The effect of sodium chlorate and nitrapyrin on the nitrification mediated nitrosation process in soils

Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH ₄ ⁺ –¹⁵N was added to the soil in the absence and the presence of NaClO₃, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg^–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter.     

Binding of nitrite-N on polyphenols during nitrification

Summary The conversion of substantial amounts of ammonia nitrogen into organic nitrogen as a direct result of nitrification at neutral pH-values, was monitored in soil suspensions amended with ammonium nitrogen. The influence of the chemolithotrophic nitrifying bacteria was verified by applying nitrapyrin as a selective inhibitor in control experiments. In addition, the role of phenolic compounds was examined by adding α naphthol. The factors influencing the nitrification processi.e. pH, NH ₄ ⁺ −N, NO ₂ ⁻ −N, NO ₃ ⁻ −N were measured during a 60 days incubation period. Nitrification started to be active after 5 and 10 days in the normal and the naphthol spiked soil suspensions respectively; it was inhibited in the nitrapyrin controls. Parallel with nitrification, formation of organic nitrogen was observed. The humic matter fractions were extracted and analyzed by I.R. spectroscopy which revealed the valence vibration ranges of nitro and nitroso groups fixed in different positions on aromatic compounds, both for normal and naphthol spiked samples. High resolution gas chromatography combined with mass spectroscopic analysis indicated the formation of nitrosonaphtholes. In addition a novel organic nitro compound was identifiedi.e. an azido nitro benzene. No nitrogen was fixed in the samples treated with nitrification inhibitor. A mechanism for the fixation of nitrite nitrogen during nitrification is proposed.     

Nitrogen transformations in limed and nitrogen fertilized soil in Norway spruce stands

Nitrogen transformations in the soil, and the resulting changes in carbon and nitrogen compounds in soil percolate water, were studied in two stands of Norway spruce (Picea abies L.). Over the last 30 years the stands were repeatedly limed (total 6000 kg ha^–1), fertilized with nitrogen (total about 900 kg ha^–1), or both treatments together. Both aerobic incubations of soil samples in the laboratory, and intact soil core incubations in the field showed that in control plots ammonification widely predominated over nitrification. In both experiments nitrogen addition increased the formation of mineral-N. In one experiment separate lime and nitrogen treatments increased nitrification, in the other, only lime and nitrogen addition together had this effect. In one experiment immobilization of nitrogen to soil microbial biomass was lower in soil only treated with nitrogen. Soil percolate water was collected by means of lysimeters placed under the humus layer and 10 cm below in the mineral soil. Total N, NH₄-N and NO₃-N were measured, and dissolved organic nitrogen was fractioned according to molecular weight. NO₃-N concentrations in percolate water, collected under the humus layer, were higher in plots treated with N-fertilizer, especially when lime was also added. The treatments had no effect on the N concentrations in mineral soil. A considerable proportion of nitrogen was leached in organic form.     

The effects of stem girdling on biogeochemical cycles within a mixed deciduous forest in eastern Tennessee

Summary Nitrogen mineralization and net nitrification rates were 3–7 times greater in soil incubations from a girdled Liriodendron tulipifera (L.) stand than in a control stand. Neither litter nor root extracts had an inhibitory effect on nitrogen mineralization or nitrification rate. A lack of nitrification inhibitors also was demonstrated by the fact that ammonium added to the control stand was completely converted to nitrate upon incubation. Additions of sucrose increased CO₂ evolution and decreased nitrogen mineralization and nitrification rates in the girdled plot soil, suggesting that nitrification could be effectively controlled by competition for NH ₄ ⁺ supplies by heterotrophic soil organisms. CO₂ evolution rates during incubation showed that heterotrophic as well as nitrifier activities were greater in the girdled plot soil than in the ungirdled plot soil, but the ratio of C to N mineralized was lower in the girdled plot soil. These results collectively indicate that nitrification is regulated by the availability of NH ₄ ⁺ in these stands, and that the latter is strongly regulated by heterotrophic demand for N.Operated by Union Carbide Corporation for the U.S. Department of Energy     

The effect of harvesting intensity on the fate of applied 15N-ammonium to the organic horizons of a coniferous forest in N. Wales

