Saccharidic compounds as soil redox effectors and their influence on potential N2O production

Cellulose, xylan, and glucose were compared in waterlogged soil as modifying factors of the redox potential (Eh), of the quantity of reducing equivalents, and of the soil capacity to produce N₂O and CO₂. During the study period (168 h) soils supplied with glucose and xylan showed a higher Eh decrease than the control soil and the soil treated with cellulose. In samples taken after 0, 24, 48, and 168 h, the soils supplied with C showed a higher number of reducing equivalents than the control soil did. These quantities were not correlated with Eh values, nor with N₂O production. N₂O production was increased compared with the control soil over the entire experimental period in the glucose-amended soils but only after 48 h in the xylan-amended soils and not until 168 h in the cellulose-treated soils. The CO₂:N₂O ratio was consistently higher than the theoretical value of 2, suggesting that denitrification and CO₂ production via fermentation occurred simultaneously. Moreover, this ratio was highly correlated with the Eh values. We conclude that more research is needed to explain the role of soil redox intensity (Eh) and capacity (quantity of redox species undergoing reduction) in the expression of soil denitrification-fermentation pathways.     

Denitrification Rate and Controlling Factors for Accumulated Nitrate in the Deep Subsoil of Intensive Farmlands: A Case Study in the North China Plain

Denitrification in subsoil(to a depth of 12 m) is an important mechanism to reduce nitrate(NO_3~-) leaching into groundwater.However, regulating mechanisms of subsoil denitrification, especially those in the deep subsoil beneath the crop root zone, have not been well documented. In this study, soil columns of 0–12 m depth were collected from intensively farmed fields in the North China Plain. The fields had received long-term nitrogen(N) fertilizer inputs at 0(N0), 200(N200) and 600(N600) kg N ha~(-1) year~(-1). Main soil properties related to denitrification, i.e., soil water content, NO_3~-, dissolved organic carbon(DOC), soil organic carbon(SOC),pH, denitrifying enzyme activity(DEA), and anaerobic denitrification rate(ADR), were determined. Statistical comparisons among the treatments were performed. The results showed that NO_3~- was more heavily accumulated in the entire soil profile of the N600 treatment, compared to the N0 and N200 treatments. The SOC, DOC, and ADR decreased with increasing soil depth in all treatments,whereas considerable DEA was observed throughout the subsoil. The long-term fertilizer rates affected ADR only in the upper 4 m soil layers. The ADRs in the N200 and N600 treatments were significantly correlated with DOC. Multiple regression analysis indicated that DOC rather than DEA was the key factor regulating denitrification beneath the root zone. Additional research is required to determine if carbon addition into subsoil can be a promising approach to enhance NO_3~- denitrification in the subsoil and consequently to mitigate groundwater NO_3~- contamination in the intensive farmlands.     

The effect of some carbon substrates on denitrification rates and carbon utilization in soil

Summary Soil was amended with a variety of carbon sources, including four soluble compounds (glucose, sucrose, glycerol and mannitol) and two plant residues (straw and alfalfa).. Potential denitrification rates, measured both as N₂O accumulation and NO₃ ^– disappearance, were compared, and the predicted values of available C, measured as CO₂ production and water-extractable C, were assessed.The two measures of denitrification agreed well although N₂O accumulation was, found to be most sensitive. Soil treated with the four soluble C compounds resulted in the same rate of denitrification although glycerol was not as rapidly oxidized. Alfalfa-amended soil produced a significantly higher rate of denitrification than the same amount of added straw. CO₂ evolution was found to be a good predictor of denitrification over the first 2 days of sampling, but neither measure of available substrate C correlated well with denitrification rate beyond 4 days, when NO₃ ^– was depleted in most treatments. The data with alfalfa-amended soil suggested that denitrifiers used water-extractable C. materials produced by other organisms under anaerobic conditions.     

Methane and water soluble iron production under controlled soil pH and redox conditions

Abstract

When a soil is flooded, iron (Fe) reduction and methane (CH₄) production occurred in sequence as predicted by thermodynamics. The dissolution and precipitation of Fe reflected both soil pH and soil redox potential (Eh). The objective of our experiment was to determine both CH₄ production and Fe reduction as measured by Fe in solution in a flooded paddy soil over a wide range of closely controlled pH and Eh conditions. The greatest release of CH₄ gas occurred at neutral soil pH in combination with low soil redox potential (‐250 mV). Production of CH₄ decreased when soil pH was lowered in combination with an increase in the soil redox potential above ‐250 mV. Highest concentration of ferrous‐iron (Fe²⁺) under reducing conditions occurred when soil pH was lowered. Thus Fe reduction influenced CH4 formation in the flooded paddy soil. Results indicated that CH₄ production was inhibited by the process of ferric‐iron (Fe³⁺) reduction.     

