Modeling paddy field subsurface drainage using HYDRUS-2D

All of steady and non-steady subsurface drainage equations were developed mostly based on water flow pattern in an ordinary field conditions. However, subsurface drainage in a paddy field is quite different from subsurface drainage in an ordinary field. Thus, it is necessary to develop new equations and mathematical models to design subsurface drainage system in a paddy field. The objective of this study was to apply the HYDRUS-2D model, based on the Richard’s equation, to simulate water flow under subsurface drainage in a paddy field for various drain depths (0.5, 0.75 and 1.0 m) and spacings (7.5 and 15.0 m), surface soil textures (clay loam and silty clay loam) and crack conditions. Simulation results were compared with two well-known drainage equations. The maximum drainage rate was obtained under 7.5-m spacings and 1-m depth. With increasing drain spacings, the drainage rate decreased. Drain spacings had more effect on drainage rate and water pressure head as compared to drain depth. Drainage rates calculated by the Hooghoudt’s and Murashima and Ogino’s equations were much lower than those calculated by the Richard’s equation. The Hooghoudt’s equation, developed for ordinary fields, did not perform well for paddy fields. This study also proved the importance of cracks in subsurface drainage system of paddy fields. HYDRUS-2D stands as a robust tool for designing subsurface drainage in a paddy field.     

Evaluation of HYDRUS-2D model to simulate the loss of nitrate in subsurface controlled drainage in a physical model scale of paddy fields

Agriculture is a major source of nitrogen usage and release to environment. Due to the effect of water movement on solute transport, investigating the effect of different management scenarios of irrigation and drainage could be useful for reducing nitrate loss and environmental pollution. This study is a scientific attempt to assess the ability of HYDRUS-2D model to simulate the effect of subsurface controlled drainage on nitrate loss of paddy fields. So, two physical models with difference in depth of subsurface controlled drainage (40 and 60 cm) were constructed. The tanks were filled with loam silty soil texture and then transplanted rice. 90 kg/ha potassium nitrate fertilizer was added in two stages of rice growth. Mid-season drainage was applied 26 days after transplantation. After 17 days, drains were closed again and applied flooded irrigation with 5-cm water stagnant layer above soil surface. During experiment, nitrate concentration of drain water was measured. HYDRUS-2D was calibrated with measured data in 60 cm drain depth and validated with 40 cm drain depth. HYDRUS-2D could simulate nitrate concentration with the coefficient of determination 0.95 and 0.89 in calibration and validation stages, respectively. The comparison between the volume of drain water and nitrate concentration from the drains in the depths of 40 and 60 cm indicated lower nitrate load in depth of 40 cm. The results obtained proved that the presence of hardpan layer in depth of 25 cm rather than the absence of it causes increase in 3 % of average nitrate concentration and reduce in 17 % of water discharge.     

High-yield SRI in West Java by decomposing straw in waterlogged paddy field

System of Rice Intensification (SRI) often achieved higher yield than conventional practice. We identified the high-yielding farmers from the yield records of 1909 paddy fields belonging to an organic farmers’ association. Farmers whose yields were from 8.4 to 10.4 t ha^?1 were interviewed and their fields surveyed. Their yields had increased by an estimated average of 40% following the adoption of SRI practices. They applied 2–12 t ha^?1 of compost. Compared to the conventional practice, they shortened seedling age at transplanting from 27.4 to 17.6 days and reduced the number of seedlings per hill from 4–6 to 2–3, while hill spacing remained unchanged. Instead of intermittent irrigation which is recommended in standard SRI, they kept shallow flooding of 1–2 cm. Although they applied a lot of compost, no correlation was found between the amount of compost application and the yields. Instead, high-yielding farmers returned rice straw into waterlogged paddy after harvest, which presumably is an ideal condition for biological nitrogen fixation. This may occur around rice straw during decomposition under waterlogged condition and might supplement the negative nitrogen balance, thereby enabling the high yield as compared with conventional practices where the fresh rice straw is removed and/or burned.     

