Nitrogen transformation rates are affected by cover soil type but not coarse woody debris application in reclaimed oil sands soils

Forest floor mineral soil mix (FMM) and peat mineral soil mix (PMM) are cover soils commonly used for reclamation of open‐pit oil sands mining disturbed land in northern Alberta, Canada; coarse woody debris (CWD) is another source of organic matter for land reclamation. We investigated net nitrogen (N) transformation rates in FMM and PMM cover soils near and away from CWD 4–6 years after oil sands reclamation. Monthly net nitrification and N mineralization rates varied over time; however, mean rates across the incubation periods and microbial biomass were greater (p < 0.05) in FMM than in PMM. Net N mineralization rates were positively related to soil temperature (p < 0.001) and microbial biomass carbon (p = 0.045). Net N transformation rates and inorganic N concentrations were not affected by CWD; however, the greater ¹⁵N isotope ratio of ammonium near CWD than away from CWD indicates that CWD application increased both gross N mineralization/nitrification (causing N isotope fractionation) and gross N immobilization (no isotopic fractionation). Microbial biomass was greater near CWD than away from CWD, indicating the greater potential for N immobilization near CWD. We conclude that (1) CWD application affected soil microbial properties and would create spatial variability and diverse microsites and (2) cover soil type and CWD application had differential effects on net N transformation rates. Applying FMM with CWD for oil sands reclamation is recommended to increase N availability and microsites.     

Gross mineralization and plant N uptake from animal manures under non-N limiting conditions, measured using 15N isotope dilution techniques

To utilise wisely the manure resource, a better understanding of the processes that control the breakdown of organic N to inorganic N (mineralization) is required. ¹⁵N isotope dilution techniques should allow estimates of plant N uptake and gross mineralization from organic manures under non-N limiting conditions to be made. In natural systems the study of organic nitrogen breakdown to inorganic nitrogen, mineralization, is confounded by the processes of nitrification, nitrate leaching, gaseous N losses and plant N uptake. The ¹⁵N isotope dilution approach allows measurement of gross mineralization independently of these processes. Greenhouse experiments were conducted to determine plant N uptake from organic manures under non-N limiting conditions using the soil pre-labelling isotope dilution approach. The soil was pre-labelled with ¹⁵N and maize plants were then grown on the control treatments (no organic amendment) or on the manure treatments. The principle is thus that the control crop has a ¹⁵N abundance which reflects the ¹⁵N status of the soil and the treatment crop has a ¹⁵N enrichment diluted by the contribution of mineralized unlabelled manure N. Using this technique, it was estimated that maize plants derived 17 and 34% of their N from sewage sludge and turkey manure, respectively. The soil pre-labelling isotope dilution approach allowed yield-independent estimation of nitrogen derived from manures under non-N limiting conditions. Estimates of gross N mineralization were made to determine the breakdown of manure under field conditions. Results suggested that there was a rapid mineralization of turkey manure N in the initial weeks after application, in the order of 50 kg N ha^?1, which tailed off in the following weeks. The technique suggested that the soil used in the study had an extremely low basal mineralization rate, and a high nitrification rate.     

Soil N forms and gross transformation rates in Chinese subtropical forests dominated by different tree species

Background and aims

Knowledge related to extent of differing soil N forms and N transformation rates in subtropical southern China is severely limited. Accordingly, the purpose of this study was to investigate if and how tree species of different foliage types (coniferous, deciduous, and evergreen broadleaved) influence N forms and microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) content as well as gross N transformation rates in the organic and mineral soils of three distinct subtropical forests in China.

Methods

Chloroform fumigation extraction was used to determine MBC and MBN content while ¹⁵N-isotope dilution techniques were used to measure gross N transformation rates. Canonical correspondence analysis (CCA) was used to quantify relationships between soil chemical characteristics and changes in soil N transformation rates.

Results

Soil N forms, MBC and MBN content, and N transformation rates were found to be significantly different between tree species. Deciduous forest soil exhibited the highest N transformation rates. Soil N transformation rates were closely associated with total soil C and N and MBC and MBN content.

Conclusions

Soil substrate quantity and soil microbial activity play a more important role in soil N transformation processes than does soil quality in China’s subtropical forests. Tree species type should therefore be taken into account when trying to determine ecosystem N cycling.     

Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]

Elevated atmospheric carbon dioxide concentrations [CO₂] is projected to increase forest production, which could increase ecosystem carbon (C) storage. This study contributes to our broad goal of understanding the causes and consequences of increased fine‐root production and mortality under elevated [CO₂] by examining potential gross nitrogen (N) cycling rates throughout the soil profile. Our study was conducted in a CO₂‐enriched sweetgum (Liquidambar styraciflua L.) plantation in Oak Ridge, TN, USA. We used ¹⁵N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm. Our objectives were twofold: (1) to determine whether N is available for root acquisition in deeper soil and (2) to determine whether elevated [CO₂], which has increased inputs of labile C resulting from greater fine‐root mortality at depth, has altered N cycling rates. Although gross N fluxes declined with soil depth, we found that N is potentially available for roots to access, especially below 15 cm depth where rates of microbial consumption of mineral N were reduced relative to production. Overall, up to 60% of potential gross N mineralization and 100% of potential net N mineralization occurred below 15 cm depth at this site. This finding was supported by in situ measurements from ion‐exchange resins, where total inorganic N availability at 55 cm depth was equal to or greater than N availability at 15 cm depth. While it is likely that trees grown under elevated [CO₂] are accessing a larger pool of inorganic N by mining deeper soil, we found no effect of elevated [CO₂] on potential gross or net N cycling rates. Thus, increased root exploration of the soil volume under elevated [CO₂] may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO₂.     

In situ gross nitrogen transformations differ between temperate deciduous and coniferous forest soils

Despite long-term enhanced nitrogen (N) inputs, forests can retain considerable amounts of N. While rates of N inputs via throughfall and N leaching are increased in coniferous stands relative to deciduous stands at comparable sites, N leaching below coniferous stands is disproportionally enhanced relative to the N input. A better understanding of factors affecting N retention is needed to assess the impact of changing N deposition on N cycling and N loss of forests. Therefore, gross N transformation pathways were quantified in undisturbed well-drained sandy soils of adjacent equal-aged deciduous (pedunculate oak (Quercus robur L.)) and coniferous (Scots pine (Pinus sylvestris L.)) planted forest stands located in a region with high N deposition (north Belgium). In situ inorganic ¹⁵N labelling of the mineral topsoil (0–10?cm) combined with numerical data analysis demonstrated that (i) all gross N transformations differed significantly (p?<?0.05) between the two forest soils, (ii) gross N mineralization in the pine soil was less than half the rate in the oak soil, (iii) meaningful N immobilization was only observed for ammonium, (iv) nitrate production via oxidation of organic N occurred three times faster in the pine soil while ammonium oxidation was similar in both soils, and (v) dissimilatory nitrate reduction to ammonium was detected in both soils but was higher in the oak soil. We conclude that the higher gross nitrification (including oxidation of organic N) in the pine soil compared to the oak soil, combined with negligible nitrate immobilization, is in line with the observed higher nitrate leaching under the pine forest.     

氮稳定同位素技术在陆地生态系统氮循环研究中的应用

在过去几十年中, 氮(N)稳定同位素技术的发展提高了人们对于陆地生态系统氮循环的认识。该文回顾了氮稳定同位素技术在研究生态系统氮循环中的历史, 综述了最近十多年来氮稳定同位素技术在陆地生态系统氮循环研究中的典型案例, 包括利用氮同位素自然丰度鉴定植物氮来源、指示生态系统氮状态和量化过程速率, 利用¹⁵N标记技术示踪氮的去向和再分布等。该文同时指出这些应用中存在的问题, 以及在陆地生态系统上氮稳定同位素技术今后研究的重点发展方向。     

Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ¹⁵N) data, patterns of ¹⁵N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum ¹⁵N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The ¹⁵N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling ¹⁵N in soil profiles, fractionation against ¹⁵N during N transfer by mycorrhizal fungi to host plants, leads to ¹⁵N-depleted plant litter at the soil surface and ¹⁵N-enriched nitrogen of fungal origin at depth. The preferential preservation of ¹⁵N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large ¹⁵N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ¹⁵N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.     

