Soil Nitrogen Turnover Research Articles

Integrated rather than organic farming history facilitates soil nitrogen turnover and N2O reduction in a green rye – silage maize cropping sequence

AbstractSoil gross mineral N production and consumption processes are crucial regulators of plant productivity and N loss from croplands. Substituting synthetic fertilizers by integrating legumes in cultivation systems is common in organic farming, but research on its long-term impact on dynamics of gross soil N transformation and associated environmental N loss is scarce. In particular, studies at a temporal resolution that allows for a mechanistic understanding of long-term effects of organic farming are missing. Therefore, we determined gross N turnover rates of ammonification, nitrification, and ammonium and nitrate immobilization at monthly temporal resolution during a full green rye-maize cropping sequence. Measurements were carried out at sites with same pedo-climatic background but organic farming (OF) and integrated farming (IF) history. During green rye growing, N turnover rates for OF and IF were low and not significantly different, likely owing to low temperatures. During silage maize growing, IF exhibited significantly higher average N turnover rates of 1.86, 4.46, and 5.57 mg N kg⁻1 dry soil d⁻1 for gross ammonification, ammonium immobilization, and nitrate immobilization, respectively, compared to OF values of 1.11, 1.80, and 2.90 mg N kg⁻1 dry soil d⁻1. The significantly higher N turnover rates were likely due to higher soil organic C, N and microbial biomass which result from different long-term management practices. Especially the increased immobilization potential on the IF site contributed to significantly lower area-scaled N₂O emissions (1.45 vs. 4.36 kg N ha⁻1) during periods of high nitrification. This shows that for low SOC soils, integrated farming history with high C return enhances soil N cycling and reduces the risk of N losses in the form of N2O emission.

Effects of tillage patterns and stover mulching on N2O production, nitrogen cycling genes and microbial dynamics in black soil

Stover-covered no-tillage (NT) is of great significance to the rational utilization of stover resources and improvement of cultivated land quality, and also has a profound impact on ensuring groundwater, food and ecosystem security. However, the effects of tillage patterns and stover mulching on soil nitrogen turnover remain elusive. Based on the long-term conservation tillage field experiment in the mollisol area of Northeast China since 2007, the shotgun metagenomic sequencing of soils and microcosm incubation were combined with physical and chemical analyses, alkyne inhibition analysis to elucidate the regulatory mechanisms of NT and stover mulching on the farmland soil nitrogen emissions and microbial nitrogen cycling genes. Compared with conventional tillage (CT), NT stover mulching significantly reduced the emission of N2O instead of CO2, especially when 33% mulching was adopted, and correspondingly the nitrate nitrogen of NT33 was higher than that of other mulching amounts. The stover mulching was associated with higher total nitrogen, soil organic carbon and pH. The abundance of AOB (ammonia-oxidizing bacteria)-amoA (ammonia monooxygenase subunit A) was substantially increased by stover mulching, while the abundance of denitrification genes was reduced in most cases. Under alkyne inhibition, the tillage mode, treatment time, gas condition and interactions between them noticeably influenced the N2O emission and nitrogen transformation. In CT, NT0 (no mulching) and NT100 (full mulching), the relative contribution of AOB to N2O production was markedly higher than that of ammonia oxidizing archaea. Different tillage modes were associated with distinct microbial community composition, albeit NT100 was closer to CT than to NT0. Compared with CT, the co-occurrence network of microbial communities was more complex in NT0 and NT100. Our findings suggest that maintaining a low-quantity stover mulching could regulate soil nitrogen turnover toward proficiently enhancing soil health and regenerative agriculture, and coping with global climate change.

Effect of substitution of chemical fertilizer by bioinoculants on plant performance and rhizospheric bacterial community: case study with Cajanus cajan.

Improper nutrient management is one of the major limitations linked with cultivation of Cajanus cajan. This calls for an urgent need for a promising alternative, employing both bioinoculants and chemical fertilizer. Present study attempted to understand the impact of bioinoculants {Azotobacter chroococcum, Bacillus megaterium, and Pseudomonas fluorescens (ABP)} as their mono-inoculations, triple-inoculation, and their combination with different doses of fertilizer on (a) plant parameters, (b) soil nitrogen (N) economy, (c) resident bacterial community, (d) genes and transcripts involved in N cycle, and to evaluate the extent to which fertilizer could be replaced by ABP without compromising on grain yield. Bradyrhizobium sp. was used in all the treatments (as it was recommended for C. cajan). Combined application of bioinoculants and 75% of recommended dose of fertilizer (RDF) led to 1.28-fold enhancement in grain yield as compared to RDF alone. Apart fromexerting a positive impact on grain yield, the combined application of ABP and fertilizer led to an improvement in soil fertility, and modified the culturablerhizospheric bacterial community involved in N cycle. Integrated use of bioinoculants and fertilizer led to better N substrate utilization and hence, metabolic diversity when compared with application of fertilizer alone. An increase in the transcripts of nifH gene at the harvest stagein the soil treated with ABP alone and its combination with fertilizer, over individual treatment withfertilizer was observed. The combined use of ABP and fertilizer shaped the resident bacterial community towards a more beneficial community, which helped in increasing soil nitrogen turnover and hence, soil fertility as a whole.

