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Soil Science Society of America Journal 65:270-278 (2001)
© 2001 Soil Science Society of America

DIVISION S-10-WETLAND SOILS

Quantifying Soil Hydromorphology of a Rice-Growing Ultisol Toposequence in Taiwan

Zeng-Yei Hseua and Zueng-Sang Chenb

a Dep. of Environmental Engineering and Science, National Pingtung University of Science and Technology, Pingtung 912, Taiwan, ROC
b Graduate Institute of Agricultural Chemistry, National Taiwan Univ., Taipei 106, Taiwan

Corresponding author (soilchen{at}ccms.ntu.edu.tw)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seasonally irrigated flooding and fluctuation of groundwater level control the redoximorphic features, saturation, and reduction of rice-growing Ultisols in different landscape positions of Taiwan. The objective of this study was to quantify soil hydromorphology of an Ultisol toposequence with different saturation and reduction conditions. Three study soils, along a toposequence at the red earth terrace, were selected for monitoring of the water table, matric potential, and redox potential (Eh) at various soil depths in 1996 and 1997. The three soils are Houhu (Typic Plinthaquult) in the toeslope, Hsinwu (Typic Plinthaquult) in the footslope, and Lungchung (Plinthaquic Paleudult) in the lower backslope. The Houhu and Lungchung soils were seasonally flooded for rice (Oryza sativa L.) production and produced perched water tables from March to October. The Hsinwu soil, where rice has not been planted since 1991, still revealed seasonally high water levels and perched water tables during the study period. Redox concentrations originally occurred as soft masses and concrete nodules associated with seasonally high water levels, but irrigation and drainage processes also influenced the development of redoximorphic features. The abundance of Fe–Mn concretions and Fe depletions increased as cycling of oxidation and reduction conditions increased in rice production. The durations of saturation and reduction in the Btv horizons of the Houhu soil in the toeslope position were more than 80% of the year, and the soil had {approx}10% of Fe–Mn concretions. The Btv horizons of the Hsinwu soil in the footslope position were saturated for 50% of the year and reduced for 25% of the year, and the soil had {approx}20% Fe–Mn concretions. The Btv horizons of the Lungchung soil in the lower backslope position were saturated for 40% of the year and reduced for only {approx}10% of the year, and the soil had 15% Fe–Mn concretions. The Houhu and Hsinwu soils had anthraquic conditions and the Lungchung soil, with less reduction, was proposed as having oxyaquic conditions as defined in U.S. soil taxonomy.

Abbreviations: PVC, polyvinyl chloride


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SEASONAL FLOODING AND DRAINAGE cycles control the saturation and reduction of rice-growing (paddy) soils. Saturation and reduction are the common soil characteristics in Taiwan's arable lands, and various redoximorphic features occur with anthraquic or oxyaquic conditions (Hseu and Chen, 1994, 1996, 1999). The concept of aquic conditions was introduced in the Keys to Soil Taxonomy to assess seasonal wetness throughout the soil profile rather than just using indicators within the upper 50 cm of the pedon for moisture category (Soil Survey Staff, 1992). Alternating cycles of reduction and oxidation in soils over prolonged periods, and the consequent mobility and accumulation or depletion of Fe and Mn, result in the formation of redoximorphic features (Fanning and Fanning, 1989; Vepraskas, 1992). Past studies have tested the application of redoximorphic features as soil saturation and reduction indicators in various pedogenic environments. Iron and Mn concretions were found to be about 7.5% in the upper B horizon of the soils along several hydrosequences in Bavaria, Germany (Schwertmann and Fanning, 1976). In a study of five fine-silty soils in Indiana, Fe depletions with chroma 2 indicated for >30% saturation of the year at a depth of 1 m (Franzmeier et al., 1983). In hardwood forest areas in Louisiana, soils with chroma <2 and redox concentrations just below the A horizon generally indicated saturation and reduction >25% of the growing season (Faulkner and Patrick, 1992).