Coupled nitrification-denitrification associated with liquid manure in a gel-stabilized model system

Summary Following the application of liquid manure to soil, the development of the two processes, nitrification and denitrification, was studied in a two-phase model system. A saturated mixture of manure and soil, stabilized with silica gel, was overlain by an aerobic soil phase. Profiles of the redox potential pH, inorganic N, dissolved organic C, nitrification and denitrification potentials, and phospholipid concentrations for an estimate of microbial biomass were measured during a 20-day period. NH ₄ ⁺ diffusing into the aerobic soil was oxidized within 10 mm of the interface, but with only a small accumulation of NO ₂ ^- and NO ₃ ^- . It was estimated that N equivalent to approximately 70% of the NH ₄ ⁺ originally present in the manure was lost through coupled nitrification-denitrification. The potentials for nitrification and denitrification increased 40-and 20-fold, respectively, around the interface. Maximum values were recorded after 14 days. Within 0–5 mm of the anaerobic zone, apparent generation times for NH ₄ ⁺ -oxidizing bacteria of 1.1–1.8 days were estimated between day 1 and day 7. The phospholipid concentration profiles suggested that the biomass within 2 mm on either side of the interface was stimulated throughout the 20-day period.     

Low soil C:N ratio results in accumulation and leaching of nitrite and nitrate in agricultural soils under heavy rainfall

Nitrate (NO^-₃) and nitrite (NO₂^-) leaching threatens groundwater quality.Soil C:N ratio,i.e.,the ratio of soil organic carbon to total nitrogen,affects mineralization,nitrification,and denitrification;however,its mechanism for driving soil NO^-₃ and NO^-₂ accumulation and leaching remains unclear.Here,a field investigation in a fluvo-aquic soil and a soil column experiment were performed to explore the relation...     

区分土壤中硝化与反硝化对N2O产生贡献的方法

土壤是产生N₂O的最主要来源之一。硝化和反硝化反应是产生N₂O的主要机理，由于硝化和反硝化微生物同时存在于土壤中，因而硝化和反硝化作用能同时产生N₂O。N₂O的来源可通过使用选择性抑制剂，杀菌剂以及加入的标记底物确定。通过对生成N₂O反应的每一步分析，主要从抑制反应发生的催化酶和细菌着手，总结了测量区分硝化、反硝化和DNRA反应对N₂O产生的贡献方法。并对¹⁵N标记底物法，乙炔抑制法和环境因子抑制法作了详细介绍。     

高氮和NO2-对中亚热带森林土壤N2O和NO产生的影响

利用15N同位素标记方法,研究在两种水分条件即60％和90％ WHC下,添加硝酸盐(NH4NO3,N 300 mg kg-1)和亚硝酸盐(NaNO2,N 1 mg kg-1)对中亚热带天然森林土壤N2O和NO产生过程及途径的影响.结果表明,在含水量为60％ WHC的情况下,高氮输入显著抑制了N2O和NO的产生(p＜0.01);但当含水量增为90％ WHC后,实验9h内抑制N2O产生,之后转为促进.所有未灭菌处理在添加NO2-后高氮抑制均立即解除并大量产生N2O和NO,与对照成显著差异(p＜0.01),在60％ WHC条件下,这种情况维持时间较短(21 h),但如果含水量高(90％ WHC)这种情况会持续很长时间(2周以上),说明水分有效性的提高和外源NO2-在高氮抑制解除中起到重要作用.本实验中N2O主要来源于土壤反硝化过程,而且加入未标记NO2-后导致杂合的N2O(14N15NO)分子在实验21 h内迅速增加,表明这种森林土壤的反硝化过程可能主要是通过真菌的“共脱氮”来实现,其贡献率可多达80％以上.Spearman秩相关分析表明未灭菌土壤NO的产生速率与N2O产生速率成显著正相关性(p＜0.05),土壤含水量越低二者相关性越高.灭菌土壤添加NO2-能较未灭菌土壤产生更多的NO,但却几乎不产生N2O,表明酸性土壤的化学反硝化对NO的贡献要大于N2O.     