Nitrate sources and the effect of land cover on the isotopic composition of nitrate in the catchment of the Rhône River

ABSTRACTThe Rhône River originates in the high Alps and drains an intensely cultivated and industrialised catchment before it discharges to the Gulf of Lion. We investigated the interaction of catchment geomorphology with nitrate sources (atmosphere, agriculture, and nitrification of soil organic matter) and removal processes in large and diverse watersheds on the basis of dual nitrate isotope signatures in river water.In March 2015, we took surface water samples along the Rhône River, including its main tributaries, and measured nutrient concentrations and the stable isotopic composition of nitrate (δ15N, δ18O and Δ17O), and water (δ18O-H2O).Results show that high altitude regions are dominated by nitrate from nitrification in pristine soils and atmospheric deposition, while nitrate in the downstream Rhône River originates mainly from nitrification of agricultural/urban sources. Parallel increases in δ15N and δ18O reflect the influence of primary production. Previous studies suggested robust correlations between land use and . Based on our observation that nitrate δ15N values at higher altitudes are lower than expected, we assume that lower nitrate δ15N values likely reflect limited nitrate consumption and lower soil nitrogen turnover rates. We propose that correlation between land use and nitrate δ15N is sensitive to slope and geomorphology.

重度放牧对欧亚温带草原东缘生态样带土壤氮矿化及其温度敏感性的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 重度放牧对欧亚温带草原东缘生态样带土壤氮矿化及其温度敏感性的影响 DOI: 10.5846/stxb201810082172 作者: 作者单位: 中国农业科学院草原研究所,中国农业科学院草原研究所,中国农业科学院草原研究所,中国农业科学院草原研究所,中国农业科学院草原研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点基础研究发展计划（973计划）项目（2014CB138806）；国家重点研发计划政府间国际科技创新合作重点专项（2016YFE0116400）；中央级公益性科研院所基本科研业务费专项（1610332016013） Effects of heavy grazing on soil nitrogen mineralization and temperature sensitivity along the Eastern Eurasia Steppe Transect Author: Affiliation: Grassland Research Institute，Chinese Academy of Agricultural Sciences,Grassland Research Institute，Chinese Academy of Agricultural Sciences,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:以欧亚温带草原东缘生态样带为平台，以样带上未放牧和重度放牧配对样地为研究对象，开展重度放牧对欧亚温带典型草原土壤氮矿化及其温度敏感性的影响研究。结果表明：（1）在室内培养条件下，土壤氮积累量和土壤净氮矿化速率呈现出干燥度指数较大的样点显著大于干燥度指数最低的样点（P < 0.05）。在相对湿润的样点，土壤氮素矿化周转速率较快；（2）重度放牧对不同样点土壤氮积累量和土壤氮矿化速率的影响是不同的。在干燥度指数较高样点，重度放牧样地土壤铵态氮减少量和速率较未放牧样地低（P < 0.05），重度放牧显著降低了土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率（P < 0.05）；在干燥度指数较低样点，重度放牧样地土壤铵态氮减少量和速率较未放牧样地高（P < 0.05），重度放牧对土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率影响不大（P > 0.05）；（3）土壤氮矿化作用温度敏感性（Q10）变化范围在1.61-2.06，重度放牧对Q10无显著影响。随着纬度的升高，Q10呈升高趋势。Q10与基质质量指数以及表观活化能与基质质量指数均呈显著的负相关关系（P < 0.05）；（4）土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率对重度放牧的响应比与干燥度指数呈极显著负相关（P < 0.01），重度放牧对欧亚温带典型草原土壤氮矿化的影响受气候条件（温度和降水）的调控。 Abstract:Effects of heavy grazing on soil nitrogen mineralization and temperature sensitivity in the Eastern Eurasian Steppe were studied using laboratory incubation methods by comparing non-grazing plots with heavy grazing plots at different sites. The results showed that accumulations of soil nitrogen and soil nitrogen mineralization rates were significantly higher in sites with a higher aridity index (AI) than sites with a lower AI (P < 0.05), which indicated that soil nitrogen turnover rates were high in relatively humid sites. In sites with a higher AI, the amount and rate of soil ammonium nitrogen reduction in heavy grazing plots were lower than that of non-grazing plots (P < 0.05), and heavy grazing significantly decreased accumulations of soil nitrate nitrogen (Anit), accumulations of soil inorganic nitrogen (Amin), soil nitrification rate (Rnit), and net nitrogen mineralization rate (Rmin) (P < 0.05). In sites with lower AI, the amount and rate of soil ammonium nitrogen reduction in heavy grazing plots were higher than that of non-grazing plots (P < 0.05), and heavy grazing had little effect on Anit, Amin, Rnit, and Rmin (P>0.05). The temperature sensitivity of net nitrogen mineralization (Q10) ranged from 1.61 to 2.06 and heavy grazing had little effect on Q10. Q10increased with increasing latitude and showed a significant negative correlation with the substrate quality index (P < 0.05). There was a significant negative correlation between substrate quality index and apparent activation energy (P < 0.05). Furthermore, the response ratios of Anit, Amin, Rnit, and Rmin to heavy grazing were significantly negatively correlated with AI (P < 0.01). Our results emphasized that the effects of heavy grazing on soil nitrogen mineralization in the Eastern Eurasian Steppe was regulated by climate (temperature and precipitation). 参考文献 相似文献 引证文献

