Volatilization of Ammonia Derived from Fertilizer and Its Reabsorption by Coffee Plants

Abstract

Volatilization of ammonia derived from nitrogen (N) fertilizers and its possible reabsorption by crops depend on specific soil, climate, and atmospheric conditions, as well as the method of fertilizer application and plant architecture. In an experiment carried out in Piracicaba, State of São Paulo, Brazil, the volatilization of ammonia derived from urea, ammonium sulfate, and natural soil were quantified using static semi‐open N‐ammonia (NH₃) collectors. Fertilizers were top‐dressed under the plant canopy on top of dead leaf mulch. In another experiment, the reabsorption of the volatilized ammonia by plants was quantified using ¹⁵N‐labeled urea. Results showed, as expected, that volatilization derived from urea was seven times more intense in relation to ammonium sulfate, whose volatilization was very low, and slightly more than the natural volatilization from soil at pH 5.3. The loss of ammonia from the ammonium sulfate was very low, little more than twice of that of the natural soil. Through isotopic labeling, it was verified that 43% of the volatilized N‐NH₃ was reabsorbed by coffee plants, which gives evidence that volatilization losses are greatly reversed through this process.     

Ammonia volatilization from Vertisols

Farmers want to minimize losses of nitrogen (N) by volatilization of ammonia when adding fertilizers and improve fertilizer recovery of N by plants. We aimed to quantify the losses of N through NH₃ volatilization as affected by soil moisture content, type of fertilizer, and placement method in Vertisols in Kenya, and conducted three experiments for the purpose under controlled conditions in the laboratory. We found that NH₃-N losses were greatest at 80% water holding capacity, which we ascribed to the ready availability of water to dissolve the fertilizer at that water content. The soil's cation exchange capacity (CEC) did not influence volatilization, whereas the soil's pH indicated the potential of the soil to volatilize ammonia. Ammonia losses from the fertilizers were in the order urea > ammonium sulphate > ammonium nitrate applied. Incorporating fertilizer within the 0–5 cm soil layer more than halved NH₃ volatilization but did not prevent it completely. These results indicate that soil pH, rather than CEC, is the main inherent characteristic influencing ammonia volatilization from Vertisols. Ammonium-based fertilizers should be incorporated within the 0–5 cm soil layer, or preferably somewhat deeper, to avoid losses via NH₃ volatilization, particularly in alkaline soils. Nitrate fertilizers are preferable to urea where the risks of NH₃ volatilization are large, provided due consideration is given to denitrification risks.     

The interdependence of ammonia volatilization and denitrification as nitrogen loss processes in flooded rice fields in the Philippines

Summary The relative importance of ammonia volatilization and denitrification as loss processes following the application of urea to flooded rice by the traditional method was assessed at four sites with different characteristics in the Philippines. The effect of reducing ammonia loss on denitrification and total N loss was also studied. The total N loss was determined by a ¹⁵N-balance method and ammonia volatilization was assessed by a bulk aerodynamic method following the application of urea to small plots (4.8×5.2 m). As run-off was prevented and leaching losses were negligible, the denitrification loss was assessed as the difference between total N loss and ammonia loss. When urea was broadcast into the floodwater at transplanting, the ammonia loss varied from 10% to 56% of the applied N. Loss was smallest at Aguilar where wind speeds were low and the greatest at Mabitac where floodwater pH values and temperatures were high and the winds were strong. The ammonia loss was reduced at all sites by incorporating the urea into the soil by harrowing. However, the reduction achieved varied markedly between sites, with the largest reduction (from 56% to 7% loss of the applied N) being observed at Mabitac. The total N lost from the basal application into the floodwater ranged from 59% to 71% of the applied N. Incorporating the urea by harrowing reduced the total N loss at two sites, increased the total N loss at the third site, and had no effect at the fourth site. The denitrification losses ranged widely (from 3% to 50% of the applied N) when urea was broadcast into the floodwater at the four sites. The denitrification loss was low when the ammonia loss was high (Mabitac) and high when the ammonia loss was low (Aguilar). Reducing ammonia losses by incorporating the urea into the flooded soil resulted in increased denitrification losses at three of the sites and appeared to have no effect on denitrification at the fourth site. The results show that reducing the ammonia loss by incorporating urea into the soil does not necessarily result in reduced total N loss, and suggest that the efficiency of fertilizer N will be improved only when both N-loss processes are controlled simultaneously.     

