Organic matter stabilization in two Andisols of contrasting age under temperate rain forest

Recent studies with Andisols show that the carbon (C) stabilization capacity evolves with soil age relative to the evolution of the mineral phase. However, it is not clear how soil mineralogical changes during pedogenesis are related to the composition of soil organic matter (SOM) and ¹⁴C activity as an indicator for the mean residence time of soil organic matter (SOM). In the present study, we analyzed the contribution of allophane and metal–SOM complexes to soil C stabilization. Soil organic matter was analyzed with solid-state ¹³C nuclear magnetic resonance spectroscopy. Additionally, the soil was extracted with Na-pyrophosphate (Al_p, Fe_p) and oxalate (Al_o, Si_o, and Fe_o). Results supported the hypothesis that allophane plays a key role for SOM stabilization in deep and oldest soil, while SOM stabilization by metal (Al and Fe) complexation is more important in the surface horizons and in younger soils. The metal/C_p ratio (C_p extracted in Na-pyrophosphate), soil pH, and radiocarbon age seemed to be important indicators for formation of SOM–metal complexes or allophane in top- and subsoils of Andisols. Changes in main mineral stabilization agents with soil age do not influence SOM composition. We suggest that the combination of several chemical parameters (Al_p, Fe_p and C_p, metal/C_p ratio, and pH) which change through soil age controls SOM stabilization.     

Phosphate sorption in relation to aluminum and iron oxides of oxisols from Ghana

Abstract

Phosphate (P) sorption characteristics of six natural Ghanaian Oxisols, selected because of their hydrological and topographical suitability for agriculture, were evaluated. Availability of P appears to be adequate for half of the soils as suggested by the Bray P1 test and determination of the standard P requirement (SPR), i.e., the amount of P sorbed at a concentration of 0.2 ppm P (6.46 μM). The SPR was found to be very closely related to P_max (Langmuir P sorption capacity), which in turn, was significantly correlated with oxalate‐extractable aluminum (Al) (Al_o) and iron (Fe) (Fe_o) and related (not significantly) to the difference between dithionite‐citrate‐bicarbonate‐extractable Fe (Fe_d) and oxalate‐extractable Fe. Accordingly, P_max is fairly well predicted by the model of Borggaard: P_ca]e=0.211#lbÀl_o+0.115#lbFe_o+ 0.05#lb(Fe_d‐Fe_o)+0.3, except for one soil strongly enriched in Fe oxides, mainly goethite. This goethite was found by X‐ray diffraction analysis to consist of crystals larger than normally found for pedogenic Fe oxides. The difference between P_max and P_calc for this soil could, therefore, be attributed to the occurrence of these large Fe oxide crystals, because P sorption will decrease with increasing crystal size (decreasing specific surface area).     

Stabilization of organic matter by soil minerals — investigations of density and particle‐size fractions from two acid forest soils

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle‐size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X‐ray spectroscopy (SEM‐EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF‐soluble, or located in the density fraction d >1.6 g cm^—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm^—3 were closely correlated to the contents of oxalate and dithionite‐citrate‐bicarbonate‐extractable Fe (r² = 0.93 and 0.88, P <0.001). SEM‐EDX analyses suggested associations of OC with aluminum whereas silicon‐enriched regions were poor in OC. The specific surface area and the microporosity of the soil mineral matrix after destruction of organic matter were less closely correlated to OC than the extractable iron fractions. This is possibly due to variable surface loadings, depending on different OC inputs with depth. Our results imply that subsoils are important for the storage of OC in terrestrial ecosystems because of intimate association of organic matter with secondary hydrous aluminum and iron phases leading to stabilization against biological degradation.     