Maize Stover Biochar Accelerated Urea Hydrolysis and Short-term Nitrogen Turnover in Soil

The study was carried to determine the labile carbon contents in biochar pyrolyzed under different temperatures and its effect on urea hydrolysis rate. Corn stovers were pyrolyzed at highest temperature of 300°C, 500°C and 700 °C to produce fresh biochar. Soil incubation experiment was conducted with biochar application rate of 0% and 2% with presence of urea. The results showed that biochars accelerated urea hydrolysis, and high temperature pyrolyzed biochar have more significant effect on soil pH enhances and acceleration of urea hydrolysis than biochar pyrolyzed at low temperature. Moreover, biochar produced at 300°C contains relative high concentration of labile carbon, and 43-64% of labile carbons were oxidized within 40 days incubation. The labile carbon in biochars also leaded microbes thrives and resulted in accelerate short-term N turnover. i.e. at early stage of incubation, fresh biochar increased mineralization of soil N by 79-449 mg•kg−1, and matured biochar by 30-61 mg•kg−1, but microbial immobilization effect was observed in fresh biochar-amended soil at the end of incubation. We concluded that aged biochar is suitable for simultaneous soil amendment with urea rather than newly produced biochar as it can promote available N accumulation in short time thus increase the risk of inorganic nitrogen leaching loss.

Net and Gross Nitrogen Turnover in Soil Amended with Acidified and Differently Dried Solids from Biogas Digestate

Core Ideas Acidification decreased the degradability of digestate solids. Acidification stabilized the gross N mineralization pattern with time. Dried acidified solids showed the highest potential N fertilizing value. Low drying temperature promoted net N mineralization from unacidified solids. The use of dried digestate solids from biogas plants as organic fertilizers may be disadvantaged by considerable N losses during thermal treatment. Acidification can decrease NH3 emissions during drying, but little is known about the degradability and N release pattern of acidified and dried digestate solids. In a laboratory incubation, C mineralization and gross and net N mineralization from digestate solids subjected to acidification (to pH 5.5) and drying (70 or 160°C) were monitored for 160 d at 15°C. Gross N mineralization rates were determined using the 15N isotopic pool dilution technique, and gross N transformation rates were calculated with the FLUAZ model. Irrespective of further treatment, acidified solids showed up to 50% reduced cumulative CO2–C respiration throughout the incubation and consequently decreased gross N mineralization rates compared with unacidified solids. Moreover, acidification of digestate solids delayed the nitrification dynamics during the first 3 wk of incubation. Throughout the experimental period, the acidified dried digestate solids had the highest inorganic N contents among all treatments. It was evident that acidification increased the stability of the organic matter and enhanced the availability of N on application to soil.

Gross Nitrogen Turnover of Natural and Managed Tropical Ecosystems at Mt. Kilimanjaro, Tanzania

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH4 + immobilization 38.92, NO3 − immobilization 10.74, and DNRA 1.54 µg N g−1 SDW d−1) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups.

