Water-soluble organic materials in paddy soil ecosystem

Abstract

A column experiment was conducted to analyse the composition of organic materials in the leachate from the plow layer and their fate in the subsoil. Water-soluble organic materials in the leachate were fractionated by insoluble polyvinylpyrrolidone (PVP) and ion exchange resins. The content of total organic carbon (TOC) in the leachate increased by the addition of rice straw (RS) to the plow layer soil sample. The leachate contained a constant amount of PVP-adsorbed Fraction, while that of the PVP-non-adsorbed Fraction changed during the 45 day incubation period. In the fractionation using ion exchange resins, the fraction adsorbed onto the anion exchange resin was the major one.

By the connection of a subsoil column to the plow layer soil column with RS, the TOC content in the leachate decreased by percolation into the subsoil sample. In the Anjo soil sample (Yellow Soil), the decrease occurred throughout the incubation period, and about 90% of the PVP-adsorbed Fraction in the leachate decreased by percolation into the subsoil sample. In the Fukushima soil sample (Gray Lowland Soil), the TOC content decreased in the early and middle periods of incubation, while in the late period the decrease was negligible. This decrease of the TOC content by percolation into the subsoil sample was mainly due to retention in the subsoil sample of the Anjo soil, while in the Fukushima soil sample it was due to decomposition and retention. It was considered that easily decomposable organic materials like organic acids were decomposed in the early to middle periods of incubation, while in the late period the contents of such substances in the leachate from the plow layer soil sample with RS were small and the decrease of TOC was negligible.     

CH4 production potential in a paddy soil exposed to atmospheric CO2 enrichment

Abstract

An anaerobic incubation experiment was conducted to investigate methane (CH₄) production potential in soil samples collected from a paddy field after exposure to free-air CO₂ enrichment (FACE). The FACE experiment with two CO₂ levels, ambient and ambient + 200 p.p.m.v CO₂ during the rice growing season, was conducted at Shizukuishi, Iwate Prefecture, Japan. The soil was a wet Andosol. Soil samples were taken from the surface (0–1 cm) and the sub-surface (1–10 cm) soil layers 2 months after rice harvest. Sub-samples of the fresh soils were put into glass bottles and submerged under N₂ gas headspace during the incubation. The results showed that, prior to incubation, the contents of total C and dissolved organic C (DOC) were significantly greater in FACE soil than ambient soil. During the incubation, CH₄ production potential was approximately 2–4-fold higher in FACE soil than ambient soil and approximately 500–1,000-fold greater in surface soil than sub-surface soil. In general, the FACE soil contained more DOC than ambient soil, particularly in the surface soil layer. These findings suggest that FACE treatment exerted long-term positive effects on CH₄ production and increased organic C content in this paddy soil, particularly in the surface soil layer.     

长期有机养分循环利用对红壤稻田土壤供氮能力的影响

通过15年的田间定位试验结合盆栽试验,研究了长期有机养分循环利用和不同化肥配施对红壤稻田土壤供氮能力的影响。结果表明,土壤有机碳、全氮、微生物生物量氮(MB-N)和土壤氮的矿化量与生物吸氮量有极显著的正相关关系,是良好的土壤供氮能力指标。长期有机养分循环利用或配合化肥施用能显著提高土壤有机碳、全氮含量和氮的矿化量,提高幅度分别为20.1%4～0.9%、0.460～.60.g/kg和55.0%(6周);明显提高土壤MB-N含量,提高幅度平均为70.3%。长期纯化肥处理对土壤碳、氮库的积累和氮的矿化量的提高作用甚微。盆栽试验表明,长期施用氮肥和氮、磷、钾肥土壤供氮量提高量极小,与长期不施肥相比提高幅度分别为2.1%和6.2%,而有机养分循环利用能显著提高土壤供氮量,提高幅度为33.7%8～9.0%。随着有机养分循环利用和NPK肥配合程度的提高,土壤供氮量提高幅度呈上升的趋势。     