¹⁵N-ammonium sulphate equivalent to 0.5 kg N/ha was added as a tracer to lysimeters containing the organic horizons of an acid forest soil. The effect of logging debris (brash), vegetation and second rotationPicea sitchensis seedlings on the amount of the¹⁵N found in various soil, vegetation and leachate pools was followed over a period of 60 days. Transformation of¹⁵N-ammonium to nitrate occurred within 24 hours. Although total nitrate leachate losses were high, tracer-derived nitrate represented only 0.4%–4.2% of the applied¹⁵N-ammonium. The atom % excess of the KCI-extractable organic-N pool was initially lower than for the inorganic species but due to the large pool size, consistently represented 3–6% of the applied¹⁵N-ammonium. The similarity of the atom % excess of the ammonium and nitrate pools indicated an autotrophic nitrification pathway.A significant proportion of the¹⁵N-ammonium passed through the microbial biomass which contained between 16 and 48% of the¹⁵N-ammonium 2 days after addition of the¹⁵N-ammonium. This nitrogen was in a readily available form or short-term pool for the first two weeks (with no change in the overall biomass pool), after which the nitrogen appeared to become transformed into more stable compounds representing a long-term pool. Total recovery of the¹⁵N was between 68% and 99% for the different treatments. The presence of brash reduced microbial immobilisation of the¹⁵N-ammonium and total retention in the organic matter. This is suggested to be a consequence of greater nitrification and denitrificatiion rate in organic horizons beneath a brash covering due to different microclimatic conditions.     

氨基酸添加对亚热带森林红壤氮素转化的影响

为探究氨基酸氮形态对亚热带土壤氮素含量及转化的影响,选择建瓯市万木林保护区的山地红壤为对象,采用室内培养实验法,通过设计60%和90%WHC两种土壤含水量并添加不同性质氨基酸,测定了土壤中铵态氮、硝态氮、可溶性有机氮的含量和氧化亚氮的释放量,分析了可溶性有机碳、土壤p H值的大小变化及其与氮素的相互关系。结果表明:与对照处理相比,氨基酸添加显著增加了土壤NH_4~+-N含量并使土壤p H值升高,且在一定程度上解除了高含水量(90%WHC)对NH_4~+-N产生的抑制,其中甲硫氨基酸的效果最为明显。酸性、碱性、中性氨基酸对土壤NO_3~--N含量和N_2O释放影响不显著,但甲硫氨基酸可显著抑制土壤硝化从而导致NH_4~+-N的积累,并在培养前期抑制土壤N_2O产生而在培养后期促进N_2O释放,总体上促进N_2O释放。60%WHC的氨基酸添加处理较90%WHC条件下降低土壤可溶性有机氮的幅度更大。氨基酸对土壤氮素转化的影响与带电性关系较小,而可能与其分解产物密切相关。可见,不同性质氨基酸处理对森林土壤氮素含量及转化存在不同程度的影响,且甲硫氨基酸对土壤氮素转化的影响机理值得深入研究。     

Is growth of soil microorganisms in boreal forests limited by carbon or nitrogen availability?

To study whether the biomass of soil microorganisms in a boreal Pinus sylvestris-Vaccinium vitis-idaea forest was limited by the availability of carbon or nitrogen, we applied sucrose from sugar cane, a C₄ plant, to the organic mor-layer of the C₃–C dominated soil. We can distinguish between microbial mineralization of the added sucrose and respiration of endogenous carbon (root and microbial) by using the C₄-sucrose as a tracer, exploiting the difference in natural abundance of ¹³C between the added C₄-sucrose (¹³C –10.8) and the endogenous C₃–carbon (¹³C –26.6 ). In addition to sucrose, NH₄Cl (340 kg N ha^–1) was added factorially to the mor-layer. We followed the microbial activity for nine days after the treatments, by in situ sampling of CO₂ evolved from the soil and mass spectrometric analyses of ¹³C in the CO₂. We found that microbial biomass was limited by the availability of carbon, rather than nitrogen availability, since there was a 50% increase in soil respiration in situ between 1 h and 5 days after adding the sucrose. However, no further increase was observed unless nitrogen was also added. Analyses of the ¹³C ratios of the evolved CO₂ showed that increases in respiration observed between 1 h and 9 days after the additions could be accounted for by an increase in mineralization of the added C₄–C.     