Colloidal stability in some tropical soils of southeastern Nigeria as affected by iron and aluminium oxides

The stability of microaggregates in soils as opposed to its dispersion is a very important soil phenomenon that checks degradation arising from unguided tillage and soil erosion. Ten soils from southeastern Nigeria were sampled from their typical A and B horizons for the study. The aim was to identify the extent of colloidal stability of the soils and the forms of Fe and Al oxides in the soils contributing to their stability. The soils are mostly Ultisols and Inceptisols formed on sandstones and shale parent materials. The soils are low in soil basic cations including the soil organic carbon (SOC). The major clay mineral is kaolinite while the soil is acid in reaction. The various forms of soil Fe and Al oxides are high with the total forms of Fe and Al being most dominant and > dithionite extracted Fe and Al > oxalate extracted Fe and Al > pyrophosphate extracted Fe and Al. The water-dispersible clay and silt (WDC) and (WDSi) which are index of dispersion in most soils are low to medium thus reflecting in the low to medium dispersion ratio (DR). The clay flocculation index (CFI) and aggregated silt + clay (ASC) were moderate to high implying the high potential stability of the soils. Soil organic carbon did not seem to be contributing much to the stability of the microaggregates while oxalate and pyrophosphate extractable Fe (Fe_ox, Fe_p) and to some extent total Al (Al_t) were among the different forms of oxides that act as aggregating agents. We propose here that rather than SOC acting as a disaggregating agent in the soils, it might have acted in association with these oxides in a linkage or bridge such as C–P–OM–C to ensure stability of the soils.     

Sorption of phosphorus by some surface soils from Quebec in relation to their properties

Abstract

Surface horizons from Podzolic and Gleysolic soils were collected in various parts of the province of Quebec, Canada, and equilibrated with various amounts of KH₂PO₄ in 0.01 M CaCl₂ for 48 hours. P sorption data conformed to the linear form of the Langmuir and Freundlich equations. P solubility isotherms showed evidence of hydroxyapatite formation in most samples studied, whereas equilibration solutions of only few samples were saturated with respect to either dicalcium phoshate dihydrate or octocalcium phosphate. These reaction products were associated to soil pH and levels of added phosphate. The average values of the Langmuir sorption maximum for these studied Gleysolic and Podzolic samples were 763 and 1096 μg/g respectively. These values were higher than those obtained by the segmented and modified Freundlich models.

Relationships between the soil characteristics and P sorption parameters were evaluated by regression analysis. Among all variables, oxalate‐extractable Fe plus Al content of the Podzolic samples and the ratio of oxalate—extractable Al to clay of the Gleysolic samples gave the best significant correlation coefficients. Furthermore, soil pH and various ratios such as pyrophosphate‐extractable Fe and Al, oxalate‐extractable Fe and organic matter to clay were found to be significantly correlated only with the P sorption parameters of the Gleysolic samples.     

Controls on carbon accumulation and storage in the mineral subsoil beneath peat in Lakkasuo mire, central Finland

What processes control the accumulation and storage of carbon (C) in the mineral subsoil beneath peat? To find out we investigated four podzolic mineral subsoil profiles from forest and beneath peat in Lakkasuo mire in central boreal Finland. The amount of C in the mineral subsoil ranged from 3.9 to 8.1 kg m^?2 over a thickness of 70 cm and that in the organic horizons ranged from 1.8 to 144 kg m^?2. Rates of increase of subsoil C were initially large (14 g m^?2 year^?1) as the upland forest soil was paludified, but decreased to < 2 g m^?2 year^?1 from 150 to 3000 years. The subsoils retained extractable aluminium (Al) but lost iron (Fe) as the surrounding forest podzols were paludified beneath the peat. A stepwise, ordinary least‐squares regression indicated a strong relation (R² = 0.91) between organic C concentration of 26 podzolic subsoil samples and dithionite–citrate–bicarbonate‐extractable Fe (negative), ammonium oxalate‐extractable Al (positive) and null‐point concentration of dissolved organic C (DOC_np) (positive). We examined the ability of the subsoil samples to sorb dissolved organic C from a solution derived from peat. Null‐point concentration of dissolved C (DOC_np) ranged from 35 to 83 mg l^?1, and generally decreased from the upper to the lower parts of the profiles (average E, B and C horizon DOC_np concentrations of 64, 47 and 42 mg l^?1). The DOC_np was positively correlated with percentage of soil C and silt and clay content. The concentration of dissolved organic C in pore water in the peat ranged from 12 to 60 mg l^?1 (average 33 mg l^?1), suggesting that the sorptive capacity of the subsoil horizons for C had been exhausted. We suggest that the increase of C contents in the subsoil beneath mires is related to adsorption of dissolved organic C and slow mineralization under anaerobic conditions.     

