生物硝化抑制剂——一种控制农田氮素流失的新策略

农业生产中氮肥的施用是影响全球氮素循环的一个重要因素,在促进作物增产的同时,也对生态环境产生了重要的影响。由于铵态氮肥在旱地中很容易经过硝化作用转变为硝态氮,其中一小部分为植物所吸收,而大量的硝态氮被淋失,或经反硝化作用进入大气,造成土壤氮素严重损失。自然界中一些植物的根系能够分泌抑制硝化作用的物质,被称为生物硝化抑制剂,因而可以显著提高土壤氮素利用率。本文阐述了有关生物硝化抑制剂的由来、分泌调节、作用机制及其应用潜力,并探讨了其在农业生产中氮素高效管理等方面的应用前景。     

高粱在不同氮源处理下分泌生物硝化抑制剂的差异

高粱根系能分泌抑制土壤亚硝化细菌的物质,称为生物硝化抑制剂.在铵态氮存在时,这类物质会大量分泌,而以硝态氮为氮源时则分泌很少.目前还不清楚,这是因为不同氮素代谢的差异所引起的,还是根系吸收铵态氮后根际酸化影响了硝化作用.因此本研究通过砂培试验,用铵硝营养处理高粱根系,并且控制pH,最后收集根系分泌物,利用一种荧光标记的亚硝酸细菌来测定高粱分泌的硝化抑制剂活性,计算其分泌速率.结果发现,铵态氮是导致根系大量分泌硝化抑制剂的主要原因,并且在根际pH＜6时,硝化抑制剂的活性与分泌速率随着根系分泌氢离子数量的增加而增强.研究表明,铵态氮营养下高粱根系分泌生物硝化抑制剂高于硝态氮营养可能是高粱根系保护根际铵氧化,提高氮素利用率的一个重要生理机制.     

生物反硝化抑制剂原花青素对土壤氮转化及植物生长的影响

我国农业生产氮肥投入量大，但利用率低，氮素损失严重。其中，由反硝化过程产生的氮素损失占比最高可达50%以上，有效调控土壤反硝化过程对于减少农业生态系统氮素损失、降低氮素环境污染具有重要意义。生物反硝化抑制剂(biodenitrification inhibitors,BDIs)是一类植物分泌的次生代谢产物，其中的原花青素已被证实可促进土壤氮素储存并增加作物产量，有望成为一种高效且绿色的氮素调控物质。本文系统梳理了BDIs的发现及其反硝化抑制机制，总结了目前国内外BDIs研究领域的主要进展，并对未来研究方向进行了展望，以期为BDIs施用技术及产品的开发、农产品质量的提升及现代农业绿色健康发展提供借鉴。     

高粱分泌硝化抑制物对羟基苯丙酸与质子泵的关系研究

硝化作用是导致农田氮素损失的一个重要原因,自然界中一些植物根系能分泌抑制土壤氨氧化细菌的物质,统称为生物硝化抑制剂。高粱根系分泌的对羟基苯丙酸(简称MHPP),具有很好的抑制效果。为了研究MHPP的分泌机制,本实验用铵态氮与硝态氮两种不同氮素形态水培高粱幼苗,在一定时间内收集根系分泌物。将根系分泌物通过高效液相色谱法(HPLC)测定其中的MHPP含量,通过微量滴定法测定氢离子含量。同时,将根系采样后,用两相法分离根系细胞膜,测定质子泵活性。结果表明,在铵态氮营养下,高粱分泌MHPP的速率在3.52μmol g-1 d-1左右,而硝态氮营养下几乎不分泌MHPP。在铵态氮营养下,根系分泌氢离子的速率为16.49μmol g-1 d-1,而硝态氮营养下没有氢离子的分泌。而且,随着根系分泌物收集液中铵离子浓度的增加,也促进了根系分泌MHPP和氢离子的速率。因此,铵态氮营养促进了根系分泌MHPP。进一步研究发现,虽然在表观上MHPP的分泌受氮素形态影响,但是在这一过程中MHPP分泌与氢离子的分泌速率是偶合的。而细胞分泌氢离子是由细胞膜上质子泵活性决定的。因此,本实验结果发现,MHPP的分泌实质上是通过根系细胞膜质子泵活性来调控的:在没有氮源的情况下,只要通过壳梭孢菌素(Fusicoccin)或钒酸盐(Vanadate)改变质子泵活性就可以影响MHPP的分泌。所有实验数据的统计结果发现,MHPP分泌与质子分泌之间的比例大约为1∶4,且相关系数r=0.98。因此,铵态氮营养下高粱根系大量分泌生物硝化抑制剂MHPP在一定程度上是由于铵离子的吸收刺激了质子泵活性提高所引起的,这也是防止根际铵态氮被氧化、提高氮素利用率的一个重要生理机制。     

