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Volcanic ash-derived soils are important globally for their C sequestration potential and because they are at risk of compaction and degradation due to land use change. Poorly or non-crystalline minerals impart enormous capacity for soils to store and stabilize C, but also unusual chemical and physical properties that make quantifying meaningful soil C pools challenging. Here, we optimize a soil physical fractionation method to effectively assess soil organic matter dynamics in volcanic ash soils by first comparing three common methods for Andisols of the same soil series under three land uses. Components of those methods that (1) effectively isolated C pools of different size and turnover and (2) demonstrated potential sensitivity to land use change were then modified for a final, combined method. The isolation of C pools corresponding to fundamental mechanisms of protection within aggregates and organo-mineral control on the stabilization of C, which often function to the extreme in volcanic ash soils, underlie these modifications. Combined application of ultrasonic energy to disrupt aggregates and the removal of light fractions with sequential high density fractionation successfully isolated multiple C pools that ranged in radiocarbon-based turnover time from 7 to 1,011 year in the surface 0–15 cm of mineral soil in an undisturbed, native forest. Soil C accumulates as a result of high, continuous input that cycles through a transitional (century-scale) organo-mineral pool and then either becomes occluded and protected within aggregates (multiple centuries) or enters a continuum of organo-mineral and non-crystalline mineral-dominated pools (from centuries to millennium-scale). Comparison of relative C pool sizes and C isotope signature among soils from native forest, grazed pasture, and managed Eucalyptus plantation revealed the potential for making accurate, direct measurements of soil C change over time with land use and management change or disturbance regime.

Volcanic ash soil aggregates can be disaggregated using heat under wet conditions. This study aimed to investigate factors affecting the disaggregation of volcanic ash soil aggregates in a field with organic cattle manure (M plot) and a field with chemical fertilizer (F plot) that were exposed to heat. The two-step wet sieving method, in which aggregates were sieved twice at different water temperatures for different times, was used to investigate the disaggregation caused by heat. It was found that increasing the temperature during the second sieve was more effective in disaggregating aggregates than extending the second-step sieve time. When the water temperature was increased to 80 °C, macroaggregates became more vulnerable, especially those in the F plot. The total carbon (TC) remaining in the soil aggregates was also measured after sieving. Although the TC content in aggregates decreased after sieving, there was only a minor relationship between decreasing TC content and the degree of disaggregation. This suggests that aggregates were not disaggregated by eluting binding agents containing carbon contents, but by partial breakage of the binding agent and/or the peeling of particles with binding agents from the aggregates.

This study was conducted to assess bacterial species richness, diversity and community distribution according to different fertilization regimes for 16 years in citrus orchard soil of volcanic ash. Soil samples were collected and analyzed from Compost (cattle manure, 2,000 kg/10a), 1/2 NPK+compost (14-20-14+2,000 kg/10a), NPK+compost (28-40-28+2,000 kg/10a), NPK (28-40-28 kg/10a), 3 NPK (84-120-84 kg/10a), and Control (no fertilization) plot which have been managed in the same manners with compost and different amount of chemical fertilization. The range of pyrosequencing reads and OTUs were 4,687–7,330 and 1,790–3,695, respectively. Species richness estimates such as Ace, Chao1, and Shannon index were higher in 1/2 NPK+compost than other treatments, which were 15,202, 9,112, 7.7, respectively. Dominant bacterial groups at level of phylum were Proteobacteria, Acidobacteria, and Actinobacteria. Those were occupied at 70.9% in 1/2 NPK+compost. Dominant bacterial groups at level of genus were Pseudolabrys, Bradyrhizobium, and Acidobacteria. Those were distributed at 14.4% of a total of bacteria in Compost. Soil pH displayed significantly closely related to bacterial species richness estimates such as Ace, Chao1 (p<0.05) and Shannon index (p<0.01). However, it showed the negative correlation with exchangeable aluminum contents (p<0.05). In conclusion, diversity of bacterial community in citrus orchard soil was affected by fertilization management, soil pH changes and characteristics of volcanic ash.

Key message The CO 2 effect on the root production of a broad-leaved community was insignificant when grown in brown forest soil, however, it was positively large when grown in volcanic ash soil. We evaluated the root response to elevated CO 2 fumigation of 3 birches ( Betula sp.) and 1 deciduous oak ( Quercus sp.) grown in immature volcanic ash soil (VA) or brown forest soil (BF). VA is a nutrient-poor, phosphorus-impoverished soil, broadly distributed in northern Japan. Each species had been exposed to either ambient (375–395 μmol mol −1 ) (aCO 2 ) or elevated (500 μmol mol −1 ) (eCO 2 ) CO 2 during the daytime (more than 70 μmol m −2  s −1 ) over 4 growing seasons. The results suggest that eCO 2 did not cause an increase in total root production when the community had grown in fertile BF soil, however, it did cause a large increase when the community was grown in infertile VA soil. Yet, carbon allocation to plant roots was not affected by eCO 2 in either the BF or VA soils. Rhizo-morphogenesis appeared to occur to a greater extent under eCO 2 . It seems that the saplings developed a massive amount of fine roots under the VA and eCO 2 conditions. Unexpectedly, eCO 2 resulted in a larger total root mass when the community was grown in VA soil than when grown in BF soil (eCO 2  × VA vs. eCO 2  × BF). These results may hint to a site-specific potential of communities to sequester future atmospheric carbon. The growing substance of plants is an important factor which root response to eCO 2 depends on, however, further studies are needed for a better understanding.