Veneman et al. (1998) reviewed the literature dealing with the relationship between soil morphology and moisture regime from the last 50 yr. Their review found a gradual shift from a descriptive format to a more quantitative approach that linked moisture regimes to specific morphological indicators. Genthner et al. (1998) used 33 pedons in an Upper Coastal Plain landscape in Virginia to establish simple linear regression to quantify the relationships between (i) seasonal high water tables and the depth to the shallowest Fe depletions or Fe-depleted matrices of Munsell chroma and (ii) the seasonally high water tables and the depth to the shallowest Fe concentrations of Munsell chroma . West et al. (1998) indicated that redoximorphic features observed in soils on summits and upper backslopes occur higher in the profile than expected from duration of saturation of soils along the transects of the Dougherty Plain in southwest Georgia. Hseu and Chen (1996) have identified anthraquic and oxyaquic conditions in these soils by the presence of redox depletions and concentrations, and they have correlated these features with hydromorphic features and saturation and reduction status. They found that the soils had high chroma matrix colors, Fe concentrations (chroma 6), Mn concentrations (chroma 1), and Fe depletions (chroma 1). These features were found even in horizons that were saturated for considerable periods of time, but were reduced for a much shorter time (Hseu and Chen, 1996). Current research efforts in soil moisture and redoximorphic features should focus on the quantitative relationship between redoximorphic features and the frequency, duration, and intensity of saturation and reduction events. Quantitative descriptions of redoximorphic features associated with hydromorphology should be established for the soil drainage classes and soil management techniques on rice-growing soils in Taiwan. The objectives of this study were (i) to define the depth, frequency, and duration of saturation and the period of reduction at the 200-cm depths of three soils with anthraquic conditions in northern Taiwan; (ii) to interpret the formation of redoximorphic features related to human activity and landscape position along a toposequence; and (iii) to quantify the soil hydromorphology of rice-growing soils along a toposequence.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
The study area is located inside the Chungli Terrace in northern Taiwan, {approx}40 km southwest of Taipei City (Fig. 1) . The elevation ranges from 20 to 40 m above sea level. The soil developed on an alluvial terrace from the Quaternary period, with a minimum thickness of 5 m of alluvial materials (Ho, 1986). The Chungli Terrace has a slope between 1 and 7%, going down gently from the eastern hill land to the western seashore. Finer alluvial materials overlie cobbles in the terrace and groundwater usually perches at the contact between these two layers. Climatic data show the mean air temperature is 27°C in summer and 13°C in winter. Average annual rainfall in 1996 and 1997 was 1350 mm, which is slightly lower than the mean value ({approx}1560 mm) from the last decade. Monthly rainfall in winter has been much less than in the other seasons for the last 14 yr (Fig. 2) . The annual rainfall in this area almost exceeds the annual evapotranspiration. The agricultural lands on the Chungli Terrace have been used for rice production since 1950s. Each year, rice is harvested twice during the March to October growing season, and the soils are fallowed in winter. These soils are seasonally flooded by the irrigation water and saturated or reduced by the groundwater.



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  		Fig. 1. Site location and landscape position of the three study soils. (The dotted line of seasonal high water level is exaggerated.)

 


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  		Fig. 2. Mean monthly rainfall and evaporation in the study area from 1984 to 1997. (Vertical bars indicate the standard deviations.)

 
Three representative rice-growing soils with plinthite along a toposequence were selected for this study. Each study site consisted of a 7 by 7 m plot that was monitored in 1996 and 1997. The soil pedon was excavated near the plot. All three pedons were close ({approx}2 km) to the seashore and were classified as Typic Plinthaquult (Houhu), Typic Plinthaquult (Hsinwu), and Plinthaquic Paleudult (Lungchung) (Soil Survey Staff, 1998). All three soils are classified as Acrisols according to the FAO system (FAO/UNESCO, 1988). The Lungchung soil is the farthest from the seashore ({approx}4 km) and is located at the highest elevation in lower backslope position of the toposequence. The Hsinwu soil is located at the footslope position. The Houhu soil is the nearest to the seashore and has the lowest elevation in the toeslope position.

Soil Analyses
Soil pits were excavated in August 1996. We described soil morphological characteristics, including redoximorphic features, based on the quantification method (Vepraskas, 1992) and the soil survey manual (Soil Survey Staff, 1993). A simple tracing technique was used to improve the quantification of redoximorphic features within the soil profile. Several 10 by 10 cm transects at representative areas within soil horizons were used to quantify the redoximorphic features. The redoximorphic features were recorded on transparent plastic strips. The 10 by 10 cm transparent plastic strip was digitized, and the number, size, and abundance (%) of each type of redoximorphic feature were calculated and recorded within the 10 by 10 cm area.