Soil properties influencing the denitrification potential of Flemish agricultural soils

The denitrification potential of the soil horizons between 0- and 90-cm depth of 20 agricultural fields, representative of the most frequent combinations of agricultural crops and soil textures in Flanders (Belgium), and the factors affecting the denitrification potential were studied in the laboratory under controlled conditions. The denitrification potential in the presence of an added soluble C and N source was measured at 15°C after saturation of air-dried soil samples with water. The denitrification potential of the lower horizons was generally negligible compared to the upper horizons. The lower denitrification potential of the deeper horizons could partially be explained by their limited C availability. The denitrification potential of the upper horizons strongly depended on texture. Based on this parameter the soils could be divided into three groups: soils with a high clay content (>30% clay) were characterised by a high denitrification potential (>8.33 µg N g^-1 dry soil day^-1); soils with medium texture had a medium denitrification potential, between 0.41 and 7.25 µg N g^-1 dry soil day^-1; and soils with a high sand content (>80% sand) had a low denitrification potential (<2.58 µg N g^-1 dry soil day^-1). In most cases, extending the saturation period during pre-incubation increased the denitrification potential. Comparison of the denitrification potential of the upper horizons with and without addition of a soluble C source showed that the denitrification potential of the upper horizons of these soils was limited by their percentage of endogenous C. The measured denitrification potentials indicate that denitrification losses in soils high in clay content can be important when NO₃ ^- concentrations are high.     

Oxidation-reduction regimes in some Oxisols of tropical China

The oxidation-reduction regimes of some Oxisols of tropical China were investigated on the basis of the Eh values and the amounts of reducing substances, both measured by newly devised methods. The Eh values in the surface layers of soil profiles under natural forest and commercial forest were 400–550 mV, lower by 100–200 mV than the deeper horizons. The concentrations of reducing substances in the surface layers corresponded to 0.5–5.0 · 10^?5M of Mn²⁺. In the cultivated layer of a submerged paddy soil the concentration of reducing substances was as high as 13·10^?5M, with an Eh value of less than 100 mV. There was a negative correlation between the Eh value and the logarithm of the concentration of reducing substances, showing that the intensity aspect and the capacity aspect of the oxidation-reduction status of a soil are closely interrelated.     

氰在土壤中的吸附与转化

氰(C2N2)是一种具有替代溴甲烷潜力的新熏蒸剂,明确C2N2在土壤中的吸附与转化行为对C2N2的安全应用具有重要的意义.利用静态法研究了C2N2在土壤中的吸附与转化过程,土壤对C2N2的吸附速率与土壤的理化性质有关,受环境温度、土壤含水量和土壤生物的影响较少,与熏蒸浓度无关.采用气相色谱(GC)和流动注射分析仪(FIA)测定了土壤对C2N2的吸附率和C2N2在土壤中的可能转化产物.结果表明,土壤对C2N2的吸附能力很强,熏蒸2h吸附率在75％以上,熏蒸24 h吸附率在98％以上,并可快速转化为HCN、NH4+和NO3-.其中,大约20％的C2N2转化为NH4+和NO3-,13％的C2N2转化为HCN.熏蒸48 h未检测到NO2-.HCN在土壤中不稳定,可进一步转化为其他含氮化合物.     

Nitrogen Inputs with Different Substrate Quality Modified pH,Eh, and N Dynamics of a Paddy Soil Incubated under Waterlogged Conditions

Synthetic fertilizer, livestock manure, and green manure are the typical nitrogen (N) sources in agriculture. This study was conducted to investigate the effects of different N sources on soil chemical environment and N dynamics. Changes in pH, redox potential (Eh), and concentration and δ¹⁵N of dissolved N [ammonium (NH₄⁺), nitrate (NO₃^?), organic N, and total N] of soils treated with urea (U), pig manure compost (PMC), and hairy vetch (HV) were investigated in an incubation experiment under waterlogged conditions. The patterns of pH, Eh, and N concentration reflected both a greater mineralization potential of N derived from U than that from HV and PMC and easier decomposability of HV than PMC. The δ¹⁵N further suggested that nitrification was more active for U than for HV- and PMC-treated soils and that N loss via NH₃ volatilization and denitrification would be greater for HV than U and PMC treatments.     

