Lowland Paddy Soils Research Articles

Effects of climate, soil and water source on the distribution of bioavailable iron in low-land paddy soils of Sri Lanka

Iron (Fe) is an essential mineral element of plants. However, the current status of bioavailable Fe in lowland paddy fields in Sri Lanka is not known. This study was conducted to examine the (i) spatial distribution of bioavailable Fe concentration, and (ii) interactive effects of agro-climatic zone (ACZ), soil order, and water source in determining the bioavailable Fe concentration in lowland paddy fields in Sri Lanka. A total of 6003 soil samples representing six ACZs, six soil orders, and three water sources were collected using a stratified random sampling approach. Bioavailable Fe was extracted using 0.01 M CaCl2 and measured using Inductively Coupled Plasma Mass Spectrophotometry. Bioavailable Fe concentration ranged between 0.0003 and 339 mg kg−1 with a mean of 49.4 mg kg−1. Samples collected from the Wet zone, particularly the Wet zone Low country, had higher Fe concentrations than those reported in other ACZs (p < 0.05). Among the soil orders, Histosols, Inceptisols and Ultisols had higher Fe concentrations while Entisols, Alfisols and Vertisols reported lower values (p < 0.05). Paddy fields receiving water from major irrigation systems in Wet zone Low country had more Fe. Moreover, Fe concentration was not correlated with rice crop productivity (p > 0.05). The spatial maps of bioavailable Fe generated in this study could be useful for making area-specific agronomic and administrative decisions based on ACZs, soil orders and water sources used in paddy cultivation in Sri Lanka.

Carbon Sequestration in Low Land Paddy Soils: Effect of Certain Cultural and Nutrient Management Practices: A Review

Carbon(C) is the only key to running in this worldly life and without carbon, nothing can be ensured, but the amount and form of C in different spheres of the earth make numerous changes. Changes in the carbon levels cause the lives of all living things. Soil carbon flux directly or indirectly affects the global climate and thus agriculture productivity. To ensuring the human rations, protection is intended for the rising populace worldwide, where the critical challenges in the agriculture sector are inevitable. Improved soil and nutrient supervisions and cultural practices are very imperative to tackling these troubles. Augmenting the productivity of various agro-ecosystems, soil productiveness, and carbon accretion via certain approaches become a must concern towards sustainable food production. “Paddy soils form the huge area of artificial swamplands on the earth, and serves as food basket for the world population also responsible for sequestering soil organic carbon potentially”. Rice accounts for around 9-10 % of the total cropland area globally, and their environmental conditions are responsible for storing organic carbon in soil, methane (CH4) production, and emit nitrous oxide (N2O) in meager amount. The present review signifies the present and future potential agricultural management practices, particularly soil and plant nutrition and their effects on soil organic carbon storage (SOCS) and carbon sequestration (CS) by paddies grown under submerged conditions compared to other crops. Increasing carbon inputs and reducing SOC losses in low land paddy soils need attention as its concern with GHGs that implies direct causes of global climate. As future direction, life-cycle assessments of certain practices in low land paddy soils helps in assessing the carbon footprints and sustaining the crop productivity consequently mitigating climate change. With this view, this review study was taken to the life of carbon in the terrestrial ecosystem and its accumulation in low land paddy soils moderated by cultural and nutrient management practices adapted for rice production in low lands.

Changes in lowland paddy soil fertility in the Philippines after 50 years of the Green Revolution

