The effect of soil flooding on the transformation of Fe oxides and the adsorption/desorption behavior of phosphate

As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast‐China were studied in anaerobic incubation experiments (submerging with water in N₂ atmosphere). Soil flooding significantly increased oxalate‐extractable Fe (Fe_ox), mainly at the expense of dithionite‐soluble Fe (Fe_DCB), as well as oxalate‐extractable P (P_ox), but decreased the ratio of P_ox/Fe_ox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was P_ox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non‐flooded soils. There are obviously significant differences in the nature of both, the Fe_ox and P_ox fractions under non‐flooded and flooded conditions. The degree of the changes in Fe_ox, P_ox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non‐flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often‐reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.     

Oxalate-soluble compounds of Al, Fe, and Si in soils of cold humid regions as a function of weathering

Original and published data on the contents of X-ray amorphous oxalate-soluble compounds of Al, Fe, and Si in mesomorphic eluvial soils of cold, moderately cold, and moderately warm continental humid and semihumid regions are generalized. The groups of soils developed from mafic igneous, metamorphic, and pyroclastic rocks are considered. It is shown that the content of oxalate-soluble oxides (OSOx) in the horizons of their maximal accumulation varies from less than 1% to 20–30%; the Al_ox/Fe_ox ratio varies from 1 to 6.5. The leading factor dictating the amount and quality of the OSOx in the soils is the presence or absence of volcanic glass in the parent materials. The boundary between the soils with and without volcanic glass corresponds to the OSOx content of 5% and the Al₂O_3ox/Fe₂O_3ox ratio equal to 2. These criteria are more reliable than the Al_ox/Fe_ox ratio used by foreign soil scientists to specify Andosols (Al_ox/Fe_ox > 2). The contents of oxalate-soluble oxides of Al and Fe do not depend on the total contents of these oxides in the parent material and seem to be related to the presence of these elements in minerals with different resistance to weathering. Under the natural conditions described in this paper, the content of OSOx shows a very weak response to zonal (temperature-controlled) climatic changes and/or to changes in the degree of humidity and the continentality of the climate.     

Proposal for advanced classification of brown forest soils in Japan with reference to the degree of volcanic ash additions

Abstract

The advanced classification of brown forest soils (BFS) is based on the specific properties of these soils, including the acid ammonium oxalate extractable aluminum (Al_ox) and lithic fragment contents, as well as their vertical distributions in the soil profile. In the present study, these properties were used to classify BFS, resulting in four types: (1) H-Al_ox-NGv BFS, (2) H-Al_ox-Gv BFS, (3) M-Al_ox BFS, (4) L-Al_ox BFS. H-Al_ox-NGv BFS is derived from volcanic ash characterized by a high Al_ox content and no lithic fragment, whereas L-Al_ox BFS is derived from weathered bedrock and has a low Al_ox content. H-Al_ox-Gv BFS and M-Al_ox BFS are derived from mixtures of volcanic ash and weathered bedrock. H-Al_ox-Gv BFS is characterized by high Al_ox content and many lithic fragments, whereas M-Al_ox BFS has moderate Al_ox content. H-Al_ox-NGv BFS and black soils (BLS) develop from accumulated volcanic ash, as indicated by declining Al_ox and clay content with decreasing depth in the surface horizons, as a result of successive additions of less-weathered volcanic ash to the soil surface.     

High gradient magnetic separation of some soil clays from Nigeria, Brazil and Colombia.

High gradient magnetic separation was used to fractionate the clay from some tropical soils. Acid-oxalate-extractable iron (Fe_ox) and aluminium (Al_ox) and total carbon were measured in the whole clay, the magnetic fraction and the tailings. The magnetic separation resulted in a wider range of concentrations of these elements than in the whole clays. In each of the clays Fe_ox was greater in the magnetic fraction than in the tailings; Al_ox was more variable. Carbon was also concentrated in the magnetic fraction suggesting that it is associated more with Fe_ox than Al_ox. The relationships between Fe_ox, Al_ox and carbon depend on soil classification and soil age.     