Denitrification losses from puddled rice soils in the tropics

Summary Although denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N₂+N₂O)-¹⁵N from ¹⁵N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N₂+N₂O)-¹⁵N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the ¹⁵N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N₂+N₂O)-¹⁵N from added nitrate. In the absence of runoff and leaching losses, the amount of (N₂+N₂O)-¹⁵N evolved from ¹⁵N-labeled nitrate was consistently less than the unrecovered ¹⁵N in ¹⁵N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N₂+N₂O)-¹⁵N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N₂+N₂O)-¹⁵N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total ¹⁵N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and ¹⁵N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.     

Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N₂O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm ¹⁵N tracer experiments were performed to evaluate the effect of common wetland plants (Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the ¹⁵N balance method. In two experiments (wet and reflooded soil conditions) the N₂O and N₂ emissions were also directly determined.Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the ¹⁵N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77-95% of the ¹⁵N-nitrate added) and the transformation into organic N compounds was very low (1.8% of ¹⁵N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20-25% of the ¹⁵N nitrate added due to plant uptake. The N₂ emissions exceeded the N₂O emissions by a factor of 10-20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N₂O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N₂O and N₂ formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N₂O emissions and no emission via the shoots was observed.     

A model of ammonia volatilization from applied urea. III. Sensitivity analysis, mechanisms, and applications

A sensitivity analysis of the model described in Part I showed that the proportion of N lost as ammonia from surface applied urea is very sensitive to the initial pH of the soil, its pH buffer capacity, the rate of urea application, and the soil urease activity. Under the conditions tested, the diffusion of bicarbonate ion to the soil surface, to neutralize the acid generated when NH₄⁺ is volatilized as NH₃, appeared to be the main process controlling the rate of ammonia volatilization. The amount of ammonia volatilized was not very sensitive to the value of the transfer coefficient between the soil surface and the atmosphere, nor to the soil moisture status if this was around field capacity. Adsorption of ammoniacal-nitrogen was less important than the soil pH buffer capacity in influencing the ammonia volatilization. Further applications and extensions of the model are discussed.     

Ammonia volatilization following urea application at maize fields in the East African highlands with different soil properties

Use of nitrogen (N) fertilizer is underway to increase in Sub-Saharan Africa (SSA). The effect of increasing N rates on ammonia (NH₃) volatilization—a main pathway of applied-N loss in cropping systems—has not been evaluated in this region. In two soils (Alfisols, ALF; and Andisols, AND) with maize crop in the East African highlands, we measured NH₃ volatilization following urea broadcast at six rates (0–150 kg N ha^?1) for 17 days, using a semi-open static chamber method. Immediate irrigation and urea deep placement were tested as mitigation treatments. The underlying mechanism was assessed by monitoring soil pH and mineral N (NH₄⁺ and NO₃^?) concentrations. More cumulative NH₃-N was volatilized in ALF than in AND at the same urea-N rate. Generally, higher urea-N rates increased proportional NH₃-N loss (percent of applied N loss as NH₃-N). Based on well-fitted sigmoid models, simple surface urea application is not recommended for ALF, while up to 60 kg N ha^?1 could be adopted for AND soils. The susceptibility of ALF to NH₃ loss mainly resulted from its low pH buffering capacity, low cation exchange capacity, and high urease activity. Both mitigation treatments were effective. The inhibited rise of soil pH but not NH₄⁺ concentration was the main reason for the mitigated NH₃-N losses, although nitrification in the irrigation treatment might also have contributed. Our results showed that in acidic soils common to SSA croplands, proportional NH₃-N loss can be substantial even at a low urea-N rate; and that the design of mitigation treatments should consider the soil’s inherent capacity to buffer NH₃ loss.     