The use of mean pool abundances to interpret 15N tracer experiments

Details are presented of a simple mathematical framework that allows ¹⁵N tracer experiments to be interpreted in terms of the main processes of the soil/plant nitrogen cycle. The calculations, all of which can be performed on a scientific calculator, yield the rates of gross mineralization and nitrification and the crop nitrogen uptake occurring as ammonium and nitrate. Two procedures are presented. One requires paired experiments with labelled ammonium and unlabelled nitrate as one treatment, and unlabelled ammonium and labelled nitrate as the other. The second procedure requires only the labelled ammonium, unlabelled nitrate treatment. Example calculations are presented using actual experimental data. The interpretative procedure uses the fact that the rate of isotopic dilution in an ammonium pool labelled with ¹⁵N is a function of the rate at which unlabelled ammonium is introduced into the pool via mineralization. Similarly, the rate of isotope dilution in an ¹⁵N labelled nitrate pool is a function of the rate at which unlabelled nitrate is introduced into the pool via nitrification.     

Permafrost nitrogen status and its determinants on the Tibetan Plateau

It had been suggested that permafrost thaw could promote frozen nitrogen (N) release and modify microbial N transformation rates, which might alter soil N availability and then regulate ecosystem functions. However, the current understanding of this issue is confined to limited observations in the Arctic permafrost region, without any systematic measurements in other permafrost regions. Based on a large‐scale field investigation along a 1,000 km transect and a laboratory incubation experiment with a ¹⁵N pool dilution approach, this study provides the comprehensive evaluation of the permafrost N status, including the available N content and related N transformation rates, across the Tibetan alpine permafrost region. In contrast to the prevailing view, our results showed that the Tibetan alpine permafrost had lower available N content and net N mineralization rate than the active layer. Moreover, the permafrost had lower gross rates of N mineralization, microbial immobilization and nitrification than the active layer. Our results also revealed that the dominant drivers of the gross N mineralization and microbial immobilization rates differed between the permafrost and the active layer, with these rates being determined by microbial properties in the permafrost while regulated by soil moisture in the active layer. In contrast, soil gross nitrification rate was consistently modulated by the soil content in both the permafrost and the active layer. Overall, patterns and drivers of permafrost N pools and transformation rates observed in this study offer new insights into the potential N release upon permafrost thaw and provide important clues for Earth system models to better predict permafrost biogeochemical cycles under a warming climate.     

Threshold responses of soil gross nitrogen transformation rates to aridity gradient

The responses of soil nitrogen (N) transformations to climate change are crucial for biome productivity prediction under global change. However, little is known about the responses of soil gross N transformation rates to drought gradient. Along an aridity gradient across the 2700 km transect of drylands on the Qinghai-Tibetan Plateau, this study measured three main soil gross N transformation rates in both topsoil (0–10 cm) and subsoil (20–30 cm) using the laboratorial ¹⁵N labeling. The relevant soil abiotic and biotic variables were also determined. The results showed that gross N mineralization and nitrification rates steeply decreased with increasing aridity when aridity was less than 0.5 but just slightly decreased with increasing aridity when aridity was larger than 0.5 at both soil layers. In topsoil, the decreases of the two gross rates were accompanied by the similar decreased patterns of soil total N content and microbial biomass carbon with increasing aridity (p < .05). In subsoil, although the decreased pattern of soil total N with increasing aridity was still similar to the decreases of the two gross rates (p < .05), microbial biomass carbon did not change (p > .05). Instead, bacteria and ammonia oxidizing archaea abundances decreased with increasing aridity when aridity was larger than 0.5 (p < .05). With an aridity threshold of 0.6, gross N immobilization rate increased with increasing aridity in wetter region (aridity < 0.6) accompanied with an increased bacteria/fungi ratio, but decreased with increasing aridity in drier region (aridity > 0.6) where mineral N and microbial biomass N also decreased at both soil layers (p < .05). This study provided new insight to understand the differential responses of soil N transformation to drought gradient. The threshold responses of the gross N transformation rates to aridity gradient should be noted in biogeochemical models to better predict N cycling and manage land in the context of global change.     