有机物料输入稻田提高土壤微生物碳氮及可溶性有机碳氮

土壤微生物量碳、氮和可溶性有机碳、氮是土壤碳、氮库中最活跃的组分,是反应土壤被干扰程度的重要灵敏性指标,通过设置相同有机碳施用量下不同有机物料处理的田间试验,研究了有机物料添加下土壤微生物量碳（soil microbial biomass carbon,MBC）、氮（soil microbial biomass nitrogen,MBN）和可溶性有机碳（dissolved organic carbon,DOC）、氮（dissolved organic nitrogen,DON）的变化特征及相互关系。结果表明化肥和生物碳、玉米秸秆、鲜牛粪或松针配施下土壤微生物量碳、氮和可溶性有机碳、氮显著大于不施肥处理（no fertilization,CK）和单施化肥处理,分别比不施肥处理和单施化肥平均高23.52%和12.66%（MBC）、42.68%和24.02%（MBN）、14.70%和9.99%（DOC）、22.32%和21.79%（DON）。化肥和有机物料配施处理中,化肥+鲜牛粪处理的微生物量碳、氮和可溶性有机碳、氮最高,比CK高26.20%（MBC）、49.54%（MBN）、19.29%（DOC）和32.81%（DON）,其次是化肥+生物碳或化肥+玉米秸秆处理,而化肥+松针处理最低。土壤可溶性有机碳质量分数（308.87 mg/kg）小于微生物量碳（474.71 mg/kg）,而可溶性有机氮质量分数（53.07 mg/kg）要大于微生物量氮（34.79 mg/kg）。与不施肥处理相比,化肥和有机物料配施显著降低MBC/MBN和DOC/DON,降低率分别为24.57%和7.71%。MBC和DOC、MBN和DON随着土壤有机碳（soil organic carbon,SOC）、全氮（total nitrogen,TN）的增加呈显著线性增加。MBC、MBN、DOC、DON、DOC+MBC和DON+MBN之间呈极显著正相关（P<0.01）。从相关程度看,DOC+MBC和DON+MBN较MBC、DOC、MBN、DON更能反映土壤中活性有机碳和氮库的变化,成为评价土壤肥力及质量的更有效指标。结果可为提高洱海流域农田土壤肥力,增强土壤固氮效果,减少土壤中氮素流失,保护洱海水质安全提供科学依据。     

Changes in natural 15N abundance in paddy soils under different,long-term soil management regimes in the Tohoku region of Japan

Abstract

Long-term temporal changes in natural ¹⁵N abundance (δ¹⁵N value) in paddy soils from long-term field experiments with livestock manure and rice straw composts, and in the composts used for the experiments, were investigated. These field experiments using livestock manure and rice straw composts had been conducted since 1973 and 1968, respectively. In both experiments, control plots to which no compost had been applied were also maintained. The δ¹⁵N values of livestock manure compost reflected the composting method. Composting period had no significant effect on the δ¹⁵N value of rice straw compost. The δ¹⁵N values increased in soils to which livestock manure compost was successively applied, and tended to decrease in soils without compost. In soils to which rice straw compost was successively applied, the δ¹⁵N values of the soils remained constant. Conversely, δ¹⁵N values in soils without rice straw compost decreased. The downward trend in δ¹⁵N values observed in soils to which compost and chemical N fertilizer were not applied could be attributed to the natural input of N, which had a lower δ¹⁵N value than the soils. Thus, the transition of the δ¹⁵N values in soils observed in long-term paddy field experiments indicated that the δ¹⁵N values of paddy soils could be affected by natural N input in addition to extraneous N that was applied in the form of chemical N fertilizers and organic materials.     

Soil organic matter dynamics in lands continuously cultivated with maize and soybean in Heilongjiang Province,Northeast China: Estimation from natural 13C abundance

Abstract

Northeast China is the main production area of maize and soybean in China. In the present study, the rates of decomposition and replacement of soil organic carbon (SOC) were estimated using the soil inventory collected since 1991 from long-term maize and soybean cultivation plots in Heilongjiang Province, Northeast China, to evaluate the sustainability of the present cultivation system. The total carbon (C) content in soil was stable without any significant changes in the plots (approximately 28.5 g C kg^?1). The δ¹³C value of soil organic matter under continuous maize cultivation increased linearly with an annual increment of 0.07 from ?23.9 in 1991, which indicated that approximately 13% of the initial SOC was decomposed during the 13-year period of maize cultivation, with a half-life of 65 years. Slow decomposition of SOC was considered to result from the low annual mean temperature (1.5°C) and long freezing period (170–180 days year^?1) in the study area. In contrast, the amount of organic C derived from maize increased in the soil with a very slow annual increment of 0.17 g C kg^?1, probably because of the removal of all the plant residues from the plots. Based on the soil organic matter dynamics observed in the study plots, intentional recycling/maintenance of plant residues was proposed as a way of increasing soil fertility in maize or soybean cultivation.     