ABSTRACT The Green Revolution (GR) made a major contribution to increasing the rice yield in the Philippines from 1.3 t ha−1 in 1966 to 4.0 t ha−1 in 2018, but the changes in paddy soil fertility that occurred during this 50-year period remain unknown. We collected 37 soil samples from the plow layers of paddy fields in Luzon, Leyte, Panay, and Mindanao in the Philippines in 2016 and 2017 (the ‘2010s’) and compared their fertility-related soil properties with published data for soil samples collected from the same or nearby sampling sites in 1969 (the ‘1960s’). Paddy soils in the Philippines had relatively high exchangeable calcium (Ca) and magnesium (Mg) content, cation exchange capacity (CEC), available silicon (Si) content, and total carbon (C) and nitrogen (N) content, reflecting their indigenous pedological background. We found that the 2010s soils had a lower available N content, particularly in wetter regions, despite the high input of N fertilizers, whereas the available phosphorus (P) showed a six-fold increase from the 1960s to the 2010s. The total potassium (K) content had significantly decreased during this period, possibly due to the mining effect of intensive farming with K-deficient fertilization. The 2010s soils contained a greater silt content and higher proportion of smectite in the clay fraction, which may have been transported from upstream via irrigation water. The increase in smectite corresponded with a significant increase in the cation exchange capacity, despite the organic C and clay contents tending to decrease, and these increases in smectite and soil pH may have contributed to the increase in the available fraction of Si that had not been applied as fertilizers. These findings demonstrate that paddy soil fertility in the Philippines changed in many ways over this 50-year period, largely owing to the direct impact of GR implementation in paddy fields but also potentially due to offsite effects from upland fields. The environmental impact of the nutrient loss and soil organic matter degradation behind the fertility improvement should be assessed holistically for the sustainable development of rice paddy ecosystems. (337 words)

Co-application of poultry-litter biochar with Azolla has synergistic effects on CH4 and N2O emissions from rice paddy soils

Poultry-litter biochar and Azolla as green manure amendments are reported to enhance paddy soil fertility and rice yields. However, whether their co-application in lowland rice paddies has synergistic effects and whether those benefits are accompanied by greenhouse gas (GHG) emissions remains unknown. The objective of this study was to determine the effects of poultry-litter biochar (hereafter: biochar) and its co-application with Azolla as green manure (hereafter: Azolla), on the simultaneous methane (CH4) and nitrous oxide (N2O) emissions from a lowland paddy soil planted with rice during a single rice growing season in Tsuruoka, Yamagata, Japan. Biochar and Azolla amendments were applied once before rice was transplanted at a density of 20 t ha−1 and 133.9 kg N ha−1, respectively. Compared with NPK, NPK + biochar, and Azolla only treatments, Azolla and biochar co-application (i.e., Azolla + biochar) significantly increased CH4 emissions by 33%–197.6% in the early stages of rice growth (before 63 days after transplanting, DAT), but did not significantly influence CH4 emissions at both late rice growth stages (after 63 DAT,) and whole rice growth period (112 DAT). Conversely, Azolla + biochar significantly reduced N2O emissions by 83.0%–97.1% before 63 DAT, and by 76.4%–95.9% during the whole rice growth period at 112 DAT, with a significantly high interaction between biochar and fertilizer amendments. There were no significant N2O emission differences among all treatments after 63 DAT. Additionally, Azolla + biochar significantly increased rice grain yield by 27.3%–75.0%, and consequently, decreased both yield-equivalent CH4 emissions by 24.7%–25.0% and N2O emissions by 81.8%–97.7%. Our findings suggest that the co-application of poultry-litter biochar and Azolla as green manure offers a novel approach to increase rice yield while reducing the emissions of non-carbon dioxide greenhouse gases.

Comparison of Different Approaches on Determining Nitrogen Balance in a Lowland Paddy Soil

Comparison of Different Approaches on Determining Nitrogen Balance in a Lowland Paddy Soil

The Effect of Biofertilizer on The Diversity of N2O Reducing Bacteria in Paddy Fields of Sukabumi, Indonesia