氮添加对樟子松人工林氮转化及相关功能基因丰度的影响

土壤氮(N)的有效性是影响土壤微生物群落结构以及土壤氮循环的重要因子。为探索N添加对樟子松人工林氮素转化及N功能基因(NFGs)表达的影响及其作用机制,该文以塞罕坝千层板林场的樟子松人工林为研究对象,进行了2年的氮添加处理,设置4个不同氮添加水平0、1、5、10 g N·m^-2·a^-1,分别记作N0、N1、N5、N10,采用功能基因微阵GeoChip 5.0系统及室内土壤培养法,探讨了土壤NFGs对氮添加的反应及其对氮转化过程的影响。结果表明:(1)与N0相比,中低N添加处理(N1、N5)促进了氨化(ureC、nirA、nrfA)、硝化(amoA)和反硝化(norB)相关基因的相对丰度,高N处理(N10)则抑制了所有NFGs的表达。(2)相关分析表明,N1、N5的促进作用与土壤有机碳(SOC)、硝态氮(NO₃^--N)和微生物生物量碳(MBC)显著相关,N10处理显著降低了所有氮转化过程NFGs的相对丰度,这种负面影响与溶解性有机碳(DOC)、MBC含量的减少有关。(3)与氮转化基因丰度规律趋势相似,N1和N5处理显著增加了净N硝化、净N矿化以及N₂O的排放速率,但N10促进作用不明显,表明氮添加对氮转化的促进作用存在阈值。(4)多元回归分析进一步表明,amoA-AOB和MBC是影响净N硝化的关键因素,ureC、nirK和MBC是影响净氮矿化的关键因素,narG、nirS是影响N₂O排放的关键因素。综上,N添加可提高促进樟子松人工林的氮转化及提高部分特定酶功能基因的相对丰度,但氮添加水平存在阈值,当施用10 g N·m^-2·a^-1时,氮转化受到抑制,添加5 g N·m^-2·a^-1是促进樟子松人工林土壤N转化的较佳水平。     

Crop utilization and fixation of added ammonium in soils of the West Indies

NH₄ ⁺-fixation by inorganic and organic soil components and crop utilization of fertilier nitrogen was studied in a number of Carbbean soils using¹⁵N fertilizers. At moderate rates of nitrogen application, NH₄ ⁺-fixation by clays during several-week laboratory incubations was rapid and highly vaiable, ranging from less than 10% to over 70% of the NH₄ ⁺ added. The 2: 1 lattice types were the most reactive, and the process were almost complete by one week after fertilization. Fixation increased with rate of NH₄ ⁺-N application and was higher at elevated temperatures in soils that were allowed to air-dry during incubation. NH₄ ⁺-N fixation was more active in the fulvic fractions of the soil organic matter than in the humuc fractions (25–69%vs0–3% of the added NH₄ ⁺ was fixed in each, respectively). There was little incorporation of fertilizer-N by the N-containing fractions of soil organic matter. Plant uptake of added NH₄ ⁺-N in greenhouse pot experiments showed that a greater percentalte of fertilizer-N was taken up by Sudan grass (Sourghum sudanese) at a fertilizer rate of 40 kg NH₄ ⁺-N ha^?1 than at a rate of 200n kg NH₄ ⁺N ha^?1. howver, the recovery was low, ranging from 10 to 25 percent of that applied. In field experiments with maize (Zea mays), urea-N was rapidly lost when applied to soils in a wet tropical environment. At normal rates of application (100 kg urea-N ha^?1) only about half of the fertilizer was utilized by the crop. Mulches did not significantly affect the fate of added nitrogen; however, mulching did result in increased yields for dry-season cropping, due probably to water conservation effects. There is good indication that for conditions in Trinidad, NH₄ ⁺-N is better utilzed and less subject to unidentified losses than is urea. Addition of fertilizer-N resulted in crop uptake of important quantities of native soil nitrogen. The Caribbean Andepts were outstanding in that the showed very little NH₄ ⁺-fixation under all experimental conditions and very little tendency for apparent nitrification of added NH₄ ⁺-N.     