Irrigation management transfer (IMT) and system of rice intensification (SRI) practice in the Philippines

SRI practices can usually increase the yield of paddy without using special varieties of rice or chemicals. High yield of paddy can be simply achieved through the combination of transplanting single seedlings about 8–10 days old with just two leaves on 40 cm × 40 cm spacing, providing organic fertilizer, and intermittent irrigation. No pesticides are applied. Historically, integrated management of water and crop (particularly rice) in the Philippines, had its beginning with the Spaniards, but more formal events were recorded in the 1950s by Margate (1954) in “Rice: 100 Cavans (50 kg/cavan) per Hectare.” This was followed by the Water Management Manual released in the 1970s by the Asian Development Bank (ADB) and National Irrigation Administration (NIA). Recently, the Southern Philippines Irrigation Sector Project (SPISP), a joint ADB–NIA effort, has commenced trials with the system of rice intensification (SRI) practice. One of the most specific features of SRI is the intermittent irrigation system which requires assured water supply at necessary timing. It is easier to attain assured water supply on timely manner through irrigation management transfer (IMT) which encourages the empowerment of fair water distribution. This article analyzes the merit of IMT for SRI.     

Investigating the reduction of sodium adsorption ratio from agricultural waste by rice husk in batch scale and physical model of drain envelop

Sodium adsorption ratio (SAR) is one of the water quality indexes that whose is important due to reuse or depletion to environment. Solutes in drain water can be controlled by adsorption, chemical or biological reaction, organic envelope of drainage. Rice husk is the common option of drainage envelops in paddy fields. In this study, the ability of reduction of SAR by rice husk was evaluated in batch scale and physical model of drain envelops. In the batch experiments, the adsorption of SAR parameters was investigated by adding 2 g of rice husk into a 100 ml of sodium chloride solution. The results indicated that rice husk absorbed calcium, magnesium and sodium, respectively. By increasing the temperature, contact time and pH, adsorption of calcium, magnesium and sodium was increased; however, the higher concentration of sodium in soil solution reduced the percentage of adsorption. In a more realistic state, physical models of subsurface drainage in the paddy fields were made. Drainage envelope treatments included of rice husk (H), combination of 20 and 60 % of husk with gravel (H₂₀G₈₀ and H₆₀G₄₀) and a pipe without envelope (NE). Due to higher drain discharge and more sodium removal (lower SAR in drain water), treatment H with the discharge of 16.2 ml/min and SAR of 1.27 (meq/l)^0.5 was better in comparison with other treatments.     

Smooth bromegrass seed yield and yield component responses to seeding rates and row spacings in two climates

Successful grass seed production depends on identifying a suitable environment for the species and proper agronomic practices. Previous research on many species has addressed identifying appropriate agronomic practices for grass seed production, but these studies have not evaluated the effects of environment. By conducting the same experiments in Jiuquan, China (a desert climate) and Tongliao, China (a semiarid continental monsoon climate), the effects of environment, seeding rate, row spacing and their interactions were determined for smooth bromegrass (Bromus inermis Leyss) seed production. Three seeding rates (.3, .5, and .7 g m^?1 pure live seed) and four row spacings (30, 50, 70, and 90 cm) were evaluated over three years. Jiuquan had comparable seed yield (SY) and greater thousand-seed weight (TSW) than Tongliao. Three-year average SY decreased with increased row spacings at both sites. Results suggest that in both climates, successful smooth bromegrass seed production was possible, but greater TSW is predicted for desert climates with good irrigation conditions than in semiarid continental monsoon climates due to greater sunshine duration (574 h compared with 527 h) and low relative humidity during seed development (48% vs. 66%). A seeding rate of .3 g m^?1 and a row spacing of no wider than 30 cm appears to be adequate for smooth bromegrass seed production in these research locations and in similar ecological regions around the world.     