The effects of long-term superphosphate application on soil organic matter content and composition from an intensively managed New Zealand pasture

Changes in quantity and composition of soil organic matter (SOM) in pasture receiving annual superphosphate (SP) applications for 41 years at 0 (control), 188 and 376 kg SP ha^?1a^?1 were investigated in soil samples collected from 0–75 and 75–150 mm depths by determining total carbon (TC), total nitrogen (TN), biomass C (BC), biomass N (BN) and subjecting the soils to sequential extraction using cold water, hot water, a mixture of hydrochloric (0.1 M HC1) and hydrofluoric (0.3 M HF) acids (HCl/HF), and sodium pyrophosphate (Na₄P₂O₇) followed by sodium hydroxide (NaOH) for extracting labile and stable SOM fractions. There were significant differences in some SOM fractions between control (0) and SP treatments (188 and 376), especially in the topsoil (0–75 mm) but these were not observed between the two SP treatments. Soil TN (0–75 mm), BN and BN: TN ratio (0–75 and 75–150 mm depths) and the proportion of hot–water–extractable C (HC) in soil TC (HC:TC) (0–75 mm) were significantly greater in the SP treatments than in the control. HC1/HF extractable C and the proportions of soil TC as HC1/HF extractable C (HC1/HF extractable C: TC) were smaller in the topsoil of SP treatments than in the control. Similar results were observed in humin N: TN ratio and the proportions of soil TC as cold–water–extractable carbohydrate (CWcC: TC) and of soil HC as hot–water–extractable carbohydrate (HWcC : HC). Increases in the proportion of labile fraction in SOM were reflected in values of BN, BC: BN, BN: TN, HWcC : HC and HC: TC whereas decreases in the proportion of stable fraction in SOM were found in humin N: TN and HCl/HF–extractable C: TC ratios. Increases in labile SOM (BN and N–containing compounds such as amino acids and amino sugars, which were extractable by hot water but were not present as carbohydrate) and decreases in stable SOM (HC1/ HF–extractable C and humin fraction) in soils under pastures treated with annual SP applications compared with the control were attributed to pasture improvement and the amelioration of P and S deficiency, resulting in a greater return of plant residues and animal excreta and also an increase in clover growth and associated biological N₂ fixation. The additional labile SOM in SP treatments compared with that of the control was not associated with the soil mineral Al and Fe components.     

Land-use/cover change effects and carbon controls on volcanic soil profiles in highland temperate forests

The stabilization of SOM by Al–humus complexes and non-crystalline minerals is a key issue to explain the soil-C variability and the biogeochemical processes that determine the fate of soil C following land-use/cover change (LUCC) in volcanic landscapes. In an altitudinal gradient of volcanic soils (2550–3500 masl), we quantified the total soil C (C_T) concentrations and stocks in soil pits sampled by genetic horizons. We performed analyses at landscape and local scales in order to identify and integrate the underlying environmental controls on C_T and the effects of LUCC. We selected four sites, two on the upper piedmont, one on the lower mountain slope and one on the middle mountain slope at Cofre de Perote volcano (eastern central Mexico) where temperate forests are the natural vegetation. At each site we selected three to five units of use/cover as a chronosequence of the LUCC pathways. In each soil horizon chemical characteristics (i.e. N, C/N ratio, pH, exchangeable bases) were determined and mineralogical properties were estimated from selective Al, Fe and Si oxalate and pyrophosphate extractions (i.e. the Alp/Alo ratio, the active Al related to non-crystalline minerals as Al_o ? Al_p, the allophane concentration, and the non-crystalline Al and Fe minerals as Al_o + 1/2Fe_o). At landscape scale, the Al–humus complexes were strongly related to the C_T concentration in topsoil (A horizons) but this relationship decreased with depth. In turn, the non-crystalline minerals and the C/N ratio explained the variability of the C_T concentrations in C horizons. At local scale, C_T concentrations and stocks were depleted after conversion of forest to agriculture in Vitric Andosols at the upper piedmont but this was not observed in Silandic Andosols. However, in Vitric Andosols the reduction of the C_T stocks is partially recovered throughout the regeneration/reforestation processes. The results suggest that the lower vulnerability of Silandic Andosols than Vitric Andosols to changes in the C_T after LUCC is due to the higher levels of SOM stabilized by Al–humus complexes and non-crystalline minerals in the Silandic soils. Furthermore, the importance of the allophane to explain the C_T stocks in the Silandic Andosols of the middle slopes suggests that the C_T stabilized by this mineral fraction in the subsoil adds an important fraction of the C_T to the estimates of the stocks.     

