Validation of the DNDC-Rice model by using CH4 and N2O flux data from rice cultivated in pots under alternate wetting and drying irrigation management

The DNDC (DeNitrification-DeComposition)-Rice model, one of the most advanced process-based models for the estimation of greenhouse gas emissions from paddy fields, has been discussed mostly in terms of the reproducibility of observed methane (CH₄) emissions from Japanese rice paddies, but the model has not yet been validated for tropical rice paddies under alternate wetting and drying (AWD) irrigation management, a water-saving technique. We validated the model by using CH₄ and nitrous oxide (N₂O) flux data from rice in pots cultivated under AWD irrigation management in a screen-house at the International Rice Research Institute (Los Baños, the Philippines). After minor modification and adjustment of the model to the experimental irrigation conditions, we calculated grain yield and straw production. The observed mean daily CH₄ fluxes from the continuous flooding (CF) and AWD pots were 4.49 and 1.22?kg?C?ha^?1?day^?1, respectively, and the observed mean daily N₂O fluxes from the pots were 0.105 and 34.1?g?N?ha^?1?day^?1, respectively. The root-mean-square errors, indicators of simulation error, of daily CH₄ fluxes from CF and AWD pots were calculated as 1.76 and 1.86?kg?C?ha^?1?day^?1, respectively, and those of daily N₂O fluxes were 2.23 and 124?g?N?ha^?1?day^?1, respectively. The simulated gross CH₄ emissions for CF and AWD from the puddling stage (2 days before transplanting) to harvest (97 days after transplanting) were 417 and 126?kg?C?ha^?1, respectively; these values were 9.8% lower and 0.76% higher, respectively, than the observed values. The simulated gross N₂O emissions during the same period were 0.0279 and 1.45?kg?N?ha^?1 for CF and AWD, respectively; these values were respectively 87% and 29% lower than the observed values. The observed total global warming potential (GWP) of AWD resulting from the CH₄ and N₂O emissions was approximately one-third of that in the CF treatment. The simulated GWPs of both CF and AWD were close to the observed values despite the discrepancy in N₂O emissions, because N₂O emissions contributed much less than CH₄ emissions to the total GWP. These results suggest that the DNDC-Rice model can be used to estimate CH₄ emission and total GWP from tropical paddy fields under both CF and AWD conditions.     

Growth and yield of lowland rice as affected by integrated nutrient management and cultivation method under alternate wetting and drying water regime

The effects of integrated nutrient management, cultivation method, and variety on root and shoot growth, grain yield and its components of lowland rice under alternate wetting and drying (AWD) irrigation were evaluated. Treatments included were three varieties (Pathumthani 1, RD57, and RD41), three cultivation methods [dry direct seeding, wet direct seeding, and transplanting], and three nutrient combinations [100% NPK (160?kg ha^?1), 50% NPK (80?kg ha^?1) + 50% FYM (5 t ha^?1), and 100% FYM (10 t ha^?1)] under AWD. Root dry matter of RD41 and RD57 was reduced by 12–25% at the 100% NPK and 100% FYM compared with the 50% NPK + 50% FYM. Panicle number, panicle length, and 1000-grain weight were higher at the 50% NPK + 50% FYM. Application of the 50% NPK + 50% FYM could be a feasible option under AWD irrigation; however, benefits may vary with varieties and cultivation methods.     

Incorporation of rice straw mitigates CH4 and N2O emissions in water saving paddy fields of Central Vietnam

This study evaluated the effects of rice straw and water regimes on CH₄ and N₂O emissions from paddy fields for two rice growing seasons (summer 2014 and spring 2015). Water regimes included alternating wet–dry irrigation (AWD) maintained at three levels (–5 cm, – 10 cm and –15 cm) in comparison to continuous flooding irrigation (CF). Rice straw (5 t ha^–1) was incorporated into the top soil (0 – 15 cm), distributed and burned in situ. Results showed that using burned in situ rice straw was found to reduce seasonal cumulative CH₄ emission (24–34% in summer; 18–28% in spring), N₂O emission (21–32% in summer; 22–29% in spring) and lower rice yield (8–9%) than rice straw incorporation into top soil. AWD methods reduced the amount of CH₄ production (22.6–41.5%) and increased N₂O emission (25–26%) without any decrease in rice yield. Rice straw incorporation into the top soil with AWD had higher water productivity (23–37%) than rice straw when burned in situ with CF. The results conclude that AWD and rice straw management can be employed as mitigation strategy for CH₄ and N₂O emissions from paddy fields in Central Vietnam.     