Resistance of mineral soils to Fe(III) reduction

Gleying and enhancement of hydromorphism in wetland soils due to Fe(III) reduction entail a series of degradation processes. The resistance of wetlands to degradation can be calculated from the content of potentially reducible iron, Fe(III)_pr, which is found from the van Bodegom equation taking into account the contents of oxalate-soluble iron Fe_ox and dithionite-soluble iron Fe_dit in the soil. In addition, this makes it possible to distinguish relict and actual gleysols. The van Bodegom equation is applicable to soils from which the oxalate solution extracts only amorphous and poorly crystallized iron compounds, which are quickly reduced by Fe-reducing bacteria. These soils have a low proportion of Fe(II) (no more that 15% of the total iron), as well as an accumulative profile distribution of Fe_ox. The van Bodegom equation is unsuitable for calculating the Fe(III)_pr content in soils with a high proportion of Fe(II) and a nonaccumulative profile distribution of Fe_ox.     

Comparison of different models for phosphate sorption as a function of the iron and aluminium oxides of soils

Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into zccount. Oxalate-extractable P, (P_ox), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Fe_ox) and aluminium (Al_ox). A significant relation between crystalline iron (Fe_d– Fe_ox) and total P sorption was not found. Reversibly adsorbed phosphate (P_i), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.     

Estimation of the phosphorus sorption capacity of acidic soils in Ireland

The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be α (Al_ox + Fe_ox), where Al_ox and Fe_ox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil^?1, and a value of 0.5 is commonly used for the scaling factor α in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (P_ox) so that DPS = P_ox/PSC. The relation between PSC and Al_ox, Fe_ox and P_ox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Al_ox and Fe_ox extracted, but there was also a negative correlation with P_ox. When PSC was calculated as the sum of the measured sorption after 252 days and P_ox, the multiple regression of PSC on Al_ox and Fe_ox gave the equation PSC = 36.6 + 0.61 Al_ox+ 0.31 Fe_ox with a coefficient of determination (R²) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Al_ox and Fe_ox did not overlap, indicating a significantly larger regression coefficient of P sorption on Al_ox than on Fe_ox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Al_ox and Fe_ox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R² by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0–3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water‐soluble P than either the Olsen or Morgan tests for P in soil.     

Assessing alternative approaches to predicting soil phosphorus sorption

Twenty‐five pasture soils were sampled from high‐rainfall zones of southeastern Australia to examine relationships between soil properties, and between soil properties and P buffering capacity (PBC) measures. Correlations between PBC values and soil properties were generally poor, with the exception of oxalate‐extractable Al (Al_ox) (r ≥ 0.97). Predictions of PBC were further improved when clay, as well as Al_ox, was included in a linear regression model (r² ≥ 0.98). When Al_ox and oxalate‐extractable Fe were excluded from the modelling exercise, a more complex three‐term linear regression model, including pH_H2O, exchangeable H and cation exchange capacity, adequately fitted both PBC values of the 25 soils examined in this study (r² ≥ 0.76). However, the Al_ox, Al_ox plus clay and the three‐term models gave poor predictions of the PBC values when the models were validated using 28 independent soils. These results emphasize the importance of model validation, because predictive models based on soil properties were not robust when tested across a broader range of soil types. In comparison, direct measures of PBC, such as single‐point P sorption measures, are more practical and robust methods of estimating PBC for Australian soils.     

Evaluation of preflooding effects on iron extractability and phytoavailability in highly calcareous soil in containers