Dinitrogen and N2O emissions in arable soils: Effect of tillage, N source and soil moisture

A laboratory investigation was performed to compare the fluxes of dinitrogen (N₂), N₂O and carbon dioxide (CO₂) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with ¹⁵N-labeled fertilizers (100 mg N kg⁻¹ soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N₂O and CO₂ fluxes were measured by a gas chromatography (GC) system while total N₂ and N₂O losses and their ¹⁵N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N₂ and N₂O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N₂+N₂O, N₂O and CO₂ from the NT soils than from the CT soils. Compared with the addition of nitrate (NO₃⁻), the addition of ammonium (NH₄⁺) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH₄⁺ on the N₂ and/or N₂O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N₂O emission under the lower moisture condition. Total and fertilizer-induced emissions of N₂ and/or N₂O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO₂ fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N₂O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO₃⁻ form of N fertilizer would reduce potential N₂O emissions from arable soils.     

Chemical fixation of ammonia to soil organic matter after application of urea

Chemical fixation of NH₃ to soil organic matter was studied in two Swedish soils with different contents of organic matter: a clay soil with 2.3% C and an organic soil with 36.6% C. ¹⁵N‐labelled urea was applied at different rates to both sterilized and non‐sterilized soils. After 10 days, the soils were extracted and washed with K₂SO₄ and determined for total N and atom% ¹⁵N excess. Urea N was recovered as non‐extractable N in sterilized soil corresponding to 9.7% of supplied ^l5N‐labelled urea in the organic soil and 2.2% in the clay soil. Since no biological immobilization is thought to occur in the sterile soil, this non‐extractable N is suggested to be chemically fixed to soil organic matter. Owing to urea hydrolysis in the clay soil, pH increased from 6.3 to 9.3 and in the organic soil from 5.7 to 6.9 and 8.8, respectively, at the low and high urea supply.     

Soil properties affecting volatilization of ammonia from soils treated with urea

Abstract

The effects of various soil properties on ammonia (NH₃) volatilization from soils treated with urea were studied by measuring the NH₃ evolved when 20 soils selected to obtain a wide range in properties were incubated at ‐0.034 mPa soil moisture potential and 30°C for 10 days after treatment with urea. The nitrogen (N) volatilized as NH₃ from these soils represented from 0 to 65% of the urea‐N applied and averaged 14%. Simple correlation analyses showed that loss of NH₃ was negatively correlated (P<0.1%) with cation‐exchange capacity, silt content, and clay content and was positively correlated (P <0.1%) with sand content. Loss of NH₃ was also negatively correlated with total nitrogen content (P<1.0%), organic carbon content (P<1.0%), hydrogen ion buffering capacity (P<5.0%), and exchangeable acidity (P<5.0%), and was positively correlated with calcium carbonate equivalent (P <1.0%) and with soil pH after incubation with urea (P<1.0%), but was not significantly correlated with initial soil pH or soil urease activity. Multiple linear regression analyses indicated that the amount of urea N volatilized as NH₃ from the 20 soils studied increased with increase in sand content and decreased with increase in cation‐exchange capacity. They also indicated that soil texture and cation‐exchange capacity are better indicators of potential loss of urea N as NH₃ from soils fertilized with urea than are hydrogen ion buffering capacity or initial soil pH.     

