Toru Mitsuno

Assessment of Root Zone Nitrogen Leaching as Affected by Irrigation and Nutrient Management Practices

et al., 1995), center-pivot fertigation (Spalding et al., 2001), groundwater table control (Drury et al., 1997), and reMultiple or split N fertilizer applications during a single cropping duced till or no-till practices (Power et al., 2001). Whatperiod is a recommended practice for controlling N (specifically ever management practices may be adopted, it is imporNO3–N) leaching into groundwater. Here, we demonstrate the benefit of split fertilizer applications in two typical upland soils of Japan tant to consider the simultaneous transport of water and (sand and Andosol) using a combination of a laboratory experiment N to evaluate the potential for NO3 groundwater poland modeling. Soil water flow and N transport properties of the soils lution. were measured using standard procedures. Transient N and water Numerous models have been proposed for modeling transport experiments were conducted in cylindrical soil columns unthe transport of N in soils. The conceptual N model by der single (lumped) and split NH4–N applications. The column experiTanji et al. (1977) is very simple with few input data ments were successfully simulated using Richards’ equation and an and is based on the principles of mass balance and steady advection–dispersion model with equilibrium nonlinear sorption constate. The extended conceptual model proposed by Tanji ditions and first-order transformation for N speciation. Using the et al. (1979) is also simple and is applicable to transient model for the two soils, several irrigation and fertilizer management conditions. The Nitrate Leaching and Economic Analyscenarios were then simulated based on 1992 through 2000 meteorological data to investigate the long-term effects of lumped and split sis Package (NLEAP) model combined with GIS is used fertilization schedules for a representative set of crop and irrigation to identify potential NO3 hot spots in shallow alluvial conditions. In comparison with lumped applications, split fertilizer aquifers under irrigated agricultural areas (Shaffer et al., applications were found to consistently reduce the amount of N leach1995, 1996; Follett, 1995). The SOILN model simulates ing, even though year-to-year differences of N leaching reductions daily N and C fluxes in agricultural systems, including between sand and Andosol were significant. For unstressed crops, the plant growth and N uptake (Jabro et al., 2001). actual reduction in N leaching are shown to depend on the timing of Nitrate leaching is considered to occur mainly during precipitation and irrigation events, on soil type, and on plant N uptake high precipitation or during irrigation; hence, transient behavior. However, across all scenarios, two split applications instead dynamic models of water and N transport and N transof a single, lumped application reduced the N leaching fraction by formation are more adequate to evaluate the risk of approximately one-third. In the sandy soil, a three-way split resulted in further leaching reductions compared with the two-way split. SixNO3 leaching into groundwater under various water and way split applications did not result in further N leaching improvefertilizer management scenarios. The Water Heat and ments in either sand or Andosol. After adjusting N application rates Nitrogen Simulation Model (WHNSIM), which simuto account for reduced N use efficiency, N leaching rates for unstressed lates transient water, heat, and N movements, including crops under lumped fertilization were found to be several times higher N transformations, satisfactorily predicted NO3 concenthan under split applications. tration in the soil solution and N uptake originating from experimental sites (Huwe and Totsche, 1995). Modeling of urea, NH4, and NO3 transport and transforG NO3 contamination is a common probmations conducted by Ma et al. (1999) includes urea lem in field crops and dairy areas of Japanese upand NH4 adsorption, urea diffusion and hydrolysis, diflands. Quantitative, technical information is needed to fusion of NH4 and NO3, nitrification, and denitrification help farmers make management decisions that support in flooded soil. Antonopoulos and Wyseure (1998) evalprofitable yields while avoiding environmental degradauated the transient water movement, mass transport, tion. Various management practices have been proposed and N transformations of restored and undisturbed soil. to control NO3 leaching. These include, for example, crop The HYDRUS code (Šimůnek et al., 1998, 1999) simulates rotation (Delgado et al., 2001), controlled-release fertilwater, heat, and solute movement in oneand two-dimenizer (Paramasivam et al., 2001), fine-tuned irrigation and sional variably saturated media. The solute transport equaNmanagementbasedonsoil testingprograms(Poweretal., tions incorporate the effects of zero-order production, 2001) or chlorophyll meter readings of the crop (Schepers first-order degradation, and first-order decay and production reactions that provide the required coupling between the solutes involved in the sequential first-order chain. K. Nakamura and T. Mitsuno, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, Japan, 606Several long-term studies have been conducted to 8502; T. Harter, Dep. of Land, Air and Water Resources, University of evaluate the effects of proposed best management pracCalifornia, Davis, CA, 95616; Y. Hirono, National Institute of Vegetable tices on NO3–N leaching, aquifer water quality, and crop and Tea Science, Kanaya, Shizuoka, Japan, 428-8501; H. Horino, Diviyields (e.g., Pang et al., 1997a, 1997b; Delgado et al., sion of Agriculture, Osaka Prefecture University, 1-1 Gakuen-cho, Sa2001; Jaynes et al., 2001; Paramasivam et al., 2001). Most kai, Osaka, Japan, 599-8531. Received 25 May 2004. Original Research Paper. *Corresponding author (ThHarter@ucdavis.edu). existing models have been evaluated on an annual or a crop season basis (e.g., Jabro et al., 2001; Delgado et Published in Vadose Zone Journal 3:1353–1366 (2004). al., 2001). Long-term studies are effective for evaluating