Chemical fractionation of seven heavy metals (Cd, Cu, Fe, Mn, Ni, Pb, and Zn) in selected paddy soils of Iran

The content and fractionation of seven heavy metals (Cd, Cu, Fe, Mn, Ni, Pb, and Zn) were determined in 28 surface soil samples (0–20 cm) of agricultural topsoil from Isfahan Province in central Iran. The order of abundance of metals in the soils was Fe (1240.4 mg kg^?1) > Mn (95.7 mg kg^?1) > Pb (51.6 mg kg^?1) > Zn (23.8 mg kg^?1) > Ni (13.4 mg kg^?1) > Cu (7.0 mg kg^?1) > Cd (2.8 mg kg^?1). Iron, Mn, Ni, Pb, and Zn existed in paddy soils mainly in Fe-Mn oxides (53.6 %, 65.2 %, 40.4 %, 40.8 %, 53.3 %, respectively), whereas Cu and Cd occurred essentially as residual mineral phase (41.4 %) and carbonate (36.1 %), respectively. The mobile and bioavailable fractions of Cd, Cu, Fe, Mn, Ni, Pb, and Zn in paddy soils averaged 48.8, 20.8, 0.79, 29.2, 28.5, 41.1, and 24.8 %, respectively, which suggests that the mobility and bioavailability of the seven metals probably decline in the following order: Cd > Pb > Mn ≥ Ni > Zn > Cu ? Fe, suggesting greater contribution of anthropogenic Cd. As Cd in soil is easily accumulated by plants through the root system, the concentration of Cd in these paddy soils could be a concern to human health.     

Application of machine-learning models for diagnosing health hazard of nitrate toxicity in shallow aquifers

There is a growing concern about health hazards linked to nitrate (NO₃) toxicity in groundwater due to overuse of nitrogen fertilizers in rice production systems of northern Iran. Simple-cost-effective methods for quick and reliable prediction of NO₃ contamination in groundwater of such agricultural systems can ensure sustainable rural development. Using 10-year time series data, the capability of adaptive neuro-fuzzy inference system (ANFIS) and support vector machine (SVM) models as well as six geostatistical models was assessed for predicting NO₃ concentration in groundwater and its noncarcinogenic health risk. The dataset comprised 9360 water samples representing 26 different wells monitored for 10 years. The best predictions were found by SVM models which decreased prediction errors by 42–73 % compared with other models. However, using well locations and sampling date as input parameters led to the best performance of SVM model for predicting NO₃ with RMSE = 4.75–8.19 mg l^?1 and MBE = 3.3–5.2 mg l^?1. ANFIS models ranked next with RMSE = 8.19–25.1 mg l^?1 and MBE = 5.2–13.2 mg l^?1 while geostatistical models led to the worst results. The created raster maps with SVM models showed that NO₃ concentration in 38–97 % of the study area usually exceeded the human-affected limit of 13 mg l^?1 during different seasons. Generally, risk probability went beyond 90 % except for winter when groundwater quality was safe from nitrate viewpoint. Noncarcinogenic risk exceeded the unity in about 1.13 and 6.82 % of the study area in spring and summer, respectively, indicating that long-term use of groundwater poses a significant health risk to local resident. Based on the results, SVM models were suitable tools to identify nitrate-polluted regions in the study area. Also, paddy fields were the principal source of nitrate contamination of groundwater mainly due to unmanaged agricultural activities emphasizing the importance of proper management of paddy fields since a considerable land in the world is devoted to rice cultivation.     

Forage yield and chemical composition of canola (Brassica napus L.) as affected by sowing methods

