Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

Deforestation of native tropical forests has occurred extensively over several decades. The plantation of fast-growing trees, such as <italic>Acacia</italic> spp., is expanding rapidly in tropical regions, which can contribute to conserve the remaining native tropical forests. To better understand belowground biogeochemical cycles and the sustainable productivity of acacia plantations, we assessed the effects of vegetation (acacia plantations vs. native forests) and soil types (Oxisols vs. Ultisols) on soil properties, including the diversity and community structures of bacteria- and fungi-colonizing surface and subsurface roots and soil in the Central Highlands of Vietnam. The results in surface soil showed that pH was significantly higher in acacia than in native for Oxisols but not for Ultisols, while exchangeable Al was significantly lower in acacia than in native for Ultisols but not for Oxisols. Bacterial alpha diversity (especially within phylum Chloroflexi) was higher in acacia than in native only for Oxisols but not for Ultisols, which was the same statistical result as soil pH but not exchangeable Al. These results suggest that soil pH, but not exchangeable Al, can be the critical factor to determine bacterial diversity. Acacia tree roots supported greater proportions of copiotrophic bacteria, which may support lower contents of soil inorganic N, compared with native tree roots for both Oxisols and Ultisols. Acacia tree roots also supported greater proportions of plant pathogenic <italic>Mycoleptodiscus</italic> sp. but appeared to reduce the abundances and diversity of beneficial ECM fungi compared with native tree roots regardless of soil types. Such changes in fungal community structures may threaten the sustainable productivity of acacia plantations in the future.

DOI： 10.3389/fmicb.2021.735121

PubMed

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apsoil.2021.103884

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apsoil.2021.104027

Authorship：Lead author,　Corresponding author  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The forest-savanna transition zone is widely distributed on nutrient-poor Oxisols in Central Africa, and a population explosion has led to the rapid cultivation of these vegetation types in this zone. To reveal and compare the effects of short-term (3 years) cultivation on the soil nutrient status of the forest and savanna vegetation in this area, we evaluated microbial nutrient limitation and availability by conducting hourly measurements of soil microbial respiration after the addition of glucose in combination with nitrogen (N) and/or phosphorus (P) to soils that were collected from a forest site (FOR), a savanna site (SAV), as well as cropland for 3 years derived from a forest (Crop-F) and a savanna (Crop-S), in eastern Cameroon. The N addition had little effect on the pattern of microbial respiration rate for the FOR and Crop-F sites, indicating N rich for microbes. In contrast, N addition resulted in the increases in maximal respiration rates after the exponential increase for the SAV and Crop-S sites, indicating microbial N limitation, and cultivation accelerated the soil N depletion. Furthermore, we observed that P addition resulted in the increase in the maximal respiration rates, indicating microbial P limitation for all sites, except for FOR site. Since the cultivation significantly affected the microbial properties only in the forest ecosystem, such as the increase in the microbial specific growth rate and the decreased microbial C:N and C:P ratios, these changes would induce the P limitation for Crop-F. These results indicate that (1) the FOR site was a N-rich ecosystem for soil microbes, and 3 years of cultivation in the Crop-F site did not alter the high soil N status but induced microbial P limitation, with the changes in the microbial properties, and that (2) the SAV site was N and P limited for soil microbes, and 3 years of cultivation clearly decreased the soil N availability.

DOI： 10.1080/00380768.2018.1517585

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

There are contrasting results regarding the effect of repeated drying-rewetting (DRW) cycles on microbial biomass carbon (MBC), and two fundamentally different mechanisms have been postulated. The first is a microbial stress mechanism which will reduce MBC as stress-sensitive microbes die, and the second is a substrate supply mechanism which will increase MBC through the release of a microbial substrate from non-biomass soil organic carbon (C). However, the balance of these two mechanisms has not been fully examined for various soils with different climatic histories, especially for soils from humid areas. We hypothesized that soils subjected to fewer DRW events would be largely affected by the stress mechanism. Therefore, the effect of repeated DRW cycles on MBC and C dynamics was investigated and compared to that of a moist control treatment for four soils with different DRW histories. The first DRW significantly reduced the MBC for soils with less DRW but not for soils with more DRW. However, when comparing the sizes of MBC after 28 days of four DRW cycles and the moist treatment, the result was not related to the microbial resistance of each DRW. Cumulative respired CO2-C over a 4-day moist period after each DRW was always significantly greater than that in the moist treatment even when the MBC was not reduced by the DRW. The results with no change in MBC suggested that the substrate supply mechanism rather than the stress mechanism would be essential for the effect of repeated DRW cycles on MBC and C dynamics.