&lt;p class="IsiAbstrakInggris"&gt;&lt;span lang="EN-GB"&gt;Some of the methanotrophic bacteria and N&lt;sub&gt;2&lt;/sub&gt;O reducing bacteria have been proven to be able to support the plant growth and increase the productivity of paddy. However effect of the methanotrophic and N&lt;sub&gt;2&lt;/sub&gt;O reducing bacteria application as a biofertilizer to indigenous N&lt;sub&gt;2&lt;/sub&gt;O reducing bacteria is still not well known yet. The aim of this study was to analyze the diversity of N&lt;sub&gt;2&lt;/sub&gt;O reducing bacteria in lowland paddy soil based on a &lt;/span&gt;&lt;em&gt;&lt;span lang="EN-GB"&gt;nosZ&lt;/span&gt;&lt;/em&gt;&lt;span lang="EN-GB"&gt; gene. Soil samples were taken from lowland paddy soils in Pelabuhan Ratu Sukabumi, West Java, Indonesia. There were two treatments for the paddy field soil, ie. biofertilizer-treated field 20% fertilizer (50 kg/ha) with the addition of biofertilizer and 100% fertilizer. PCR amplification of &lt;/span&gt;&lt;em&gt;&lt;span lang="EN-GB"&gt;nosZ &lt;/span&gt;&lt;/em&gt;&lt;span lang="EN-GB"&gt;gene was successfully conducted using &lt;/span&gt;&lt;em&gt;&lt;span lang="EN-GB"&gt;nosZF&lt;/span&gt;&lt;/em&gt;&lt;span lang="EN-GB"&gt; and &lt;/span&gt;&lt;em&gt;&lt;span lang="EN-GB"&gt;nosZR&lt;/span&gt;&lt;/em&gt;&lt;span lang="EN-GB"&gt; primer pair. Denaturing Gradient Gel Electrophoresis (DGGE) process was conducted at 150 V for 5.5h. There were three differences &lt;/span&gt;&lt;em&gt;&lt;span lang="EN-GB"&gt;nosZ&lt;/span&gt;&lt;/em&gt;&lt;span lang="EN-GB"&gt; bands were sequenced. The phylogenetic analysis showed that they were close to uncultured bacteria. Microbial diversity in the biofertilizer-treated field was higher than that of in the 100% fertilizer-treated field. The biofertilizer treatment has higher in microbial diversity than that of applied non-biofertilizer paddy fields. This research might have impact in the application of biofertilizers due to the emission of N&lt;sub&gt;2&lt;/sub&gt;O as a green house gas from paddy fields farming activity. The biofertilizer has great potential application in sustainable environmental friendly agriculture systems.&lt;/span&gt;&lt;/p&gt;

Metagenomic assessment of methane production-oxidation and nitrogen metabolism of long term manured systems in lowland rice paddy

Biochemical pathways of methanogenesis and methanotrophy coupled with carbon (C)-nitrogen (N) metabolism were studied in long term (13years) manured systems in lowland rice paddy through metagenomics approach. Manured systems included in this study were, control (exclusion of application of any manure), farm yard manure (FYM, @5Mgha−1yr−1) and green manuring (GM with Sesbania aculeata). Metagenomic sequence data revealed the dominance of C decomposing bacterial communities' like Proteobacteria, Planctomycetes, Actinobacteria, Firmicutes, Acidobacteria, in manure amended soils as compared to control. Diversities for assimilatory and dissimilatory N-fixing microorganisms at phylum level were found higher under GM as compared to rest. Two genera responsible for methanogenesis, viz. Methanolobus and Methanotorris were absent in manured systems as compared to control. The acetoclastic and serine pathway was found as the predominant pathway for methanogenesis and methanotrophy, respectively, in tropical lowland rice paddy. Abundance reads of enzymes were in the range of 254–445 in the acetoclastic methanogenesis pathway. On the other hand, these were varied from 165 to 216 in serine pathway of methanotrophy. Lowland paddy soil exhibited higher functional and structural diversities in manured systems as compared to unamended control in respect to labile C pools and CH4 production. Methane (CH4) emission was 31% higher in FYM system than GM. However, nitrous oxide (N2O) emission was found 25% higher in GM as compared to FYM. As a whole, bacterial diversities were higher under FYM system in tropical lowland rice paddy as compared to GM and unamended systems.

