Hiromi Imoto

Contribution of shallow groundwater rapid fluctuation to soil salinization under arid and semiarid climate

Rising saline shallow groundwater and associated soil salinization problems are widespread especially in arid and semiarid areas. There have been numerous studies on groundwater-associated salinity, but more information is required on the effects of groundwater frequent and high fluctuations on soil salinization. In the present study, laboratory experiments and numerical simulations using HYDRUS-1D model were carried out for this purpose. The experimental and modeling results showed that groundwater fluctuation caused not only the accumulation of more salt in the soil profile compared to stable groundwater, but also an enhancement of the mechanism. Water table fluctuation induced a much greater spreading of the bromide (Br) tracer within the column than the constant water table. The Br content was on average five orders of magnitude greater under a fluctuating water table than under a constant one. Further, the numerical simulations showed that an increase in the groundwater fluctuation frequency brought about an increase in soil surface salinization under the same evaporation boundary conditions. Additional simulations with HYDRUS-1D were used to study the effects of various management strategies on soil salinization induced by shallow groundwater. Hence, by reducing the evaporation rate through the application of surface mulching, a significant reduction of salt concentration at the soil surface was observed. Moreover, frequent irrigations with small quantities were effective to reduce soil surface salt accumulation induced by saline shallow groundwater.

Evaluation of water-saving rice-winter crop rotation system in a suburb of Tokyo.

Abstract Water-saving rice-winter crop rotation systems were repeated for 4 cycles from 2000 to 2004 in an urban area, Nishitokyo, Japan, to assess the effects of water-saving (i.e. non-flooded vs. flooded) on grain yield of rice (Oryza sativa L.) and chemical constituents of percolating water. The effects of pre-rice winter cropping compared with fallow on rice yield were also examined. The pre-cultivated crops were wheat (Triticum aestivum L.), italian ryegrass (Lolium multiflorum Lam.) or spinach (Spinacea oleracea L.) with their above-ground parts removed, chinese milk vetch (Astragalus sinicus L.) or rapeseed (Brassica napus L.) with their above-ground parts incorporated before rice transplanting. Neither winter cropping effects nor its interaction with water-saving were significant for rice yield, although the yield after rapeseed incorporation tended to be 9 % higher than that after fallow. In 2001, 2003 and 2004, when more than 70% of irrigation water was saved in the non-flooded trial, average yield in non-flooded trial was 58 % of flooded trial, but water productivity increased (from 0.10 to 0.16 kg m-3). Among the 3 years, yield in non-flooded trial was highest in 2004 when the amounts of irrigation and total water supply was larger, the frequency of dry spells was the lowest, and 2 seedlings were transplanted per hill. The nitrate and nitrite concentrations in the percolating water were far below the environmental standard values by WHO. The study showed that incorporation of winter crops had no negative effects on water-saving rice production at least for the first 4 years, and that under extreme water-saving, irrigation and planting methods could minimize yield reduction.

Effects of nanoscale pores in soils on carbon evolution under extremely dry conditions

Abstract Soil carbon evolution under the lowest moisture conditions varies considerably among experimental systems/techniques, leading to discrepancies in the estimations from carbon dynamics models under low moisture conditions. We focused our study on clarifying the regulating factors of soil carbon evolution under the lowest moisture conditions by conducting laboratory experiments under precisely controlled conditions. Nanoscale porosity and surface properties of these soils were determined to analyze the role of residual water in the carbon evolution processes in dry soils. Laboratory incubation showed that the carbon evolution from a microporous (D < 2 n m) volcanic soil proceeded even at -100 U kg-1 water potential (INT) in contrast to the carbon evolution from a phyllosilicate alluvial soil. Pore-size estimation and 1H-NMR spectroscopy showed that the carbon evolution at -100 U kg-1 WP proceeded through the utilization of nanopore-water in soils. Batch sorption experiment suggested that the surface affinity of the soils to dissolved organic matter (DOM) had enhanced carbon evolution by attracting DOM into hydrophilic spheres of the soil at -100 U kg-1 MT. Solid-state IIGNMR of organic matter samples (incubated in the absence of soils) suggested that the chemical alteration of the samples was significant for aliphatic components, while the alteration was not observed in the samples incubated at -100 U kg-1 WP. This fact also indicated the contribution of nanoscale pores in the volcanic components to carbon evolution. Application of the experimental results to several biogeochemical models revealed that both volumetric water content and MT are required to estimate carbon evolution under low moisture conditions. A micro habitat model showed that the carbon evolution at -100 U kg-1 WP could be attributed to extracellular enzymatic processes or other abiotic processes rather than to the activities of living microorganisms.