Discriminating the influence of soil texture and management-induced changes in macropore flow using soft X-rays

Soft X-ray radiography, a nondestructive technique, was employed to examine macropore flow characteristics in soils of different management : paddy, upland field, and forest. A constant-head saturated hydraulic conductivity experiment was conducted using the soft X-ray apparatus. A contrast medium, CH 2 I 2 , was applied as a tracer to obtain contrast images of macropore flow. The visualization efficiency has been increased by the low-energy system that enables high-contrast images to be obtained as a result of a wider range of mass attenuation coefficients. CH 2 I 2 has a larger attenuation coefficient than bulk soil and has a kinematic viscosity similar to water. It was introduced dropwise to allow movement with the water flow. Macropore flow was captured using a soft X-ray TV camera, whereas flow paths were photographed on X-ray films. Light duralumin, was used for the sample holder instead of the conventionally used stainless steel in order not to cancel the efficiency induced by the contrast medium. We found that the highest resolution obtained was 42.3 μm. This corresponded well to the calculated value of 30 μm at an energy level of 60 to 70 keV. The resultant images showed that only about 30% of potentially available macropores conducted water flow. Macropore flow paths were affected by land management: straight isolated cylindrical paths in paddy field soils; a network of tortuous paths in upland field soils; and round cloudy interaggregate paths in forest soils. Macropore flow velocity was estimated from CH 2 I 2 movement. Because the flow area was restricted to a small number of macropores, the Reynolds number of flow ranged from 51.6 to 88.6. The flow was in a transition region from laminar to turbulent flow. Under actual conditions of macropore flow, the assumption of laminar flow was not realized. Both structural and dynamic analyses of macropore flow explained the flow characteristics successfully.

Characteristics of effluent load from a watershed including irrigation ponds

This paper describes an attempt to estimate the mass budget of irrigation ponds within a watershed and the possibilities to control the effluent load of nutrients from the watershed located in the Kyotanabe area. The paddy field lots were irrigated by the ponds and several mountain streams. In the study watershed, there were two ponds located on the upper and lower sides of a paddy area. Water could be pumped up from the lower pond to the upper pond as necessary. At the ponds, the total amount of nitrogen and phosphorus in the inflow loads including the sediment release was larger than those in the outflow loads. In SS, the effect of load reduction in the ponds was high. It is assumed that organic load reduction was not expected in the ponds. The study result indicates that it is possible to remove the nutrients in the ponds when a larger amount of water is pumped up than when the ponds are only in irrigation use. Moreover, the removal capacities of the nutrients could probably be increased in both ponds by controlling the amount of sediment releases.