DOI： 10.1016/j.apsoil.2017.07.023

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER  

The fates of carbon (C) input to soils widely vary under different nutrient availability. In this study, both initial substrate-induced respiration (SIR) rates and substrate-induced growth responses to nitrogen (N) and/or phosphorus (P) additions were investigated together because the two approaches had been assessed separately in previous studies. A wide range of Japanese, Thai and Kazakh soils with different land uses were used, including acidic Japanese forest soils with low N availability, which has not been fully examined. We hypothesized that microbes in Japanese forest soils would mainly respond to N addition. The results showed that N addition decreased initial SIR rates to 75-87% for the acidic Japanese and Thai forest soils probably due to the increases in substrate C use efficiencies, but had no effect for other soils. P addition tended to increase initial SIR rates. In addition, P but not N addition resulted in the increases of specific microbial growth rates for all tested soils. In conclusion, soil microbes responded to P addition with enhanced their growth rates and initial SIR rates, whereas they responded to N addition with reduced initial SIR rates probably through efficient use of C only for the Japanese and Thai forest soils. Therefore, nitrogen availability will have a crucial role in microbial use of substrate C especially for acidic forest soils with low N availability.

DOI： 10.1016/j.ejsobi.2017.10.002

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Many surface soils, including those that have rarely experienced dry conditions, may experience more frequent and/or intense drying-rewetting (DRW) events in the future. Such DRW events are likely to induce large and sudden CO2 pulses derived from increased substrate availability owing to their release from non-biomass soil organic C (SOC) and microbial biomass C (MBC); however, few studies have investigated respiration rates following DRW at high time resolutions (e.g., 1 h) or in soils from humid areas. In this study, DRW effects on the dynamics of respiration rates at hourly intervals for 12 h and substrate-induced respiration (SIR) rates were investigated. Soils previously subjected to different DRW frequencies were collected from forest and arable or grassland sites in Japan (humid), Thailand (semi-humid) and Kazakhstan (semi-arid). The relatively humid Japanese and Thai soils were further subjected to five DRW cycles in the laboratory to compare the effects of the first and fifth DRW. Respiration rates after the first and fifth DRW and first DRW to the Japanese and Thai forest soil, respectively, were not fitted by models employing exponentially-decaying functions, and were initially similar to SIR rates. In such cases, C-saturated conditions for surviving microbes would occur probably because of increased substrate availability following release from dead MBC, suggesting the importance of incorporating microbial parameters into SOC models. Respiration rates after DRW to other samples decreased exponentially to a constant rate. In such cases, when increases in the labile and stable C pools by DRW were estimated by the respiration kinetics, the labile C pool would be primarily derived from dead MBC. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsoil.2016.02.010

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Microbial nitrogen (N) immobilization following the addition of organic materials to soils regulates soil N availability, which affects plant growth and N leaching from soils. In this study, the potential for microbial N immobilization was evaluated by short-term incubation experiments following the addition of available carbon (C) under non-limiting conditions of N and phosphorus (P) to seven Japanese arable and forest soils. Glucose was added as a model substrate at concentrations close to microbial biomass C. The forest soils had lower pH and smaller increases in respiration rates after the glucose addition, and higher organic and biomass C compared to the arable soils. Microbial N immobilization, estimated by net decreases in extractable N, was significantly correlated with the concentrations of added glucose and was on average 43mgNg(-1) glucose C during 3- and 7-day incubation for all soils. Net increases in biomass N measured by the chloroform fumigation-extraction method using the common conversion factor of 0.54 at 3 and 7days after the glucose addition were lower than the microbial N immobilization for all soils, and the biomass N accounted for a smaller portion of immobilized N in the arable soils than in the forest soils. Therefore, the present study suggests that microbial N immobilization would be dependent on the concentrations of available C in organic materials and higher than the increases in biomass N, especially for arable soils when organic materials are added to soils under non-limiting conditions of N and P.