Effects of long-term application of various organic amendments on soil particulate organic matter storage and chemical stabilisation of vertisol soil

Current understanding of the effects of long-term application of various organic amendments on soil particulate organic matter (POM) storage and chemical stabilisation remains limited. Therefore, we collected soil samples from the soil profile (0–100?cm) under six treatments in a 31-year long-term fertilisation experiment: no fertiliser (CK), mineral fertilisers (NPK), mineral fertilisers plus 3.8 or 7.5?t?ha^?1?year^?1 (fresh base) the amount of wheat straw (1/2SNPK and SNPK) and mineral fertilisers plus swine or cattle manure (PMNPK and CMNPK). Long-term incorporation of wheat straw and livestock manure amendments significantly (p?<?0.05) increased crop yield and sustainable yield index, and POM storage compared with CK and NPK treatments. The mole ratios of H/C in the POM under organic amendment treatments significantly (p?<?0.05) decreased by 13.8% and 37.1%, respectively, compared with the NPK treatment. Similarly, solid state NMR spectroscopy showed that the O–alkyl carbon content of POM was greatly decreased, whereas aromatic carbon contents and alkyl to O–alkyl carbon ratios were substantially increased under PMNPK and CMNPK treatments. In conclusion, we recommend long-term livestock manure application as a preferred strategy for enhancing POM quantity and quality (chemical stability), and crop yield of vertisol soil in northern China.     

长期定位试验条件下的水稻田土壤有机质含量变化研究

依托湖北武汉、重庆北碚、湖南望城、湖南祁阳、江西南昌、浙江杭州6个水稻土壤肥力长期定位试验历史样品及数据,分析和讨论了土壤有机质含量变化趋势及对施化肥和有机肥的响应差异。施有机肥提升土壤有机质含量显著高于施化肥的效果。施化肥NPK处理,6个试验点土壤有机质含量都呈现提升趋势;但是,有机质平均年增量、有机质累计增量与累计有机肥施用量的比值都是逐年下降的,固定施肥方法提高土壤有机质含量是有限的,最高达到平衡点,施化肥的有机质含量的平衡点低于施有机肥的,土壤有机质含量提升不仅对施有机肥有响应,而且与累积产量也有一定的相关关系。     

氮肥、土壤湿度和温度对稻田土壤甲烷氧化的影响

Effects of nitrogen fertilizer,soil mosture and temperature and temperature on methane oxidation in paddy soil were investigated under laboratory conditions.Addition of 0.05 g N kg^-1 soil as NH4Cl strongly inhibited methane oxidation and addition of the same rate of KCl also inhibited the oxidation but with more slight effect,suggesting that the inhibitory effect was partly caused by increase in osmotic potential in microorganism cell,Not only NH4^ but also NO3^- greatly affected methane oxidation.Urea did not affect methane oxidation in paddy soil in the first two days of incubation,but strong inhibitory effect was observed afterwards.Methane was oxidized in the treated soil with an optimum moisture of 280 g kg^-1 ,and air-drying inhibited methane oxidation entirely.The optimum temperature of methane oxidation was about 30℃ in paddy soil.while no methane oxidation was observed at 5℃or 50℃。     

Compound-specific δC and δH analyses of plant and soil organic matter: A preliminary assessment of the effects of vegetation change on ecosystem hydrology

Here we present δ¹³C and δ²H data of long-chained, even-numbered (C₂₇-C₃₁) n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland-woodland vegetation sequence in central Queensland, Australia. Our data show that δ¹³C values of the C4 grassland species were heavier relative to those of C3 tree species from the woodland (Acacia leaves) and woody grassland (Atalaya leaves). However, n-alkanes from the C4 grasses had lighter δ²H values relative to the Acacia leaves, but showed no significant difference in δ²H values when compared with C3 Atalaya leaves. These results differ from those of previous studies, showing that C4 grasses had heavier δ²H values relative to C3 grasses and trees. Those observations have been explained by C4 plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water (“Two-layered soil-water system”). By comparison, our data suggest that ecosystem changes (vegetation “thickening”) can significantly alter the soil hydrological characteristics. This is shown by the heavier δ²H values in the woodland soil compared with lighter δ²H values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated δ²H values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter δ²H values (avg. −46‰) compared with water accessed by trees in the woodland vegetation (avg. −7‰). These data suggest that in semi-arid environments the “two-layer” soil water concept might not apply. Furthermore, our data indicate that compound-specific δ²H and δ¹³C analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems.     

