Abstract. The soil hydraulic parameters for analyzing soil water movement can be determined by fitting a soil water retention curve to a certain function, i.e., a soil hydraulic model. For this purpose, the program "SWRC Fit," which performs nonlinear fitting of soil water retention curves to 5 models by Levenberg-Marquardt method, was developed. The five models are the Brooks and Corey model, the van Genuchten model, Kosugi's log-normal pore-size distribution model, Durner's bimodal pore-size distribution model, and a bimodal log-normal pore-size distribution model propose in this study. This program automatically determines all the necessary conditions for the nonlinear fitting, such as the initial estimate of the parameters, and, therefore, users can simply input the soil water retention data to obtain the necessary parameters. The program can be executed directly from a web page at http://purl.org/net/swrc/; a client version of the software written in numeric calculation language GNU Octave is included in the electronic supplement of this paper. The program was used for determining the soil hydraulic parameters of 420 soils in UNSODA database. After comparing the root mean square error of the unimodal models, the van Genuchten and Kosugi's models were better than the Brooks and Corey model. The bimodal log-normal pore-size distribution model had similar fitting performance to Durner's bimodal pore-size distribution model.

The causes of soil alkalinization in the Songnen Plain of Northeast China were mainly analyzed from two aspects, natural and anthropogenic. Natural factors of alkalinization are parent materials, topographic positions, freeze-thaw action, wind conveyance, water properties and semi-arid/sub-humid climate. Some of them were always being neglected, such as freeze-thaw action and wind conveyance. Anthropogenic causes are mainly population pressure, overgrazing and improper agricultural and economic policies. In recent decades, overgrazing played a main role in secondary soil alkalinization, which led to the decline of Leymus chinensis grasslands. Now, the alkalinization is very severe, and more than 3.2 9 10 6 ha area has been affected by salt, which becomes one of the three largest sodic-saline areas in the world.

Natural wetlands are a significant source of atmospheric methane, an important greenhouse gas. Compared with numerous papers on measurements of methane emission from natural wetland surfaces, there are few reports on methane configuration and distribution within wetland soil profiles. By using a newly designed gas sampler, we succeeded in collecting free-phase gas from beneath the water table down to 120 cm in a peat. The volumetric percentage of methane in the gas phase increased with depth and was generally more than 50% beneath the zone within which the water table fluctuates. The volume of the gas phase in the peat beneath the water table was estimated to be from 0 to 19% with significant variation with depth, suggesting uneven distribution of gas bubbles. Using the volume ratio of the gas and liquid phases and methane concentration data in the gas phase, as well as assuming that methane was in equilibrium (based on Henry's Law between the two phases), we calculated that *60% of the methane accumulates in the form of bubbles. These results suggest the importance of ebullition in methane emission, which might be a major cause for the reportedly large variation of methane emission in both space and time. Most importantly, our results show the need to consider gaseous-phase methane for understanding the production, transport and emission mechanisms of methane in wetlands, which has been overlooked to date.

Microorganisms can clog pores in soils and decrease hydraulic conductivity and infiltration. We did three column experiments to clarify the effects. In all three columns, glucose solution of 50 μg cm−3 was percolated for 120 days, and both the saturated hydraulic conductivity, Ks, and the volume ratio of the gas phase, a, were measured continuously. The Ks decreased rapidly for the initial 10 days, and it slowly decreased for the following 110 days. By adding chloramphenicol to the second column as bactericide and cycloheximide to the third column as fungicide, we observed clogging by bacteria and fungi, respectively, bacterial clogging proceeding more rapidly than the fungal clogging. The volume of the gas phase increased and reached the maximum value of 30.6% after 103 days from the beginning of percolation. This large amount of gas was retained in the soil pores as bubbles and occluded the pathways of water, resulting in the decrease in Ks. When the percolating solution was changed to sodium azide (a strong biocide), after 120 days the volume of the gas phase decreased rapidly, and Ks increased simultaneously.