The pH of the air-dried samples (<2 mm) was measured in a mixture of soil and deionized water (1:1, w/v) with a glass electrode (McLean, 1982). Bulk density was determined by the core method (Blake and Hartge, 1986). Micromorphological characteristics of the different horizons for three soils were described in Hseu and Chen (1999).

Hydrological Monitoring
The following data were recorded at biweekly intervals from January 1996 to December 1997: (i) water table level; (ii) soil water tension at depths of 25, 75, 100, and 200 cm; and (iii) soil redox potential at depths of 25, 75, 100, and 200 cm. All data were recorded in triplicates. Because the cultivation of lowland rice forms the compact subsurface plowed layer in the soils, perched water appears in the study area during the growing seasons. We used constructed wells to measure the apparent water table and used a piezometer to determine the perched water table. Tensiometers and platinum electrodes were used to monitor the matric potential and redox potential. Wells were constructed from 6 cm i.d. by 225 cm long polyvinyl chloride (PVC) pipe perforated in 2-cm intervals on the sides from the well bottom to the soil surface (200-cm total) at each study site. Wells were wrapped with 1-mm-diam. opening (20-mesh) nylon screen to prevent soil from filling in the wells. The upper end of each well was covered with a PVC cap, and a small hole was drilled in the cap's side to facilitate air entry. The piezometers were constructed from 1.9-cm-i.d. PVC pipe and 10-cm well screen as described by Hudnall and Wilding (1992). A piece of geofabric was glued onto the screen to avoid clogging and to close the end of the pipe. The other end of the piezometer was covered with a PVC cap with a small hole in its center. Triplicate nested piezometers were installed at each site at 25-, 50-, and 75-cm depths to determine if water tables were perched. Tensiometers were constructed by epoxying a 100-kPa standard ceramic cup to one end of a piece of 1.5-cm-i.d. PVC and epoxying a short piece of plexiglass tube to the other end. Tensiometer were made and calibrated by the Department of Soil and Environmental Science, National Chung Hsing University, Taichung, Taiwan. The sensitivity of tensiometer was 2 kPa. They were filled with distilled water and capped with rubber septa. Triplicate tensiometers were installed at predetermined depths. Redox potentials were measured with a Pt electrode and saturated calomel reference electrode in soil cores (8-cm diam.) collected at depths of 25, 75, 100, and 200 cm. The Eh values were determined in the field with a portable voltmeter. The voltage readings were recorded after the drift decreased to an equilibrated value, which changed within 5 mV in 10 s. The voltage readings were adjusted by adding +244 mV in order to correct for the reference electrode potential. The presence of reducing conditions (Eh < 250 mV) at various depths was calculated at pH 5.5. The presence of reducing condition was coincided with the onset of Fe reduction seen on an Eh–pH phase diagram (Collins and Buol, 1970). Rainfall information was adopted from the climate data of the nearest meteorological station from the study area.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Saturation and Reduction Conditions
During 1996 and 1997, only the Houhu and Lungchung soils were planted for rice production, but there were perched water tables at all three soils during the growing season (Fig. 3, 4, and 5) . When the soils were irrigated in March, shallow groundwater was separated from perched water by the unsaturated compact layer. The unsaturated zones of the three soils generally occurred at depths between 50 and 75 cm identified by field observation and bulk density data (Table 1). During the fallow period from November to February, the perched water tables gradually disappeared due to lack of irrigation and high evapotranspiration from October to December (Fig. 2). In other words, all three soils were unsaturated within the upper 2 m during the fallow season.



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  		Fig. 3. Water table depths, matric potential, and redox potential at the Houhu soil during the 2-yr monitoring period

 


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  		Fig. 4. Water table depths, matric potential, and redox potential at the Hsinwu soil during the 2-yr monitoring period

 


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  		Fig. 5. Water table depths, matric potential, and redox potential at the Lungchung soil during the 2-yr monitoring period

 

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  		Table 1. Morphological characteristics of three soil pedons in the study area

 

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  		Table 2. Semi-quantitation of redoximorphic features in the study area.{dagger}

 
Houhu Soil
Because the Houhu soil had the lowest elevation at the toeslope position along the hillslope (Fig. 1), the apparent water tables were the closest to the soil surface (Fig. 3). This soil was flooded throughout the March to October growing seasons, except during the brief drainage in July preceding the first harvest. Higher rainfall in summer and autumn also contributed to rising water tables (Fig. 2). During these periods, the apparent water tables were within 50 cm and ranged from 100 to 150 cm during the fallow period (November to next February) (Fig. 3). Because of very poor drainage at the Houhu soil, the matric potentials at the depth of 25 cm were already >+5 kPa when flooding started at the beginning of the growing season. During the growing seasons, the matric potential values dropped to zero due to the equilibrium of soil and irrigated water (Fig. 3). The monitoring data indicated that the saturation percentage of the year at the different soil depths was as follows: 25 cm, 60%; 75 cm, 70%; 100 cm, 75%; and 200 cm, 90%.