Tillage effect on organic carbon in a purple paddy soil

The distribution and storage of soil organic carbon （SOC） based on a long-term experiment with various tillage systems were studied in a paddy soil derived from purple soil in Chongqing, China. Organic carbon storage in the 0-20 and 0-40 cm soil layers under different tillage systems were in an order： ridge tillage with rice-rape rotation （RT-rr） 〉 conventional tillage with rice only （CT-r） 〉 ridge tillage with rice only （RT-r） 〉 conventional tillage with rice-rape rotation （CT-rr）. The RT-rr system had significantly higher levels of soil organic carbon in the 0-40 cm topsoil, while the proportion of the total remaining organic carbon in the total soil organic carbon in the 0-10 cm layer was greatest in the RT-rr system. This was the reason why the RT-rr system enhanced soil organic carbon storage. These showed that tillage system type was crucial for carbon storage. Carbon levels in soil humus and crop-yield results showed that the RT-rr system enhanced soil fertility and crop productivity. Adoption of this tillage system would be beneficial both for environmental protection and economic development.     

有机物料与化肥配施提高黄泥田水稻产量和土壤肥力

农业有机物料具有资源化再利用的特点,与化肥配施既可以保证作物产量,也可以提升地力。为了建立最适宜的南方低产黄泥田培肥模式,该文在浙江金衢盆地开展3年田间试验研究化肥与不同有机物料(菇渣、紫云英、牛粪和秸秆)配施对水稻产量和土壤肥力的影响。结果表明:1)有机物料与化肥配施可以显著提高水稻产量,化肥+菇渣、化肥+紫云英、化肥+牛粪和化肥+秸秆处理下,水稻三年的平均产量分别比单施化肥提高了9.7%、9.5%、12.3%和9.5%;2)有机物料与化肥配合施用,土壤有机质、土壤养分(全氮、有效磷、速效钾和CEC)及土壤容重较单施化肥处理有一定程度的改善,其中,化肥+牛粪效果最明显,有机质质量分数提高了12.5%,土壤有效磷质量分数提高了37.7%,CEC提高了16.1%;3)与单施化肥相比,化肥+菇渣、化肥+牛粪处理下5 mm机械稳定性大团聚体分别提高了10.4%和6.7%,各配施处理均显著降低了团聚体破坏率。总得来讲,连续三年有机物料与化肥配施较单施化肥处理提高了水稻产量、改善了土壤肥力状况、增加了土壤团聚体稳定性,其中又以牛粪与化肥配合施用效果最佳。     

Priming effect of the addition of maize to a Japanese volcanic ash soil and its temperature sensitivity: a short-term incubation study

ABSTRACT

The response of soil organic matter (SOM) to global warming is a crucial subject. However, the temperature sensitivity of SOM turnover remains largely uncertain. Changes in the mineralization of native SOM, i.e., priming effect (PE) may strongly affect the temperature sensitivity of SOM turnover in the presence of global warming. We investigated the direction and magnitude of the PE in a Japanese volcanic ash soil at different temperatures (15°C, 25°C, and 35°C) using a natural ¹³C tracer (C4-plant, maize leaf) in a short-term (25 days) incubation study. In addition, we evaluated the temperature sensitivity expressed as Q₁₀ value with and without the addition of maize to the soil and their relations to PE. We found that positive PE occurred at each temperature condition and tended to increase with decreased temperature, and these PE results were consistent with the microbial biomass at the end of the incubation period. CO₂ emission from control soil (without maize) increased with increasing temperature (Q₁₀ = 2.6), but CO₂ emission from the soil with added maize did not significantly change with increasing temperature (Q₁₀ = 1.0). This was caused by the suppression of CO₂ emission from the soil with increasing temperature (Q₁₀ = 0.9). On the other hand, soil-originated CO₂ emission clearly increased with increasing temperature (Q₁₀ = 3.4) when Q₁₀ values were calculated on the assumption that the temperature and substrate supply increase at the same time (from 25°C). These results suggest that not only the temperature increase but also the labile carbon supply may be important for the temperature sensitivity of Japanese volcanic ash soil.     