川西北高寒草地沙化过程中土壤氮素变化特征

草地沙化是我国最严重的环境问题之一,但关于草地沙化过程中氮素变化特征的研究报道多集中于干旱半干旱地区,而半湿润地区的相关报道还比较缺乏。通过野外调查,研究了川西北半湿润地区高寒沙质草地沙化过程中土壤氮素变化特征。结果表明,草地沙化对0—100cm土层土壤氮素具有显著影响,全氮、碱解氮、铵态氮(NH_4~+-N)、硝态氮(NO_3~--N)和微生物量氮(MBN)均呈现极显著下降的变化特征,极度沙化阶段较未沙化阶段分别减少了73.95%、77.72%、76.75%、79.77%和84.12%。其中,0—20cm土层变化最显著,全氮、碱解氮、NH_4~+-N、NO_3~--N和MBN含量分别减少了86.43%、83.52%、82.11%、88.82%和91.77%。随着土层深度增加,不同程度沙化草地土壤氮素含量及其变化量逐渐减少;草地沙化过程中,不同沙化阶段土壤氮素损失数量不尽相同,其中,以轻度沙化阶段氮素损失最严重,全氮、碱解氮、NH_4~+-N、NO-3-N和MBN含量分别降低了41.18%、35.17%、46.74%、43.46%和46.88%。草地沙化过程中,土壤全氮、碱解氮、NH_4~+-N、NO_3~--N和MBN含量与土壤粉粒、粘粒含量和植被群落盖度均呈极显著正相关特征,与土壤沙粒含量呈极显著负相关特征。研究区土壤氮素损失与风蚀选择性吹蚀土壤粉粒、粘粒及地表植物覆盖状况逐渐变差密切相关,因此该区域治沙的关键是采取措施降低风蚀对地表土壤吹蚀作用,提高沙化草地地表植被覆盖。同时,还应及时对沙化前期阶段及潜在沙化的草地进行生态治理,从而避免草地沙化继续恶化。     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

大兴安岭北部天然针叶林土壤氮矿化特征

采用顶盖埋管法对大兴安岭地区天然针叶林(樟子松林、樟子松-兴安落叶松混交林和兴安落叶松林)土壤铵态氮(NH~+_4-N)、硝态氮(NO~-_3-N)、净氮矿化速率进行研究,并探索土壤理化性质与氮矿化之间的相关性,为大兴安岭地区森林生态系统土壤养分管理及森林经营提供帮助。结果表明:观测期内(5—10月)3种林型土壤无机氮变化范围为31.51—70.42 mg/kg,以NH~+_4-N形式存在为主,占比达90%以上,且与纯林相比混交林土壤无机氮含量较高。3种林型土壤净氮矿化、净氨化、净硝化速率月变化趋势呈V型,7、8月表现为负值,其他月份为正值。净氮矿化速率变化范围樟子松林为-0.54—1.28 mg kg~(-1) d~(-1)、樟子松-兴安落叶松混交林为-0.13—0.55 mg kg~(-1) d~(-1)、兴安落叶松林为-0.80—1.05 mg kg~(-1) d~(-1)。土壤净氨化过程在土壤氮矿化中占主要地位,占比达60%以上。3种林型土壤净氮矿化、净氨化及净硝化速率垂直差异显著,0—10 cm土层矿化作用明显高于10—20 cm土层(P0.05)。土壤氮矿化速率与土壤含水量、土壤有机碳含量、土壤C/N、枯落物全氮含量和枯落物C/N均存在显著相关性。不同类型的森林土壤及枯落物的质量也存在差异,进而影响土壤氮矿化特征。     