用于黑土的稳定性氯化铵的适宜硝化抑制剂和氮肥增效剂组合

【目的】本文研究添加不同种类硝化抑制剂的高效稳定性氯化铵氮肥在黑土中的施用效果，旨在筛选出适合旱作黑土的高效稳定性氯化铵态氮肥。【方法】在氯化铵中分别添加硝化抑制剂3,4-二甲基吡唑磷酸盐 (DMPP)、双氰胺 (DCD)、2-氯-6-三甲基吡啶 (Nitrapyrin，CP)、氨保护剂 (N-GD) 和1种氮肥增效剂 (HFJ) 及其组合，制成9种稳定性氯化铵氮肥。以不施氮肥 (CK) 和施普通氯化铵 (CK-N) 为对照，以9种稳定性氯化铵为处理进行了等氮量盆栽试验。在玉米苗期、大喇叭口期、灌浆期和成熟期测定了土壤中铵态氮和硝态氮含量，在玉米成熟期测定植株生物量、籽粒产量和氮素含量，计算铵态氮肥的表观硝化率、硝化抑制率、氮肥农学效率、氮肥偏生产力。【结果】1) 与CK-N处理相比，9个处理均显著提高玉米的产量，HFJ的效果均为最显著，可增加玉米籽粒产量3.99倍，提高氮肥吸收利用率4.98倍，显著高于8个硝化抑制剂处理 (P < 0.05)。CP + DMPP和CP + DCD处理提高玉米籽粒产量1.90～2.11倍，两个处理之间无显著差异；CP + DMPP玉米生物量显著高于CP处理，而与DMPP和DCD处理无显著差异；CP + DMPP玉米氮肥吸收利用率显著高于CP和DMPP处理，显著提高3.71倍 (P < 0.05)；2) CP + DMPP和CP + DCD土壤中铵态氮含量提高2.09～2.42倍，且显著高于CP、DMPP和DCD处理 (P < 0.05)，而硝态氮含量和土壤表观硝化率均显著降低24%和66%～68%，与CP和DCD处理存在显著差异 (P < 0.05)；苗期CP + DMPP和CP + DCD硝化抑制率高达23.9%～24.3%，显著高于CP和DCD (P < 0.05)。【结论】在黑土中，氯化铵中添加硝化抑制剂组合的硝化抑制率显著高于添加单一抑制剂，能够有效减缓土壤中铵态氮向硝态氮的转化，减少土壤中氮素损失，降低环境污染。CP + DMPP组合玉米的氮肥吸收利用率显著高于CP + DCD组合。氮肥增效剂HFJ显著增加玉米的氮素吸收量，提高氮肥利用率，从而使玉米获得高产并获得较高的收获指数和经济系数。因此，综合考虑产量和抑制硝化作用等因素，黑土区氯化铵作为玉米生产用氮肥时，建议首选添加氮肥增效剂HFJ来保证作物的高产和氮肥高利用率，也可以添加硝化抑制剂组合CP + DMPP，或者CP + DCD制备稳定性氯化铵来提高氯化铵的增产效果和氮肥利用率，减少氮素损失，降低环境污染。     