Podzol development with time in sandy beach deposits in southern Norway

The coastal areas of SE Norway provide suitable conditions for studying soil development with time, because unweathered land surfaces have continuously been raised above sea level by glacio‐isostatic uplift since the termination of the last ice age. We investigated Podzol development in a chronosequence of six soils on sandy beach deposits with ages ranging from 2,300 to 9,650 y at the W coast of the Oslofjord. The climate in this area is rather mild with a mean annual temperature of 6°C and an annual precipitation of 975 mm (Sandefjord). The youngest soil showed no evidence of podzolization, while slight lightening of the A horizon of the second soil (3,800 years) indicated initial leaching of organic matter (OM). In the 4,300 y–old soil also Fe and humus accumulation in the B horizon were perceptible, but only the 6,600 y–old and older soils exhibited spodic horizons. Accumulation of OM in the A horizons reached a steady state in <2,300 y, while in the B horizons OM accumulated at increasing rates. pH dropped from 6.6 (H₂O)/5.9 (KCl) in the recent beach sand to 4.5 (H₂O)/3.8 (KCl) within approx. 4,500 y (pH_H2O)/2,500 y (pH_KCl) and stayed constant thereafter, which was attributed to sesquioxide buffering. Base saturation showed an exponential decrease with time. Progressive weathering was reflected by increasing Fe_d and Al_d contents, and proceeding podzolization by increasing amounts of pyrophophate‐ and oxalate‐soluble Fe and Al with soil age. These increases could be best described for most Fe and Al fractions by exponential models. Only the increasing amounts of Fe_p could be better described by a power function and those of Fe_o by a linear model.     

Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents

Oxidative treatment can isolate a stable organic matter pool in soils for process studies of organic matter stabilization. Wet oxidation methods using hydrogen peroxide are widely used for that purpose, but are said to modify poorly crystalline soil constituents. We investigated the effect of a modified NaOCl oxidation (pH 8) on the mineral composition of 12 subsoils (4.9–38.2 g organic C kg^?1) containing varying amounts of poorly crystalline mineral phases, i.e. 1.1–20.5 g oxalate‐extractable Fe kg^?1, and of different phyllosilicate mineralogy. Post‐oxidative changes in mineral composition were estimated by (i) the determination of elements released into the NaOCl solution, (ii) the difference in dithionite‐ and oxalate‐extractable Si, Al and Fe, and (iii) the specific surface areas (SSAs) of the soils. The NaOCl procedure reduced the organic C concentrations by 12–72%. The amounts of elements released into the NaOCl extracts were small (≤ 0.14 g kg^?1 for Si, ≤ 0.13 g kg^?1 for Al, and ≤ 0.03 g kg^?1 for Fe). The SSA data and the amounts of dithionite‐ and oxalate‐extractable elements suggest that the NaOCl oxidation at pH 8 does not attack pedogenic oxides and hydroxides and only slightly dissolves Al from the poorly crystalline minerals. Therefore, we recommend NaOCl oxidation at pH 8 for the purpose of isolating a stable organic matter pool in soils for process studies of organic matter stabilization.     

