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.     

Nitrogen fixation,biomass production,and nutrient uptake by annual Sesbania species in an alkaline soil

Summary In three field trials conducted during the summer season of 1986, 1987 and 1989 in an alkaline soil, 17 accessions of annual Sesbania spp. were evaluated for nodulation, N₂ fixation (acetylene reduction assay), dry weight of roots and shoots, woody biomass production, and nutrient uptake. At 50 days after sowing all the accessions were effectively nodulated (average 36.4 root nodules plant^-1) with a high nodule score (3.4). There was a lot of variation in nodule volume and mass and in acetylene reduction activity but not in N content (5.2%). N uptake in shoots, roots and nodules averaged 639, 31, and 13 mg plant^-1, respectively, and much of the fixed N remained in shoots. Accessions of S. cannabina complex performed better than others. S. rostrata had poor root nodulation but exhibited excellent stem nodulation (300 nodules plant^-1) even though not inoculated with Azorhizobium sp. Average concentrations of N, P, K, S, Ca, and Mg in the shoots were high, at 3.2, 0.28, 1.5, 0.28, 1.5, and 0.4% respectively, and Na was low (0.15%), reflecting the usefulness of Sesbania spp. as an integrated biofertilizer source. Green matter production was 26.0 Mg ha^-1 (5.9 Mg dry matter) and N uptake was 158 kg ha^-1, 54 days after sowing. Average woody biomass of six accessions at maturity, 200 days after sowing, was high (19.9 Mg ha^-1), showing its potential for shortterm firewood production. Total nutrient uptake for production of woody biomass (200 days of growth) was no more demanding than growing the plant to the green-manuring stage of 50–60 days' growth.     

Litter decomposition and nitrogen mineralization in newly formed gaps in a Danish beech (Fagus sylvatica) forest

Gap formation is suggested as an alternative forest management approach to avoid extreme changes in the N cycle of forest ecosystems caused by traditional management practises. The present study aimed to investigate the effect of gap formation on N availability in beech litter and mineral soil on sites, which experienced only little soil disturbance during tree harvest. N pools, litter decomposition, and N mineralization rates in mineral soil were studied in two gaps (17 and 30 m in diameter) in a 75-year-old managed European beech (Fagus sylvatica L.) forest in Denmark and related to soil temperature (5 cm depth) and soil moisture (15 cm depth). Investigations were carried out during the first 2 years after gap formation in measurement plots located along the north-south transect running through the centre of each gap and into the surrounding forest.An effect of gap size was found only for soil temperatures and litter mass loss: soil temperatures were significantly increased in the northern part of the large gap during the first year after gap formation, and litter mass loss was significantly higher in the smaller gap. All other parameters investigated revealed no effect of gap size. Nitrification, net mineralization, and soil N concentrations tended to be increased in the gaps. Cumulative rates of net mineralization were two fold higher in the gaps during the growing season (June-October), but a statistically significant increase was found only for soil NH₄-N concentrations during this period. Forest floor parameters (C:N ratios, mass loss, N release) were not significantly modified during the first year after gap formation, neither were the total C content nor the C:N ratio in mineral soil at 0-10 cm depth.     

不同氮肥及肥料组合对芹菜生长和硝酸盐含量的影响

通过田间试验研究了氮肥品种组合及不同肥料配施对芹菜生长和硝酸盐含量的影响.结果表明氮肥中的氯化铵、硫酸铵及其配合施用有利于降低芹菜体内硝酸盐含量,减少土壤NO3--N积累;肥料组合正交试验结果表明几种肥料对芹菜产量影响的主次关系为尿素＞硫酸钾＞石灰＞油枯,对芹菜硝酸盐积累影响的主次关系为尿素＞油枯＞硫酸钾＞石灰.施用油枯和石灰,芹菜硝酸盐显著积累,但其用量水平间差异不显著.随着尿素施用量的增加,芹菜硝酸盐含量大幅度增加.钾肥能提高芹菜的产量并降低硝酸盐含量.统计分析表明油枯1125 kg/hm2,尿素495 kg/hm2,硫酸钾600 kg/hm2,石灰0 kg/hm2的配合施用在促进芹菜高产方面最为适宜.     