施用生物炭抑制塌陷区复垦土壤硝化作用

生物炭具有培肥土壤和影响土壤氮素转化的效应,但对于不同肥力尤其是极低肥力的采煤塌陷复垦区土壤氮素转化方面缺乏研究。该文采用室内恒温控湿好气培养的方法,研究生物炭在不同氮肥水平下对肥力差异较大的两种土壤(肥力高的菜地土壤、肥力极低的塌陷区复垦土壤)硝化作用的影响。试验设2种氮肥水平、3种生物炭施用量。结果发现,相对于菜地土壤,塌陷区复垦土壤硝化作用缓慢,土壤最大硝化速率仅为菜地土壤的17.32%,且最大硝化速率出现的时间延迟4.2 d。高氮条件下,土壤硝化作用进行得较慢,施入生物炭后对硝化作用的抑制增强,并使土壤硝化加速阶段延长6 d(塌陷区复垦土壤)至11 d(菜地土壤)。塌陷区土壤尤其在高氮条件下最大硝化速率出现的时间明显随生物炭添加量增加而逐渐延迟,而土壤最大硝化速率不受生物炭及氮肥水平的影响。但菜地土壤土壤最大硝化速率值、最大硝化速率出现的时间值显著受氮肥水平及生物炭施用量影响。因此,生物炭对硝化作用的抑制主要表现在硝化作用的加速阶段,抑制强度受氮肥水平和土壤类型交互作用影响。     

农田土壤硝化—反硝化作用与N2O的排放

在北京潮土上研究了冬小麦夏玉米轮作体系下土壤硝化反硝化作用以及N2O排放情况。结果表明，小麦生育期土壤温度及含水量降低，无论是反硝化损失氮量还是土壤的N2O生成排放量均不高。土壤的N2O生成排放量与反硝化氮量相当或低于反硝化氮量。玉米生育期土壤温度升高以及孔隙含水量的较大的改善，反硝化损失氮量、N2O生成排放量有明显上升。通常情况下土壤反硝损失氮量与N2O排放氮量基本处于同一水平。在玉米十叶期追肥后的较短时间内，N2O总排放量明显高于反硝化损失氮量，说明至少在这一阶段中，硝化作用在北方旱地土壤N2O的排放中发挥了主要作用。在评价北方旱地农田土壤氮素硝化反硝化损失中，硝化作用的氮素损失是不可忽视的重要方面。     

轮作不同高粱品种阻控设施菜田氮素损失潜力研究

为缓解华北地区设施菜田夏季休闲期氮素淋洗损失严重等问题,提高氮素利用率及土壤生物多样性,结合高粱的生物学特征,选取晋杂12等6种北方常见高粱品种为供试品种,以常规填闲作物甜玉米为对照,筛选适宜作为北京设施菜田夏季填闲作物的高粱品种。结果表明,从生物量、根系及养分带走量等方面来看,晋杂12较其他品种均占优势,成熟期生物量为11.2 t·hm-2,0~45 cm总根长为299 m,作物地上部氮素带走量为139 kg N·hm-2,均显著高于常规填闲作物甜玉米,可以替代甜玉米推广使用。同时研究发现冀梁2号品种在尿素施用后土壤铵态氮含量明显高于其他品种,可能是作物根系分泌的生物硝化抑制剂减缓了硝化过程。与甜玉米相比,高粱可能通过释放生物硝化抑制剂和根系在土壤剖面生物提氮共同起到减少氮素损失的作用。     