蚂蚁筑巢对西双版纳热带森林土壤有机氮矿化的影响

蚂蚁作为生态系统工程师能够调节土壤微生物及理化环境,进而对热带森林土壤有机氮矿化速率及其时间动态产生显著影响。以西双版纳白背桐热带森林群落为研究对象,采用室内需氧培养法测定土壤有机氮矿化速率,比较蚁巢和非蚁巢土壤有机氮矿化速率的时间动态,揭示蚂蚁筑巢活动引起土壤无机氮库、微生物生物量碳及化学性质改变对有机氮矿化速率时间动态的影响。结果表明：（1）蚂蚁筑巢显著影响土壤有机氮矿化速率（P<0.01）,相较于非蚁巢,蚁巢土壤有机氮矿化速率提高了261%;（2）土壤有机氮矿化速率随月份推移呈明显的单峰型变化趋势,即6月最大（蚁巢1.22 mg kg^-1 d^-1、非蚁巢0.41 mg kg^-1 d^-1）,12月最小（蚁巢0.82 mg kg^-1 d^-1、非蚁巢0.18 mg kg^-1 d^-1）;（3）两因素方差分析表明,不同月份及不同处理对土壤有机氮矿化速率、NH₄-N及NO₃-N产生显著影响（P<0.05）,但对NO₃-N的交互作用不显著;（4）蚂蚁筑巢显著提高了无机氮库（NH₄-N与NO₃-N）、微生物生物量碳、有机质、水解氮、全氮及易氧化有机碳等土壤养分含量,而降低了土壤pH值;（5）回归分析表明,铵态氮和硝态氮对土壤有机氮矿化速率产生显著影响,分别解释87.89%、61.84%的有机氮矿化速率变化;（6）主成份分析表明NH₄-N、微生物生物量碳及有机质是影响有机氮矿化速率时间动态的主要因素,而全氮、NO₃-N、易氧化有机碳、水解氮及pH对土壤有机氮矿化速率的影响次之,且pH与土壤有机氮矿化速率呈显著负相关。总之,蚂蚁筑巢活动主要通过影响土壤NH₄-N、微生物生物量碳及有机质的状况,进而调控西双版纳热带森林土壤有机氮矿化速率的时间动态。研究结果将有助于进一步提高对土壤氮矿化生物调控机制的认识。     

Glycine uptake in heath plants and soil microbes responds to elevated temperature, CO2 and drought

Temperate terrestrial ecosystems are currently exposed to climatic and air quality changes with increased atmospheric CO₂, increased temperature and prolonged droughts. The responses of natural ecosystems to these changes are focus for research, due to the potential feedbacks to the climate. We here present results from a field experiment in which the effects of these three climate change factors are investigated solely and in all combinations at a temperate heath dominated by heather (Calluna vulgaris) and wavy hair-grass (Deschampsia flexuosa).Climate induced increases in plant production may increase plant root exudation of dissolved organic compounds such as amino acids, and the release of amino acids during decomposition of organic matter. Such free amino acids in soil serve as substrates for soil microorganisms and are also acquired as nutrients directly by plants. We investigated the magnitude of the response to the potential climate change treatments on uptake of organic nitrogen in an in situ pulse labelling experiment with ¹⁵N¹³C₂-labelled glycine (amino acid) injected into the soil.In situ root nitrogen acquisition by grasses responded significantly to the climate change treatments, with larger ¹⁵N uptake in response to warming and elevated CO₂ but not additively when the treatments were combined. Also, a larger grass leaf biomass in the combined T and CO₂ treatment than in individual treatments suggest that responses to combined climate change factors cannot be predicted from the responses to single factors treatments.The soil microbes were superior to plants in the short-term competition for the added glycine, as indicated by an 18 times larger ¹⁵N recovery in the microbial biomass compared to the plant biomass. The soil microbes acquired glycine largely as an intact compound (87%), with no effects of the multi factorial climate change treatment through one year.     

Consequences of liming and gypsum top-dressing on nitrogen and carbon dynamics in acid forest soils with different humus forms