Denitrification,N2-fixation and fermentation during anaerobic incubation of soils amended with glucose and nitrate

Nitrate and glucose additions were investigated for their role in the C and N dynamics during anaerobic incubation of soil. A gas-flow soil core method was used, in which the net production of N₂, N₂O, NO, CO₂, and CH₄ under a He atmosphere could be monitored both accurately and frequently. In all experiments clayey silt loam soil samples were incubated for 9 days at 25 °C. Addition of nitrate (50 mg KNO₃-N kg^-1 soil) had no effect on total denitrification and CO₂ production rates, while the N₂O/N₂ ratio was affected considerably. The cumulative N₂O production exceeded the cumulative N₂ production for 6 days in the treatment with nitrate addition, compared to 1.2 days in the unamended treatment. Glucose addition stimulated the microbial activity considerably. The denitrification rates were limited by the growth rate of the denitrifying population. During denitrification no significant differences were observed between the treatments with 700 mg glucose-C kg^-1 and 4200 mg glucose-C kg^-1, both in combination with 50 mg KNO₃-N kg^-1. The N₂ production rates were remarkably low, until NO _inf3 ^sup- exhaustion caused rapid reduction of N₂O to N₂ at day 2. During the denitrification period 15–18 mg N kg^-1 was immobilised in the growing biomass. After NO _inf3 ^sup- shortage, a second microbial population, capable of N₂-fixation, became increasingly important. This change was clearly reflected in the CO₂ production rates. Net volatile fatty acid (VFA) production was monitored during the net N₂-fixation period with acetate as the dominant product. N₂-fixation faded out, probably due to N₂ shortage, followed by increased VFA production. In the high C treatment butyrate became the most important VFA, while in the low C treatment acetate and butyrate were produced at equal rates. During denitrification no VFA accumulation occurred; this does not prove, however, that denitrification and fermentation appeared sequentially. The experiments illustrate clearly the interactions of C-availability, microbial population and nitrate availability as influencing factors on denitrification and fermentation.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

不同初始氧化还原条件下土壤中重金属的运移研究Ⅰ：单一Cd、Cu、Zn的土柱实验

以重金属离子镉、铜、锌为研究对象,通过室内土柱混合置换实验,分析了五种不同氧化还原电位对Cd、Cu、Zn在土壤中运移的影响。结果表明,氧化条件的土壤中Cd的穿透曲线(BTCs)峰值大小的顺序为Eh=490 mV>350 mV>240 mV;还原条件的土壤中Cd的BTCs的峰值为Eh=20 mV>100 mV>240 mV。在氧化条件下,高Eh能明显促进Cu在土壤中的迁移;还原条件下,低Eh会抑制Cu的迁移,抑制程度与还原剂浓度无关。与原土(氧化还原电位240 mV)相比,氧化性土壤中Zn运移较快,表现为相对浓度峰值较高,而不同的还原条件对Zn的迁移影响并不明显。三种重金属Cd、Cu、Zn出流液的pH均表现为先下降后上升最后趋于平缓,氧化条件土壤出流液的pH小于还原条件;而Eh正好相反。     

Assessment of the soil phosphorus–mobilization potential by microbial reduction using the Fe(III)‐reduction test

The mobility of soil P is greatly influenced by the redox potential (Eh), which depends on the reducing activity of soil microorganisms. Standard extraction methods for the determination of the mobile soil P disregard the P mobilization caused by the influence of microorganisms on Eh, while P test methods that include soil microbial activities are lacking. Thus, the Fe(III)‐reduction test was investigated for its suitability to determine the P fraction that is mobilized in soil under reducing conditions (P_Red). In this test, the soil‐microbial reducing activity is measured from the microbial Fe(III) reduction combining a bioassay with 7 d incubation and a chemical extraction using 1M KCl. After the incubation, Eh in 26 different soil samples ranged from –282 to –123 mV. The concentration of P_Red in the soil samples ranged from concentrations below the limit of detection to 84.9 mg kg^–1 and was on average of all soil samples by a factor of 2.4 to 18 smaller than the P fractions determined by standard soil P–extraction methods. As standard agronomic and environmental P extractants, respectively, water (P_H2O), dithionite citrate bicarbonate (P_Dith), ammonium oxalate (P_Ox), ammonium lactate (P_AL), double lactate (P_DL), and sodium bicarbonate (P_Olsen) were selected. The P_Red fraction was not correlated with P_AL, P_DL, P_olsen, and the degree of P saturation, but with P_H2O (r = 0.43*), P_Dith (r = 0.60***), and P_Ox (r = 0.61***). Furthermore, P_Red depended on the concentration of amorphous Fe oxides (Fe_Ox, r = 0.53**) and was closely correlated with the concentration of microbially reduced Fe (Fe_Red, r = 0.94***). This indicated the influence of the Fe(III)‐reducing activity of soil microorganisms on P mobilization. In subsoils, low in Fe(III)‐reducing activity, no P was released by the Fe(III)‐reduction test, which was in contrast to the results from the other chemical extraction methods. Additional alterations of the microbial activity by inhibiting and activating amendments, respectively, clearly affected the microbial Fe(III)‐reducing activity and the associated release of P_Red. Thus, P_Red, determined by the Fe(III)‐reduction test, might be termed as the fraction that is potentially released from soil by microbial reduction.     

