Kozue Sawada

Different effects of pH on microbial biomass carbon and metabolic quotients by fumigation-extraction and substrate-induced respiration methods in soils under different climatic conditions.

Abstract Soil microbial biomass C, the metabolic quotient ([qCO2] 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 qCO2 measured using SIR-biomass C (SIR-qCO2) was negatively correlated with soil pH, whereas the qCO2 measured using FE-biomass C (FE-qCO2) 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-qCO2 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-qCO2 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-qCO2 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.

Soil microorganisms have a threshold concentration of glucose to increase the ratio of respiration to assimilation

Abstract 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.

Threshold concentrations of glucose to increase the ratio of respiration to assimilation in a Japanese arable soil and a strongly acidic Japanese forest soil

Abstract 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 (qCO2) 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 ‘flushes’, such as may occur in the rhizosphere or in the vicinity of plant residues.

Potential nitrogen immobilization as influenced by available carbon in Japanese arable and forest soils

Abstract 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 43 mg N g−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 7 days 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.

Effects of the Application of Digestates from Wet and Dry Anaerobic Fermentation to Japanese Paddy and Upland Soils on Short-Term Nitrification

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.

Simulating short-term dynamics of non-increasing soil respiration rates by a model using Michaelis-Menten kinetics

Abstract 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 for a Kazakh forest, a Japanese forest and a Japanese arable soils, none of which show any increase with time, 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 K 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.

In situ short-term dynamics of CO2 flux and microbial biomass after simulated rainfall in dry croplands in four tropical and continental ecosystems

Abstract 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 (qCO2) 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 qCO2 but not in MB, causing a clear but short C flush in all W plots (10–37 h), while the WG treatment caused an increase both of qCO2 and MB, resulting in substantially longer and larger C flush in the WG plots (ca. 100 h). 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 24 h, 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.

Soil Fertility and Soil Microorganisms

Classification of soil potential productivity, the classification criteria, and standards for evaluating the soil fertility in Japan are introduced in this chapter. Increasing the carbon sequestration in soil during the process of biomass production is of great importance to improve the soil fertility for producing more biomass. The modified Rothamsted Carbon model that has been improved for evaluating major arable soils in Japan, i.e. Andosols and paddy soils, will also be included. Soil microbes play crucial roles in soil functions, especially nutrient cycling, and thus contribute significantly to sustainable crop production. General aspects of soil microbes are discussed in order to provide an understanding of the role that soil microbes play in crop production. Because soils with higher nitrogen-supplying capacity, expressed as the rate of microbial mineralization of soil organic matter, are considered to have higher soil fertility; microbial mediation to enhance soil fertility is also discussed.

Effects of 3-year cultivation on the soil nutrient status in a tropical forest and savanna of Central Africa, as determined by the microbial responses to substrate addition

ABSTRACT 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.