南京郊区番茄地中氮肥的气态氮损失

采用田间试验研究了番茄地施用化学氮肥后的氨挥发、反硝化损失和N2O排放及其影响因素。氨挥发采用通气密闭室法测定，反硝化损失（N2＋N2O）采用乙炔抑制-土柱培养法测定，不加乙炔测定N2O排放。结果表明，番茄生长期间全部处理均未检测到氨挥发，其原因是土表氨分压低于检测灵敏度，较低的氨分压是由于表层土壤的铵态氮浓度和pH都不高所致。在番茄生长期间，对照区即来自有机肥和土壤本身的反硝化损失和N2O℃排放量相当高，反硝化损失总量高达N29．6kghm^-2，N2O排放量为N7．76kghm^-2。施用化学氮肥显著增加了反硝化损失和N2O排放，3个施用化学氮肥处理的反硝化损失变化在N40．8～46．1kghm^-2之间，占施入化肥氮量的5．50％～6．01％；N2O排放量为N13．6～17．6kghm^-2，占施入化肥氮量的2．62％～4．92％；与尿素相比，包衣尿素未能显著减低反硝化损失和N2O排放。施用尿素的处理在每次追肥后，耕层土壤均会出现NO3^--N高峰，继之的反硝化和N2O排放高峰。反硝化速率与土壤含水量呈极显著正相关。总的看来，番茄生长期间没有氨挥发，而硝化反硝化是氮素损失的重要途径之一。     

Control of ammonia volatilization with N‐(N‐butyl) thiophosphoric triamide in loamy sands

Abstract

In a laboratory study, ammonia (NH₃) was trapped from 10 g soil units treated with 10 mg urea‐N, 10 mg urea‐N plus 50 ug N‐(n‐butyl) thiophosphoric triamide (NBPT), or 10 mg urea‐N plus 50 ug phenyl‐phosphorodiamidate (PPD). The soil was a Dothan loamy sand with pH levels adjusted to 6.0, 6.5, and 6.9 prior to N application. After 12 days, NBPT reduced NH₃ volatilization 95 to 97%, while PPD reduced it 19 to 30%. Although NH₃ loss was positively related to initial soil pH, there was no interaction between pH and urease inhibitor. In a field study, NH₃ was trapped in semi‐closed chambers from 134 kg N/ha surface applied to corn (Zea mays L.) 6 weeks after planting. Nine days after N application, NH₃ losses were 20.5, 1.5, 1.5, and 0.2 kg N/ha from urea, urea plus 0.25% NBPT, urea plus 0.50% NBPT, and ammonium nitrate, respectively. Covariance analysis showed that percent organic matter was negatively related to NHL losses. The soil properties, initial pH, CEC, and percent sand, did not vary enough to affect NH₃ volatilization. In conclusion, in both the laboratory and the field, NBPT exhibited strong control of NH₃ volatilization, and could thereby prevent significant loss of surface‐applied urea‐N to crops.     

Availability of soil and fertilizer nitrogen to wheat (Triticum aestivum L.) following rice-straw amendment

Summary A pot experiment was conducted to study the N availability to wheat and the loss of ¹⁵N-labelled fertilizer N as affected by the rate of rice-straw applied. The availability of soil N was also studied. The straw was incorporated in the soil 2 or 4 weeks before a sowing of wheat and allowed to decompose at a moisture content of 60% or 200% of the water-holding capacity. The wheat plants were harvested at maturity and the roots, straw, and grains were analysed for total N and ¹⁵N. The soil was analysed for total N and ¹⁵N after the harvest to determine the recovery of fertilizer N in the soil-plant system and assess its loss. The dry matter and N yields of wheat were significantly retarded in the soil amended with rice straw. The availability of soil N to wheat was significantly reduced due to the straw application, particularly at high moisture levels during pre-incubation, and was assumed to cause a reduction in the dry matter and N yields of wheat. A significant correlation (r=0.89) was observed between the uptake of soil N and the dry matter yield of wheat with different treatments. In unamended soil 31.44% of the fertilizer N was taken up by the wheat plants while 41.08% of fertilizer N was lost. The plant recovery of fertilizer N from the amended soil averaged 30.78% and the losses averaged 45.55%     

Reducing ammonia loss from urea and improving soil-exchangeable ammonium retention through mixing triple superphosphate, humic acid and zeolite