Effects of N sources and methane concentrations on methane uptake potential of a typical coniferous forest and its adjacent orchard soil

After removal of the above-ground plant debris three different soil layers were taken from a typical coniferous forest and its adjacent orchard in Numata City, Japan. The potentials of soil CH₄ uptake at two initial CH₄ concentrations were studied under aerobic conditions in the laboratory, along with inhibition of soil CH₄ oxidation by urea-N or KNO₃-N addition. Due to long-term N inputs, the CH₄ uptake of the upper mineral layer of the orchard soil was 25.4% and 87.7% lower than that of the surface forest soil at 2.4 and 12.6 l l^–1 CH₄, respectively. Methane uptake of the forest soil decreased with increasing soil depths at two CH₄ levels. However, maximal CH₄-consuming activity occurred in the 9- to 23-cm depth of the orchard soil at 12.6 l l^–1 methane. Nitrogen additions in the form of KNO₃ or urea at the rate of 200 g N g^–1 soil substantially reduced soil CH₄ uptake in the upper and sub-surface mineral layers at both sites, except that the addition of KNO₃-N had no apparent inhibitory effect on the CH₄ uptake in the 9- to 23-cm depth of the orchard soil. A strong inhibitory effect of NO₃^– addition on the CH₄ uptake, in contrast to NH₄⁺, occurred in the surface forest soil. The use of KNO₃-N, as compared to urea or urea plus a nitrification inhibitor (dicyandiamide), resulted in a lower potential to cause inhibition of CH₄ oxidation in the 0- to 23-cm depth of the orchard soil.     

The effect of rhizobiophages on<Emphasis Type="Italic"> Sinorhizobium meliloti</Emphasis> -<Emphasis Type="Italic"> Medicago sativa</Emphasis> symbiosis

Using Tn5 mutagenesis of lysogenic Sinorhizobium meliloti strain L5–30 (L54), three neomycin-resistant transconjugants differing in phage-resistance profiles were isolated. Two of them increased the dry weight of plants and were capable of establishing root nodules, whereas the third one was ineffective. The bacterium-phage interaction did not have observable consequences in the Medicago sativa - S. meliloti symbiosis because it did not affect the number of nodules on M. sativa or plant dry weight.     

Bi-directional nitrogen transfer in an intercropping system of peanut with rice cultivated in aerobic soil

Three separate greenhouse experiments were conducted to determine the bi-directional N transfer in a peanut and rice intercropping system using the direct ¹⁵N foliar feeding technique at N application rates of 15, 75 and 150 kg ha^–1. When peanut was used as the ¹⁵N donor plant, the atom % ¹⁵N in the rice shoot was consistently higher than in control rice, indicating that ¹⁵N transfer from peanut to the associated rice crop occurred. The percentage of N transfer (%NT) from peanut to the associated rice was 9.9%, 5.7% and 4.2% at the three N application rates, respectively. The N transferred from peanut to rice was 22.6, 15.5 and 8.2 mg N plant^–1, accounting for 10.9%, 6.4% and 3.1% of the total N accumulated in rice plants at the three N application rates, respectively. When rice functioned as the ¹⁵N donor plant, the %NTs were 4.4%, 2.1% and 1.4% and represented about 5.2%, 3.4% and 2.4% of total N accumulated in peanut shoot at the three N application rates, respectively. The net directional N transfer was from peanut to rice and this was calculated by the difference in the bi-directional transfers and was mainly due to peanut root decomposition. Thus, the %NTs were 10.7%, 6.3%, 5.1% and 3.5% on 28 July (the day on which peanut shoots were cut), 8 August, 28 August and 8 September, respectively, and correspondingly, the N transferred from peanut to rice represented 6.0%, 5.8%, 5.1% and 3.2% of the total N accumulated in the rice plants.     

Biotic and abiotic processes of nitrogen immobilization in the soil-residue interface