Soil organic carbon and its sorptive preservation in central Germany

Soils are increasingly viewed as a potential sink for atmospheric carbon. However, their use to meet CO₂ emission reductions is problematic for there are knowledge gaps regarding the mechanisms involved in the sequestration of organic carbon (OC). There is evidence which suggests that OC concentrations are controlled by the mineralogy and related specific surface area (SSA) of a given soil. The goal of this study was to examine the importance of sorptive mechanisms on OC stabilization. The objectives were (i) to determine the SSA, clay mineralogy and dithionite‐ and oxalate‐extractable Fe and Al concentrations of several soils, and (ii) to analyse how these variables are related to OC concentrations. Five soils were sampled and analysed: two Umbrisols, a Stagnic Acrisol, an Anthrosol/Vertisol/Gleysol‐Chernozem and a Gleysol (FAO terminology), all located in Hesse, Germany. Oxalate‐extractable Fe and Al were found to be the best predictors of OC concentrations in the soils examined. Specific surface area correlated significantly with the OC content of the B and C horizons of one Umbrisol and the entire profile of the Anthrosol/Vertisol/Gleysol‐Chernozem. The relationship between SSA and OC concentrations is likely to be restricted to certain soils and might be a product of the sorptive capacity of Fe and Al oxides. We can assume that the available mineral surface area on oxides is a limiting factor in terms of a soil's capacity to sequester organic carbon. As such, attention should be paid to soil mineralogy and how this might limit the use of soils as a sink for atmospheric CO₂.     

Molybdenum fractions and mobilization kinetics in acid forest soils

We extracted molybdenum (Mo) from eight acid forest soils (19 A, E, and B horizons) in NE-Bavaria and from one site in the Ore Mountains, using (1) anion exchange-resin, (2) 0.2 M ammonium oxalate, and (3) ascorbic acid/ammonium oxalate. The Mo concentrations in the anion exchange-resin fraction varied between 5 and 28 μg kg^-1. Oxalate-extractable Mo ranged from 44 to 407 μg kg^-1 and after reduction of iron (Fe) with ascorbic acid, 135 to 1071 μg Mo kg^-1 were extracted. The lowest concentrations of Mo were measured in acid and sesquioxide impoverished E horizons. The total concentrations of Mo in spruce needles correlated with ion exchange resin extractable Mo, indicating that this fraction represents Mo readily available to plants. The Mo and Fe dissolution kinetics during oxalate extraction were studied on 8 of the soil samples to obtain further information on Mo mobilization. Oxalate extractable iron (Fe_o) was mobilized within a few hours. A first order equation was applicable to the Fe dissolution kinetics with the rate constants ranging between 0.9 and 9.0 h^-1. The mobilization of Mo occurred in two distinct stages. An initially rapid dissolution was followed by a further increase in extractable Mo but with slower kinetics. A combined first order-diffusion equation was found to be appropriate for modelling the results. The first order rate constants for Mo mobilization ranged from 0.6 to 11.4 h^-1. However, correlations between the rates of reaction of Mo and Fe could not be established, indicating that Mo is either not distributed equally along Fe minerals or that there is another pool, possibly the organic substance of the soil, from which Mo is extractable by oxalate.     

EXTRACTABLE SESQUIOXIDES IN SIX MEDITERRANEAN SOILS DEVELOPED ON BASALT AND SCORIA

Iron, Al, and Mn were extracted by oxalate and dithionite from two Brown Mediterranean Soils, two Red Mediterranean Soils, one Vertisol and one Gley soil, all derived from basalt or scoria in the sub-humid and humid Mediterranean regions of the Golan Heights. Ratios of oxalate: dithionite extracted iron (Fe_o:Fe_d) were low in all soils, indicating that the predominant form of free iron is crystalline. Fe_o accumulates in the argillic B horizons of the Mediterranean soils, while Fe_d accumulates in the surface horizons. A large part of the free iron oxide in the surface horizons of Mediterranean soils is associated with non-clay fractions. While manganese behaves in a manner somewhat similar to that of iron, no definite trends could be discerned in the vertical distribution of free aluminium. In the Vertisol, Fe_o and Mn_o accumulate in the subsoil. Fe_d and Mn_d increase slightly with soil depth. In the Gley soil, amorphous iron accumulates in the surface horizon, total free iron in the bottom horizon. Both amorphous and total free Mn had been depleted from the upper horizons of the Gley soil.     