DOI： 10.1080/00380768.2015.1075364

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The wet-dry cycles of soil primarily drive carbon (C) dynamics in dry croplands that mainly experience sporadic rainfall events. We evaluated the in situ short-term (hourly) dynamics of soil carbon dioxide (CO2) efflux and microbial biomass, to compare the significance of a single rainfall event with/without C substrate to reveal the effects of a single rainfall on the soil C dynamics in clayey dry croplands in four different climates and ecosystems. The experiments were conducted on four clayey dry croplands as follows: Thailand (TH) and Tanzania (TZ) in tropical climates, and Kazakhstan (KZ) and Hungary (HG) in continental climates. Hourly measurements of soil CO2 efflux, in situ microbial biomass (MB) and in situ microbial activity (qCO(2)) were conducted after the application of simulated rainfall (W plots) and rainfall/glucose (WG plots) treatments. We also evaluated the easily mineralizable carbon (EMC) by incubation. The rainfall treatment caused an increase in the qCO(2) but not in MB, causing a clear but short C flush in all W plots (10-37h), while the WG treatment caused an increase both of qCO(2) and MB, resulting in substantially longer and larger C flush in the WG plots (ca. 100h). The ratio of the cumulative soil CO2 flux caused by rainfall treatment to EMC was larger in TH-W and TZ-W plots (8.2 and 4.9%, respectively) than in the KZ-W and HG-W plots (2.9 and 1.1%, respectively). In addition, applied glucose was more heavily mineralized in the TH-WG and TZ-WG plots (15.0 and 9.7%, respectively) than in the KZ-WG and HG-WG plots (6.4 and 3.4%, respectively), because of the different MB increment patterns for the first 24h, i.e., immediate and large MB increments in TH and TZ, but not in KZ and HG. These results reveal a possible mechanism that causes the rapid decomposition of soil organic carbon and applied organic matter in the dry tropical cropland.

DOI： 10.1080/00380768.2015.1018800

Web of Science

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPANESE SOC MICROBIAL ECOLOGY, DEPT BIORESOURCE SCIENCE  

Wet and dry anaerobic fermentation processes are operated for biogas production from organic matter, resulting in wet and dry digestates as by-products, respectively. The application of these digestates to soil as fertilizer has increased in recent years. Therefore, we herein compared the effects of applying wet digestates (pH 8.2, C/N ratio 4.5), dry digestates (pH 8.8, C/N ratio 23.4), and a chemical fertilizer to Japanese paddy and upland soils on short-term nitrification under laboratory aerobic conditions. Chloroform-labile C, an indicator of microbial biomass, was only minimally affected by these applications, indicating that a small amount of labile N was immobilized by microbes. All applications led to rapid increases in NO3-N contents in both soils, and ammonia-oxidizing bacteria, but not archaea may play a critical role in net nitrification in the amended soils. The net nitrification rates for both soils were the highest after the application of dry digestates, followed by wet digestates and then the chemical fertilizer in order of decreasing soil pH. These results suggest that the immediate effects of applying digestates, especially dry digestates with the highest pH, on nitrate leaching need to be considered when digestates are used as alternative fertilizers.

DOI： 10.1264/jsme2.ME14080

Web of Science

PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：the Japanese Society of Soil Microbiology  

DOI： 10.18946/jssm.67.1_32

CiNii Books

Other Link： http://agriknowledge.affrc.go.jp/RN/2010852139

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Society of Soil Microbiology  