Previous pot cropping and laboratory incubation experiments were consistent with field observations showing that temporary flooding before cropping can increase the availability of soil Fe to plants. To study the effect of temporary flooding on changes in soil Fe phytoavailability we used 24 highly calcareous, Fe chlorosis–inducing soils to carry out a pot experiment where peanut and chickpea were successively grown after flooding for 30 d. At the end of the cropping experiment, the preflooded soil samples exhibited higher concentrations of acid oxalate‐, citrate/ascorbate‐ and diethylenetriaminepentacetic acid (DTPA)–extractable Fe (Fe_ox, Fe_ca, and Fe_DTPA, respectively) than the control (nonflooded) samples. Also, Fe_ox and Fe_ca exhibited no change by effect of reflooding of the cropped soils or three wetting–drying cycles in freeze‐dried slurries of soils previously incubated anaerobically for several weeks. Leaf chlorophyll concentration (LCC) in both peanut and chickpea was greatly increased by preflooding. The best predictor for LCC was Fe_ox, followed by Fe_ca and Fe_DTPA. The LCC–soil Fe relationships found suggest that the Fe species extracted by oxalate and citrate/ascorbate from preflooded soils were more phytoavailable than those extracted from control soils. However, the increased phytoavailability of extractable Fe forms was seemingly limited to the first crop (peanut). Flooding dramatically increased Fe_DTPA; however, high Fe_DTPA levels did not result in high LCC values, particularly in the second crop. Therefore, this test is a poor predictor of the severity of Fe chlorosis in preflooded soils.     

N2O and NO production in various Chinese agricultural soils by nitrification

Eleven types of agricultural soils were collected from Chinese uplands and paddy fields to compare their N₂O and NO production by nitrification under identical laboratory conditions. Before starting the assays, all air-dried soils were preincubated for 4 weeks at 25 °C and 40% WFPS (water-filled pore space). The nitrification activities of soils were determined by adding (NH₄)₂SO₄ (200 mg N kg⁻¹ soil) and incubating for 3 weeks at 25 °C and 60% WFPS. The net nitrification rates obtained fitted one of two types of models, depending on the soil pH: a zero-order reaction model for acidic soils and one neutral soil (Group 0); or a first-order reaction model for one neutral soil and alkaline soils (Group 1). The results suggest that pH is the most important factor in determining the kinetics of soil nitrification from ammonium. In the Group 1 soils, initial emissions (i.e. during the first week) of N₂O and NO were 82.6 and 83.6%, respectively, of the total emissions during 3 weeks of incubation; in the Group 0 soils, initial emissions of N₂O and NO were 54.7 and 59.9%, respectively, of the total emissions. The net nitrification rate in the first week and second-third weeks were highly correlated with the initial and subsequent emissions (i.e. during the second and third weeks), respectively, of N₂O and NO. The average percentages of emitted (N₂O+NO)-N relative to net nitrification N in initial and subsequent periods were 2.76 and 0.59 for Group 0, and 1.47 and 0.44 for the Group 1, respectively. The initial and subsequent emission ratios of NO/N₂O from Group 0 (acidic) soils were 3.77 and 2.52 times, respectively, higher than those from Group 1 soils (P<0.05).     

The influence of soil processes on carbon isotope distribution and turnover in the British uplands

Understanding the natural variation of carbon within the soil, and between soil types, is crucial to improve predictive models of carbon cycling in high and mid-latitude ecosystems in response to global warming. We measured the carbon isotope distributions (¹²C, ¹³C and ¹⁴C) in soil organic matter (SOM) from Podzols, Brown Podzolic soils and Stagnohumic Gleysols from the British uplands, which were then compared with the total amounts and turnover of carbon in these soils. We did so by sampling at 2-cm intervals down six profiles of each soil type. The average amount of carbon stored in the top 28 cm of the Stagnohumic Gleysols is twice that of the other two soils. The ¹³C content and ¹⁴C age show a general increase with depth in all soils, and there is also a significant correlation between isotopic variation and the main pedogenic features. The latter suggests that soil-forming processes are significant in determining the carbon isotope signatures retained in SOM. Organic matter formed since 1960 is not found below 5 cm in any of the soils. Evidently organic detritus in the surface layers (LF and Oh) is rapidly mineralized. This accords with our modelled net annual C fluxes which show that more than 80% of the CO₂ emanating from these soils is derived from the top 5 cm of each profile. Although these soils contain much carbon, they do not appear to assimilate and retain SOM rapidly. The mean residence time of most of their carbon is in the 2–50 years range, so the soils are fairly ineffective sinks for excess CO₂ in the atmosphere. Under the predicted future ‘greenhouse’ climate, likely to favour more rapid microbial decomposition of organic materials, these soils are a potential source of CO₂ and are therefore likely to accelerate global warming.     