Organic matter content of a lowland paddy soil as affected by plant growth and urea fertilization

Soil organic matter (SOM) plays an important role in determining the land productivity. The effect of rice plant and inorganic fertilizer on the SOM content with time and soil depth in a lowland paddy field was investigated in this study. Two factors (fertilizer: with urea, without urea, and plant: with rice plant, without rice plant) with four treatment combinations were arranged in a randomized complete block design in a low land paddy field at a minor irrigation tank in Kurunagala district in Sri Lanka during the 2013/2014 Maha season. The soil samples were collected at 0 - 8, 8 - 16, 16 - 24 and 24 - 32 cm depths at 27, 43, 65 and 98 days after sowing (DAS) and SOM was measured using a standard method. Data were analyzed as a factorial design. The interaction between fertilizer and soil depth found to be significant (p=0.030). At the depth of 24 - 32 cm, the SOM content was significantly higher without urea than with urea. Fertilizer also showed a significant (p=0.024) interaction with DAS. The SOM content at 43 DAS was higher with urea than without urea while it is reverse at 65 DAS. Therefore, urea application has an influence on SOM content at different depths and time.

Bulk soil properties as determinants of the compression strength of puddled lowland paddy soils in Sri Lanka

Soil strength/penetration resistance of lowland puddled soil is an important parameter for designing farm machinery. This study was aimed to estimate the overall penetration resistance of lowland puddled soils through the assessment of bulk soil parameters. Penetration resistance and bulk soil parameters including moisture content, bulk density, particle density, porosity, texture and organic matter content were measured under real field condition and evaluated to identify the suitable determinants to explain the variation of soil penetration resistance and their relationships. Results revealed that measured penetration resistance by Eijkelkamp hand penetrometer and bulk soil parameters except particle density, notably varied with weed controlling methods, depth of the soil and the time. Penetration resistance showed a significant relationship with bulk density (BD), moisture content (MC) and porosity (PO) at .05 α level as using the relationships 4017.87- 44.72 MC -1669.83 BD, 2115.65-44.18 PO and 3383.78-58.09 PO in 0 – 10 cm, 10 – 20 cm and 20 – 30 cm depths, respectively. Tropical Agricultural Research Vol. 26 (3): 561 – 568 (2015)

Soluble selenium content of agricultural soils in Japan and its determining factors with reference to soil type, land use and region

To evaluate labile selenium (Se) content in agricultural soils in Japan and to investigate its determining factors, 178 soil samples were collected from the surface layer of paddy or upland fields in Japan and their soluble Se contents were determined. Two grams of soil was extracted with 20 mL of 0.1 mol L−1 sodium sulfate (Na2SO4) solution for 30 min in boiling water, and the released Se was reduced to Se (IV) after organic matter decomposition. The concentration of Se (IV) was then determined by high performance liquid chromatography (HPLC) with a fluorescence detector after treatment with 2,3-diaminonaphthalene (DAN) and extraction with cyclohexane. Soluble Se content ranged from 2.5 to 44.5 μg kg−1 with geometric and arithmetic means of 11.4 and 12.8 μg kg−1, respectively, and corresponded to 3.2% of the total Se on average. The overall data showed log-normal distribution. In terms of soil type, Non-allophanic Andosols and Volcanogenous Regosols had relatively high soluble Se content, and Wet Andosols and Lowland Paddy soils had relatively low soluble Se content. In terms of land use, upland soils had significantly higher soluble Se content than paddy soils (p < 0.01). The soluble Se content had significant positive correlation with total organic carbon (TOC) content of the extract, soil pH and total Se content (p < 0.01). In conclusion, total Se content in combination with soil pH was the main determining factor of the soluble Se content of agricultural soils in Japan.

Factors controlling organic amendment application rate and long-term change in application rate in Japanese paddy field using longitudinal questionnaire survey dataset (the Basic Soil Environment Monitoring Project, Stationary Monitoring, 1979–1998)