The sowing method of spring‐type canola (Brassica napus L. var. oleifera) for forage has a major influence on its productivity and agronomic management. A field experiment was conducted in Matamoros, Coahuila, Mexico, during two growing seasons (2008–2009 and 2009–2010) to determine dry matter (DM), crude protein (CP) and net energy for lactation (NE_L) yields, as well as canola forage chemical composition as a function of six sowing methods. The treatments assessed were broadcast sowing and five different row spacings: 0·19, 0·38, 0·57, 0·76 and 0·95 m (double row, 0·20 m apart). In the first year, with a mean growing season temperature of 17·2°C, sowing methods did not affect DM yield, but CP and NE_L content and yield were higher in 0·19‐m row spacing. The mean temperature in the second year (13·5°C) was slightly lower than the long‐term mean (14·8°C) in the region, resulting in the highest DM (8840 kg ha^?1), CP (2486 kg ha^?1) and NE_L yields (51 103 MJ ha^?1) with 0·19‐m row spacing. In row‐sowing methods with over 0·19‐m row spacing, DM, CP and NE_L yields decreased by 19·3–39·7, 20·4–42·1 and 21·2–42·7% respectively. Results indicate that sowing methods significantly affected canola forage productivity.     

Effects of temperature and soil moisture on gross nitrification and denitrification rates of a Chinese lowland paddy field soil

Alternate wetting and drying (AWD) irrigation is widely adopted to save water in rice production. AWD practice shifts lowland paddy fields from being continuously anaerobic to being alternately anaerobic and aerobic, thus affecting nitrogen (N) transformations in paddy field soils. Using the barometric process separation technique, a large number of soil cores sampled from lowland paddy field soil profiles were measured for gross nitrification and denitrification rates under different temperature and soil moisture conditions. The gross nitrification and denitrification rates vary with rice growth stages and range between 1.18–30.8 and 0.65–13.54 mg N m^?3 h^?1, respectively. Results indicate that both gross nitrification and denitrification rates increased with the increase in temperature in all three studied soil layers. Gross nitrification rates significantly decrease with increasing soil moisture while denitrification rates increase, and different soil layers demonstrated different rates of variation to the increase in soil moisture. Gross nitrification rates in the cultivated horizon layer decreased more sharply with the increase in soil moisture. High soil water content is favorable to denitrification of all soil layers.     

Influence of solar drying and storage conditions on microstructure,crack propagation and nano-hardness of paddy and wheat

The influence of hybrid solar drying (HSD) and storage conditions on microstructure, crack propagation, nano-hardness and milling indices of paddy and wheat grains were investigated. Milling yield and head rice yield of dried paddy was 71.48% and 72.42%, which was further increased by 1–1.26% and 3.12–4.65%, respectively. Flour yield from dried wheat was found to be 77.30% and was reduced by 3.5–7.7% after 180 days of storage. Maximum nano-hardness of 0.15 ± 0.02 GPa was obtained for rice stored at 5 °C, whereas, for wheat, nano-hardness, elastic modulus, and peak load values gradually reduced with a storage time of 180 days. Micro-X ray computed tomography images revealed the pore size of paddy and wheat samples to be in the range of 0.01–0.8 mm³. Micrographs showed a compact paddy surface, whereas wheat endosperm witnessed cell disruption and agglomeration.     

Effect of plant spacing and nitrogen fertilizer levels on the growth,dry‐matter yield and nutritive quality of Columbus grass (Sorghum almum stapf) in southwest Nigeria

A field experiment was conducted in 2006 and 2007 to determine the agronomic performance and nutritive value of Sorghum almum for introduction in the derived savannah area of Nigeria. The experiment was arranged in a 2 × 4 factorial design with 2 plant spacings (0·5 × 0·5 m and 1·0 × 1·0 m) and 4 nitrogen (N) fertilizer levels (0, 60, 120 and 180 kg N ha^?1). Plant height, tiller number, leaf proportion, biomass yield and nutritive value of the herbage were evaluated as part of the search for alternatives (especially drought tolerant) to local forages for dry season feeding of ruminants. Herbage yield data were tested for linear, quadratic and cubic trends to identify the optimal fertilizer levels for both spacings. Spacing × N interactions (P < 0·05) were observed for plant height and tiller number in both years. Agronomic performance was marginally better in 2007 compared with 2006. The maximum dry‐matter (DM) yield of 3500 and 3740 kg ha^?1 for the more dense row spacing (0·5 × 0·5 m) was achieved at N fertilizer levels of 144 and 149 kg N ha^?1 for 2006 and 2007 respectively. For the less dense (1·0 × 1·0 m) row spacing, the maximum DM yield of 3020 and 3240 kg ha^?1 was achieved at 51 and 97 kg N ha^?1 for 2006 and 2007 respectively. The crude protein content of the grass ranged from 61 to 89 g kg^?1 DM, while the neutral detergent fibre (NDF) content ranged from 700 to 734 g kg^?1 DM. The ability of S. almum to persist into the second year in this region is seen as a promising index as persistence is one of the characteristics of a good forage plant. Considering the exorbitant price of N fertilizer, less dense row spacing with N fertilizer rate in the range of 50–100 kg N ha^?1 is hereby recommended for this region.     