Potential Iron and Phosphate Mobilization During Flooding of Soil Material

This study shows that mobilization of phosphate from soils under anaerobic conditions can be intimately coupled with reductive dissolution of iron from iron oxides. Among four soil samples from the reclaimed Skjernå estuary in Denmark incubated anaerobically and amended with glucose, 28–39% of the dithionite-citrate-bicarbonate-extractable iron and 10–25% of the oxalate-extractable phosphorus (P_ox) were released to the soil solution after 31 days. Significant correlation (r = 0.992**) between the molar ratio P_ox/(Fe_ox + Al_ox) for the aerobic samples and (P_{P sol}/Fe_sol) (the molar ratio between phosphate and iron in solution during anaerobic incubation), indicates that the phosphate saturation status of the soil is an important determinant of the amount of phosphate released during flooding of moderately acid soils.     

Predicting the degree of phosphorus saturation using the ammonium acetate–EDTA soil test

As a result of the important role played by phosphorus (P) in surface water eutrophication, the susceptibility of soils to release P requires evaluation. The degree of phosphorus saturation, assessed by oxalate extraction (DPS_ox), has been used as an indicator. However, most laboratories do not include DPS_ox in routine soil tests because of cost and time. This study evaluates the suitability of the ammonium acetate extraction in the presence of EDTA (AAEDTA), the standard soil test P (STP) in Wallonia (Southern Belgium), to predict DPS_ox; we also compared it with the Mehlich 3 extraction. Ninety‐three topsoil samples were collected in agricultural soils throughout Wallonia. Good correlations were found between the AAEDTA and the Mehlich 3 methods for P, Fe and Al (r = 0.85, 0.77 and 0.86, respectively). An exponential relationship was found between P_AAEDTA and DPS_ox. Results of principal component analysis and regression demonstrated that STP can be used to predict DPS_ox (r = 0.93) after logarithmic transformation. Soil test Al was also a good indicator of the P sorption capacity (PSC_ox) of soils (r = 0.86). Including the clay fraction in regression equations only slightly improved the prediction of PSC_ox (r = 0.90), while other readily available data (such as pH or organic carbon) did not significantly improve either DPS_ox or PSC_ox predictions.     

Evolution of organic matter added to soils under cultivation conditions

The development of an organic matter (OM) based on mixed sheep manure and peat, when it was incorporated into soils as fertilizer, was studied. The experiment was carried out in soils under almond tree culture, with drip irrigation and non irrigation regimes. Two doses, 10 and 4.5 kg tree^–1, were assayed. Changes in the humic acid fraction one year after incorporation into soils showed oxidation and enrichment in condensed structures, as observed by an increase of the O*:H* ratio and a decrease of the H*:C* ratio, and also by FTIR spectra. The oxidative process was more significant in the coarser textured and also in the non‐irrigated soil. The evolution of the ratios C_ext:C_ox and C_HA:C_FA throughout the culture cycle was followed by sampling and chemical analysis of different forms of organic carbon. Evolution of C_ext:C_ox showed a uniform humification state in the irrigated soil, and a significant decrease in the non‐irrigated soil, at the beginning of the experiment. Curves of C_HA:C_FA evolution showed changes attributed to mineralization or drainage of the fulvic acids fraction, giving a maximum in spring in both soils and a final increase at the end of the cycle by drainage only in the irrigated soil.     

Organic C levels of Ap horizons in North German Pleistocene sands as influenced by climate,texture, and history of land‐use

