Control of gaseous nitrogen losses from urea applied to flooded rice soils

This paper reports field experiments designed to determine whether the two main processes responsible for nitrogen (N) loss from flooded rice (ammonia volatilization and denitrification) are independent or interdependent, and glasshouse studies which investigated the effect of soil characteristics on gaseous nitrogen loss.In the first field experiment ammonia (NH₃) loss from the floodwater was controlled using algicides, biocides, frequent pH adjustment, shade or cetyl alcohol, and the effect of these treatments on total N loss and denitrification was determined. Most treatments reduced NH₃ loss through their effects on algal growth and floodwater pH. Total gaseous N loss (54% to 35%) and NH₃ loss (20% to 1.2%) were affected similarly by individual treatments, indicating that the amount lost by denitrification was not substantially changed by any of the treatments.In a subsequent field experiment NH₃ and total N loss were again affected similarly by the treatments, but denitrification losses were very low. In control treatments with different rates of urea application, NH₃ and total N loss were each a constant proportion of the urea applied (NH₃ loss was 17% and total N loss was 24%). These results indicate that techniques which reduce NH₃ loss can be expected to reduce total gaseous N loss.The glasshouse experiment showed that gaseous N losses could be reduced by draining off the floodwater, and incorporating the urea into the 0–0.05 m soil layer before reflooding. Even with this method, losses varied widely (6–27%); losses were least from a cracking clay and greatest from a coarse sand which allowed the greatest mobility of the applied N. Incorporation of applied urea can therefore be expected to prevent losses more successfully from clay soils with high ammonium retention capacity.     

Ammonia volatilization from flooded soils

Ammonia volatilization from flooded soils has been studied for over half a century. In reviewing the literature on this subject, it becomes clear that there is no consensus on the importance given to this loss mechanism. In part, the differences of opinion can be explained by the fact that ammonia losses were studied in different environments, but to a great extent it seems due to the wide diversity of techniques used to study this loss mechanism.The many factors that influence ammonia volatilization from flooded soils are chemical, biological, and environmental in nature. These various factors are reviewed in depth and discussed with respect to their implications for measurement techniques and for soil, fertilizer, and water management.The major objective of this paper is to familiarize the reader with the most current developments in thinking about the mechanisms and extent of ammonia loss and hopefully to stimulate meaningful research on ammonia volatilization from flooded soils. Such research should be conducted in a wide range of agroclimatic conditions utilizing measurement techniques that are valid or for which the limitations are clearly understood. A better appreciation for the importance of ammonia volatilization will provide the impetus to research and development in fertilizer technology and management aimed at preventing such losses.     

Effect of soil clay mineralogy on the efficiency of ammonium sulfate in flooded rice

The effect of soil clay mineralogy on the efficiency of (NH₄)₂SO₄ in flooded rice was investigated in a greenhouse pot trial with four clayey soils of diverse clay mineralogies (x-ray amorphous, montmorillonite, vermiculite, beidellite). KCl (75 kg K ha^–1) and triple superphosphate (25 kg P ha^–1) were incorporated in the soil with and without (NH₄)₂SO₄ (100 kg N ha^–1) before transplanting 1-week-old IR-36 rice seedlings which were then grown to maturity under flooded conditions. Efficiency of (NH₄)₂SO₄ was inferred from the response of agronomic characteristics such as tiller number, height, grain and straw yields to NH₄ fertilization.The results showed greatest efficiency of (NH₄)₂SO₄ on the x-ray amorphous soil, followed by montmorillonitic soil; efficiency was much lower on the vermiculitic and negligible on the beidellitic soil.Soil clay mineralogy may be an important factor in the reduced efficiency of NH₄ (or NH₄-forming) fertilizers in certain rice soils.     

2. Nitrogen transformations in flooded rice soils

A review is made of the recent literature pertaining to the reactions and processes that soil and fertilizer N undergo in lowland rice soils in relation to the improved N management and overal N economy of lowland rice soils. Topics discussed include: nitrogen leaching, ammonium fixation and release, ammonia volatilization, N₂ fixation, mineralization-immobilization, nitrification-denitrification, dissimilatory nitrate reduction, urea hydrolysis, critical pathways for control of nitrogen loss.     