DOI： 10.18946/jssm.65.1_41

CiNii Books

Other Link： http://agriknowledge.affrc.go.jp/RN/2010810824

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

Short-term dynamics of soil respiration rates over time measured at hourly intervals during less than 12 h after microbial substrates were added to soils can be classified into first- order, zero-order and growth-associated types. To simulate the zero-order type respiration rates, a model using Michaelis-Menten kinetics is proposed, because this kinetics model includes a maximum respiration rate but no increase in microbial biomass. In this model, soil respiration by microorganisms was assumed to be the sum of the mineralization of easily available substrates (R), which include both added glucose (G) and substrates released by disturbance such as a mixing treatment (D) and constant mineralization under steady state conditions. By analyzing the short-term dynamics of previously published respiration rates, which show no increase with time, for a Kazakh forest, a Japanese forest and a Japanese arable soils, the parameter values of z(r) and D(0), which indicate the ratio of respired to utilized R and the initial concentration of D, respectively, were estimated. This allowed simulation of the decreasing concentrations of R and estimation of the parameters V(max) and M in the Michaelis-Menten equation. Simulations using the obtained parameter values matched the measured data well. Correlation coefficients (r(2)) and root mean square errors (RMSE) indicated that the simulations usually matched the measured data, which included not only zero-order respiration rates but also first- order respiration rates. Therefore, the proposed model using Michaelis-Menten kinetics can be used to simulate the short-term dynamics of respiration rates, which show no increase over time, when easily available substrates would be added in soils.

DOI： 10.1111/j.1747-0765.2010.00524.x

Web of Science

J-GLOBAL

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

It is widely recognized that the efficiency of substrate C use in acid and/or disturbed soils by soil microorganisms is relatively low based on the observation that metabolic quotients (qCO(2)) are usually high in these soils. In the present study, threshold concentrations of glucose, at which the ratio of respiration to assimilation by soil microorganisms began to increase, were comparatively analyzed using two soils differing in pH and disturbance, a Japanese arable soil disturbed by cultivation and a strongly acidic Japanese forest soil. Varying concentrations of glucose C, generally less than those in the microbial biomass C, were added to the two soils. The ratio of respired to utilized glucose C remained at approximately 20% when lower concentrations of glucose were added and respiration rates did not increase (zero-order types), whereas the ratio increased when the concentrations of added glucose exceeded a certain level and respiration rates increased (growth-associated types). The substrate-induced respiration rate a few hours after the addition of glucose increased only for the growth-associated types, although chloroform-labile C increased in both types as the concentrations of added glucose increased. The results clearly confirmed the presence of a threshold concentration of glucose above which the ratio of respiration to assimilation increased. The threshold concentrations in Japanese arable and forest soils were lower than the concentration previously reported in a moderately acidic Kazakh forest soil. The lower threshold concentrations observed in the Japanese arable and forest soils are considered to result from different microbial growth characteristics after the addition of glucose linked with a shorter lag period before the exponential increase of the respiration rate and a lower ratio of substrate induced respiration rate to biomass C, respectively. The results suggest that the efficiency of substrate C use in acid and disturbed soils is relatively low in situations where higher concentrations of substrates are occasionally supplied with temporal C &apos;flushes&apos;, such as may occur in the rhizosphere or in the vicinity of plant residues.

DOI： 10.1111/j.1747-0765.2009.00400.x

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

Soil microbial biomass C, the metabolic quotient ([qCO(2)] respiration rate to biomass ratio) and growth characteristics, such as lag period and specific respiration increment after the addition of glucose to the soil, were determined for 19 surface soils varying widely in pH and land use under different climatic conditions in Asia. The soil samples included natural undisturbed forest and grassland soils, and disturbed soils affected by cultivation or slash and burn agricultural practices. Although chloroform labile C was significantly correlated with the substrate-induced respiration (SIR) rate, the ratio of chloroform labile C to SIR was negatively correlated with soil pH. The SIR-biomass C to organic C ratio was significantly correlated with soil pH, but this correlation was not observed between pH and the biomass C to organic C ratio using the fumigation-extraction (FE) method. Similarly, the qCO(2) measured using SIR-biomass C (SIR-qCO(2)) was negatively correlated with soil pH, whereas the qCO(2) measured using FE-biomass C (FE-qCO(2)) was not correlated with pH. These results were in contrast to several reports on the significant correlations between soil pH and FE-biomass C to organic C ratio and FE-qCO(2) using soil samples collected from a relatively narrow range of climatic conditions. Therefore, it could be concluded that soil pH can indirectly affect the FE-biomass C to organic C ratio and FE-qCO(2) by affecting the quality and decomposition of litter and soil organic matter, but has a more direct effect on the SIR-biomass C to organic C ratio and SIR-qCO(2) by inhibiting the mineralization of glucose in acid soils. Although the lag period and the specific respiration increment were not well correlated with any measured variables, the lag period was significantly lower in the disturbed soils than in the natural undisturbed soils. This suggests that the lag period after glucose addition could be used as a good indicator of disturbance.