We evaluated the contents of organic carbon (C_org) of Ap horizons from 11 North German study areas along a Southeast to Northwest precipitation gradient with respect to their general levels and as related to C : N ratio, soil texture (clay content), bulk soil density, climate, and historical land‐use since 1780. The focus was on sandy soils, with the largest group of samples originating from 308 km² of the Fuhrberg catchment north of Hannover/Lower Saxony. Data from loess areas were used for comparisons. Major aims were (1) to quantify current C_org stocks, (2) to provide data on site‐specific, steady‐state C_org levels in old arable soils, and (3) to identify the main controls of C_org levels in the studied sands. The mean C_org content in sandy, well‐drained, old Ap horizons (uplands, > 200 years under cultivation, near steady‐state) increased with precipitation from < 8 g kg^—1 in the dry eastern parts of the study area (530 mm year^—1, 8.3°C) to 25 g kg^—1 in the moist Northwest (825 mm year^—1, 8.4°C). The C_org levels in lowlands which have been drained for more than 40 years were approximately 3 g kg^—1 higher than those of uplands under a similar climate. The factor clay content had no predictive value because low contents were associated with high C_org levels. Large proportions of refractory organic matter in sands resulting from specific features of historical land‐use and soil development (calluna heathland, heath plaggen fertilization, podzolization) appeared to be the most probable reason for such high C_org levels. However, the high C_org levels of these old arable sites were still exceeded by those of younger arable areas formerly under continuos grassland. A chrono‐sequence suggested that a period of about 100 years is necessary until a new steady‐state C_org level is established after conversion of grassland into arable land. Elevated C_org levels in current Ap horizons were also found for former woodland and heathland soils. The main conclusion is that sands can contain a lot of stable organic matter, sometimes more than finer textured soils.     

Phosphate sorption by Egyptian,Ethiopian and German soils and P uptake by rye (Secale cereale L.) seedlings

Phosphate sorption was studied in samples (0 - 20 cm depth) of five soils from Egypt (pH 7.4 - 8.7), four soils from Ethiopia (pH 3.9 - 5.3) and six soils from Germany (pH 3.3 - 7.2). Sorption parameters were calculated according to Pagel and Van Huay (1976) and according to Langmuir (Syers et al., 1973). Phosphate sorption parameters and oxalate extractable Fe and Al (Fe_ox, Al_ox) were related to the phosphate uptake by young rye plants in Neubauer pot experiments. P sorption parameter after Pagel and Van Huay (A) correlated significantly positively with the Fe_ox and Al_ox content in acid (r = 0.73) as well as in calcareous soils (r = 0.89) if the whole equilibrium concentration range (0 - 14 mg P/L) was considered. The relations calculated after Langmuir (B) were similar. P uptake by rye in acid soils was negatively correlated with the affinity constant n (r = ?0.76, (A)). In calcareous soils, a negative correlation between P uptake and affinity constant was calculated in the lower P equilibrium range (0 - 2.8 mg P/L) only for (B). Thus, P uptake decreased with increasing strength of P bonding to soil. From these results it is concluded that phosphate sorbed to Fe/Al oxides is an important P source for plants in acid and calcareous soils.     

Zinc Sorption in Selected Soils

Abstract

The adsorption of nutrient elements is one of the most important solid‐ and liquid‐phase interactions determining the retention and release of applied plant nutrients and the efficiency of fertilization. The study showed that the soils with high cation exchange capacity (CEC), CaCO₃, organic matter contents, and heavy texture adsorbed more zinc (Zn). The alkaline soils from Pakistan adsorbed more Zn than English acidic soils. Langmuir and Freundlich isotherm fit was excellent, and r² values for the Langmuir isotherm were highly significant (r²=0.84 to 0.99). The Langmuir b values, representing the adsorptive capacity of a soil, increased as the texture fineness increased in the soil, with increases in the concentration of adsorptive material (such as organic matter and CaCO₃) and with increases in CEC and pH. The alkaline soils from Pakistan had higher bonding energy constant and higher log Kf values than the acidic English soils. Sequential extraction of Zn in these soils showed that most of the Zn was held in CaCO₃ pool in the alkaline soils, whereas in acidic soils adsorbed Zn was in exchangeable form.     