Effect of liming on dehydrogenase and rhodanese activity of flooded and nonflooded acid sulfate soils

Addition of calcium oxide (lime) powder enhanced dehydrogenase and rhodanese activity of two acid sulfate soils from Kerala, locally known aspokkali andkari under nonflooded and flooded conditions. The enhanced dehydrogenase activity in both non-flooded and flooded soils after lime addition was probably due to the increase in microbial population. But, enhanced rhodanese activity in lime-amended nonflooded and flooded soils was probably due to the enzyme activation by CaO.     

The fate of algal nitrogen in a flooded soil system

Algal N labelled with ¹⁵N added to a flooded soil in laboratory columns without plants was studied to determine the changes over time in the fate of N assimilated by algae and to study how its fate is affected by (a) exclusion of light simulating complete closure of the rice canopy, and (b) addition of fertilizer-NH₄^*. In the light but with no added fertilizer-N there was little net mineralization of the added algal N during the first 4 weeks, but after 8 weeks 42% had been mineralized, of which 95% was denitrified. Exclusion of light caused net mineralization to proceed more rapidly in the first 4 weeks due to the death of algal cells and lowered reassimilation. After 8 weeks 51% had been mineralized, of which 54% was denitrified, 16% volatilized and 30% was present as KCl exchangeable NH₄⁺-N. Application of fertilizer-NH₄⁺ apparently caused mineralization of 25% of the algal N within one week but the results were probably affected by pool substitution in which labelled N mineralized to NH₄⁺-N was diluted with fertilizer – NH⁺₄ and then immobilized leaving more labelled NH₄–N in the mineral pool. After 8 weeks, 42% of algal N had been mineralized, of which 69% was estimated to have been denitrified, 19% lost through NH₃ volatilization and 12% remained as extracted NH₄⁺+NO^-₃. Uptake of N by a rice crop would reduce the gaseous losses. Algal N was mineralized quickly enough to be available during the growing season of a rice crop and, depending on field conditions, algae may have a role in assimilating N and protecting it from loss as well as being a major driving force for NH₃ volatilization through diurnal increases in pH.     

Phosphorus status of Philippine rice soils with different clay mineralogical characteristics

In an effort to relate clay mineralogies of some tropical rice soils to their P supplying status, clay mineral compositions of 214 wetland rice soils from the Philippines were determined by x-ray diffractometry, followed by Olsen P estimations on the airdried soil samples.Soils containing halloysite, kaolinite, and x-ray amorphous components (allophane and imogolite) were found deficient in available P (Olsen P < 10 mg per kg), whereas those containing montmorillonite, beidellite, vermiculite, and hydrous mica were adequate in available P (Olsen P > 10 mg per kg).     

Significance of soil depth on nitrogen transformations in flooded and nonflooded systems under laboratory conditions

The influence of different depths of repacked soil cores on changes in N transformation processes was studied with a subtropical semi-arid soil amended with 100 mg N kg^-1 of Sesbania green manure (GM) or fertilizer (NH₄)₂SO₄ for 35 days under flooded and nonflooded conditions. Shallow soil depth enhanced the rate of nitrification, particularly where aeration was impeded in flooded soils. However, the opposite occurred for denitrification as the relative predominance of underlying anoxic zone increased with increase in soil depth. Nitrate produced in the thin oxic surface soil layer and overlying water in flooded soils was subsequently lost via denitrification, more rapidly where carbon was supplied by added GM. Decomposition of GM was rapid and apparent recovery of applied 100 mg GM-N kg^-1 soil as mineral N after 35 days in flooded soils was 8, 26, 30 and 38% in 1.25-, 2.5-, 5.0- and 7.5-cm deep soil cores, respectively. Soil ammonium-N declined rapidly after an initial rise during decomposition of GM in soil in the shallow soil depth. In contrast, no such decline in NH ₄ ⁺ -N was observed in deep soil cores. In conclusion, the use of shallow soil depths during laboratory incubations can lead to variable results under flooded conditions. To simulate field conditions for obtaining reliable and quantitative information regarding N transformations in soils under flooded conditions, soil depths of 7.5 cm or greater should be used for laboratory incubations and growth chamber studies.     