The aim of this study was to understand the effects of lime and gypsum on nitrogen and carbon turnover of the soil. A pot experiment was conducted in parallel with a field experiment which was set up in 1989 in a declining forest of the French Ardennes. A dystric cambisol, associated with a moder and mull humus separately, was used to study changes in the soil chemistry as a result of added lime and gypsum top-dressing.The lime was applied to the surface of an acid mull humus of an oak (Quercus petraea) stand and of a moder humus of a spruce (Picea abies) stand. A quantity of 2.8 t ha^-1 equivalent CaO was supplied as CaCO₃, CaCO₃+MgO and CaSO₄.2H₂O. The experiment was installed in an open-air nursery for 20 months, during which the organic carbon and nitrogen in the solution were analysed monthly. They were analysed in the solid phase after 20 months. At the end of this period the changes in the soil and leachate depended mainly on the type of the material added.The leachate was enriched with nitrogen from the third month of the experiment under lime treatments and in the control. The same pattern was found under the two humus types but the magnitude was higher in soil with a mull humus. The nitrogen was mostly leached as NO₃ ^--N in the carbonate treatments and in the control, whereas it was predominantly NH₄ ⁺-N under gypsum. The NO₃ ^--N was 50% higher than NH₄ ⁺-N in the control and CaCO₃, CaCO₃+MgO treatments. In the CaSO₄ treatment this phenomenon was reversed. The leaching of organic carbon was greater under gypsum than under the other treatments whatever the humus.In the solid phase of the soil (organic layers) the organic carbon and nitrogen concentration decreased significantly after liming, especially in the mull humus. Consequently it induced a decrease in C:N ratio of about 18% with respect to the control.     

The role of monoterpenes in soil nitrogen cycling processes in ponderosa pine

The effects of select monoterpenes on nitrogen (N) mineralization and nitrification potentials were determined in four separate laboratory bioassays. The effect of increasing monoterpene addition was an initial reduction in NO₃ ^–-N production (nitrification inhibition), followed by a reduction in the sum of NH₄ ⁺-N and NO₃ ^–-N (inhibition of net N mineralization and net immobilization at high monoterpene additions. Monoterpenes could produce this pattern by inhibiting nitrification, reducing net N mineralization, enhancing immobilization of NO₃ ^–-N relative to NH₄ ⁺-N, and/or stimulating overall net immobilization of N by carbon-rich material.Initial monoterpene concentrations in the assay soils were about 5% of the added amount and were below detection after incubation in most samples.Potential N mineralization-immobilization, nitrification, and soil monoterpene concentrations were determined by soil horizon for four collections from a ponderosa pine (Pinus ponderosa) stand in New Mexico. Concentrations of monoterpenes declined exponentially with soil depth and varied greatly within a horizon. Monoterpene content of the forest floor was not correlated with forest floor biomass. Net N mineralization was inversely correlated with total monoterpene content of all sampled horizons. Nitrification was greatest in the mineral soil, intermediate in the F-H horizon, and never occurred in the L horizon. Nitrification in the mineral soil was inversely correlated with the amount of monoterpenes in the L horizon that contain terminal unsaturated carbon-carbon bonds (r ² = 0.37, P 0.01). This pattern in the field corresponded to the pattern shown in the laboratory assays with increasing monoterpene additions.     

Soil nitrogen heterogeneity in a Dehesa ecosystem

The C mineralization and N transformations during the decomposition of sunflower stalks (Helianthus annuus L.) and wheat straw (Triticum aestivum L.) with and without addition of (NH₄)₂SO₄ (27.53 atom% ¹⁵N) were studied in a Vertisol. Soil samples were incubated under aerobic conditions for 224 days at 22 °C. The plant residues were added at a rate of 5.2 g kg^-1 soil. Nitrogen was applied at a rate of 50.7 mg N kg^-1 soil. Carbon dioxide emission and inorganic N content in soil were periodically determined. Gross N immobilization and remineralization were calculated on the basis of the isotopic dilution technique. At the end of the incubation period a ¹⁵N balance was established. Respectively, 68 and 45% of the applied residue-C mineralized from the sunflower stalks and wheat straw after 224 days. Both crop residues caused losses of up to 25% of added ¹⁵N after 224 days of incubation. These ¹⁵N losses were about three times larger than in the control soil, and were probably due to denitrification. The net immobilization of soil derived N following residue incorporation was largest in the case of wheat straw, depleting all soil inorganic N. In the wheat straw treatment with added (NH₄)₂SO₄ soil inorganic N remained available, resulting in an enhanced initial C mineralization and N immobilization compared to the treatment without added N. In the case of the sunflower stalks, the high inorganic N content of the stalks suppressed the effects of N addition on C mineralization and N immobilization/mineralization. Gross N immobilization amounted to 31.9 and 28.2 mg N g^-1 added C after 14 days for wheat straw and sunflower stalks, respectively. At the end of the incubation, about 35% of the newly immobilized N was remineralized in both plant residue treatments. Gross N immobilization plotted against decomposed C suggests that fairly uniform C-N relationships exist during the decomposition of divers C substrates. The results demonstrate that low fertilizer N use efficiencies may be expected in a wheat-sunflower cropping system with incorporation of crop residues, as the fertilizer N applied becomes largely immobilized in the soil organic fraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