Effect of controlled irrigation and drainage on nitrogen leaching losses from paddy fields

The effect of controlled irrigation and drainage on N leaching losses from paddy fields was investigated by controlling root zone soil water content and water table depth using a lysimeter equipped with an automatic water table control system. Three treatments that combined irrigation and drainage managements were implemented: controlled irrigation (CI) + controlled water table depth 1 (CWT1), CI + controlled water table depth 2 (CWT2), and flooding irrigation (FI) + actual field water table depth (FWT). Controlled irrigation and drainage had significant environmental effects on the reduction of NH₄ ⁺–N and NO₃ ^?–N leaching losses from paddy fields by decreasing water leakage. The NH₄ ⁺–N leaching losses from CI + CWT1 and CI + CWT2 were 3.68 and 4.45 kg ha^?1, respectively, which significantly reduced by 59.2 and 50.7 % compared with FI + FWT (9.02 kg ha^?1). The NO₃ ^?–N leaching losses from CI + CWT1 and CI + CWT2 were 0.88 and 0.43 kg ha^?1 with a significant reduction of 45.2 and 73.2 %, respectively, compared with FI + FWT (1.61 kg ha^?1). The application of CI + CWT1 can be a pollution-controlled water management method of reducing N leaching losses from paddy fields.     

Modelling of groundwater recharge potential from irrigated paddy field under changing climate

Groundwater recharge from irrigated paddy field under various projected climate change scenarios was assessed using HYDRUS-1D model. Recharge flux, root water uptake, evaporation and surface runoff were simulated on daily time step for the growing period of paddy. Crop evapotranspiration and effective rainfall during the simulation period were estimated to be 301.9 and 269.4 mm, respectively. Cumulative bottom flux, root water uptake, evaporation and surface runoff were 69.2, 23.2, 30.8 and 0.0 cm for sandy loam and 37.2, 23.0, 30.8 and 0.7 cm for clay loam soils, respectively. Simulation results showed that the groundwater recharge potentials in sandy loam and clay loam soils with paddy crop are 69.2 and 37.2 cm, respectively. Cumulative recharge under various climate change scenarios from paddy field varied from 63.9 to 74.4 cm, 33.7 to 39.8 cm, 29.3 to 35.4 cm and 27.1 to 34.3 cm from land units A1 (sandy loam), B1 (clay loam with slight salinity), C1 (clay loam with moderate saline and slight sodic) and D1 (clay loam with strong saline and sodic), respectively. Cumulative recharge flux under the scenarios in which increase in relative humidity along with decrease in duration of sunshine hours was associated with rise in average temperature and wind speed, groundwater recharge would increase by 7.4 %. Cumulative recharge flux under the scenarios which were based on rise in temperature along with the increase in rainfall, groundwater recharge would increase by 0.2–3.9 %. Simulation results also showed that cumulative recharge would decrease under all those scenarios, which were based on rise in temperature only.     