Using data from a longitudinal survey (the Basic Soil Environment Monitoring Project, Stationary Monitoring, 1979–1998, launched by the Ministry of Agriculture, Forestry and Fisheries of Japan), it was found that livestock possession, soil type, utilization form of paddy fields and full-/part-time status of farmers were important factors controlling application rates of organic amendments (OA) in Japanese paddy fields. When data points were categorized in terms of these four factors, application rate of livestock waste compost (LWC) on the basis of fresh weight (FW) varied remarkably. Application rates of LWC decreased during the survey period, whereas rice straw residue (RSR) application rates increased. The smallest LWC application rate (average 0.3 ± 0.2 Mg FW ha−1 for the period between 1994 and 1998) was found among rice (Oryza sativa L.) single-cropping, poorly drained lowland paddy soils (PDLPS) including Histosols, with full-time farmers possessing no livestock. Even among rice single-cropping, the application rate was clearly larger (average 10.6 ± 0.5 Mg FW ha−1 for the period between 1979 and 1983) for non-poorly drained lowland paddy soils (non-PDLPS), with full-time farmers possessing livestock. Much greater than this was the LWC application rate for converted paddy fields (average 29.9 ± 4.3 Mg FW ha−1 for the period between 1989–1993), non-PDLPS, with full-time farmers possessing livestock. Accordingly, this study emphasizes the importance of categorizing data points at least in terms of soil type, utilization form of paddy fields, livestock possession and part- or full-time farming status when constructing an inventory, exploring changes in OA application rate, and making policy.

Heavy metal contamination of agricultural soils around a chromite mine in Vietnam

In Vietnam, the Co Dinh mine is the largest chromite mine in the country. Mining, ore dressing and disposal of the tailings provide obvious sources of heavy metal contamination in the mine area. The present study examined the influence of chromite mining activities on the adjacent lowland paddy field by investigating heavy metal and As levels in the mine tailings, sediments, paddy soils and water. At paddy fields located near the mine tailings, the total contents of Cr, Co and Ni were 5,750, 375 and 5,590 mg kg−1, and the contents of their water-extractable form were 12.7, 1.16 and 32.3 mg kg−1, respectively. These results revealed severe contamination of lowland paddy soils with Cr, Co and Ni as a result of mining activity, suggesting serious health hazards through agricultural products, including livestock in this area. The principal source of the pollution was sediment inflow owing to the collapse of the dike, which was poorly constructed by heaping up soil. Moreover, water flowing out from the mining area was also polluted with Cr and Ni (15.0–41.0 and 20.0–135 μg L−1, respectively). This might raise another problem of heavy metal pollution of watercourses in the area, indicating the need for further investigation and monitoring of fluctuations of water quality with seasonal changes.

Measurement and modeling of soil water regime in a lowland paddy field showing preferential transport

Paddy is commonly grown under flooded or submerged condition in which substantial amount of water is lost by different processes. Puddling is traditionally done to reduce water loss from lowland rice fields. Since the very step of puddling, rice root zone undergoes structural changes leading to the formation of a layered profile having a hydraulically less-conductive plow sole below the root zone. However, studies have shown that soil cracking and the presence of preferential flow paths in puddle fields defeat this purpose. Description of soil water regime in such a dynamic soil requires an in situ measurement method for soil hydraulic properties. A field experiment was conducted in twelve 30 m 2 plots during the rainy seasons (June to October) of 2004 and 2005 to evaluate a suitable method for estimating soil hydraulic properties of lowland paddy soil. Results showed that piezometric (pressure) heads installed in different soil layers responded to the drying and wetting cycles typically followed in transplanted rice and are observed as a part of monsoon climate in eastern India. The Marquardt–Levenberg algorithm built in the HYDRUS-1D simulation environment was used to inversely estimate soil hydraulic parameters. Estimated parameters revealed larger hydraulic conductivity for the compacted plow sole than those published in literature, which may have resulted from alternate wetting and drying typically observed under monsoon climate and earthworm burrows observed in our experimental field. Results from simulation studies suggest that both the single- and dual-porosity models could simulate water flow considerably well in lowland paddy field although the latter described pressure head time series data slightly better in about 50% of simulations. Similar performance of the single- vs. dual-porosity model may have resulted from estimating a seasonally mean soil hydraulic properties which include the effect of both preferential flow and matrix flow as the specific soil and boundary conditions prevailed. While water may have preferentially transported through the macropores during the wetting cycles in a near-saturated soil, it would have dominantly moved through soil matrix during the drying cycles. This study shows that simple piezometers may be combined with a simulation model to estimate hydraulic properties of different soil layers in a lowland paddy field.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Pedo-transfer function for saturated hydraulic conductivity of lowland paddy soils