The interface between decaying plant residues and soil is a hotspot for microbial immobilization of soil inorganic N. Recent studies on forest and grassland soils have demonstrated that rapid abiotic immobilization of inorganic N is also induced by the presence of plant residues. We, therefore, examined (1) how N immobilization varies with distance from the soil-residue interface and (2) whether abiotic immobilization occurs in agricultural soils. Spatiotemporal changes of N immobilization in the soil-residue interface were evaluated using a box that enabled soil to be sampled in 2 mm increments from a 4 mm-thick residue compartment (RC). The RC was filled with paddy soil containing ground plant residue (rice bran, rice straw or beech leaves) uniformly at a rate of 50 g dry matter kg⁻¹. Soil in the surrounding compartments contained no residue. After aerobic incubation for 5, 15 and 30 days at 25 °C, soils in each compartment were analyzed. After 5 days, significant depletion of inorganic N occurred throughout a volume of soil extending at least 10 mm from the RC in all residue treatments, suggesting extensive diffusion of inorganic N towards the RC. The depletion within 10 mm of the RC amounted to 5.0, 4.3 and 3.4 mg for rice bran, rice straw and beech leaf treatment, respectively. On the other hand, microbial N had increased significantly in the RC of the rice bran and rice straw treatments (11 mg and 5.5 mg, respectively) and insignificantly in the RC of the beech leaf treatment (0.06 mg). This increase amounted to 221% (rice bran), 129% (rice straw) and 1.7% (beech leaves) of the decrease in inorganic N within 10 mm of each RC. Thereafter the rate of N mineralization exceeded that of immobilization, and inorganic N levels had recovered almost to their original level by 15 days (rice bran) and 30 days (rice straw and beech leaves). These results suggested the predominance of biotic immobilization in soil near rice bran and rice straw and of abiotic immobilization in soil near beech leaves. No significant increase in both microbial and soluble organic N in the vicinity of beech leaves after incubation for 5 days further suggested that the abiotic process was responsible for the transformation of inorganic N into the insoluble organic N.     

Nitrogen deposition and dissolved organic carbon production in northern temperate forests

Deposition of anthropogenic nitrogen (N) alters the decomposition of organic matter in forest ecosystems by changing the expression of key microbial enzymes. We investigated the effects of experimental N deposition on dissolved organic matter (DOM) in soils of three forest ecosystems representative of the upper Great Lakes region: the sugar maple/basswood (SMBW), sugar maple/red oak (SMRO) and white oak/black oak (WOBO) ecosystems. Mineral soil samples were collected on five dates from ambient and N-amended plots (80 kg N ha⁻¹ yr⁻¹) in three replicate stands of each forest type. DOM was extracted (2:1, water:soil) from each soil sample and analyzed for dissolved organic carbon (DOC). DOC concentration was significantly greater in the N-amended soils (on average: 24% higher for SMBW, 9% for SMRO, and 40% for BOWO). In June and October 2002, bioassays were performed to assess N treatment effects on the composition of DOM and its interacting bacterial community. Within each site, DOM extracts from the ambient and N-amended plots were reciprocally inoculated with bacteria from each plot. After a 48 h incubation at 20 °C, community activity in each microcosm was profiled by measuring 10 extracellular enzyme activities (EEA). MANOVA showed that ecosystem type, sampling date, DOM source (ambient or N-amended plot) and inoculum source (ambient or N-amended plot) all had significant effects on bioassay EEA. Post hoc tests (Tukey's HSD) found significant reductions in oxidative enzyme activity as a result of the N treatment. In general, the bioassay results corroborated a previous report describing losses in soil oxidative enzyme activity in response to N saturation. However, it is not clear whether increased DOC concentration is the direct result of reduced oxidative activity.     

控释氮肥对水稻的增产效应及提高肥料利用率的研究

田间试验在日本山形县鹤冈市进行 ,探讨了树脂包膜类控释氮肥基施与尿素、硫铵分次施肥对单季稻的增产效应 ;应用15N示踪法研究了不同施肥处理水稻的吸氮模式及肥料利用率。结果表明 ,移栽稻以尿素与控释氮肥 (LPS-100)按 1∶1混合基施产量最高 ,分别比尿素和硫铵分次施肥增产23.6%和9.2% ;直播稻以LP-100基施产量最高 ,分别增产 9.2%和4.0% ;控释肥基施 ,水稻自移栽 (或直播 )至幼穗分化期吸氮量明显高于尿素或硫铵分次施肥。15N示踪研究表明 ,最高分蘖前水稻从尿素和硫铵基肥中吸收的氮素只占基肥氮总吸收量的 33%～ 45% ,水稻从基肥中吸收氮素可延续至抽穗期 ,而从LP-100中吸氮可持续至收获期。15N示踪法测得的氮素总回收率 ,以尿素处理最高 ,硫铵和LP-100接近 ;差减法测得的氮素总回收率以LP-100为最高 ,尿素 /LPS-100、尿素、硫铵三者接近 ;基肥的氮素回收率以LP-100硫铵 尿素。尿素 /LPS-100和LP-100一次基施 ,氮素生理效率和农学效率明显高于尿素和硫铵 ,是高产的主要原因