The influence of seepage from canals and paddy fields on the groundwater level of neighboring rotation cropping fields: a case study from the lower Ili River Basin,Kazakhstan

In the large-scale irrigation schemes of the lower Ili River Basin of Kazakhstan, crop rotation combines paddy rice and non-rice crops. Continuous irrigation is practiced in paddy fields, whereas other crops are sustained from groundwater after only limited early irrigation. The water table in non-rice crops is raised by seepage from canals and the flooded paddy fields. We investigated the areal extent to which the groundwater level of non-irrigated fields is influenced by seepage from canals and paddy fields by examining the relationship between distance (from canal and paddy field) and groundwater level in upland fields. The groundwater level was influenced for up to 300 and 400 m from the canals and paddy fields, respectively. Geographic information system analysis of crop and canal patterns in the 11 selected years showed that if the zone of influence is 300 and 400 m from the canals and paddy fields, respectively, the groundwater level of most of the area of upland fields was raised by seepage. We conclude that the water supply to cropping fields by seepage from irrigation canals and paddy fields is adequate, but the spatial distribution of the paddy fields may be an important factor that needs more attention to help improve water use efficiency in this irrigation district.     

Drainage and irrigation performance of hybrid ditches in converted paddy fields under winter wheat cultivation in Hokkaido

Paddy fields converted into winter wheat fields in Hokkaido, Japan, receive extremely high snowfall, creating a risk of flood damage to crops in spring due to waterlogging of snowmelt runoff and poor drainage. Meanwhile, in June there is relatively little rainfall, and a lack of moisture inhibits winter wheat growth. Therefore, we developed a method involving a series of 30-cm-deep ditches in agricultural fields to be used for drainage during the flood-prone period and for furrow irrigation during the dry period using water drawn from the canals that feed the paddy fields. The ditches are called ‘hybrid ditches’ as they are able to perform both drainage and irrigation functions. In this study, we investigated the optimal construction timing and spacing for hybrid ditches. We also evaluated their ability to improve the drainage and irrigation of winter wheat. We found that the optimal timing for digging hybrid ditches is immediately after sowing, and the inter-ditch spacing for irrigation should be 15 m or less. The hybrid ditches promoted increased soil temperature and healthy development of wheat plants by improving drainage during the flood-prone period. In addition, water was successfully supplied via the hybrid ditches to irrigate the fields in June. Under experimental conditions in which rainfall was excluded, grain yield was 10% higher and percent protein content was more than 1% point greater in the irrigated plot compared with the non-irrigated plot. Grain yield was also observed to increase by 3–29% in demonstration tests conducted at local farms. From these results, we conclude that hybrid ditches are capable of improving the growth and yield of winter wheat by improving drainage and providing irrigation in converted paddy fields in Hokkaido.     

Cooperative effects of sand application and flushing during the sensitive stages of rice on its yield in a hard saline–sodic soil

Application of sand can ameliorate rice paddy fields converted from saline–sodic land. However, the requirement of huge amount of sand has been limiting its practical application. In this study, flushing during saline sodic-sensitive stages of rice plant growth was incorporated into the ameliorating system to reduce the sand usage. A split-plot design was adopted with sand application (SA) with two levels as main plots and flushing during the sensitive stages (FL) with two levels as subplots in a hard saline–sodic soil, Northeast China. Four treatments included CK (no-sand, no-flush flooding), NF (non-sand, flush flooding), SN (sand, no-flush flooding), and SF (sand, flush flooding). The results showed that both SA and FL significantly affected all the investigated yield parameters. The combined effect of SA and FL on the grain yield was additive in the first year in respect of the effect on panicle density and seed weight per panicle; while it showed synergistic effect on the seed weight per panicle and grain yield in the second year. The rice yield in different treatments was in the order of SF > SN > NF > CK in both years, with the highest yield (4.37 t ha^?1) obtained by SF treatment in the second year. Our results demonstrate that half the traditional amount of sand in combination with water-flushing during the saline–sodic-sensitive growth stages of rice is sufficiently effective in ameliorating saline–sodic soil and thereby enhancing rice grain yield in saline–sodic paddy fields.     