Chemical characteristics of glomalin-related soil protein (GRSP) extracted from soils of varying organic matter content

Glomalin is described in the literature as a N-linked glycoprotein and the putative gene product of arbuscular mycorrhizal fungi (AMF). Since the link between glomalin and various protein fractions in soil is not yet clearly defined, glomalin-related soil protein (GRSP) more appropriately describes glomalin's existence in natural organic matter (NOM). The objective of this study was to examine the chemical characteristics of GRSP present in several mineral and organic soils of varying organic carbon content. GRSP was isolated using high temperature sodium citrate extraction followed by either trichloroacetic acid (TCA) or hydrochloric acid (HCl) precipitation. GRSP was characterized by quantitative solid-state ¹³C DPMAS NMR, infrared (IR) spectroscopy, elemental analysis, and the Bradford assay for protein content. GRSP accounted for 25% and 52% of total C in the mineral soils and organic soil, respectively. Molar C/N and H/C ratios reveal that GRSP has less nitrogen than bovine serum albumin (BSA), and that GRSP extracted from the Pahokee peat soil possessed a more unsaturated, and thus aromatic character relative to the mineral soil GRSP, respectively. GRSP's high aromatic (42-49%) and carboxyl (24-30%) carbon contents and low aliphatic (4-11%) and carbohydrate-type carbon contents (4-16%) suggests that GRSP does not resemble a typical glycoprotein. In fact, the NMR spectra of GRSP closely resemble that of humic acid. GRSP extracted from mineral and organic soils possessed the same NMR fingerprint regardless of the precipitation method used (i.e., either TCA or HCl). It is likely that the current GRSP extraction methods, because of their similarity to the method used to extract humic acid, are coextracting both materials.     

Influence of flooding on exchangeability and release of stable and radioactive cesium in contaminated paddy soil

ABSTRACT

An incubation experiment was conducted to clarify how soil flooding influences the mobility of radioactive cesium (RCs: ¹³⁴Cs and ¹³⁷Cs) in paddy soil after aging, focusing on the effects of ammonium increase and soil reduction. We used two contaminated paddy soils (A and B, both Gleyic Fluvisols) aged for 26 months after the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant accidents, and analyzed both the RCs and stable cesium (¹³³Cs). The soils were incubated in a flooded condition, with or without the addition of ammonium fertilizer, for 1, 15 and 30 d, and in an unflooded condition for 30 d. After the incubation periods, we quantified ¹³³Cs concentration in the soil solution, ¹³³Cs and RCs released from soil in 0.01 mol L^–1 calcium chloride solution as parameters of release intensity, and ¹³³Cs and RCs extracted with 1 mol L^?1 ammonium acetate solution as exchangeable quantities, and analyzed their relationship with ammonium content and redox condition in soil. The increase of ammonium by both ammonification and fertilizer application promoted release of exchangeable ¹³³Cs and RCs to the soil solution and calcium solution. When ammonium content became low during unflooded incubation, exchangeable ¹³³Cs and RCs themselves decreased. When soil reduction progressed with flooded incubation, however, exchangeable ¹³³Cs and RCs decreased, despite high ammonium content. To estimate the influence of soil reduction on the exchangeability of RCs, soil A was sequentially extracted with sodium hydrosulfite (a reducing agent) and ammonium acetate solutions. Compared with a control treatment using sodium sulfate instead of sodium hydrosulfite, the total RCs extracted by the reducing treatment was 42% lower, indicating that soil reduction decreases RCs exchangeability. Through these analyses of ¹³³Cs and RCs, we conclude that soil flooding influences the mobility of aged RCs through two opposite effects: the release of exchangeable RCs into soil solution is enhanced with increased ammonium, while the exchangeable RCs itself decreases due to soil reduction.     