水分对林地和草地土壤氮初级转化速率的影响

水分含量是与土壤氮转化相关微生物活性的重要影响因素。本研究以黑龙江省北安市的草地和林地土壤为对象,通过室内培养试验,利用¹⁵N同位素标记技术和FLUAZ数值优化模型研究60%和100%田间持水量(WHC)条件下土壤氮初级矿化速率、初级固定速率、初级硝化速率和初级反硝化速率,以探讨土壤氮初级转化速率对水分含量变化的响应,阐明不同水分条件下土壤中氮的产生、消耗、保存机制及其生态环境效应。结果表明: 土壤水分变化不影响草地和林地土壤氮初级矿化速率和铵态氮固定速率,水分含量由60% WHC增加至100% WHC后显著增加了林地土壤的初级硝化速率,但对草地土壤的初级硝化速率没有显著影响。60% WHC条件下草地和林地土壤的初级反硝化速率可以忽略不计,水分含量增加至100% WHC后土壤初级反硝化速率显著提高,且草地土壤的初级反硝化速率显著低于林地土壤。100% WHC条件下林地土壤初级硝化速率与铵态氮固定速率比值(gn/ia)和N₂O排放量均显著高于60% WHC;100% WHC条件下草地土壤的N₂O排放量显著高于60% WHC,但两个水分条件下的gn/ia值无显著差异。表明短期内水分含量的增加可能会增加草地和林地土壤氮转化的负面环境效应,且对林地土壤的影响尤为显著。     

硝化抑制剂DCD、 DMPP对褐土氮总矿化速率和硝化速率的影响

采用15N库稀释-原位培养法研究了硝化抑制剂DCD、DMPP对华北盐碱性褐土氮总矿化速率和硝化速率的影响.试验在山西省运城市种植玉米的盐碱性土壤上进行,设单施尿素、尿素+DCD、尿素+DMPP 3个处理.结果表明:施肥后2周,DCD、DMPP分别使氮总矿化速率和氮总硝化速率减少了25.5％、7.3％和60.3％、59.1％,DCD对氮总矿化速率的影响显著高于DMPP,两者对氮总硝化速率的影响无显著差异;而在施肥后7周,不同硝化抑制剂对氮总硝化速率的影响存在差异.施肥后2周,3个处理的土壤氮总矿化速率和硝化速率分别是施肥前的7.2 ～10.0倍和5.5 ～21.5倍;NH4+和NO3-消耗速率分别是施肥前的9.1 ～12.2倍和5.1 ～8.4倍,这是由氮肥对土壤的激发效应所致.硝化抑制剂使氮肥更多地以NH4+形式保持在土壤中,减少了NO3-的积累.土壤氮总矿化速率和总硝化速率受硝化抑制剂的抑制是N2O减排的主要原因.     

硝化抑制剂DCD、DMPP对褐土氮总矿化速率和硝化速率的影响

采用¹⁵N库稀释-原位培养法研究了硝化抑制剂DCD、DMPP对华北盐碱性褐土氮总矿化速率和硝化速率的影响.试验在山西省运城市种植玉米的盐碱性土壤上进行,设单施尿素、尿素+DCD、尿素+DMPP 3个处理.结果表明：施肥后2周,DCD、DMPP分别使氮总矿化速率和氮总硝化速率减少了25.5%、7.3%和60.3%、59.1%,DCD对氮总矿化速率的影响显著高于DMPP,两者对氮总硝化速率的影响无显著差异;而在施肥后7周,不同硝化抑制剂对氮总硝化速率的影响存在差异.施肥后2周,3个处理的土壤氮总矿化速率和硝化速率分别是施肥前的7.2～10.0倍和5.5～21.5倍;NH₄⁺和NO₃^-消耗速率分别是施肥前的9.1～12.2倍和5.1～8.4倍,这是由氮肥对土壤的激发效应所致.硝化抑制剂使氮肥更多地以NH₄⁺形式保持在土壤中,减少了NO₃^-的积累.土壤氮总矿化速率和总硝化速率受硝化抑制剂的抑制是N₂O减排的主要原因.     