Agronomic evaluation of rice cultivation systems for water and grain productivity

Field experiments were conducted during summer (March–July) and kharif (June–September), 2008 at the wetland farm, Tamil Nadu Agricultural University, Coimbatore, India, to study the performance of different rice cultivation methods on productivity and water usage using the hybrid CORH-3 as a test crop. Treatments consisted of different rice cultivation methods, namely, transplanted rice (conventional), direct sown rice (wet seeded), alternate wetting and drying method (AWD), system of rice intensification (SRI) and aerobic rice cultivation. Results revealed that maximum number of tillers m^?2, higher shoot and root length at maturity were recorded under SRI followed by transplanted rice, while aerobic rice produced lower growth parameters in both the seasons. Chlorophyll content at flowering was higher under SRI in two seasons studied (42.74 and 39.48 SPAD value, respectively) and transplanted rice compared to aerobic rice and AWD. In both summer and kharif seasons, SRI produced higher grain yield (6014 and 6682 kg ha^?1), followed by transplanted rice (5732 and 6262 kg ha^?1), while the lowest grain yield (3582 and 3933 kg ha^?1) was recorded under aerobic rice cultivation. Under SRI, 5 and 6.7% increase in grain yield and 12.6 and 14.8% water saving were noticed compared to transplanted rice, respectively, during summer and kharif seasons. In respect to water productivity, the SRI method of rice cultivation registered the highest water productivity (0.43and 0.47 kg m^?3), followed by AWD and aerobic rice cultivation. The conventional rice cultivation and direct sown rice produced lower grain yield per unit quantity of water used.     

Real-time nitrogen management in aerobic rice by adopting leaf color chart (LCC) as influenced by silicon

Leaf color chart (LCC) guides fertilizer nitrogen (N) application to rice as per requirement of the crop on the basis of a critical leaf color. Two field experiments were conducted to evaluate the effect of silicon (Si) and LCC based N management in aerobic rice. Following LCC-based N management, from 60 to 90 kg N ha^?1 and 75 to 100 kg N ha^?1 with 10–40% and 25–30% less fertilizer N was used without any reduction in yield as compared to the package of practices of 100 (50 kg N ha^?1 as basal + two split of 25 kg N ha^?1) kg N ha^?1 respectively, during both the seasons. The highest grain yield was noticed with 90 kg N ha^?1 (30 kg N ha^?1 as basal + LCC-3) and 100 kg N ha^?1 (50 kg N ha^?1 as basal + two split of 25 kg N ha^?1) along with the application of calcium silicate (CaSiO₃) at 2 t ha^?1 as sources of Si and on par with 60 kg N ha^?1 (no basal + LCC-3) and 75 kg N ha^?1 (30 kg N ha^?1 as basal + LCC-3), respectively, during the season in 2008 and 2009. Higher fertilizer N use efficiency was recorded with Si and need-based N management using LCC-3 rather than recommended dose of fertilizer N.     

Optimal rate and schedule of nitrogen fertilizer application for enhanced yield and nitrogen use efficiency in dry-seeded rice in north-western India

Dry direct-seeded aerobic rice (DSR) is an emerging attractive alternative to traditional puddled transplanted rice (PTR) production system for reducing labour and irrigation water requirements in the Indo-Gangetic plains (IGP) of India. The fertilizer N requirement of DSR grown with alternate wetting and drying water management may differ from that of PTR grown under continuous flooding due to differences in N dynamics in the soil/water system and crop growth patterns. Limited studies have been conducted on optimizing N management and application schedule for enhanced N use efficiency in DSR. Therefore, field experiments were conducted over 3 years in NW India to evaluate the effects of N rate and timing of its application on crop performance and N use efficiency. Interaction effects of four N rates (0, 120, 150, and 180 kg ha^?1) as urea and four schedules of N application on yield and N use efficiency were evaluated in DSR. The N schedules included N application in three equal split doses (0, 35 and 63, and 14, 35 and 63 days after sowing, DAS) and four equal split doses (0, 28, 49 and 70; 14, 28, 49 and 70 DAS). There was no significant interaction between N rate and schedules on grain yield. Significant response to fertilizer N was observed at 120 kg N ha^?1 and economic optimum dose for three equal split doses and skipping N at sowing was 130 kg N ha^?1. Highest mean grain yield of 6.60 t ha^?1 was obtained when N was applied in three equal split doses at 14, 35 and 63 DAS which was about 8.5% higher compared with N applied in four equal split doses at 14, 28, 49 and 70 DAS. Under the best N application schedule, agronomic N use efficiency (26 kg grain kg^?1), recovery efficiency (49%) and physiological efficiency (53 kg kg^?1) were comparable to the values reported in Asia for PTR. Results from our study will help to achieve high yields and N use efficiency in DSR to replace resource intensive PTR.     