Soil distribution in valleys according to stream order

While much research has been done on predicting the occurrence of waterlogged soils in small catchments, we need to improve our knowledge of the extension of these soils in large catchments. The aim of this study is to analyse the extent of soils with redoximorphic features in the valley bottomland domain as a function of the stream order.This study concerns a 10,000 km² catchment (River Vilaine, Armorican massif, north-western France) where valley bottomland soils commonly associated with hydromorphic characteristics cover up to 20% of the basin area. To describe the catchment organisation, we used the stream order classification of Strahler to test the behaviour of the topographic index in a large range of landscape morphological settings. Two methods were used to define the extent of the hydromorphic zone (HZ) in each valley bottomland according to stream order: (i) A field study based on mapping the HZ according to the occurrence of redoximorphic features along 60 transects; (ii) a modelling approach linking a DEM-derived topographic index to the digitized stream network of the River Vilaine.In view of the topographic factors, progressive valley widening may represent an enhancing factor of HZ extent. Thus, simple topographic index modelling predicts an increase in waterlogging in high-order channel settings (orders 6–7). By contrast, field mapping suggests that HZ extent remains stable with increasing order and decreases significantly for high-order settings (orders 6–7). Therefore, topographic index modelling appears effective in upper catchment settings (1st, 2nd and 3rd order). On the contrary, modelling efficiency is limited in high-order settings where the indices prove to be inappropriate: in such contexts, interactions between adjacent hillslope and HZ are of secondary importance. Along the longitudinal profile of the catchment, soil material near the streams shifts from having a colluvial origin in low-order to an alluvial origin in high-order settings. In high-order settings, the fine-scale valley bottomland topography and the spatial organisation of deposits control waterlogging duration and possibly play a major role in HZ extent. Finally, the integration of stream order data should considerably improve the efficiency of modelling the spatial distribution of soils over large catchments.     

Einfluß der Bodeneigenschaften auf die Freisetzung der gelösten organischen Substanz (DOM) aus dem Oberboden

Influence of Soil Properties on the Release of Dissolved Organic Matter (DOM) from the Topsoil A percolation experiment over a period of three month with small monoliths from forest and grassland soils varying in their anthropogenic changes was carried out to identify, to weigh and to quantify important soil properties for DOM release from the topsoil. Quality of soil organic matter determines the amount of DOM released from the topsoil if the soils have a low ability to adsorb and to precipitate DOM. Favorable conditions for high DOC concentrations in the soil solution are wide C/N ratios in the soil and in the hot water soluble fraction, a high soil content of hot water soluble organic carbon and a high portion of hot water soluble organic carbon in the total organic carbon content. Anthropogenic changes of the soils are effective to DOM release via changing quality of soil organic matter. Long dry periods and large water fluxes can further increase DOM release. The effects of soil organic matter can be disguised in soils with a high DOM retention capacity (high CEC, high content of exchangeable bases, Fe_ox and Fe_d). Then adsorption and precipitation determine DOM release from the topsoil and contribute to a decrease of DOM release.     

Volcanic ash additions control soil carbon accumulation in brown forest soils in Japan

Abstract

The objective of the present study was to clarify the influence of volcanic ash addition on soil carbon stocks and the carbon accumulation process in brown forest soils (BFS) in Japan. The degree of volcanic ash addition to the soil was estimated according to the acid ammonium oxalate extractable aluminum (Al_ox) and lithic fragment contents, and their vertical distribution patterns. The BFS was classified in order of increasing volcanic ash influence on the soil into the following types: high Al_ox content with no gravel (H-Al_ox-NGv), high Al_ox with a high gravel content (H-Al_ox-Gv), moderate Al_ox (M-Al_ox), and low Al_ox (L-Al_ox), and then analyzed for carbon content, carbon amount, carbon stock, Al_ox amount and pyrophosphate extractable aluminum (Al_py) amount. The correlation between the carbon and Al_py amounts and the relationship between the Al_py and Al_ox amounts in the BFS samples indicated that the amount of carbon is determined by Al—humus complex formation, which is defined by the active Al generated from additional volcanic ash in BFS soil samples of BFS. Therefore, soils with thicker horizons and greater amounts of Al_ox had higher carbon levels in deeper horizons. For this reason, soil carbon stocks at depths of 0–30 cm and 0–100 cm, and in the effective soil depth of BFS, were larger and followed the order H-Al_ox-NGv = H-Al_ox-Gv > M-Al_ox > L-Al_ox. Furthermore, successive accumulations of volcanic ash on the soil surface promoted soil carbon accumulation as a result of the development of the surface horizon in H-Al_ox-NGv BFS. Our results suggest that volcanic ash additions control the soil carbon accumulation of forest soil in Japan.