有机物料碳和土壤有机碳对水稻土甲烷排放的影响

基于30年水稻土长期施肥定位试验,在保证原有定位试验正常开展的前提下,将部分化肥处理变更为有机肥处理(或反之),通过观测一年水稻轮作周期内不同处理甲烷(CH_4)排放通量季节性变化,探讨不同肥力水稻土中外源有机碳及土壤有机碳含量对田间CH_4排放的影响。结果表明:施化肥处理和有机肥处理,水稻土全年CH_4累积排放量范围分别为1.73~4.72和35.09~86.60 g·m~(-2)。有机肥处理改施化肥后,田间土壤CH_4的排放量显著降低;化肥处理改施有机肥或有机肥处理增施有机肥后,田间土壤CH_4的排放量显著提高。外源有机碳的输入量是田间土壤CH_4年排放量的决定性因素,外源有机碳输入量(x)与水稻土CH_4年累积排放量(y)之间满足直线方程:y=0.087 7 x+3.265 7(R~2=0.965 9,n=21)。土壤有机碳同样也是影响稻田CH_4排放的因素,在不同有机碳水平的水稻土上施用等量相同化肥或有机肥,土壤有机碳含量高的水稻土都更有利于CH_4的产生。单施化肥稻田土壤CH_4排放的最主要碳源是土壤有机碳,有机碳含量(x)和水稻土CH_4年累积排放量(y)之间的指数方程:y=0.162 4 e~(0.162 2 x)(R~2=0.940 6,n=9)。有机肥可促进土壤有机碳分解释放CH_4,土壤有机碳含量相同的条件下,高量有机肥比常量有机肥的土壤有机碳分解比率高0.65%,等量相同有机肥但土壤有机碳含量不同的条件下,土壤有机碳分解比率无显著差异;同样,土壤有机碳也可促进有机物料碳分解释放CH_4,在常量有机肥或高量有机肥处理中,土壤有机碳含量高者比低者的有机物料碳分解比率分别多出3.57%和2.34%。     

配施有机肥对潜育化水稻土的培肥效果

为探索潜育化水稻土的培肥措施,通过田间试验研究了有机无机肥配施对潜育化低产水稻土产量和土壤养分有效性的影响。结果显示:连续2季施用有机肥后,潜育化水稻土产量显著提高,以早稻配施紫云英-晚稻配施猪粪处理的产量最高。添加有机肥降低了潜育化水稻土耕层土壤p H值。早稻配施紫云英,晚稻配施猪粪处理的有机质提高明显,比NPK处理提高了15.22%;早稻配施紫云英,晚稻配施猪粪处理的潜育化水稻土全氮含量比单施化肥处理提高4.19%;早稻配施紫云英,晚稻单施化肥处理的潜育化水稻土有效磷含量提高最多,比NPK处理提高了4.92%;相比于NPK处理,早稻配施紫云英,晚稻单施化肥处理的土壤速效钾含量比两季均单施化肥处理提高了13.2%。因此,潜育化水稻土适当配施有机肥可以提高土壤养分有效性,增加水稻产量,可以作为培肥改良潜育化水稻土的参考方法。     

The mineralization rate of black soil carbon in the deep layers of Japanese volcanic ash soil may be easily accelerated by labile carbon supply

ABSTRACT

The stability of black soil carbon in the deep layers of Japanese volcanic ash soil (i.e., buried A horizons) is often explained by its unique chemical (molecular structure) and physical (associated with short-range-order minerals) recalcitrance. However, the stability of black soil C in buried A horizons may be changed by labile C supply for soil microbes. Here, we hypothesized that the mineralization of black soil C in buried A horizons of Japanese volcanic ash soil could be easily accelerated by a supply of labile C (i.e., a priming effect; PE). To test our hypothesis, we investigated the direction and magnitude of the PE with a buried A horizon in Japan using ¹³C-labeled glucose (2.188 atom %) in a short-term (21 days) incubation study. We also investigated the effect of mineral nitrogen (N), which could contribute to microbial activity in this incubation study. We found that a positive PE occurred by glucose supply with (182%) or without (181%) mineral N input over the 21-day incubation, and its values were very similar to the PE ratios previously reported in other deep soils. The estimated mean residence time (MRT) of black soil C considering PE was clearly accelerated by glucose supply, regardless of mineral N input, compared with the initial soil MRT. These results strongly support our hypothesis that the mineralization rate of black soil C in buried A horizons is easily accelerated by a labile C supply, and it also demonstrates important implications for the effects of global warming on buried A horizons (e.g., increased root exudation, fine root biomass supply, and N deposition) in Japanese volcanic ash soils.