Influence of nitrogen levels and plant spacing on growth,productivity and quality of two inbred varieties and a hybrid of aromatic rice

A field investigation was conducted at the Indian Agricultural Research Institute's Research Farm during the kharif (wet) seasons of 2002 and 2003 in a split plot design with three replications, consisting of 27 treatments, namely, main plots: three varieties (PRH-10, Pusa Sugandh-3 and Pusa Basmati-1) and three plant spacings (20 × 10, 20 × 15 and 20 × 20 cm²) and sub-plots: three levels of nitrogen (0, 80 and 160 kg N ha^?1). The research results indicated that aromatic rice hybrid PRH-10 produced 33 and 6%, respectively, more grain yield than that of Pusa Sugandh-3 and Pusa Basmati-1. The appreciable higher grain yield of PRH-10 over Pusa Sugandh-3 and Pusa Basmati-1 was due to considerable improvement in most of the yield attributing characters. Application of 160 kg N ha^?1 recorded 23.7 and 26.1% more grain yield over no nitrogen application whereas it was 6.4 and 6.1% more over 80 kg N ha^?1, respectively, during first and second year of the experimentation. Wider plant spacing of 20 × 20 cm² and application of 160 kg N/ha recorded significantly higher hulling, milling and head rice recovery compared to closer spacing and zero nitrogen application.     

Soil-Test-Based Optimum Fertilizer Doses for Attaining Yield Targets of Rice under Midland Alfisols of Eastern India

Soil-test crop-response experiments on rice were conducted in the Bastar Plateau Agroclimatic Zone of Chhattisgarh during 2009–2011 to assess yield, soil, plant, and fertilizer nitrogen (N), phosphorus (P), and potassium (K) nutrient relationships and calibrate optimum fertilizer doses for attaining yield targets. Soil fertility status was poor to medium for N (194–283 kg ha^?1) and P (7.53–19.66 kg ha^?1), and medium to good for K (226–320 kg ha^?1). Based on nutrient requirements (NR, kg q^?1) and contributions from soil (CS, %), fertilizer (CF, %), and farmyard manure (CFYM, %), optimum fertilizer doses were derived. The fertilizer doses were validated for attaining yield targets of 5000 and 6000 kg ha^?1 in farmer’s fields. Rice yield within 10% deviation was attained, which indicated that soil-test-based fertilizer dose was superior. This approach could be adopted for regions with similar soil and agroclimatic conditions in other parts of the world to increase rice yields.     

Pepper Response to Inorganic and Organomineral Fertilizers in Southwestern Nigeria

Information on the combination of organomineral fertilizer (OMF) and chemical fertilizer (CF) for pepper production is scarce. The effects of CF, OMF, and their combinations on pepper were studied in Abeokuta, Nigeria. In a screenhouse, 0, 2, 4, 6, 8, 10, 12, and 14 t ha^?1 OMF and 60 kg nitrogen (N) 19.8 kg phosphorus (P), and 39.6 kg potassium (K) ha^?1 CF were applied, and to the field 12 t ha^?1 OMF (100% OMF); 60 kg N 19.8 kg P, and 39.6 kg K ha^?1 (100% CF); OMF + CF (50:50); OMF + CF (25:75); and OMF + CF (75:25) were applied. Application of 12 t ha^?1 OMF (100% OMF) increased pepper yield and N, P, and K uptake in the screenhouse, whereas OMF + CF (75:25) promoted the yield of pepper in the field. Pepper could successfully be grown with either 12 t ha^?1 OMF or OMF + CF (75:25) with the type of soil used for this experiment.     