Relieving substrate limitation-soil moisture and temperature determine gross N transformation rates

A field experiment was designed with the objective to reveal the interactions between soil moisture, temperature, total, dissolved, and phosphate buffer extractable C and N, and microbial activity in the control of in situ gross N mineralization and immobilization rates in a deciduous forest. We had three alternative hypotheses to explain variations of the gross N transformations: 1) microorganisms are C limited, 2) microorganisms are N limited, or 3) neither C nor N limit the microorganisms but moisture and temperature conditions. Each hypothesis had specific criteria to be fulfilled for its acceptance. The results demonstrated that gross N transformation rates were more dependent on and variable with soil moisture and temperature than the size of the different C and N pools. The immobilization of N was dependent on the gross mineralization rate, suggesting that the production of enzymes for mineralization of organic N and the immobilization of N from the surrounding soil is disconfirmed when the intracellular N content of the microorganisms is sufficiently high. If the microorganisms are starved for N, enzyme systems involved in both the assimilation and mineralization of N are activated. The mean in situ gross N mineralization rate was two orders of magnitude higher than the natural N deposition in the area and the N addition in the NITREX experiments, meaning that a reduction in the gross N mineralization rate of about 1% would be enough to compensate for the addition of inorganic N. This decrease would hardly be detectable given the great spatial and temporal variability of N transformation rates.     

Estimation of field soil nitrogen mineralization and nitrification rates using soil N transformation parameters obtained through laboratory incubation

We tested the potential of estimating in-field (in situ) nitrogen (N) transformation rates based on soil temperature data and N transformation parameters (Q₁₀ and N transformation rates at standard temperature) obtained through laboratory incubations at three constant temperatures for 4 weeks. This test was conducted based on a comparison between in situ measurements and estimates using soils from 16 sites across 9 regions within the Japanese archipelago. The actual in situ N mineralization and nitrification rates measured using the buried-bag method at 0–50-cm-soil depth were 111 ± 34 and 106 ± 45 kg N ha^?1 year^?1, respectively, and estimates of both the rate and the amount were largely accurate. For rate alone, estimates were accurate in the 0–10-cm soil layer for annual and seasonal averages (except for spring–summer) whereas for amount alone, estimates were accurate to depths of 50 and 30 cm for N mineralization and nitrification, respectively. Thus, estimates of the rates and amounts were approximately equal to the actual in situ rate/amount, given the wide range of prediction intervals of the field measurement data. The differences between the estimates of N transformation rates derived from hourly measured and monthly average soil temperatures were negligible. Therefore, in situ soil N transformations, which are laborious to measure in the field, have the potential to be estimated from a combination of monthly average soil temperatures and N transformation parameters, which are relatively straightforward to obtain through laboratory incubation.     

三种利用类型羊草草地氮总矿化、硝化和无机氮消耗速率的比较研究

采用同位素15N库稀释技术研究了 3种不同利用类型羊草草地土壤氮的总矿化、硝化速率以及无机氮总消耗速率 ,3种类型草地分别为 :保护区 (无人为扰动 )、割草场、过度放牧地。结果表明 :4月份过度放牧场的总矿化速率最高 ,为2 1 .3μg N/ ( g土· d) ,7月份割草场的值最高 ,为 38.5μg N/ ( g土· d) ,9月份保护区最高 ,值为 1 5 .6μg N/ ( g土· d) ,总的来看 ,保护区的总矿化速率高于其它利用类型草地 ,这与土壤有机氮的含量较高有关 ,3种类型草地铵态氮的消耗速率与总矿化速率有类似的趋势。 3种利用类型草地的氮总矿化速率均以 7月份为最高 ,分别为 36 .5、38.5、2 9.8μg N/ ( g土· d)。总硝化速率放牧场最高 ,保护区、割草场、放牧场 7月份的总硝化速率分别为 1 8.6、2 1 .4 5、35 .4 5 μg N/ ( g土·d)。 3种利用类型草地中放牧场的硝态氮含量最高 ,其消耗的速率也高于其它两种利用类型草地     

Nitrogen transformations revealed by isotope dilution in an organically fertilized hybrid poplar plantation