Even with the large temporal and spatial variations in redox potential, the seasonal fluctuation of Eh values can still apply in hydromorphological studies (Cogger et al., 1992). The Eh values reported in this study were means of triplicate observations and the coefficients of variation were less than 80% for all three soils. At the Houhu site, the soils from the surface to the depth of 200 cm were highly reduced. The Eh values at the 25-cm depth only showed oxidized states in the fallow period when groundwater tables became lower than 100 cm and water potential values dropped to below zero. The Eh values at the other soil depths of the Houhu soil remained <250 mV through the 2 yr (Fig. 3).

Hsinwu Soil
The Hsinwu soil was located in the footslope position along this toposequence. The apparent water tables ranged from 50 to 100 cm, and perched water tables were just below the 50-cm depth. Rice production at the Hsinwu site has been stopped since 1991, so the surface soil was never flooded during the monitoring period. However, the soils surrounding the site continued in rice production. The hydrology at the Hsinwu site was heavily influenced by the surrounding soils. Consequently, the apparent water tables dropped dramatically to below 180 cm when irrigation for the surrounding soils was stopped in February 1996 and 1997 (Fig. 4), but the major difference on the fluctuation of the water level between the Hsinwu and Houhu soils is that the seasonal high water tables of the Houhu soil were more influenced by the irrigation and drainage cycling. Therefore, the apparent and perched water tables of the Hsinwu soil were constantly lower than those of the Houhu soil.

Despite the lack of irrigation, the matric potential values of the Hsinwu soil were strongly influenced by rainfall and canal flow from surrounding paddy fields. The soils tended to be saturated from March to October, especially for the subsurface soils (Fig. 4). During the late fallow period in February, which is the driest part of the year, the matric potential values at the surface soil (0–25 cm) dropped to -30 kPa, coinciding well with the water table observations. A similar study of silty clay soil in Louisiana revealed the matric potential values of the soil at 25 cm dropped below -80 kPa in the dry season (Szogi and Hudnall, 1998). In March, the soils were suddenly saturated. The saturation percentage of the year at the different soil depths of Hsinwu soil was as follows: 25 cm, 40%; 75 cm, 45%; 100 cm, 50%; and 200 cm, 65%.

Because of higher landscape position and better drainage in the Hsinwu soil, the Eh values at different depths were all close to 250 mV and only indicated moderately reduced conditions from January to August in 1996 and from May to July in 1997 (Fig. 4). The Eh values at the depth of 25 cm ranged from +200 to +400 mV. The saturation percentage of the year at the different soil depths was as follows: 25 cm, 10%; 75 cm, 25%; 100 cm, 30%; and 200 cm, 10%.

Lungchung Soil
The apparent water tables at the Lungchung soil in the lower backslope along the toposequence were close to a depth of 100 cm in the growing seasons and suddenly dropped to below 200 cm in February (Fig. 5). The perched water tables at the Lungchung soil were close to the soil surface for most of the growing seasons and more than 10 cm above the ground level in September. But due to the fallow season and lower rainfall in winter, the perched water tables disappeared for {approx}2 mo from January to February in 1997.

The bulk density of the Ap horizon was 1.0 Mg m-3 and the Bw horizon was {approx}1.6 Mg m-3 (Table 1), but the Lungchung soil had the highest elevation, with convex microrelief in the transect. When irrigation stopped in February, the matric potential of the surface soil was -50 kPa. As a result, the saturation percentage of the year at the different soil depths of Lungchung soil was as follows: 25 cm, 75%; 75 cm, 45%; 100 cm, 40%; and 200 cm, 35%.