Effects of Fertilizers on Yield,Sustainability, and Soil Fertility under Rainfed Pigeon pea + Rice System in Subhumid Oxisol Soils

A long-term field experiment was conducted at the research farm of the All-India Coordinated Research Project for Dryland Agriculture, Phulbani, Orissa, India, from 2001 to 2006 to identify the best integrated nutrient-use treatments for ensuring greater productivity, profitability, sustainability, and improved soil quality in pigeon pea + rice (two rows of pigeon pea followed by five rows of rice alternately) intercropping system. In all, nine treatments, eight comprising integrated nutrient-use practices, chemical fertilizer (CF), farmyard manure (FYM), and green leaf manure (GLM) to supply nitrogen (N) at 45 kg N ha^–1 and one farmer's practice equivalent to 25 kg N ha^–1 (FYM 5 t ha^–1), were tested on a long-term basis. Results of the study revealed that 20 kg N ha^–1 (FYM) + 25 kg N (CF) gave maximum mean rice grain yield of 1.52 t ha^–1, followed by 20 kg N (GLM) + 25 kg N (urea) with grain yield of 1.51 t ha^–1. In the case of pigeon pea, 30 kg N (FYM) +15 kg N (urea) gave maximum pigeon pea grain yield of 0.94 t ha^–1, which was 34% greater than the sole application of chemical fertilizer. Pigeon pea grain yield tended to increase with increasing proportion of organic N in FYM + CF or GLM + CF combinations. Application of 20 kg N (FYM) + 25 kg N (urea) recorded maximum mean rice equivalent yield of 3.59 t ha^–1 and sustainability yield index of 59%. While studying profitability, application of 20 kg N (FYM) + 25 kg N (CF) gave maximum net returns of US$168.94 ha^–1. Impact of treatments on soil quality as assessed in terms of relative soil quality indices (RSQI) increased with increasing proportion of organic sources of N. Using an innovative and new approach, an index of integrated productivity–sustainability–profitability–soil quality performance index (I _P,S,Pr,SQ) was computed to make a precise evaluation of the treatments. Based on this index, the order of performance of the treatments was T₆ [20 N (FYM) + 25 N (CF)] (7.7) > T₇ [30 N (FYM) + 15 N (CF) (6.9)] > T₃ [20 N (GL) + 25 N (CF)] (6.8) > T₅ [10 N (FYM) + 35 N (CF) (6.6)] > T₉ [GL] (6.5) > T₈ [CF] (6.2) > T₄ [30 N (GL) + 15 N (CF)] (6.0) > T₂ [10 N (GL) + 35 N (CF)] (5.7) > T₁ [FYM at 5 t ha^–1] (4.1). Thus, the results and the methodology adopted in this study using long-term data would be very useful to researchers, farmers, land managers, and other stakeholders not only in India but also across the world under similar climatic and edaphic situations.     

Identification of Minimum Data Set Under Balanced Fertilization for Sustainable Rice Production and Maintaining Soil Quality in Alluvial Soils of Eastern India

The scarcity of non-renewable fertilizers resources and the consequences of climate change can dramatically influence the food security of future generation. Introduction of high yielding varieties, intensive cropping sequence and increasing demand of food grains day-by-day, application of recommended dose of fertilizers could not fulfill our targets due to outdated fertilizers recommendations are yet in practice. It not only alters soil quality, nutrient balance, microbial and enzymatic ecology but also affected productivity and sustainability of rice in Gangetic alluvial soils of India. The effect of fertilizers application based on “fertilizing the soil versus fertilizing the crop” which insure real balance between the applied and available soil nutrient is urgently needed. Hence, the present study was conducted during three consecutive crop seasons (2010, 2011, and 2012) to assess the effect of imbalance and balance fertilization based on initial soil test values and targeted yields, and to determine the effect of farmyard manure (FYM) when superimposed with balanced fertilizers on identification of minimum data set for the development soil quality, nutrient acquisition, and grain yield of rice. The six fertilizer treatments were laid out in a randomized block design with three replications. The treatments were: T₁-control (no fertilization), T₂-farmyard manure @ 5 t ha^?1, T₃-farmers practice (60:30:30 kg N:P₂O₅:K₂O ha^?1), T₄-precise application of mineral fertilizers based on initial soil test values (77:24:46 kg N:P₂O₅:K₂O ha^?1) for targeted grain yield of 4.0 t ha^?1, T₅-precise application of mineral fertilizers based on initial soil test values (74:23:43 kg N:P₂O₅:K₂O ha^?1) plus FYM (5 t ha^?1) for targeted grain yield of 4.0 t ha^?1 and T₆-precise application of mineral fertilizers based on initial soil test values (135:34:65 kg N:P₂O₅:K₂O ha^?1) for targeted rice grain yield of 5.0 t ha^?1. Result revealed that the targeted rice grain yield of 4.0 and 5.0 t ha^?1 was achieved in T₄ and T₆ treatments with 1.59% (4.06 t ha^?1) and –3.40% (4.83 t ha^?1) deviations, respectively. T₄, T_5, and T₆ significantly increased crop growth, nutrient uptake, available P (P_a) and K (K_a) and augmented rice grain yield by 10.6, 20.2 and 31.6%, respectively, over T₃. Microbial biomass carbon, soil respiration and enzymatic activity were enhanced significantly in T₅ as compared to T₆. Highest soil quality index was found in T₅ (0.95) followed by T₆ (0.90) and, lowest was in T₁ (0.63). The contribution of minimum data set (MDS) toward the SQI was in the descending order of ALP (30.6%) > SOC (21.5%) > K_a (11.3%) > PSM (9.68%) > N_a (8.51%). Overall, rice yield and soil quality was improved by using balance fertilization based on fertilizing the crop Vs fertilizing the soil in alluvial soils of India.     