低量施氮对小青菜生长和氮素损失的影响

采用田间试验和微区试验相结合,研究了低量施氮对小青菜(Brassica.chinensis)产量、氮肥利用率和氮素损失的影响,其中氮素总损失用15N示踪法测定,氨挥发用通气密闭室法测定,反硝化损失用乙炔抑制-原状土柱培养法测定,不加乙炔测定N2O排放。结果表明,施用氮肥显著增加了小青菜的产量和吸氮量,在75和150kg/hm2氮肥水平下,氮肥利用率分别为46.8%和39.4%。由于试验地土壤pH低(5.38),各处理的氨挥发均很低且差异不大,施用氮肥没有增加氨挥发。试验地土壤反硝化损失和N2O排放量较高,分别为N4.34kg/hm2/sup和N2.65kg/hm2,施用氮肥没有增加反硝化损失和N2O排放,表明氮源不是反硝化作用的限制因子。在N75和150kg/hm2两个施氮水平下,氮素回收率分别为103%和91.3%,并且土壤残留氮主要累积在020cm土层,表明肥料氮损失很少,这与氨挥发、反硝化损失较低的结果相吻合。     

硝化抑制剂类型和剂量对不同类型土壤硝化抑制作用机理的研究

氮肥配施硝化抑制剂是提高氮肥利用率、减少活性氮损失和降低环境代价风险的有效措施。为探讨不同硝化抑制剂类型和剂量对不同类型土壤硝化作用的机理,采用室内土壤培养试验,对3种硝化抑制剂[双氰胺(DCD)、3,4-二甲基吡唑磷酸盐(DMPP)和2-氯-6-(三氯甲基)吡啶(NP)]设置不同剂量,研究其对我国不同区域典型土壤(红壤、水稻土、潮土)硝化过程无机氮含量、土壤pH值及土壤表观硝化率变化特征的影响。结果表明,与单施硫酸铵处理相比,3种硝化抑制剂均能抑制水稻土和潮土中铵态氮向硝态氮的转化,且对潮土铵态氮向硝态氮转化抑制效果优于水稻土,而对红壤硝化作用的抑制效果均不明显,3种土壤pH值差异是影响硝化抑制剂作用效果的主因。此外,DCD和NP随着用量的增加,对水稻土和潮土的硝化抑制效果越明显,而DMPP对2种土壤硝化抑制作用无明显的剂量效应。在水稻土中,NP的抑制效果强于DMPP和DCD;在潮土中,DCD的抑制效果优于NP和DMPP,这可能是由于不同硝化抑制剂类型硝化抑制机理性的差异以及其自身特性的差异导致的。综上,针对特定土壤类型筛选适宜的硝化抑制剂类型和用量对农业优化氮肥管理、提高氮肥利用率尤为重要。     

日本氮素的循环和流向以及减少N2O散发和NO3-污染的对策

To feed an increasing population, large amounts of chemical nitrogen fertilizer have been used to produce much of our food, feed and fiber thereby increasing nitrogen levels in soils, natural waters, crop residues, livestock wastes, and municipal and agricultural wastes, with national and international concern about its potential adverse effects on environmental quality and public health. To understand these phenomena and problems, first the nitrogen cycle and the environment are described. Then recent trends for nitrogen cycling through the food and feed system, N₂O emissions from fertilized upland and paddy soils, and NO₃^- pollution in ground water in Japan are reported. Finally, mitigation strategies in Japan for reducing N₂O emission and NO₃^- pollution are proposed, including nitrification inhibitors, controlled release fertilizers, utilization of plant species that could suppress nitrification, utilizing the toposequence, government policy, and appropriate agricultural practices. Of all the technologies presented, use of nitrification inhibitors and controlled release fertilizers are deemed the most important with further development of these aspects of technologies being expected. These practices, if employed worldwide, could help reduce the load, or environmental deterioration, on the Earth''s biosphere.     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.     