Highly reduced states only existed at a depth of 25 cm, and the soils below 75 cm indicated oxidized conditions during most of the monitoring period (Fig. 5). The Eh values at the depth of 25 cm were >250 mV only during parts of fallow seasons; however, the Eh values below 75 cm were almost >300 mV throughout 1996. The surface soil had larger variations of Eh values than those at deeper depths due to the application of chemical fertilizers and the mechanical operations involved in the rice cultivation. Megonigal et al. (1993) also found larger temporal variations of redox potential in soils at a depth of 15 cm than in those at 90 cm in a seasonally flooded forest soil of South Carolina. At the Lungchung site, the reduction percentage of the year at the different soil depths was as follows: 25 cm, 60%; 75 cm, 10%; 100 cm, 10%; and 200 cm, 5%. Megonigal et al. (1993) and Calmon et al. (1998) also reported similar redox potential profiles caused by perched water tables, but their perched water resulted from fragipans in the temperate region.

Distribution of Redoximorphic Features
The three soils along the toposequence contained redox concentrations including soft masses, Fe–Mn concretions, and mangans (Table 2). All redox depletions observed in the field had soil chroma 1, shown as gray and light gray colors associated with 7.5YR and 10YR. In addition to all Btv horizons, soft masses were also found in the upper horizons above the Btv horizons of the all three soils. However, the soft masses with chromas 2, 4, and 8 in the surface and subsurface horizons usually had sharp boundaries. These morphological characteristics suggested that the soil color resulted from the mixing process of plowing the subsurface soil and the puddled surface soil for cultivating rice. These horizons included the Ap and AB horizons of the Houhu and Hsinwu soils, and the Ap, Bw, and 2A horizons of the Lungchung soils. Therefore, we avoided using the soft masses in these horizons to quantify the hydrology. In all the Bt and Btv horizons of the three soils, higher chroma (4, 6, or 8) soft masses were found, but their distributions in the pedon were irregular.

The sizes of Fe–Mn concretions were medium (5–15 mm) and coarse (>15 mm), and the contrasts were distinct and predominant. There were a variety of Fe–Mn concretion shapes in the upper parts of the Btv horizons, including angular, pellet, slice, and elliptic shapes; the lower parts of the Btv horizons contained only subangular or imperfectly round shaped Fe–Mn concretions. The maximum content of Fe–Mn concretion always occurred in the middle Btv horizons of the three pedons, especially in the Btv3 horizon of the Houhu soil (15%), the Btv3 horizon of the Hsinwu soil (30%), and the 2Btv2 horizon of the Lungchung soil (25%) (Table 2). Black mangans were clearly visible at the pedosurfaces in the Btv horizons of the Houhu and Lungchung soils. The vertical distributions of the mangan contents were irregular in the Houhu soil. A small amount of mangans was also found in the 2Btv2 horizon (100–140 cm) of the Lungchung soil.

The depths where Fe depletions occurred in the field were below the seasonal high water levels, especially for the Lungchung soil, where Fe depletions occurred below 85 cm (Table 2), and the seasonal high water level was 55–60 cm (Fig. 5). The amount of gray (10YR 6/1) and light gray (10YR 7/1) Fe depletions ranged from 5 to 20% in the three soils. They almost occurred in all Btv horizons and increased with increasing soil depths. Notably, Fe–Mn concretions and Fe depletions always coexisted in the Btv horizons, and this indicated strong Fe segregation in the study soils.

The formation of redoximorphic features in this study can be explained hypothetically as follows. First, when the parent material of the red soils was deposited on the terrace in the Quaternary Period, the soil horizons developed under humid subtropical conditions that promoted the illuviation of clay and Fe in the soil profile. Second, redox concentrations occurred as soft masses and concrete nodules associated with the fluctuations of seasonal high water tables, especially in the region close to sea level. Finally, after the soils were converted to paddy fields, a plow layer formed. Groundwater produced by precipitation and human activities led to more diverse redoximorphic features. Therefore, when the soil began to be used for rice production, tonguing plinthites including soft masses, Fe–Mn concretions, and mangans increased in the Btv horizons. Many studies of anthraquic conditions have also documented that redoximorphic features can form very quickly, in just a few weeks in some cases (Mitsuchi, 1992; Somasiri and Deturck, 1992; Hseu and Chen, 1996). The root growth of rice formed more void types in the soils. The rice roots caused Fe–Mn concretion to have different shapes in the upper parts of the Btv horizons because Fe–Mn concretions were originally formed in the voids of the soils. Iron oxides were originally accumulated in these voids, such as root channels, to form concrete nodules, which were also identified by Silva and Ojea (1991).