Explaining soil organic matter composition based on associations between OM and polyvalent cations

Site conditions and soil management determine the content and the composition of soil organic matter (SOM). Organic matter (OM) is characterized by functional groups, which preferentially interact with polyvalent cations and soil minerals. These interactions could perhaps explain the site‐specific composition of bulk SOM and a pyrophosphate‐soluble OM fraction (OM‐PY) using basic soil properties. The objective of this study was to test a simplified model for the interactions between OM and polyvalent cations (i.e., Ca, Mg, Al, Fe, and Mn) by using data from soils from long‐term field experiments. The model considered (1) OM–cation, (2) OM–cation‐mineral, and (3) OM–mineral associations and assumed that the availability of the cation's coordination sites for the interaction with OM depends on these three types of associations. The test was carried out using data (topsoil) from differently fertilized plots from three long‐term field experiments (Halle, Bad Lauchstädt, Rotthalmünster). The composition of SOM and OM‐PY was characterized by the relationship of the ratio of the C=O (i.e., here indicating both carbonylic and carboxylic groups) versus C–O–C absorption band intensities obtained from the Fourier transform infrared (FTIR) spectra with the content of exchangeable, oxalate‐, and dithionite‐extractable polyvalent cations. The assumed associations between the OM and cations and the availability of the coordination sites explained most of the variations in the C=O/C–O–C ratios of the SOM, and fewer variations in the OM‐PY, when using the site‐specific exchangeable and oxalate‐extractable cation contents. The C=O/C–O–C ratios of the OM‐PY were site‐independent for samples from plots that regularly received farmyard manure. The results suggested that a simplified model that considers the polyvalent cation content weighted by the number of coordination sites per cation according to the type of association could be used to improve the explanation of site‐specific differences in the OM composition of arable soils.     

Changes in element concentrations during Andosol formation on tephra in Japan

Andosol formation involves the rapid, abundant and in situ formation of non‐crystalline materials from tephra deposits. A large amount of humus complexed with Al also accumulates in the A horizons. As these materials are rich in Al or Fe compared to the parent tephra, the concentrations of the major and minor elements change significantly during Andosol formation. The objectives of this study were to examine how the rock type of the tephra and its weight loss during the formation process affect the changes in the element concentrations of Andosols. A total of 95 samples with different rock types from 18 pedons of Andosols in Japan were used to determine the total concentrations of 54 elements. Principal component analysis suggested that the degree of weathering and the rock types of parent tephra are the important factors in the variation of the element concentrations in Andosols. Three rock types, dacitic, andesitic and basaltic‐andesitic, were identified from the V and Zn contents of ferromagnetic minerals separated from the Andosol samples. Basaltic Andosols were identified from the abundant coloured volcanic glass and olivine in the sand fraction. Regarding concentrations of 41 elements, at least one group based on rock type was significantly different from one to three other groups at the P= 0.05 level. The content of oxalate‐extractable Si (Si_o), Al (Al_o) and Fe (Fe_o) was used as an index to show the extent of Andosol development. Relatively strong correlations between the element concentrations and Si_o, Al_o and Fe_o as well as other weathering indices were found in the andesitic samples. Among these elements, at least 27 (Be, Al, Ti, Fe, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Tl, Pb, Th and U) were enriched in the Andosols and the increases in these concentrations were related to total weight loss due to soil formation processes.     

The role of non‐crystalline Fe in the increase of SOC after long‐term organic manure application to the red soil of southern China

Because of the important role of soil organic carbon (SOC) in nutrient cycling and global climate changes, there has been an interest in understanding how different fertilizer practices affect the SOC preservation and promotion. The results from this study showed that long‐term application of manure (21 years) could increase significantly the content of SOC, total nitrogen (N) and soil pH in the red soil of southern China. The chemical structure of SOC was characterized by using solid‐state cross‐polarization magic angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy, and the aromatic C, ratio of alkyl C : O‐alkyl C, aromaticity and hydrophobicity of mineral fertilizers N, P and K plus organic manure (NPKM) and organic manure (M) treatments were less than those of mineral fertilizer nitrogen (N) and mineral fertilizers N, P and K (NPK) treatments. Both poorly crystalline (Fe_o) and organically complexed (Fe_p) iron contents were influenced significantly (P < 0.05) by different fertilizers, and it was observed that NPKM and M treatments increased the non‐crystalline Fe (Fe_o‐Fe_p) content. There was a significant (P < 0.01) positive correlation between soil organic C and non‐crystalline Fe in both the surface (0–20 cm) and subsurface (20–40 cm) soils. The results suggested that non‐crystalline Fe played an important role in the increase of SOC by long‐term application of organic manure (NPKM and M) in the red soil of southern China.     