In paddy field, soil saturated hydraulic conductivity (Ks) plays as an important component in the calculation of irrigation requirement of the water balance equation and also for irrigation efficiency. Several laboratory and field methods can be used to determine Ks. Laboratory and field determinations are usually time consuming, expensive and labour intensive. Pedo-transfer functions (PTF) serve to translate the basic information found in the soil survey into a form useful for broader applications through empirical regression of functional relationships, such as simulation modelling. Since PTFs have not been applied to paddy soils in the study area, a lot of field measurements will require high labour input to determine Ks hence high cost. This study attempts to seek a simplified method for determining Ks values based on common existing soil properties through PTF technique. Soil samples (n = 408 samples) were collected randomly depending on the soil series within the 2,300 ha Sawah Sempadan rice cultivation area. Both field work and laboratory work were carried out. The samples were then analysed for the following properties: dry bulk density (Db), soil particle percentage (Sand-S, Silt-Si and Clay-C), organic matter (OM) and geometric mean diameter (GMD). The measured Ks values were obtained by using the falling head method. The parameters were then used as inputs for developing a Ks model by regression analysis using Statistical Analysis System (SAS) package. Stepwise regression analysis was applied to determine the best fit model based on R2 and significant level. The results of the study showed that there is a high spatial variability of the saturated hydraulic conductivity in the paddy area. The best regression model for estimating Ks was based on C, Db, OM and GMD with the dependent variable (Ks) in a form of natural logarithm. The model inputs introduced by stepwise regression are commonly available therefore, this model is useful to replace the conventional method.

Relationship between vertical distributions of Bray II P content and organic P content in the Ap horizon of paddy rice fields

We examined the vertical distributions of total, Bray II and organic phosphorus (P) in the Ap horizons of five paddy rice fields including no tillage treatment. Soil samples were collected from the Ap horizons as nine thin layers of 0–1, 1–2, 2–3, 3–4, 4–6, 6–8, 8–10, 10–15 and 15–20 cm from the soil surface after harvest. In three lowland paddy soils of no-tillage Hachirogata (HA-NT), conventional-tillage Hachirogata (HA-T) and conventional-tillage Furukawa (FU-T), the total P (TP) content was relatively low. In these soils, the modified Bray II P (soil : solution = 1:20; shaking time 60s) content decreased and the organic P (OP) content increased at a depth of 0–1 cm compared with the underlying layers, possibly because of high microbial activities as suggested from the increased total C and N contents in this layer. In contrast, the decrease in Bray II P content and the increase in the OP content of the surface layer were not evident in two Andosols of the conventional-tillage Utsunomiya (UM-T) and conventional-tillage Kawatabi (KA-T). Even TP content fluctuated in the thin surface layers of 0–3 cm depending on the sampling site. Accordingly, percentages of Bray II P and OP to TP were calculated to compare the vertical distribution curves between Bray II P and OP. The percentage of Bray II P to TP of the 0–1 cm layer was lower and the percentage of OP to TP in the layer was higher than the underlying layers for HA-T, HA-NT and FU-T. The decrease in Bray II P was nearly compatible with the increase in OP for the surface of the 0–1 cm layers of HA-NT, HA-T and FU-T. This result suggests that a part of Bray II P was converted to OP in the thin surface layer of the Ap horizons in these paddy soils. This change in chemical form of P was more pronounced in the HA-NT by accumulating the effect of the microbial activities over a longer period of time because of no tillage compared with HA-T and FU-T. These findings add a new dimension to P behavior in submerged rice soils under field conditions, particularly near the soil surface. Moreover, we must be careful in soil sampling for P availability evaluation, and soil samples should represent the vertical distribution of P in the whole Ap horizon of a paddy field. Changes in OP with depth in UM-T and KA-T may have been masked by the high OP content as well as by the high TP content in these Andosols.