Key message Elevated CO 2 reduced fine root dynamics (production and turnover) of white birch seedlings, especially grown in volcanic ash soil compared with brown forest soil. Increased atmospheric CO 2 usually enhances photosynthetic ability and growth of trees. To understand how increased CO 2 affects below-ground part of trees under varied soil condition, we investigated the responses of the fine root (diameter <2 mm) dynamics of Japanese white birch ( Betula platyphylla var. japonica ) which was planted in 2010. The three-year-old birch seedlings were grown in four experimental treatments comprising two levels of CO 2 , i.e., ambient: 380–390 and elevated: 500 μmol mol −1 , in combination with two kinds of soil: brown forest (BF) soil and volcanic ash (VA) soil which has few nutrients. The growth and turnover of fine roots were measured for 3 years (2011–2013) using the Mini-rhizotron. In the first observation year, live fine root length (standing crop) in BF soil was not affected by CO 2 treatment, but it was reduced by the elevated CO 2 from the second observation year. In VA soil, live fine root length was reduced by elevated CO 2 for all 3 years. Fine root turnover tended to decrease under elevated CO 2 compared with ambient in both soil types during the first and second observation years. Turnover of fine root production and mortality was also affected by the two factors, elevated CO 2 and different soil types. Median longevity of fine root increased under elevated CO 2 , especially in VA soil at the beginning, and a shorter fine root lifespan appeared after 2 years of observation (2011–2012). These results suggest that elevated CO 2 does not consistently stimulate fine root turnover, particularly during the plant seedlings stage, as it may depend on the costs and benefits of constructing and retaining roots. Therefore, despite the other uncontrollable environment factors, carbon sequestration to the root system may be varied by CO 2 treatment period, soil type and plant age.

The possibility of using volcanic ash soils (VAS) or Andisols as a low-cost and natural adsorbent is investigated in this study for the removal of Cr (VI) from synthetic wastewater. Andisols can be used as adsorbent because they are characterized by the presence of non-crystalline secondary minerals such as allophane and imogolite that show variable charge characteristics and have the ability to retain cations and anions. The adsorption of Cr on to two VAS from Mt. Isarog and Mandalagan (B-Horizon), Philippines, was carried out at ambient temperature using batch adsorption studies. The effects of different parameters such as amount of adsorbent, contact time, initial Cr concentration and pH of the solution were investigated. The results showed that the VAS from Isarog is more effective in the removal of Cr than in Mandalagan. The maximum removal efficiency of the Isarog soil for a Cr concentration of 10 mg/L reached 89% with a dose of 20 g/L at a moderately acidic pH of 3. The Mandalagan soil on the other hand could remove only 65% at the same pH conditions and parameters. The difference in the removal of the two soils may be attributed to their physico-chemical properties in which the Isarog soil has higher clay content, porosity and lower bulk density. Isarog soil has fine particles with higher surface area and more active noncrystalline minerals and thus has higher removal efficiency than Mandalagan soil. Based on the results, the use of VAS from Isarog appears to be economical and an alternative to commercially available adsorbents for the removal of Cr from contaminated wastewater.

3-(3',4'-Dihydroxyphenyl)-l-alanine (l-DOPA), which is synthesized in velvet bean (Mucuna pruriens), inhibits plant growth. The concentration of l-DOPA in soil is reduced by adsorption and transformation reactions, which can result in the reduction of its plant-growth-inhibitory activity. To determine which part of the l-DOPA structure is involved in the adsorption and soil transformation reactions, we compared the kinetics of l-DOPA disappearance in a volcanic ash soil with that of l-phenylalanine (3-phenyl-l-alanine) and l-tyrosine (3-(4'-hydroxyphenyl)-l-alanine), compounds that are similar in structure to l-DOPA but do not have a catechol (o-dihydroxybenzene) moiety. l-Phenylalanine and l-tyrosine were not adsorbed and transformed in the soil at equilibrium pH values between 4 and 7. These results suggest that the adsorption and transformation reactions of l-DOPA in the soil involve the catechol moiety and not the amino and carboxylic acid groups, which are common to all three compounds. Like l-DOPA, (+)-catechin, another allelochemical that contains a catechol moiety, underwent adsorption and soil transformation reactions. Thus, we concluded that the concentrations of allelochemicals bearing a catechol moiety in soils will decrease rapidly owing to adsorption and transformation reactions, and this decrease will be faster in soils with a high pH value or high adsorption ability. Owing to this decrease in concentration, allelopathic phenomena may not occur.