Poorly crystalline mineral phases protect organic matter in acid subsoil horizons

Soil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X-ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon (¹⁴C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite–citrate-extractable Fe. The combination of oxalate-extractable Fe and Al explained the greatest amount of variation in stable organic C (R² = 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate-soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.     

Investigation of soils developed on volcanic materials in Nisyros Island, Greece

Soils that are forming on volcanic parent materials have unique physical and chemical properties and in most cases, on wet and humid climates, are classified as Andisols. The main purpose of this study is to examine if the soils that are forming on volcanic materials under a dry Mediterranean climate, in Nisyros Island (Greece), meet the requirements to be classified as Andisols. Soils from seven sites were sampled and examined for their main physico-chemical properties and selective dissolution analysis. Dithionite–citrate–bicarbonate (DCB) extractable Al and Fe (Ál_d, Fe_d), acid ammonium oxalate extractable Al, Fe, and Si (Ál_o, Fe_o and Si_o), and sodium pyrophosphate extractable Al and Fe (Al_p, Fe_p) were measured. In addition, Al and Si were determined after reaction with hot 0.5 M NaOH, (Al_NaOH and Si_NaOH) and with Tiron-(C₆H₄Na₂O₈S₂), (Al_T and Si_T). P-retention was also measured. The soils are characterised by coarse texture, low organic matter content, low values of cation exchange capacity (CEC), and high pH values. Values of Si_o, Al_o and Fe_o are less than 0.022%, 0.09% and 0.35% respectively, highlighting the lack of noncrystalline components. The ratio (Fe_d–Fe_o)100/Fe_d is quite high expressing the degree of crystallisation of free iron oxides. For all samples tested, values of the Al_o + 1/2Fe_o index are extremely low (< 0.24%). High Si_NaOH and Si_T (arising 2.76% and 2.18% respectively) indicate the presence of silica in amorphous forms. P-retention values are very low (< 12.6%). The results indicated the absence of noncrystalline minerals except for amorphous silica, and do not exhibit andic or vitric soil characteristics to be classified as Andisols.     

Soils and their relationship to aspect and vegetation history in the eastern Southern Alps, Canterbury High Country, South Island, New Zealand

This study focuses on soils in a mountainous catchment area located in the eastern part of the Southern Alps, South Island, New Zealand. The objective was to check the soils for non- or poorly crystalline constituents (metal organic complexes, short-range-order minerals) and if there is a relationship between pedogenesis and aspect and more recent landscape history. The morphology of the soils indicates brown soils (dystric cambisols, dystrudepts) with only few signs of podsolisation. In contrast, selected chemical properties of the soils reveal very strong weathering and leaching. Fe_o/Fe_d ratios are high and exceeding 0.6 in almost every soil horizon. Oxalate-extractable aluminium and silicon show prominent peaks in the lower subsoil horizons with 1.76–2.52% Al_o and up to 0.68% for Si_o on southern aspects. This is considerably higher than the values measured for soils on northern aspects (0.59% to 1.07% Al_o, max. 0.26% Si_o). This aspect relationship is also applying to phosphate retention reaching values of more than 90% on southern aspects and not more than 70% for northern aspects. Additionally, significant movement of organic matter in soils on southern aspects is evident by measurements of organic carbon and pyrophosphate-extractable Al and Fe. Thus soil formation can be regarded as more advanced on southern aspects. Allophane in association with organic matter can be considered as an important constituent in southern aspect subsoils being responsible for the typical andic properties. Metal-humus complexes and ferrihydrite are of subordinate significance. Considering the actual soil forming conditions under scrub-grassland (esp. soil acidity), it is unlikely that weathering and leaching is still strong enough today to allow significant podsolisation and the formation of short-range-order minerals. Under consideration of existing soil data from other studies it is proposed that these components and the podsolisation features are relicts caused by more acidic conditions under former forest cover which supported a stronger weathering and leaching.