间伐和林下植被剔除对毛竹林土壤氮矿化速率及其温度敏感性的影响

选择中亚热带毛竹人工林为研究对象,利用野外原位和室内培养相结合的方法,探讨不同间伐强度(25%间伐、50%间伐)和林下植被剔除对土壤氮矿化速率及其温度敏感性的影响。结果表明,25%间伐显著增加土壤氨化速率(P0.01),但降低硝化速率(P0.01);50%间伐显著增加土壤硝化速率(P0.01),而林下植被剔除显著降低土壤硝化速率(P0.01)。相关分析的结果表明,土壤氨化速率与有机碳(SOC)、全氮(TN)及全磷(TP)含量呈显著负相关关系;硝化速率与SOC、含水量(SWC)呈显著正相关关系,与铵态氮(NH~+_4-N)含量呈显著负相关关系。随着温度的升高,不同处理下的氨化速率均显著增加(P0.01),而硝化速率显著降低(P0.01)。25%间伐显著降低土壤净氮矿化和氨化过程的Q_(10)值,对硝化过程的Q_(10)值影响不显著;50%间伐对氨化和硝化过程的Q_(10)值影响均不显著;林下植被剔除对氨化过程的Q_(10)值影响不显著,但显著增加硝化过程的Q_(10)值。不同处理下的土壤氮矿化过程的Q_(10)值介于1.17—1.36之间。25%间伐和林下植被保留有利于毛竹林土壤氮素的供给。     

Estimation of biological nitrogen fixation associated withBrachiaria andPaspalum grasses using15N labelled organic matter and fertilizer

Summary Six pasture grasses,Paspalum notatum cv batatais,P. notatum cv pensacola,Brachiaria radicans, B. ruziziensis, B. decumbens andB. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of¹⁵N labelled organic matter or frequent small (2 kg N. ha^–1) additions of¹⁵N enriched (NH₄)₂SO₄. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in¹⁵N enrichment in plant-available nitrogen (NO ₃ ^– +NH ₄ ⁺ ) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different¹⁵N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the¹⁵N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in¹⁵N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same¹⁵N enrichment in soil derived N. Data from the final harvests of this treatment indicated thatB. humidicola andB. decumbens obtained 30 and 40% respectively of their nitrogen from N₂ fixation amounting to an input of 30 and 45 kg N.ha^–1 year^–1 respectively.     

Short-term mineralization of maize residues in soils as determined by carbon-13 natural abundance

Studies of soil organic matter equilibria must include estimates of C turnover. The objective of this study was to provide data on how the natural ¹³C abundance method can be used to determine the flow of C from C₄ residues and soil organic matter (C₃-source) in a short-term incubation. Corn residue was added at a rate of 5.7 mg C g⁻¹ soil to two soils, a clay and a sandy clay loam. During the course of a 35-day incubation in a CO₂-free system, CO₂-C and ¹³C natural abundance of the respired CO₂ were measured. About 20% of the corn residue-C added was mineralized in both soils as determined from the CO₂ respired and the ¹³C natural abundance of the respired CO₂. Mineralization of the added residues was also calculated as the difference of the total amount of the respired CO₂-C between the control and the corn residue-treated soils divided by the total amount of corn residue-C. Values were 35% for the clay soil, and 30% for the sandy clay loam soil. The difference in values calculated from the ¹³ C natural abundance and the difference method was due to mineralization of the indigenous soil organic C resulting from the addition of corn residues. Use of the natural ¹³C abundance method could determine the degree of ‘priming effect’ in soils amended with C₄-C residues. This revised version was published online in June 2006 with corrections to the Cover Date.