Evaluation of levels and methods of zinc application to rice in sodic soils

Field experiments were conducted in zinc-deficient sodic soil to study the effect of levels and methods of zinc fertilization on yield, concentration and uptake of zinc by rice. Zinc was incorporated in the soil at the rate of 0, 5.6, 11.2 and 22.4 kg Zn per ha as zinc sulfate; sprayed on the plants at 1% and 2% zinc sulfate solution; and roots of rice seedlings were dipped in 2% and 4% ZnO suspensions in water. Grain yield, zinc content and its uptake increased in all the experiments up to 22.4 kg Zn per ha. Soil applied zinc was significantly correlated with yield of rice (r = 0.80^**) and zinc uptake (r = 0.89^**). Zinc content in 45-day old plants gave a significantly higher correlation with grain yield (r = 0.84^**) than the zinc content of rice straw and grain at maturity. Roots of rice seedlings dipped in 2% or 4% zinc oxide suspension in water were not only comparable with soil application of Zn at 5.6 and 11.2 kg Zn per ha, but also proved to be more economical for sodic soils showing moderate zinc deficiency.     

The effect of the urease inhibitor N-(n-butyl) thiophosphoric triamide on the efficiency of urea application in a flooded rice field trial in Thailand

The compound N-(n-butyl) thiophosphoric triamide (nBTPT) was tested for its ability to reduce the rate of urea hydrolysis in applications of urea at 10 d after transplanting flooded rice. The rates of urea hydrolysis were relatively slow, and nBTPT caused a 1-d reduction in the rate of disappearance of urea from the floodwater. Despite this, the vapor pressures of ammonia in the floodwater were significantly lower in the plots with nBTPT than without for the first 5 d following the N application. The vapor pressures of ammonia measured in the afternoons indicate that ammonia volatilization losses were considerable from the treatments without nBTPT and low from the treatments with nBTPT. There was no nitrogen response in this wet-season crop, apparently because of the high availability of N in the soil. N conserved from ammonia volatilization losses by use of the inhibitor was apparently susceptible to denitrification loss, and 50% of the fertilizer was lost in the 37 d following the application of¹⁵N-labeled urea both with and without the inhibitor.     

Comparing the effectiveness of fertilizers

The effectiveness of different sources of the same nutrient should be compared using plots of yield versus fertilizer applied to find how much more of the less-effective source is needed to provide the same yield. This gives the same result as measuring the relative response to the two fertilizers — provided response is defined as the instantaneous slope of the yield-fertilizer curve rather than the yield produced by a given amount of fertilizer. In most cases, and especially for immobile nutrients, the relative effectiveness, measured as suggested, will not be affected by the level of application. It is reasonable to assume this simple case until this assumption can be disproved — rather than assuming a more complicated case in the first instance.     

Utilisation by upland rice of plant residue- and fertiliser-phosphorus in two tropical acid soils

A dual isotopic technique was used to assess the effects of soil type, and residues of Gliricidia sepium, without and with added fertiliser-P on the utilisation of P. Upland rice (Oryza sativa) was grown for 70 days in two tropical acid soils of different P sorbing capacity and P status. Uniformly ³²P-labelled soils were treated with inorganic fertiliser-P tagged with ³³P, Gliricidia sepium residue applied at planting and 3 weeks earlier, and in a combination of fertiliser-P and Gliricidia applied at and 3 weeks before planting. There were significant responses of shoot and root weights, and total P uptake to Gliricidia- and/or fertiliser-P addition in the Ultisol (low P status) but not the Oxisol (high P status), suggesting that P in the latter soil was not yield limiting, despite the high standard P requirement. Similarly, incorporation of Gliricidia three weeks before planting further increased shoot weight only in the Ultisol. There were generally higher proportions, quantities and percent utilisations of the Gliricidia-P and fertiliser-P in the Ultisol than in the Oxisol. Gliricidia significantly increased the utilisation of fertiliser-P only in the Ultisol. However, early application of Gliricidia increased Gliricidia-P but not fertiliser-P utilisation in the Ultisol. Added fertiliser-P did not influence Gliricidia-P utilisation.     