Biodynamics of Microbial Biomass Nitrogen in Organic Matter Amended Lowland Paddy Soil of Bangladesh

Biodynamics of Microbial Biomass Nitrogen in Organic Matter Amended Lowland Paddy Soil of Bangladesh

Effective porosity of paddy soils as an estimation of its saturated hydraulic conductivity

Soil saturated hydraulic conductivity ( K s) is an important soil physical property. Some laboratory and field methods are expensive, time consuming and labour intensive. Indirect methods such as pedo-transfer functions (PTF) are available. Effective porosity or macroporosity (Ø e) is approximately equals to porosity minus volumetric soil water content at the field capacity. According to Kozeny–Carman equation, K s could be evaluated using Ø e. Franzmeier estimated the K s from Ø e based on Ahuja et al. He found a strong relationship between Ø e and K s. This paper presents results of a study to characterize the effective porosity in lowland paddy fields and to show the possibility of using the effective porosity (Ø e) in estimating the saturated hydraulic conductivity ( K s). Soil in lowland paddy fields forms its horizon as topsoil, hardpan and subsoil. Soil samples were collected randomly within a 2300-ha rice cultivation area where there are five dominant soil series. A total of 408 soil samples were taken from 136 sampling points and at three depths, namely, the topsoil, hardpan and subsoil. K s values were measured in the laboratory using the falling head method. Soil bulk density and moisture content at −66 kPa were determined. The Ø e was then calculated using the difference of the total porosity (Ø) minus the volumetric moisture content at −66 kPa. The K s values ranged from 5.35×10 −4 to 8.77×10 −2 m day −1. The D b varied from 0.62 to 1.91 Mg m −3, and the values of the D p ranged from 1.10 to 2.89 Mg m −3. The Ø ranged from 0.17 to 0.68 m 3 m −3. The results of the Ø e of the samples in this study were obtained by calculating the difference of the total porosity and volumetric moisture content at field capacity. For clayey soils, field capacity is taken at the suction of −66 kPa. The Ø e varied from 0.05 to 0.55 m 3 m −3, with the mean value of 0.24 m 3 m −3. The regression equation of a power function shows a highly significant regression coefficient, r 2 of 0.50 ( n=400). This indicates that there is a strong relationship between K s and Ø e for the lowland paddy soils in the study area.

Selenium geochemistry of paddy soils in Yangtze River Delta

Selenium (Se) content both in total and bioavailable forms were very low (25–35 and 9.5 μg/kg, respectively) in waterlogged paddy soils in the central Yangtze River Delta. This was due to changes in geochemistry and the inherent properties of the parent materials of these lowland paddy soils. The very low Se content (23 μg/kg) in the parent material resulted in low total Se content in the soil. The main chemical changes under long-term waterlogged conditions are depletion of molecular oxygen, decrease of redox potential, and reduction of Fe (III) to Fe (II) and SeO 3 2− to Se 0. This led to low availability of Se in soils, and subsequently low Se content (29 μg/kg) in brown rice grain produced in this region. It has been suspected that low Se content in staple food might be one of the major reasons for a high infection rate of the intestine and stomach cancers and the higher death rate caused by these diseases in the region. Foliage spray of Na 2SeO 3 at early heading stage of rice plant growth improved the Se content of brown rice grain, hull, and straw, and would improve human and animal health.

Fertilizer inputs, nutrient balance, and soil nutrient-supplying power in intensive, irrigated rice systems. II: Effective soil K-supplying capacity

Based on the analysis of soil K status in 11 long-term fertility experiments in Asia, two approaches for assessing the K-supplying power of lowland paddy soils were evaluated to predict total K uptake by irrigated rice. A regression model combining commonly used static soil test parameters that appeared to integrate measures of K release from nonexchangeable forms as well as chemical factors affecting K activity in soil solution explained 72% of the crop K uptake in NP and NPK treatments. However, this approach would require determination of six soil properties and it does not provide a direct measure of K release dynamics. The second approach used mixed-bed ion exchange resin capsules to measure K release during 2-wk anaerobic incubation. The resin method provided an integrative measure of soil K status and the factors controlling K transformation and diffusion rates, which are embodied in two kinetic parameters describing the dynamics of rapid- and slow-phase K release. The resin method was sensitive to past fertilizer history and the resulting buildup or depletion of soil K reserves and it was a better predictor of total K uptake in the NP and NPK plots (r2=0.82) than static soil tests. The results also indicated that high (Ca+Mg)/K ratios may contribute to K deficiency in rice soils.