肥料添加剂降低N2O排放的效果与机理

如何降低氮肥施入农田后的N2O排放,实现氮肥增产效应的同时降低其对环境的负面影响是全球集约化农业生产中重要的科学问题,氮肥添加剂是有效途径之一。本研究采用室内静态培养法,在调节土壤水分含量和温度等环境因素的条件下,研究不同肥料添加剂对华北平原典型农田土壤N2O排放的影响及其机制。结果表明,N2O排放通量的峰值大约出现在施氮后的第24 d,肥料混施较肥料表施的出峰时间提前。与单施尿素处理相比,添加硝化抑制剂DMPP或DCD能分别降低N2O排放总量99.2%和97.1%; 添加硫酸铜对N2O排放的抑制作用不显著; 添加秸秆会增加N2O排放总量60.7%,而在添加秸秆的土壤中施加硝化抑制剂DMPP能够显著降低N2O排放量至无肥对照水平。说明华北平原农田土壤中N2O的产生主要是由硝化作用驱动,同时也可看出,添加硝化抑制剂是N2O减排的有效措施。     

Effects of application of lime nitrogen and dicyandiamide on nitrous oxide emissions from green tea fields

Abstract

The aim of this study was to assess the mitigating effects of lime nitrogen (calcium cyanamide) and dicyandiamide (DCD) application on nitrous oxide (N₂O) emissions from fields of green tea [Camellia sinensis (L.) Kuntze]. The study was conducted in experimental tea fields in which the fertilizer application rate was 544 kg nitrogen (N) ha^?1 yr^?1 for 2 years. The mean cumulative N₂O flux from the soil between the canopies of tea plants for 2 years was 7.1 ± 0.9 kg N ha^?1 yr^?1 in control plots. The cumulative N₂O flux in the plots supplemented with lime nitrogen was 3.5 ± 0.1 kgN ha^?1, approximately 51% lower than that in control plots. This reduction was due to the inhibition of nitrification by DCD, which was produced from the lime nitrogen. In addition, the increase in soil pH by lime in the lime nitrogen may also be another reason for the decreased N₂O emissions from soil in LN plots. Meanwhile, the cumulative N₂O flux in DCD plots was not significantly different from that in control plots. The seasonal variability in N₂O emissions in DCD plots differed from that in control plots and application of DCD sometimes increased N₂O emissions from tea field soil. The nitrification inhibition effect of lime nitrogen and DCD helped to delay nitrification of ammonium-nitrogen (NH₄⁺-N), leading to high NH₄⁺-N concentrations and a high ratio of NH₄⁺-N /nitrate-nitrogen (NO₃^–-N) in the soil. The inhibitors delayed the formation of NO₃^–-N in soil. N uptake by tea plants was almost the same among all three treatments.     

Potentials of nitrification inhibitors in modern N-fertilizer management

Modern agricultural practices require a new concept of N-fertilizer management in order to optimize N-utilization and avoid N-losses. Nitrification inhibitors or ?N-stabilizers”? fit very good into this conception. Dicyandiamide (DCD) is an efficient nitrification inhibitor and blocks the first step of nitrification for 1–3 months (depending on temperature). This effect is bacteriostatic (not bactericidal) and does not affect other (esp. C-heterotrophic) soil microorganisms (?biological activity”?). DCD is a non-toxic, water soluble compound and will be degraded to CO₂, NH₃ and H₂O without any residues. There are various possibilities to use DCD: addition to liquid manure temporarily prevents oxidation of ammonium nitrogen e.g. of slurry or waste water from potato starch production. In combination with inorganic fertilizers like ammonium sulfate or urea (with 10% of total-N) it enables the farmer to control NH₄-supply to crop plants in certain stages of growth and to gain certain operational advantages by less frequently split applications of N especially on sand and rendzina soils. Thus, the systematic use of nitrification inhibitors not only represents a progress in agricultural technique but also helps to substantially reduce risks concerning pollution of surface and ground waters that are sometimes inevitable consequences of agricultural production.     

Effective mitigation of nitrate leaching and nitrous oxide emissions in intensive vegetable production systems using a nitrification inhibitor,dicyandiamide

Purpose  

Vegetable production is one of the most intensive agricultural systems with high rates of nitrogen (N) fertilizer use and irrigation, conditions conducive for nitrate (NO₃⁻) leaching, and nitrous oxide (N₂O) emissions. The objective of this study was to determine the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in decreasing NO₃⁻ leaching and N₂O emissions in vegetable production systems.     