Ammonia Emissions from Anaerobically-digested Slurry and Chemical Fertilizer Applied to Flooded Forage Rice

Ammonia fluxes from application of anaerobically-digested slurry (ADS) and chemical fertilizer (CF) to flooded forage rice (Oryza sativa L.) in Japan were measured using a dynamic flow-through chamber method in lysimeters. The CF was applied at a rate of 300 N ha^?1 (three times) as ammoniacal-N fertilizer, and the ADS was applied to the lysimeters at total rates equivalent to 75, 100 and 150 kg N ha^?1, by broadcasting uniformly into the floodwater at three or six times (equal splits) between 17th June and 17th November, 2005. The emission fluxes for the first 2 days after application were very high from ADS, the highest values being 679 compared with a maximum of 156 mg N m^?2 d^?1 from CF. Most (61–93%) of the ammonia loss occurred during the first 5 days after each application of fertilizer. The total N loss as ammonia from ADS (29.6–51.7%) was much higher than from CF (12.2%). The highest fluxes were observed in August (2005) when air temperature was highest. More ammonia was lost from the ADS applied at the early stages (i.e. root taking, tiller stages) than at later stages (i.e. elongation, fruiting stages) of rice growth.     

Irrigation and Zn fertilizer management improves Zn phyto‐availability in various rice production systems

Zinc (Zn) is an important micronutrient for rice (Oryza sativa L.) production and its deficiency has been observed in various production systems. High grain Zn concentration is equally important for high rice yield and human health. In this work, the effects of Zn fertilization on seedling growth, grain yield, grain Zn concentration, and their association with root traits were studied under alternate wetting and drying (AWD), aerobic rice (AR), system of rice intensification (SRI), and continuous flooding (CF). Zinc fertilization (15 kg ha^?1) improved nursery seedlings chlorophyll and Zn concentrations, root length, and number of roots with highest values observed in CF. At harvesting, maximum plant height, panicle length, total and panicle bearing tillers, and kernel yield were found with Zn addition in AWD and CF rice systems. Mid season drainage provided at maximum tillering and Zn fertilization increased its concentration in leaves, culms, panicles, and grains under CF and AR at physiological maturity. Most of Zn applied was allocated into culms and panicles, nevertheless, a significant increase in grain Zn concentration was also observed in all production systems. Association of leaf Zn with grain Zn concentration was stronger than with culm and panicle Zn. The results indicate that Zn application after rice nursery transplanting is more important for grain Zn enrichment in all rice systems than for increase in grain yield in all systems except AWD where grain yield was also increased. More grain yield in CF and AWD as compared to SRI and AR can also be attributed to decreased spikelet sterility and to better Zn phyto‐availability in these rice systems at physiological maturity.     