Denitrification and total nitrogen gas production from forest soils of Eastern China

Reactive forms of nitrogen (Nr) are accumulating at local, regional and global levels largely due to human activities, particularly N-fertilizer production and use as well as fossil fuel combustion. This has resulted in a change in the nitrogen (N) cycle and excess Nr in the environment, which has negative environmental effects. Therefore, characterizing denitrification and the edaphic variables controlling denitrification and its products is the first step in predicting the long-term effects of Nr accumulation. In the present study, six forest soil types in different climatic zones were collected from East China and evaluated for denitrification products following a K¹⁵NO₃ amendment and subsequent incubation. The results showed that denitrification, indicated by production of nitric oxide (NO), nitrous oxide (N₂O) and dinitrogen (N₂), was higher in the studied temperate forest soils than in the studied subtropical and tropical forest soils and was negatively correlated with soil redox potential at the beginning of incubation (r = −0.94, P < 0.01), but not with soil pH. The ratios of NO/total N gas and N₂O/total N gas produced during denitrification varied among the soils, and were generally higher in the subtropical and tropical soils. Spearman's correlation analysis showed that the NO ratio was positively correlated with soil oxidation capacity (OXC) (r = 0.94, P < 0.01) and redox potential at the beginning of incubation (r = 0.86, P < 0.05), but negatively correlated with soil pH (r = −0.83, P < 0.05). The N₂O ratio was not significantly correlated with these edaphic variables, but showed a significant correlation to NO ratio (r = 0.83, P < 0.05). These results suggested that the OXC value might be the key factor affecting denitrification rates in soils. One possible explanation for these effects is that large OXC values would result in a higher level soil redox potential, thus suppressing denitrification and enhancing NO and N₂O ratios during denitrification.     

尿素和KNO3对水稻土无机氮转化过程和产物的影响 Ⅰ无机氮转化过程

用15N分别标记尿素和KNO3,研究了淹水条件下 ,黄泥土和红壤性水稻土的无机氮转化过程及尿素和KNO3对氮素转化过程的影响。结果表明 ,淹水条件下 ,土壤中存在15NH 4 的成对硝化和反硝化过程。红壤性水稻土15NH 4 硝化只检测到15NO- 2 ,但有反硝化产物15N2 生成 ,因此 ,很可能存在着好气反硝化过程。15NO- 3浓度的下降符合一级反应方程 ,黄泥土的速率常数几乎是红壤性水稻土的 1 0倍。反硝化过程和DNRA过程共同参与15NO- 3的还原。加入尿素提高土壤pH ,增加黄泥土DNRA过程对反硝化过程的基质竞争能力 ,但反硝化过程仍占绝对优势。加入尿素或KNO3改变土壤pH是导致对无机氮转化影响有所不同的主要原因 ,浓度的作用较为次要。     

Copper adsorption behavior of three Malaysian rice soils

Abstract

Copper (Cu) deficiency exists in different rice growing areas of Malaysia. A study on Cu adsorption was carried out in three Malaysian rice soils (Idris, Tebengau, and Kangar series) using six levels of Cu (0, 100, 200, 300, 400, and 500 ug g^‐1). The data on Cu adsorption were fitted into Langmuir, Freundlich, and Temkin equations. Adsorption data were also correlated with pH, cation exchange capacity, and organic matter content of the soils. The effect of Cu addition on redox potential (Eh) of the soils was also measured. The Eh values were correlated with equilibrium solution Cu concentrations. Copper adsorption increased gradually with increasing level of added Cu in all the soils. The rate of increase was the highest in Kangar series followed by Tebengau and Idris, respectively. Correlation between Cu adsorption and pH was significant (r=0.772) whereas correlation of adsorption with either organic matter content or cation exchange capacity was nonsignificant. Copper adsorption in two soils (Idris and Tebengau) fitted Langmuir, Freundlich, and Temkin equations whereas Cu adsorption in the Kangar soil fitted Freundlich and Temkin equations. Redox potential (Eh) of the soils increased gradually with increasing level of added Cu. The rate of increase was the highest in Idris followed by Kangar and Tebengau soils, respectively. The relationship between equilibrium solution Cu concentration and redox potential was significant. The results of this study indicated that copper adsorption is mainly dependent on soil pH. In soils with higher adsorption capacity, more Cu fertilizer may be needed to get immediate crop response.     

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.