Critical levels of Mn in coarse textured rice soils in India for predicting response of barley to Mn application

Green house studies of 20 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn for predicting response of barley to Mn application. Soil Mn was significantly related with both Bray's per cent dry matter yeild (r = 0.70^**) and Mn uptake (r = 0.65^**). Soil application of 25 mg Mn kg^–1 soil significantly increased yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.05 mg kg^–1 soil of DTPA extractable Mn. The critical Mn deficiency level in 45 day barley plants was 18.6 mg kg^–1 dry matter. The predictability of soil and plant critical Mn level was 91 and 80 per cent respectively.     

Effect of level of application on the relative effectiveness of rock phosphate

Responses of yellow serradella, slender serradella and subterranean clover to applications of superphosphate and Queensland apatite rock phosphate were compared in a field experiment in Western Australia. The rock phosphate was applied as a fine powder to a very sandy soil with a low buffering capacity for phosphate and for pH. At low levels of application, rock phosphate was about a tenth as effective as superphosphate — that is, ten times as much was required to give the same yield. With increasing levels of application, the relative effectiveness of rock phosphate declined to well below one per cent and hence more than 100 times as much was required to give the same yield. The decreasing relative effectiveness was incorporated into response equations. These were used to demonstrate that the decreasing relative effectiveness can be statistically tested and shown to be highly significant.A simulation study using the model of Kirk and Nye showed that a decrease in relative effectiveness with increasing level of application is especially likely when the particles of rock phosphate are very small, when the levels of application are high, and when the soil's buffering capacity for phosphate and for pH is low.     

Metal availability in contaminated soils: II. Uptake of Cd, Ni and Zn in rice plants grown under flooded culture with organic matter addition

Higher accumulation of toxic heavy metals in rice grown in contaminated soils may lead to health disorder in humans in tropical countries as rice is a staple diet. A pot experiment was conducted in a growth chamber to investigate the effect of flooding and non-flooding conditions in three soils added with4% organic matter on the concentration and uptake of Cd, Ni and Zn by rice plants (Oryza sativa L.). In flooding condition, the level of standing water was at a height of 2.5 ± 0.5 cm above the soil surface and in non-flooded culture80 ± 5% of water holding capacity was maintained. Flooding condition significantly(p < 0.05) reduced the concentration and uptake of Cd, Ni and Zn in rice grown in all three soils. The overall reduction of metal concentration in shoot at vegetative stage, and straw and polished rice at maturity, under flooding conditions was 84, 89, and 79% for Cd; 21,63and 65% for Ni; and 52, 78 and 16% for Zn, respectively. Organic matter addition significantly reduced the Ni concentration in plant parts but no such reduction was seen for Cd and Zn. Accumulation index of Cd and Zn was 82and 55% higher than that of Ni in the plant and the index of all three metals was higher in the tannery soil than the other two soils. Polished rice contained significantly lower amounts of Cd, Ni and Zn than shoot and straw. Cadmium and Ni uptake in polished rice was > 20% of the total uptake and thus it may be a concern for human health. This revised version was published online in August 2006 with corrections to the Cover Date.     