Ammonia losses from soil following fertilization with urea may be large. This laboratory study compared the effect of four different, urea–triple superphosphate (TSP)–humic acid–zeolite, mixtures on NH₃ loss, and soil ammonium and nitrate contents, with loss from surface‐applied urea without additives. The soil was a sandy clay loam Typic Kandiudult (Bungor Series). The mixtures significantly reduced NH₃ loss by between 32 and 61% compared with straight urea (46% N) with larger reductions with higher rates of humic acid (0.75 and 1 g kg^?1 of soil) and zeolite (0.75 and 1 g kg^?1 of soil). All the mixtures of acidic P fertilizer, humic acid and zeolite with urea significantly increased soil NH₄ and NO₃ contents, increased soil‐exchangeable Ca, K and Mg, and benefited the formation of NH₄ over NH₃ compared with urea without additives. The increase in soil‐exchangeable cations, and temporary reduction of soil pH may have retarded urea hydrolysis in the microsite immediately around the fertilizer. It may be possible to improve the efficiency of urea surface‐applied to high value crops by the addition of TSP, humic acid and zeolite.     

Emissions of Nitrous Oxide,Ammonia, and Carbon Dioxide from a Cambisol at Two Contrasting Soil Water Regimes and Urea Granular Sizes

The superiority of mixing and deep placement of prilled urea (PU) or urea supergranules (USG) over surface‐broadcast application for reducing nitrogen (N) loss from lowland rice is well established. In upland agricultural systems, rainfall and/or the application and loss of irrigation water from soil systems may regulate urea N transformations and gaseous losses, depending on the method of fertilizer application and the particle size. To develop further insights into these processes, experiments were carried out in a silt loam soil mixed with PU or amended with point‐placed USG at a depth of 7.5 cm. Two soil water regimes were used: around field capacity (AFC) with low evaporative conditions (depletion: 77 to 69% water‐filled pore space, WFPS) and below field capacity (BFC) with high evaporative conditions following two irrigations (depletion: 70 to 55% WFPS). The nitrous oxide (N₂O) emission was greater at AFC than at BFC, where nitrification was more rapid. The N₂O peaks appeared mostly after the disappearance of nitrite (NO₂ ^?), presumably dominated by nitrifier and/or chemodenitrification and the degree of emissions probably depended on the stability period and the reduction of NO₂ ^? induced by the soil water regimes. The relative N₂O losses from the added N were small (?0.20%) for all treatments after 21 days. The point at which 50% of its emissions (t_½) occurred was delayed up to 6 days longer than found from the application of PU. The differences between PU and USG application were likely linked with the concentrations of ammonium (NH₄ ⁺), NO₂ ^?, and pH. These high concentrations continued longer at AFC than at BFC and were limited to a distance of <5.0 cm from the application zone. Similarly, the relative losses of the added N ranged from 0.19 to 0.56% at AFC and 0.08 to 0.37% at BFC, the highest being with USG application. Based on the areas receiving equal N, the N₂O and ammonia (NH₃) emissions from USG differed marginally with PU. Carbon dioxide (CO₂) release was higher at AFC than BFC, in which the USG application probably limited microbial respiration preferentially to methane oxidation. A correlation study showed that the N₂O flux was best explained together with CO₂, nitrate (NO₃ ^?), NO₂ ^?, and WFPS (R ² = 0.67***). This indicates the influence of both auto‐ and heterotrophic microbial activities toward N₂O emission, with soil water being an important regulatory factor.     

Nitrification activity in submerged soils and its relation to denitrification loss

Summary Nitrification activity (formation of NO ₂ ^– + NO ₃ ^– per unit soil weight) was measured in the surface layer of 15 presubmerged soils incubated in petri dishes under flooded but aerobic conditions. soils with pH above 5 nitrified quickly, whereas soils with pH below this level did not nitrify or nitrified slowly. The pH values between 7 and 8.5 were optimal for nitrification. Organic-matter levels in the 15 soils of our study did not influence their nitrification activities. In a follow-up greenhouse pot study, after a period of 3 weeks, ¹⁵N-balance measurements showed that the loss of N through apparent denitrification did not follow the nitrification patterns of the soils observed in the petri dishes. Apparent denitrification accounted for 16.8% and 18.9% loss of ¹⁵N from a soil with insignificant nitrification activity and a soil with high nitrification activity, respectively. These results, thus, indicate a lack of correspondence between the nitrification activities of soil and the denitrification loss of N when the former was measured in the dark and the latter was estimated in the light. Soils that nitrified in the darkness of the incubator did not nitrify in the daylight in the greenhouse.     