Effect of continuous and intermittent irrigation methods on rice (cv. Koohrang) yield

This study was carried out to evaluate the impact of permanent and intermittent irrigation on the yield of rice (cv. Koohrang) in Lordegan rice-growing areas in Chaharmahal and Bakhtiari Province (Iran). The experiment was set up in a randomized complete block design with four treatments and four replications during 2003 and 2004. Irrigation treatments were: I1, continuous irrigation with a 5-cm water head every day; I2, alternative irrigation with a 5-cm water head every 2 days; I3, alternative irrigation with a 5-cm water head every 4 days; and I4, alternative irrigation with a 5 cm water head every 6 days. Results showed that the paddy yield was highest under treatment I2 (5655.6 kg ha^?1) and lowest under treatment I4 (3421.3 kg ha^?1). The highest rate of water use was obtained in I1 with 17 687.5 m³ ha^?1 and the lowest in I4 with 9325 m³ ha^?1. Paddy yield, plant height, panicle length and 1000-seed weight did not show a significant difference at the 5% level among I1, I2 and I3, but differed significantly to I4. Thus irrigation treatment I3 is appropriate for paddy irrigation in the Lordegan region with 10 645 m³ ha^?1 water use and 5483.7 kg ha^?1 paddy yield.     

Growth and yield of lowland rice as influenced by potassium application and cultivation method under alternate wetting and drying water regime

Besides being an essential macronutrient for plant growth, potassium (K) also acts as a stress-relieving agent against various biotic and abiotic stresses, especially water stress. An experiment consisting of three lowland Thai rice varieties (Pathumthani 1, RD57, RD41), two cultivation methods [dry direct seeding (DDS), transplanting (TP)] and four K doses (0, 80, 120,160?kg ha^?1) under alternate wetting and drying (AWD) water regime was conducted to investigate the impact of K and cultivation method on lowland rice varieties subjected to AWD. Pathumthani 1 had significantly higher grain yield at the 120?kg K ha^?1 compared with all other K doses. RD57 and RD41 had maximum grain yield at the 80 and 120?kg K ha^?1, respectively, which was significantly higher than the control; however, grain yield among different K doses was mostly similar for RD57 and RD41. K application at the rate of 120?kg ha^?1 for Pathumthani 1 and 80?kg ha^?1 for RD57 and RD41 could be a feasible option. The performance of DDS and TP was better at the 80 and 120?kg K ha^?1, respectively. Application of K at the optimum dose with proper selection of variety and cultivation method could help in sustainable rice production.     

Optimum Fertilizer Phosphorus Requirement for Maximum Productivity under Rice–Rice and Rice–Sunflower Systems in Semi-Arid Alfisols

A study was conducted at Hyderabad during 2009–11 to determine phosphorus (P) dose for rice–rice and rice–sunflower. Available P increased when 100% recommended P dose (RDP) was applied. P applied to rice gave at par yield under 100 or 75% RDP. In rice–rice, grain yield of 5668 and 5775 kg ha^?1 in kharif (5654 and 5760 kg ha^?1 in rabi) were attained with P@75 and 100% RDP. Kharif P residual effect in rabi affected rice yield. P@100/75% RDP in kharif and rabi gave grain of 5916/5973 and straw 6230/6673 kg ha^?1. P applied to sunflower revealed that yield was similar with 100 or 75% RDP. Sunflower yield was at par with P@100 or 75% RDP. 25% RDP in rice and sunflower may be reduced to attain similar yield of 100% RDP. In rice–rice, grain yield attained by 100% RDP in both seasons was 11.42t ha^?1 yr^?1, while 75% RDP gave yield of 11.45t ha^?1yr^?1.     

Is alternate wetting and drying irrigation technique enough to reduce methane emission from a tropical rice paddy?

Combination of a pre-season wet soil condition and rice straw incorporation just before transplanting, which is typical for a tropical rice double cropping, can induce a flash of methane (CH₄) emission shortly after the transplanting. The conventional practice of alternate wetting and drying (AWD) irrigation technique that typically starts at 21 days after transplanting (DAT) can hardly reduce this emission because the soil become methanogenic before the onset of AWD treatment. Field experiments were conducted in Central Luzon, Philippines, during the 2014–2017 dry rice seasons to examine the effects of the timing of rice straw/stubble incorporation on the efficacy of AWD in reducing the CH₄ emission. Two treatments of the timing of stubble incorporation were stubbles incorporated during the start of wet land preparation (S1) and stubbles incorporated during the dry fallow tillage (S2). For the water management, we compared two treatments: continuous flooding (CF) and AWD with – 15 cm threshold for irrigation. The AWD under S2 was implemented earlier at 10 DAT. We observed a significant interaction (p < 0.01) between effects of AWD and straw management on CH₄ emissions; the seasonal total CH₄ emission was reduced by AWD compared with CF by 73% under S2, while the reduction was <20% under S1. The AWD significantly (p < 0.05) increased the nitrous oxide (N₂O) emissions by 47 and 48% relative to CF under S1 and S2, respectively. The global warming potential (GWP, CH₄ + N₂O) and yield-scaled GWP were still substantially lower by 62 and 59%, respectively, in AWD than in CF under S2, but the reduction was not realized under S1 due to the relatively smaller CH₄ reduction and increased N₂O emission. The results confirm that pre-season aerobic stubble decomposition and earlier implementation of AWD enhanced AWD’s mitigation potential in reducing substantially the CH₄ emission from the tropical rice double-cropping system.     