Microbial processes responsible for nitrous oxide production from acid soils in different land-use patterns in Pasirmayang,central Sumatra,Indonesia

The most probable number (MPN) method and dicyandiamide (DCD), an autotroph-specific inhibitor, were used to identify microbial processes that produced N₂O in acid soils. Nitrous oxide production was investigated in six sites in four different land-use patterns at Pasirmayang in Jambi province, in Sumatra, Indonesia, using soils collected in October 1999, March 2000 and June 2000. The six sites were primary forests (P1 and P2), loggedover forests (L2), a young rubber plantation established after forest had been logged and burned in March 1998 (L1), a plantation of Gmelina arborea established after logging and burning of forest in August 1996 (O), and a rubber plantation of a small landholder established in 1993 (R). Very small numbers of autotrophic ammonia oxidizers and autotrophic nitrite oxidizers were detected. Heterotrophic nitrifiers were detected at all sites, with the highest MPNs at L1 and L2. In incubation experiments, the emission of N₂O from soils increased at all six sites after adding citrate, a substrate for heterotrophic nitrifiers. No correlation appeared between denitrification activity and N₂O flux. We conclude that heterotrophic nitrifiers are the main microbial contributors to N₂O emission from acid soils in Pasirmayang, Jambi.     

Urease activity and inhibition in flooded soil systems

Ammonia volatilization is an important mechanism of N loss from flooded rice soils. Inhibition of urease may delay the formation of conditions favorable to NH₃ volatilization in the floodwater, thus giving the soil and plant a better chance to compete with the atmosphere as a sink for N. The experiments reported here were designed to identify the site of urea hydrolysis in flooded soils and to attempt selective urease inhibition with some of the inhibitors reported in the literature.Studies with three flooded soils using¹⁵N-labeled urea showed that 50–60% of the urea was found in the floodwater, despite incorporation. This floodwater urea is hydrolyzed largely at the soil—floodwater interface and subsequently returns to the floodwater (> 80%) or is retained by the soil (< 20%). Of the following urease inhibitors (K-ethyl-xanthate; 3 amino-1-H-1, 2, 4-triazole; phenylphosphorodiamidate) added at 2% (w/w of urea), only the latter was able to delay the appearance of NH₃ (aq) in the flood-water and thus delay NH₃ volatilization. Use of an algicide addition to the floodwater depressed NH₃ (aq) levels during the entire period studied, but in the presence of PPD the algicide had little additional effect.     

Factors affecting the agronomic effectiveness of phosphate rock for direct application

Phosphorus (P) is critically needed to improve soil fertility for sustainable crop production in large areas of developing countries. In recent years, phosphate rock (PR) for direct application has been tested in tropical acid soils as a potential alternative to conventional water-soluble P fertilizers like single superphosphate (SSP) and triple superphosphate (TSP). Some developing countries have PR deposits which, if used to supplement other imported P fertilizers, would allow a saving of much needed foreign exchange. Solubility of P fertilizers is not the only criterion in selection of the most suitable P fertilizer. This paper discusses the results of experiments to compare the relative agronomic effectiveness (RAE) of various PR sources with respect to SSP or TSP as influenced by four important factors: PR sources, soil properties, management practices, and crop species. Under certain conditions, PRs can be agronomically effective.     

SAPO-34分子筛的合成

以吗啉和四乙基氢氧化铵为模板剂,合成SAPO-34分子筛,研究不同温度和物料比对SAPO-34分子筛合成的影响。结果表明,在180℃和n(吗啉)∶n(SiO_2)∶n(Al_2O_3)∶n(P_2O_5)∶n(H_2O)=2∶1∶1∶1∶30条件下,SAPO-34分子筛的相对结晶度达100%。     

7. The efficacy and loss of fertilizer N in lowland rice

Nitrogen fertilization is a key input in increasing rice production in East, South, and Southeast Asia. The introduction of high-yielding varieties has greatly increased the prospect of increasing yields, but this goal will not be reached without great increases in the use and efficiency of N on rice. Nitrogen enters a unique environment in flooded soils, in which losses of fertilizer N and mechanisms of losses vary greatly from those in upland situations. Whereas upland crops frequently use 40–60% of the applied N, flooded rice crops typically use only 20–40%. There is a great potential for increasing the efficiency of N uptake on this very responsive crop to help alleviate food deficits in the developing world.This article reviews current use of N fertilizers (particularly urea) on rice, the problems associated with rice fertilization, and recent research results that aid understanding of problems associated with N fertilization of rice and possible avenues to increase the efficiency of N use by rice.