Numerical modeling based on a finite element method for simulation of flow in furrow irrigation

In this study, a zero-inertia finite element model (ZIFEM) is developed and applied for simulating all phases of furrow irrigation based on Saint–Venant equations. The complexity and nonlinear behavior of the Saint–Venant equations are the major difficulty in developing a finite element model to simulate furrow irrigation. Therefore, through the Galerkin FEM approach, the model assesses the free surface flow on a variable cell length at each time step and determines the suitable element length for each individual cell and the model solves the equations by using an iterative method. Along with the free surface flow phase, the infiltration phase is estimated by the Kostiakov–Lewis equation. The ZIFEM model is verified using seven experimental data sets collected from the literature and observed data from the farm consisting of two free drainage furrows with a length of 72 m, a top width of 0.8 m, a depth of 0.25 m and a slope of 0.2%. The model accuracy is studied to simulate advance and recession trajectories and runoff by calculating the root-mean-square error (RMSE), relative error and percentage error. It is observed that in all irrigation events, the proposed model reasonably agreed with field measurements. An evaluation of the RMSE shows that in 81.25% irrigation events the ZIFEM is more accurate than the WinSRFR model. In overall, the results of the model suggest that the ZIFEM can be introduced as a potential numerical tool for analyzing and evaluating furrow irrigation.     

Mechanical insights into the effect of fluctuation in soil moisture on nitrous oxide emissions from paddy soil

Paddy fields are subjected to fluctuating water regimes as a result of the alternate drying and wetting water management, which often incurs a sensitive change in N₂O emissions from paddy soils. However, how the soil moisture regulates the emission of N₂O from paddy soil remains uncertain. In this study, three incubation experiments were designed to study the effects of constant and fluctuating soil moisture on N₂O emission and the sources of N₂O emission from paddy soil. Results showed that the N₂O emission from paddy soil at 100 % WHC (water-holding capacity) was higher than that at 40, 65, 80, 120, and 160 % WHC, indicating that 100 % WHC was the optimum soil moisture content for N₂O emission under the incubation experiment. Small peak of N₂O flux appeared when the soil moisture content from 250 % WHC decreased near to 100 % WHC, lower than that triggered by nitrogen (N) fertilization, which was mainly owing to the low NH₄ ⁺ concentration at this period. Nitrification dominated the emissions of N₂O from paddy soil at 250 % WHC (54.96 %), higher than that of nitrification-coupled denitrification (6.74 %) and denitrification (38.3 %). The contribution of denitrification to N₂O emissions (44.10 %) was equivalent to that of nitrification (44.45 %) in soil at 100 % WHC, which was higher than that of 250 % WHC treatment. In conclusion, the finding suggested that the peak of N₂O in paddy soils during midseason aeration could be attributed to the occurrence of optimum soil moisture under sufficient N availability, favorable for the production and accumulation of N₂O.     

Long-term evaluation of the BMPs scenarios in reducing nutrient surface loads from paddy rice cultivation in Korea using the CREAMS-PADDY model

Curbing nutrient loads from rice cultivation has been an issue for the water quality management of surface water bodies in the Asian monsoon region. The objectives of this study were to develop paddy BMP scenarios and to evaluate their effectiveness on nutrient loads reduction using long-term model simulation. Totally five BMP scenarios were developed based on the three paddy farming factors of drainage outlet height, fertilizer type, and application amount and were compared with conventional practices. CREAMS-PADDY model was chosen for the paddy nutrient simulation, and two-year field experimental data were used for the model calibration and validation. The validated model was used to evaluate the developed BMP scenarios for the 46 years of simulation period. The observed nutrient loads were 15.2 and 1.45 kg/ha for nitrogen and phosphorus, respectively, and mainly occurred by early season drainage and rainfall runoff in summer. The long-term simulation showed that the soil test-based fertilization and drainage outlet raising practice were the two most effective methods in nutrient loads reduction. The combination of these two resulted in the greatest loads reduction by 29 and 37 % for T-N and T-P, respectively (p value < 0.001). Overall the effectiveness of the BMP scenarios was decreased in the wet season. As the conclusion, outlet height control and soil nutrient-based fertilization were suggested as the effective practices in paddy loads reduction and their combination can be a practicable BMP scenario for the paddy nutrient management.