Soil N Losses by Denitrification Evaluated Using the 15N Tracer Method

Molecular nitrogen (N₂) and nitrous oxide (N₂O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N₂ and N₂O from potassium nitrate (K¹⁵NO₃) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the ¹⁵N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D⁺) and without (D^?) dextrose added. The treatments received 100 mg N kg^?1 (KNO₃ with 23.24 atom% ¹⁵N). N₂ emissions occurred in all of the treatments, but N₂O emissions only occurred in the D⁺ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D⁺ treatment exhibited the highest accumulated N emission (48.26 mg kg^?1). The ¹⁵N balance suggested trapping of the gases in the soil.     

Reduction in Ammonia Loss by Applying Urea in Combination with Phosphate Sources

A laboratory experiment was carried out to study the influence of 100 mg phosphorus pentoxide (P₂O₅) kg^–1 soil from various phosphate sources on ammonia losses from soils amended with urea at 200 mg nitrogen (N) kg^–1 soil. Irrespective of soil type, ammonia (NH₃) loss was significantly greater from untreated soil (control) than from the soil treated with phosphorus (P) sources. A maximum decrease in ammonia loss (56%) was observed by applying phosphoric acid followed by triple and single superphosphate. Ammonia losses were significantly greater from sandy clay loam than from clay. Rate of ammonia volatilization was maximum during the first week of incubation and became undetectable for both soils at 21 days after incubation. The addition of phosphate sources significantly decreased pH in the sandy clay loam, but in the clay a significant decrease was observed only with the phosphoric acid addition. Addition of phosphate fertilizers was beneficial in reducing NH₃ losses from urea.     

灌溉施用氮肥后氮素在土壤中的转化及淋失状况: 土柱模拟研究

A soil column method was used to compare the effect of drip fertigation (the application of fertilizer through drip irrigation systems, DFI) on the leaching loss and transformation of urea-N in soil with that of surface fertilization combined with flood irrigation (SFI), and to study the leaching loss and transformation of three kinds of nitrogen fertilizers (nitrate fertilizer, ammonium fertilizer, and urea fertilizer) in two contrasting soils after the fertigation. In comparison to SFI, DFI decreased leaching loss of urea-N from the soil and increased the mineral N (NH₄⁺-N + NO₃^--N) in the soil. The N leached from a clay loam soil ranged from 5.7% to 9.6% of the total N added as fertilizer, whereas for a sandy loam soil they ranged between 16.2% and 30.4%. Leaching losses of mineral N were higher when nitrate fertilizer was used compared to urea or ammonium fertilizer. Compared to the control (without urea addition), on the first day when soils were fertigated with urea, there were increases in NH₄⁺-N in the soils. This confirmed the rapid hydrolysis of urea in soil during fertigation. NH₄⁺-N in soils reached a peak about 5 days after fertigation, and due to nitrification it began to decrease at day 10. After applying NH₄⁺-N fertilizer and urea and during the incubation period, the mineral nitrogen in the soil decreased. This may be related to the occurrence of NH₄⁺-N fixation or volatilization in the soil during the fertigation process.     

北京地区潮土表层中NO3--N的转化积累及其淋洗损失

本试验利用渗滤池设施，采用化学分析和同位素技术相结合的方法研究了北京地区潮土表层中施用氮肥后ＮＯ３＾－－Ｎ转化积累及其１３０ｃｍ土体的淋洗状况。常规分析结果表明，在春小麦和夏玉米的生育前期可以观察到氮素明显地向ＮＯ３＾－－Ｎ的转化积累，其强度随尿素施用量的增加而明显增加，而尿素、硝铵、硫铵等不同氮肥品种处理之间有差异但大多不显著。同时夏玉米期间转化积累作用比春小麦期间强烈。＾１５Ｎ标记试验结果表明