Quantifying Soil Hydromorphology
Because contemporary redoximorphic features were located almost in the Btv horizons (approximately more than 75 cm deep) of the three soils that were not disturbed, the saturation and reduction data for the 75-, 100-, and 200-cm depths were cited to evaluate the wetness conditions of the Btv horizons associated with Fe–Mn concretions and Fe depletions. At the Houhu soil, Fe depletions in the Btv horizons ranged from 10 to 20% and Fe–Mn concretions ranged from 5 to 15%. The Btv horizons of the Houhu soils were saturated and reduced for 78 and 100% of the year. In the Hsinwu soils, Fe depletions in the Btv horizons ranged from 5 to 20% and Fe–Mn concretions ranged from 10 to 30%. The Btv horizons of the Hsinwu soils were saturated and reduced for 53 and 23% of the year. At the Lungchung soil, Fe depletions in the Btv horizons ranged from 5 to 15% and Fe–Mn concretions ranged from 5 to 25%. The Btv horizons of the Lungchung soil were saturated and reduced for 8 to 40% of the year.

The durations of saturation and reduction should be differentiated for the correlation of Fe–Mn concretion or Fe depletion with different soil moisture regimes. Faster surface and bypass flows in the Lungchung soil were caused by the convex surface in microrelief; however, the soil was reduced for less time (only {approx}1 mo of the year) than the other two soils. In contrast, the soils in all Btv horizons of the Hsinwu soil were reduced for at least {approx}3 mo of the year. Due to very poor drainage in the depression of the Houhu soil, the entire soil profile was reduced throughout the year. The soils in the Btv horizons of both the Houhu and Hsinwu soils were saturated for more than half the year, but those at the Lungchung soil were saturated for only {approx}4 mo of the year. The mean amount of Fe–Mn concretion in the Btv horizons of Hsinwu and Lungchung soils was more than 15%, and the Houhu soil was only 10%. Therefore, anthraquic conditions clearly exist in the Houhu and Hsinwu soils, as shown by saturation and reduction associated with various redoximorphic features defined in the Keys to Soil Taxonomy (Soil Survey Staff, 1998). The Lungchung soil displays the redoximorphic features and seasonal saturation, but this study proposes that its soil moisture regime is oxyaquic because the reduced time of the surface soil is <25% ({approx}3 mo) of the year to meet the requirement of anthraquic conditions.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Except during the fallow period in winter, the Houhu and Lungchung soils were seasonally flooded due to the cultivation of lowland rice from March to October, and the water levels of the Hsinwu soil were close to the soil surface because of the influence of the flooding in the surrounding soils. The depths where redoximorphic features occurred were generally below the seasonal high water levels. Redox concentrations originally occurred as soft masses and nodules associated with seasonal high water levels, but irrigation and drainage processes also influenced the development of redoximorphic features. Therefore, the abundance of Fe–Mn concretions and Fe depletions increased when rice planting began. On the contrary, the trends of soft masses, mangans and reduced matrices were irregular in the soils studied. The amount of Fe–Mn concretions at the three soils ranged from 5 to 30%, but the maximum content always occurred in the middle of the Btv horizons. Meanwhile, Fe depletions, ranging from 5 to 20%, increased with increasing the soil depth in the Btv horizons.

Landscape position is one of the factors controlling the duration of saturation and reduction in the soil. The Houhu soil in the toeslope with the lowest elevation was the most saturated and reduced among the three monitoring soils. There is no difference in Fe depletions with chroma 1 and 2 at the three soils along this toposequence. A summary of redox concentrations observed in this study can be quantified as follows. About 10% Fe–Mn concretions were correlated to more than 80% of saturation and reduction in the Houhu soil located on the toeslope position. About 20% Fe–Mn concretions were correlated to 50% of saturation and 25% reduction in the Hsinwu soil located in the footslope position. About 15% Fe–Mn concretions were correlated to 40% of saturation and 10% of reduction in the Lungchung soil located in the lower backslope position.


   ACKNOWLEDGMENTS
 
The authors thank the National Science Council of the Republic of China for providing partial financial support for this study (Grant no. NSC-86-2313-B-002-008). Mr. I.Y. Leu, T.M. Lee, and Miss P.J. Chen are appreciated for their assistance in soil sampling and monitoring, and the Taoyuan District Agricultural Improvement Station, Taiwan Provincial Government, for field assistance and collection of meteorological data.

Received for publication August 6, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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