Measuring nitrogen (N) transformations from organic fertilizers can help in selecting applications rates that provide sufficient soluble N to promote tree growth in short-rotation plantations. The objective of this study was to determine how organic fertilizers (papermill biosolids, liquid pig slurry) affected microbially-mediated N transformations in soils. Soil samples were collected from a hybrid poplar plantation before fertilization, 1 month after fertilizer application and at the end of the growing season. Net N mineralization and nitrification were evaluated during a 28 d laboratory incubation, while gross N transformations were assessed using a ¹⁵N isotope dilution technique. Pig slurry application increased soil ammonium (NH₄-N) and nitrate (NO₃-N) concentrations within 1 month, while papermill biosolids increased soil NH₄-N and NO₃-N concentrations at the end of the growing season. Gross N consumption rates were greater than gross N production rates. The NH₄-N and NO₃-N consumption rates were positively correlated with labile carbon and microbial biomass. The gross nitrification rate was 18 to 67% of the gross mineralization rate but 30% or less of the gross NH₄-N consumption rate, indicating that NH₄ consumption was overestimated by the isotope dilution technique. We conclude that N cycling in this hybrid poplar plantation was characterized by rapid consumption of plant-available N following N mineralization and nitrification.     

Evidence that Northern Pioneering Pines with Tuberculate Mycorrhizae are Unaffected by Varying Soil Nitrogen Levels

Tuberculate mycorrhizae on Pinus contorta (lodgepole pine) have previously been shown to reduce acetylene, but an outstanding question has been to what degree these structures could meet the nitrogen requirements of the tree. We compared the growth, tissue nitrogen contents, and stable nitrogen isotope ratios of P. contorta growing in gravel pits to the same species growing on adjacent intact soil. Trees growing in severely nitrogen deficient gravel pits had virtually identical growth rates and tissue nitrogen contents to those growing on intact soil that had nitrogen levels typical for the area. δ¹⁵N values for trees in the gravel pits were substantially lower than δ¹⁵N values for trees on intact soil, and isotope ratios in vegetation were lower than the isotope ratios of the soil. The form of soil nitrogen in the gravel pits was almost exclusively nitrate, while ammonium predominated in the intact soil. Discrimination against ¹⁵N during plant uptake of soil nitrate in the highly N-deficient soil should be weak or nonexistent. Therefore, the low δ¹⁵N in the gravel pit trees suggests that trees growing in gravel pits were using another nitrogen source in addition to the soil. Precipitation-borne nitrogen in the study area is extremely low. In conjunction with our other work, these findings strongly suggests that P. contorta and its microbial symbionts or associates fix nitrogen in sufficient amounts to sustain vigorous tree growth on the most nitrogen-deficient soils.     

15N自然丰度法在陆地生态系统氮循环研究中的应用

随着氮沉降的不断增加以及人们对全球变化问题的日益关注, 稳定同位素技术在全球变化研究中得到广泛的应用。因为植物和土壤的氮同位素组成记录了氮循环影响因子的综合作用, 并且具有测量简单以及不受取样时间和空间限制的优点, 所以氮同位素自然丰度法被用于氮循环的研究中。该文从氮循环过程中植物和土壤的氮分馏入手, 总结国内外相关文献, 阐述了植物和土壤氮自然丰度在预测生态系统氮饱和和氮循环长期变化趋势中的应用; 总结了利用树轮δ ¹⁵N法研究氮循环过程中应该注意的事项以及目前尚未解决的问题。     

Temporal evolution of biochar's impact on soil nitrogen processes – a 15N tracing study

Biochar addition to soils has been proposed as a means to increase soil fertility and carbon sequestration. However, its effect on soil nitrogen (N) cycling and N availability is poorly understood. To gain better insight into the temporal variability of the impact of biochar on gross soil N dynamics, two ¹⁵N tracing experiments, in combination with numerical data analysis, were conducted with soil from a biochar field trial, 1 day and 1 year after application of a woody biochar type. The results showed accelerated soil N cycling immediately following biochar addition, with increased gross N mineralization (+34%), nitrification (+13%) and ammonium (NH₄⁺) and nitrate (NO₃⁻) immobilization rates (+4500% and +511%, respectively). One year after biochar application, the biochar acted as an inert substance with respect to N cycling. In the short term, biochar's labile C fraction and a pH increase can explain stimulated microbial activity, while in the longer term, when the labile C fraction has been mineralized and the pH effect has faded, the accelerating effect of biochar on N cycling ceases. In conclusion, biochar accelerates soil N transformations in the short-term through stimulating soil microbial activity, thereby increasing N bio-availability. This effect is, however, temporary.