Volcanic ash soils retain the largest and most persistent soil carbon pools of any ecosystem. However, the mechanisms governing soil carbon accumulation and weathering during initial phases of ecosystem development are not well understood. We examined soil organic matter dynamics and soil development across a high-altitude (3,560-3,030 m) 20-kyr climate gradient on Mauna Kea in Hawaii. Four elevation sites were selected (~250-500 mm rainfall), which range from sparsely vegetated to sites that contain a mix of shrubs and grasses. At each site, two or three pits were dug and major diagnostic horizons down to bedrock (intact lava) were sampled. Soils were analyzed for particle size, organic C and N, soil pH, exchangeable cations, base saturation, NaF pH, phosphorous sorption, and major elements. Mass loss and pedogenic metal accumulation (hydroxlamine Fe, Al, and Si extractions) were used to measure extent of weathering, leaching, changes in soil mineralogy and carbon accumulation. Reactivephase (SRO) minerals show a general trend of increasing abundance with increasing rainfall. However carbon accumulation patterns across the climate gradient are largely decoupled from these trends. The results suggest that after 20 kyr, pedogenic processes have altered the nature and composition of the volcanic ash such that it is capable of retaining soil C even where organic acid influences from plant material and leaching from rainfall are severely limited. Carbon storage comparisons with lower-elevation soils on Mauna Kea and other moist mesic (2,500 mm rainfall) sites on Hawaii suggest that these soils have reached only between 1% and 15% of their capacity to retain carbon. Our results suggest that, after 20 kyr in low rainfall and a cold climate, weathering was decoupled from soil carbon accumulation patterns and the associated influence of vegetation on soil development. Overall, we conclude that the rate of carbon supply to the subsoil (driven by coupling of rainfall above ground plant production) is a governing factor of forms and amount of soil organic matter accumulation, while soil mineralogy remained relatively uniform.

Acid mine drainage (AMD) properties are high acidity, high concentration of sulfate and containing a wide range of heavy metal ions. This study was performed to remove the copper, zinc and nickel from AMD by four native natural clay minerals including bentonite, two types of volcanic ash soil (VAS-I, VAS-II) and red soil using both batch and continuous techniques. The characteristics of adsorbents were investigated by X-ray fluorescence and X-ray diffraction analysis. In batch technique, clay minerals adsorbents were used in different dosages. However, in continuous technique one natural clay mineral (volcanic ash soil) was used. Results showed that the order of capacity of adsorption by four types of minerals was: bentonite > red earth > VAS-I > VAS-II. The adsorption affinity order of the metal ions by all sorbents was Ni > Cu > Zn. In continuous technique, the critical concentration (C/C0 = 0.1) of copper, zinc and nickel in the flow rate 0.8 mL/min was 350, 160 and 170 min, respectively. Therefore, fixed-bed column experiments showed that volcanic ash soils have the characteristics of simplicity of operation, little sludge production, suitability for low concentrations, flexibility and excellence for continuous and bath operations.

In this study, the effects of soil conditioners on the growth and development of melons and the rhizosphere soil environment were explored. The optimal amount of added soil conditioner was screened to solve the practical production problems of high-quality and high-yield thin-skinned melon. The melon variety “Da Shetou” was used as the material. Under the conditions of conventional fertilization and cultivation technology management, different soil conditioners were set up for potted melons. The effects of Pastoral soil (CK), 95% Pastoral soil + 5% volcanic ash soil conditioner (KT1), 85% Pastoral soil + 15% volcanic ash soil conditioner (KT2), 75% Pastoral soil + 25% volcanic ash soil conditioner (KT3), 65% Pastoral soil + 35% volcanic ash soil conditioner (KT4), and 55% Pastoral soil + 45% volcanic ash soil conditioner (KT5) on melon yield, quality, and rhizosphere soil characteristics were investigated. The soil microbial community was analyzed using Illumina MiSeq technology. Compared to CK, KT1, KT3, KT4, and KT5, the KT2 treatment could improve the single fruit yield of melon, increasing it by 4.35%, 2.48%, 2.31%, 5.92%, and 2.92%. Meanwhile, the highest contents of soluble protein, soluble solid, and soluble sugar in the KT2 treatment were 1.89 mg·100 g−1, 16.35%, and 46.44 mg·g−1, which were significantly higher than those in the control treatment. The contents of organic matter, total nitrogen, alkali-soluble nitrogen, nitrate nitrogen, ammonium nitrogen, available potassium, and available phosphorus in melon rhizosphere soil were the highest in the KT2 treatment. Through Alpha diversity analysis, it was found that the Chao1 index, Shannon index, and ACE index were significantly higher in the KT1 treatment than in the control, while, among all groups, the Simpson index and coverage were not significantly different. The dominant bacteria in the six treated samples were mainly Actinobacteriota, Proteobacteria, Cyanobacteria, Chloroflexi, Acidobacteria, Bacteroidetes, Myxomycota, Firmicutes, Gemmatimonadota, Verrucomicrobia, and Planctomycetes, which accounted for 96.59~97.63% of the relative abundance of all bacterial groups. Through redundancy analysis (RDA), it was found that the organic matter, electrical conductivity, available phosphorus, and nitrate nitrogen of melon rhizosphere soil were the dominant factors of bacterial community change at the dominant genus level. In summary, 15% ash soil conditioner applied on melon was the selected treatment to provide a theoretical reference for the application of soil conditioner in facility cultivation.