小麦苗期施入氮肥在土壤不同氮库的分配和去向

应用盆栽试验和15N标记技术研究了小麦苗期施入N肥后土壤不同N库的动态。结果表明 ,施肥后 28d ,作物所吸收的土壤N占总吸N量的 58.1% ,吸收的肥料N占 41.9%。作物对肥料N的利用率达到 55.3% ,N肥在土壤中的残留率为 24.3% ,损失率为 20.4%。施肥后短期以NH4+-4 N存在的肥料N占施N量的 50.5% ,随着硝化作用的进行和作物的吸收 ,土壤中的NH4+-N显著下降。NO3--N在第 7d达到高峰 ,表现为先升高后降低的趋势 ,说明施肥后在 7d以前有强烈的硝化作用发生。施肥后 2d ,以固定态铵存在的肥料N占 33.7% ,至 28d ,仅占施入N量的 2.4% ,说明前期固定的铵在作物生长后期又重新释放出来供作物吸收。在施肥后第 7d ,肥料N以微生物N存在的量占施肥量的 15.2% ;至 28d来自肥料N的微生物N也几乎被耗竭 ,仅占施N量的 2.4%。随作物生长 ,肥料N在各个土壤N库中的数量均显著下降。在其它N库几乎被耗竭的情况下 ,至施肥后 28d主要以有机N的形式残留。在不种作物的条件下 ,土壤N素的矿化量很低 ,作物的吸收作用导致土壤有机N库不断矿化 ,施入N肥后 ,土壤N素的矿化量增加 ,表现为明显的正激发效应     

Influence of two “nitrogen regulators”; on soil n transformations and movement

Abstract

Experiments were conducted to assess the potential influence of a commercial product, EXTEND, on nitrogen transformations and movement in a sandy soil. Neither nitrapyrin (a commercially‐available nitrification inhibitor) nor EXTEND significantly affected the rate of NH₄ ⁺‐N or NO₃ ^‐‐N movement through a column of soil treated with urea‐ammonium nitrate liquid fertilizer. Nitrapyrin effectively inhibited nitrification, but the nitrification rate in the EXTEND treated systems were the same as control.     

Conversion of forest to agricultural land affects the relative contribution of bacteria and fungi to nitrification in humid subtropical soils

Land-use and management practices can affect soil nitrification. However, nitrifying microorganisms responsible for specific nitrification process under different land-use soils remains unknown. Thus, we investigated the relative contribution of bacteria and fungi to specific soil nitrification in different land-use soils (coniferous forest, upland fields planted with corn and rice paddy) in humid subtropical region in China. ¹⁵N dilution technique in combination with selective biomass inhibitors and C₂H₂ inhibition method were used to estimate the relative contribution of bacteria and fungi to heterotrophic nitrification and autotrophic nitrification in the different land-use soils in humid subtropical region. The results showed that autotrophic nitrification was the predominant nitrification process in the two agricultural soils (upland and paddy), while the nitrate production was mainly from heterotrophic nitrification in the acid forest soil. In the upland soils, streptomycin reduced autotrophic nitrification by 94%, whereas cycloheximide had no effect on autotrophic nitrification, indicating that autotrophic nitrification was mainly driven by bacteria. However, the opposite was true in another agricultural soil (paddy), indicating that fungi contributed to the oxidation of NH₄⁺ to NO₃^?. In the acid forest soil, cycloheximide, but not streptomycin, inhibited heterotrophic nitrification, demonstrating that fungi controlled the heterotrophic nitrification. The conversion of forest to agricultural soils resulted in a shift from fungi-dominated heterotrophic nitrification to bacteria- or fungi-dominated autotrophic nitrification. Our results suggest that land-use and management practices, such as the application of N fertilizer and lime, the long-term waterflooding during rice growth, straw return after harvest, and cultivation could markedly influence the relative contribution of bacteria and fungi to specific soil nitrification processes.