CONTROLLED-RELEASE UREA FOR RICE PRODUCTION AND ITS ENVIRONMENTAL IMPLICATIONS

Controlled-release urea (CRU) and its placement method in rice production were investigated during 2007 and 2008 seasons. Controlled-release urea was applied at 62.5, 125, and 187.5 kg nitrogen (N) ha^?1, and the urea was 187.5 kg N ha^?1. All the CRU treatments were applied to the nursery beds once, and they were brought into the paddy field during transplanting, while the urea treatment was split into three applications from the plowing to the harvest. The results showed that rice seedlings with CRUs germinated and grow well and there was no salt damage at the nursery stage. The CRU treatment with 125 kg N ha^?1 had 33% less N than urea treatment (187.50 kg N ha^?1), but it produced significantly higher grain and straw yields, higher total N uptake and total apparent N uptake efficiency. In addition, all the CRU treatments effectively decreased floodwater ammonium (NH₄ ⁺)-N and nitrate (NO₃ ^?)-N concentrations, pH, and N runoff.     

Estimation of economic threshold level of alligator weed (Alternanthera philoxeroides (Mart.) Griseb.) to Tackle Grain quality and yield Losses in Rice

Alligator weed (Alternanthera philoxeroides) is the most troublesome invasive weed in transplanted rice ecosystems worldwide. A two-year field study was conducted to estimate economic threshold levels and the season long competitive effect of six alligator weed densities (0, 2, 4, 8, 16, and 32 plants m^?2) on yield and quality traits of transplanted rice. A gradual linear incline in dry weight of alligator weed was observed with an increase in its density from 2 to 32 plants m^?2. Maximum N (15.93–18.26 kg ha^?1), P (15.10–16.46 kg ha^?1) and K (16.34–17.81 kg ha^?1) uptake by alligator weed was recorded at its density of 32 plants m^?2. More micronutrient loss to the tune of 6.53, 47.92, 19.99, and 38.42 mg kg^?1 for Cu, Fe, Zn, and Mn, respectively was observed at the same density. Increasing density of alligator weed caused more losses in paddy yield (up to 21.37–23.78%), amylose and rice grain protein contents. According to a nonlinear hyperbolic regression model, maximum paddy yield loss of rice at asymptotic value of alligator weed density was 38.8% during 2011 and 42.4% during 2012. Economic threshold value of alligator weed was estimated 1.5 and 1.3 plants per m² during 2011 and 2012, respectively.     

Relationships between microbial biomass nitrogen,nitrate leaching and nitrogen uptake by corn in a compost and chemical fertilizer-amended regosol

Abstract

To determine the relationships between microbial biomass nitrogen (N), nitrate–nitrogen leaching (NO₃-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L^?1 K₂SO₄ extractable N (extractable N), NO₃-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha^?1 containing 158 kg N ha^?1 of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO₃-N leaching were monitored in soil treated with SDC (20 Mg ha^?1) + rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha^?1 containing 0, 15, 29, 44 and 59 kg N ha^?1) and in soil treated with CF in 2001. Nitrogen was applied as (NH₄)₂SO₄ at rates of 220 kg N ha^?1 for SDC and SDC + RS treatments and at a rate of 300 kg N ha^?1 for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha^?1 at 7 days after treatment (DAT) and was maintained at 60–70 kg N ha^?1 after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO₃-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO₃-N leaching in the CF treatment was 146 kg N ha^?1, which was equal to half of the applied N, but only 53 kg N ha^?1 in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO₃-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO₃-N leaching without impacting negatively on N uptake by plants.