Applicability of soil column incubation experiments to measure CO2 efflux

Abstract Accurate measurements of CO2 efflux from soils are essential to understand dynamic changes in soil carbon storage. Column incubation experiments are commonly used to study soil water and solute transport; however, the use of column incubation experiments to study soil CO2 efflux has seldom been reported. In this study, a 150-day greenhouse experiment with two treatments (no-tillage and tillage soils) was conducted to evaluate the applicability of soil column incubation experiments to study CO2 efflux. Both the chamber measurement and the gradient method were used, and results from the two methods were consistent: tillage increased soil cumulative CO2 efflux during the incubation period. Compared with fieldwork, incubation experiments can create or precisely control experimental conditions and thus have advantages for investigating the influence of climate factors or human activities on CO2 efflux. They are superior to bottle incubation because soil column experiments maintain a soil structure that is almost the same as that in the field, and thus can facilitate analyses on CO2 behaviour in the soil profile and more accurate evaluations of CO2 efflux. Although some improvements are still required for column incubation experiments, wider application of this method to study soil CO2 behaviour is expected.

Determination of the role of entrapped air in water flow in a closed soil pipe using a laboratory experiment

Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1‐1‐1, Yayoi, Bunkyo‐ku, Tokyo 113‐8657, Japan Correspondence Takuhei Yamasaki, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1‐1‐1, Yayoi, Bunkyo‐ku, Tokyo 113‐8657, Japan. Email: yamasaki@soil.en.a.u‐tokyo.ac.jp Funding information Japan Society for the Promotion of Science, Grant/Award Number: 15H02467

Evaluation of gas diffusivity models for no-tilled and tilled volcanic ash soils

ABSTRACT Accurate quantification of soil gas diffusion is essential to understand the gas transport mechanism in soils, especially for soil greenhouse gas emissions. To date, the performance of soil gas diffusivity (Dp/D0, where Dp is the soil gas diffusion coefficient and D0 is the diffusion coefficient in free air) models has seldom been evaluated for no-tilled and tilled volcanic ash soils. In the present study, six commonly used models were evaluated for volcanic ash soils under two treatments by comparing the predicted and measured soil gas diffusivities at water potentials of pF 1.3–3. The Buckingham-Burdine-Campbell (BBC), soil-water-characteristic-dependent (SWC-dependent), and two-region extended Archie’s Law (2EAL) models showed better performance for both no-tilled and tilled volcanic ash soils, which is likely because porosity and pore size parameters of bimodal soils were taken into consideration in these models. Since the BBC model showed better accuracy than the SWC-dependent and 2EAL models and required fewer, more easily measurable parameters, this study recommends the BBC model for predicting soil gas diffusivity of volcanic ash soil under different tillage managements. In future studies, the BBC model should be further tested at water potentials of pF > 3, and may be improved by including the parameters of pore continuity and saturation.

Predicting Soil CO2 Dynamics in Arable Land of Andisol Using the SOILCO2 Model

AbstractRecently, carbon capture and storage by soils is recognized as an important function of agricultural fields. When discussing global warming mitigation such as proper agricultural practices and/or an appropriate depth of composting in agricultural lands for soil carbon storage, not only CO2 efflux, but also profiles of CO2 concentrations in soil air, are important. This study evaluated the performance of the SOILCO2 model in predicting soil CO2 dynamics, especially CO2 concentrations at various depths in an arable bare land of Andisol. We attempted to experimentally determine two soil parameters required for the CO2 production model, the optimal CO2 production rate and the parameter of a depth-dependent CO2 production function by measuring CO2 efflux from the surface and a vertical distribution of concentration of dissolved organic carbon, respectively. Soil CO2 concentrations, water contents and temperatures were continuously monitored for approximately one year at three different depths, and simu...