Permeable reactive barriers (PRBs) are an alternative technique for the biological in situ remediation of ground water contaminants. Nutrient supply via injection well galleries is supposed to support a high microbial activity in these barriers but can ultimately lead to changes in the hydraulic conductivity of the biobarrier due to the accumulation of biomass in the aquifer. This effect, called bioclogging, would limit the remediation efficiency of the biobarrier. To evaluate the effects bioclogging can have on the flow field of a PRB, flow cell experiments were carried out in the laboratory using glass beads as a porous medium. Two types of flow cells were used: a 20‐ × 1‐ × 1‐cm cell simulating a single injection well in a one‐dimensional flow field and a 20‐ × 10‐ × 1‐cm cell simulating an injection well gallery in a two‐dimensional flow field. A mineral medium was injected to promote microbial growth. Results of 9 d of continuous operation showed that conditions, which led to a moderate (50%) reduction of the hydraulic conductivity of the one‐dimensional cell, led to a preferential flow pattern within the simulated barrier in the two‐dimensional flow field (visualized by a tracer dye). The bioclogging leading to this preferential flow pattern did not change the hydraulic conductivity of the biobarrier as a whole but resulted in a reduced residence time of water within barrier. The biomass distribution measured after 9 d was consistent with the observed clogging effects showing step spatial gradients between clogged and unclogged regions.

Abstract. We develop a theory that explains the decrease in saturated hydraulic conductivity K s due to biological clogging of porous media. Experiments show that discontinuous microcolonies in fine-textured soils decrease K s more severely than biofilms do. However, most existing models for biological clogging assume that bacteria cells form biofilms which uniformly cover pore walls. We propose a mathematical model for biological clogging with a quantitative evaluation of the nonuniform microbial distribution of colonies. A series of equations describing the relation between the biological clogging and the saturated hydraulic conductivity are derived. The data of previous researchers are used to validate the model.

[1] The organic carbon of particle size <53 mm is mineralassociated organic carbon (MAOC), the measurable fraction of passive soil organic matter pool described in CENTURY model. We studied the effect of fresh organic matters (FOMs): no OM (control); chicken manure (CM): 2.12 g CM carbon kg À1 ; and leaf litter (LL): 1.81 g LL carbon kg À1 on short-term dynamics of MAOC and CO 2 evolution of two soils: Bagabag, Philippines (121°15 0 E, 16°35 0 N) and Tsumagoi, Japan (138°30 0 E, 36°30 0 N). Cumulative CO 2 evolution was significantly higher in CM-applied soils. Significant MAOC decrease in 5 -20-cm depth of Tsumagoi soil suggest short-term stable C turnover even with FOM application. Greater MAOC decline in CM-applied Bagabag soil suggest that manure application may result to bigger stable C turnover in this soil. Our results provide evidence of significant short-term stable SOC turnover, and challenge the convention that only labile SOC is involved in shortterm CO 2 evolution from soils. Citation: Dumale, W. A., Jr., T. Miyazaki, T. Nishimura, and K. Seki (2009), CO 2 evolution and short-term carbon turnover in stable soil organic carbon from soils applied with fresh organic matter, Geophys. Res. Lett., 36, L01301,

Abstract. There are two approaches available for mapping water retention parameters over the study area using a spatial interpolation method. (1) Retention models can be first fitted to retention curves available at sampling locations prior to interpolating model parameters over the study area (the FI approach). (2) Retention data points can first be interpolated over the study area before retention model parameters are fitted (the IF approach). The current study compares the performance of these two approaches in representing the spatial distribution of water retention curves. Standard geostatistical interpolation methods, i.e., ordinary kriging and indicator kriging, were used. The data used in this study were obtained from the Las Cruces trench site database, which contains water retention data for 448 soil samples. Three standard water retention models, i.e., Brooks and Corey (BC), van Genuchten (VG), and Kosugi (KSG), were considered. For each model, standard validation procedures, i.e., leave-oneout cross-validation and split-sample methods were used to estimate the uncertainty of the parameters at each sampling location, allowing for the computation of prediction errors (mean absolute error and mean error). The results show that the IF approach significantly lowered mean absolute errors for the VG model, while also reducing them moderately for the KSG and BC models. In addition, the IF approach resulted in less bias than the FI approach, except when the BC model was used in the split-sample approach. Overall, IF outperforms FI for all three retention models in describing the spatial distribution of retention parameters.