 13 may occur increases slightly with temperature and the most acidic pH value above which Al 13 may be formed has been underestimated. At T=25°C, the Al 13 polycation may be a significant Al species (4 to 5% of Al T) at pH < 4.5 if Al T > 10 −4 mol l −1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al 13 should be negligible in natural soil and surface waters and may not control either Al 3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al 13 to plants and aquatic organisms in natural conditions may be considered to be very low.

Speciation and solubility control of aluminium in soils developed from slates of the river Sor watershed (Galicia, nw Spain)

The Al species in the soid and liquid phases were studied in eight soils developed from slates in a watershed subjected to acid deposition. From soil solution data the mechanisms possibly controlling Al solubility are also discussed. The soils are acidic, organic matter rich and with an exchange complex saturated with Al. In the solid phase, more than 75% of non-crystalline Al was organo-Al complexes, mostly highly stable. In the soil solutions, monomeric inorganic. Al forms were predominant and fluoro-Al complexes were the most abundant species, except in soil solutions of pH 3, in which Al3+ predominated and sulphato-Al complexes were relatively abundant. The most stable phases were kaolinite, gibbsite and non-crystalline Al hydroxides. In most samples, Al solubility was controlled by Al-hydroxides. Only in a few cases (solutions of pH 4-5, Al3+ activity >40 µmol L-1 and SO4 content >200 µmol L-1), Al-sulphates such as jurbanite also could exert some control over Al solubility. In adition to these minerals, a possible role of organo-Al complexes or the influence of adsorption reactions of sulphate is considered, especially for samples with very low Al3+ content (<0.5 µmol L-1).

Leaf litter ash alkalinity and neutralisation of soil acidity

Soil acidification is a major factor limiting the sustainability of agricultural production systems throughout the world. Liming may not always be economically possible and therefore alternative methods or complementary methods of amelioration are required. Leaf litter collected from several tree species was examined for ash alkalinity (as an estimate of organic anion content) and ability to ameliorate an acid soil. Ash alkalinity measured by titration of the ash and excess cation values obtained by calculation as the difference between cation and anion content were correlated. Values obtained by the latter method ranged from 247 cmolc kg-1 for Melia azedarach (white cedar) to 36 cmolc kg-1 for Eucalyptus globoidea (white stringybark). There was a significant linear correlation between ash alkalinity and the Ca concentration in the litter. When added to an acid soil (pH 4.04 measured in 0.01 M calcium chloride) and incubated for 8 weeks, leaf litter raised the pH. Species differed markedly with Melia azedarach having the greatest effect. The increase in pH was proportional to the quantity of ash alkalinity (organic anions) added, expressed as calcium carbonate equivalents. Aluminium levels on the exchange complex were lowered by treatment with leaf litter through direct precipitation of a solid phase and again Melia azedarach litter was most effective. There was also indirect evidence of organo-Al complexes affecting the concentration of monomeric Al in soil treated with litter from Liquidambar styraciflua (liquidambar), Quercus robur (English oak) and Pinus radiata (radiata pine).

Magnesium uptake by Al-stressed maize plants with special emphasis on cation interactions at root exchange sites

As aluminium (Al) severely inhibits magnesium (Mg) uptake by many plant species, Mg uptake and Mg-Al interactions in maize (Zea mays L.) were studied in a series of short and long-term experiments. A relationship between Mg uptake and the degree of Mg saturation of exchange or binding sites of the root apoplast (root-CEC) was studied by growing plants in solutions containing: (i) different concentrations of Al, calcium (Ca) and hydrogen (H) ions; and (ii) a number of organic complexes of Al. In short-term experiments, Ca had little effect on the Mg nutrition of maize plants. However, with increasing concentrations of Al and H ions in nutrient solution, there was a decrease in both the degree of Mg saturation of root-CEC and Mg uptake. Effects of pH on cation (H, Al, Mg, Ca) binding at the root apoplasm were pronounced and complicated because of a simultaneous change of H ion concentration, effective root-CEC and Al speciation. The behaviour of Al as organic Al complexes differed from that supplied as aluminium chloride (AlCl3). In the presence of organo-Al complexes, less Mg was replaced from apoplastic binding sites and Mg uptake was inhibited less severely than with AlCl3. In a long-term experiment, Al-citrate, in contrast with AlCl3, was not phytotoxic to maize, expressed by the lack of any inhibition of shoot biomass production.

Relative efficiency of complexed aluminum noncrystalline Al hydroxide, allophane and imogolite in retarding the biodegradation of citric acid

The protective effect attributed to organically complexed aluminum, noncrystalline aluminum hydroxide, allophane and imogolite on the biodegradation of soil organic matter was studied by means of synthetic organo-mineral associations, with citric acid as the bound organic substance. Adsorption of citric acid on pre-existing solid Al constituents was in the order: noncrystalline Al hydroxide > allophanes > imogolite. For any constituent, the maximum lowering of the mineralization rate was observed at the highest Al/citric acid ratio. For a given Al/acid ratio, the protective effect was always higher with both noncrystalline Al hydroxide and Al bound in insoluble organo-Al associations than with allophane or imogolite. Taking into account both the intensity of the protective effects and the corresponding amounts of citric acid which were then protected, it was possible to rank the constituents studied with respect to carbon accumulation. Coflocculation of citric acid and soluble aluminum as organo-Al precipitates could lead to a strong protection of a moderate amount of organic C. Strong adsorption of citric acid by pre-existing noncrystalline Al hydroxide could lead to a very strong protection of a similar amount of organic C. Strong adsorption of citric acid either by allophane or by imogolite resulted only in a moderate protection of a low to very low amount of organic C. The following classification of the substances tested in relation to their effectiveness in decreasing the biodegradation of citric acid is proposed: noncrystalline Al hydroxide ≥ Al bound in insoluble organo-Al complexes ⪢ allophanes > imogolite. Both noncrystalline Al hydroxides and insoluble organically complexed Al rather than either allophane or imogolite should be considered as the key parameters for organic matter accumulation in andic soils. The influence of allophanic clays may result mainly from the release of Al during weathering. Their direct influence upon organic matter accumulation should not be overestimated. The protective effect was thought to result from (i) the flocculation of initially soluble organo-Al complexes, and (ii) the binding of organic molecules within either the inter-tubular spaces of imogolite threads or the inter-spherular spaces of allophane aggregates. The efficiency of noncrystalline aluminum hydroxides originates probably from a similar process. Organic matter can, thus, be protected from normal contacts with the soil enzymatic environment, leading to a lowering of its mineralization rate.