DOI： 10.1111/j.1747-0765.2009.00378.x

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL PUBLISHING  

The ratio of respiration to assimilation of glucose in soil microorganisms is reported to increase as the concentrations of added glucose increase. To explain this, a conceptual model has already been proposed in which the glucose acquired by soil microbes is incorporated into either storage compounds or storage compounds together with structural compounds only if sufficient glucose is available for growth. The model also suggested that glucose incorporation into storage compounds required less respiration and more assimilation than incorporation into structural compounds. We hypothesized that soil microbes start to invest acquired glucose into the synthesis of structural compounds when added glucose exceeds a threshold concentration. To confirm this hypothesis we investigated the patterns of glucose use after the addition of glucose C at 23 to 311% of biomass C, levels that are much lower than those tested in previous studies. Respiration rates did not increase (zero-order types) when added glucose C ranged from 23 to 47% of biomass C, but they did increase (growth-associated types) when glucose C was more than 78% of biomass C. The ratio of respiration to utilized glucose was approximately 20% for the zero-order types, but was higher than 20% and increased as the concentrations of added glucose increased for the growth-associated types. In addition, the substrate-induced respiration rate at 12 h after the glucose addition increased only for the growth-associated types, although chloroform-labile C increased in both types as the concentrations of added glucose increased. These results suggested that structural compounds were synthesized only for the growth-associated types and, thus, there was a threshold concentration of glucose over which structural compounds started to be synthesized in soil microbes and the ratio of respiration to assimilation started to increase.

DOI： 10.1111/j.1747-0765.2007.00235.x

Web of Science

Language：English   Publisher：JAPAN SOCIETY OF TROPICAL ECOLOGY  

To clarify the various functions of the fallow phase in the shifting cultivation system in northern Thailand, the fluctuation of fertility-related properties of soils throughout land-use stages was analyzed and the soil organic matter (SOM) budget was quantitatively evaluated, with special reference to soil microbial activities. The factors that have ensured the long-term sustainability of the shifting cultivation system can be summarized as follows: (1) Some soil properties relating to soil acidity improve when SOM increases in the late stages of fallow. The litter input may be supplying bases that are obtained via tree roots from further down the soil profile to the surface soil. This simultaneous increase in SOM and bases in the surface soil through forest-litter deposition in the late stages of fallow has an increasing effect on nutritional elements. (2) The decline in soil organic C during the cropping phase may be compensated by litter input during 6-7 years of fallow. With regards to overall budget, the organic matter input through incorporation of initial herbaceous biomass into the soil system after establishment of tree vegetation (approximately in the fourth year) was indispensable for maintaining the SOM level. (3) The succession of the soil microbial community from rapid consumers of resources to stable and slow utilizers, along with the establishment of secondary forest, retards further leaching loss of nitrogen (N) and enhances N accumulation in the forest-like ecosystems. It is noteworthy that during the fallow period, nitrifying activity of soil microbes, which was once activated in the cropping phase, is apparently suppressed. As a result, nitrate (NO₃^-) effluent from soil layers was remarkably decreased, even in the initial stages of fallow. The functions of the fallow phase listed above can be considered essential to the maintenance of this forest-fallow system. Agricultural production can therefore be maintained with a fallow period of around 10 years, which is somewhat shorter than widely believed. Traditional shifting cultivation in the study village is shown to be well adapted to its soil-ecological condition.

DOI： 10.3759/tropics.15.1

CiNii Books

J-GLOBAL