Climate change amplifies gross nitrogen turnover in montane grasslands of Central Europe in both summer and winter seasons.

The carbon- and nitrogen-rich soils of montane grasslands are exposed to above-average warming and to altered precipitation patterns as a result of global change. To investigate the consequences of climatic change for soil nitrogen turnover, we translocated intact plant-soil mesocosms along an elevational gradient, resulting in an increase of the mean annual temperature by approx. 2°C while decreasing precipitation from approx. 1500 to 1000mm. Following three years of equilibration, we monitored the dynamics of gross nitrogen turnover and ammonia-oxidizing bacteria (AOB) and archaea (AOA) in soils over an entire year. Gross nitrogen turnover and gene levels of AOB and AOA showed pronounced seasonal dynamics. Both summer and winter periods equally contributed to cumulative annual N turnover. However, highest gross N turnover and abundance of ammonia oxidizers were observed in frozen soil of the climate change site, likely due to physical liberation of organic substrates and their rapid turnover in the unfrozen soil water film. This effect was not observed at the control site, where soil freezing did not occur due to a significant insulating snowpack. Climate change conditions accelerated gross nitrogen mineralization by 250% on average. Increased N mineralization significantly stimulated gross nitrification by AOB rather than by AOA. However, climate change impacts were restricted to the 2-6cm topsoil and rarely occurred at 12-16cm depth, where generally much lower N turnover was observed. Our study shows that significant mineralization pulses occur under changing climate, which is likely to result in soil organic matter losses with their associated negative impacts on key soil functions. We also show that N cycling processes in frozen soil can be hot moments for N turnover and thus are of paramount importance for understanding seasonal patterns, annual sum of N turnover and possible climate change feedbacks.

Different Land Use Intensities in Grassland Ecosystems Drive Ecology of Microbial Communities Involved in Nitrogen Turnover in Soil

Understanding factors driving the ecology of N cycling microbial communities is of central importance for sustainable land use. In this study we report changes of abundance of denitrifiers, nitrifiers and nitrogen-fixing microorganisms (based on qPCR data for selected functional genes) in response to different land use intensity levels and the consequences for potential turnover rates. We investigated selected grassland sites being comparable with respect to soil type and climatic conditions, which have been continuously treated for many years as intensely used meadows (IM), intensely used mown pastures (IP) and extensively used pastures (EP), respectively. The obtained data were linked to above ground biodiversity pattern as well as water extractable fractions of nitrogen and carbon in soil. Shifts in land use intensity changed plant community composition from systems dominated by s-strategists in extensive managed grasslands to c-strategist dominated communities in intensive managed grasslands. Along the different types of land use intensity, the availability of inorganic nitrogen regulated the abundance of bacterial and archaeal ammonia oxidizers. In contrast, the amount of dissolved organic nitrogen determined the abundance of denitrifiers (nirS and nirK). The high abundance of nifH carrying bacteria at intensive managed sites gave evidence that the amounts of substrates as energy source outcompete the high availability of inorganic nitrogen in these sites. Overall, we revealed that abundance and function of microorganisms involved in key processes of inorganic N cycling (nitrification, denitrification and N fixation) might be independently regulated by different abiotic and biotic factors in response to land use intensity.

Persistence of the Fluoroquinolone Antibiotic Difloxacin in Soil and Lacking Effects on Nitrogen Turnover

The environmental risks caused by the use of fluoroquinolone antibiotics in human therapeutics and animal husbandry are associated with their persistence and (bio)accessibility in soil. To assess these aspects, we administered difloxacin to pigs and applied the contaminated manure to soil. We then evaluated the dissipation and sequestration of difloxacin in soil in the absence and presence of plants within a laboratory trial, a mesocosm trial, and a field trial. A sequential extraction yielded antibiotic fractions of differing binding strength. We also assessed the antibiotic's effects on nitrogen turnover in soil (potential nitrification and denitrification). Difloxacin was hardly (bio)accessible and was very persistent under all conditions studied (dissipation half-life in bulk soil, >217 d), rapidly forming nonextractable residues. Although varying environmental conditions did not affect persistence, dissipation was accelerated in soil surrounding plant roots. Effects on nitrogen turnover were limited due to the compound's strong binding and small (bio)accessibility despite its persistence.

Biogeochemical and ecological feedbacks in grassland responses to warming

Feedbacks can modulate the way plants respond to warming, but difficulties in detecting long-acting feedbacks complicate understanding of the climatic effects on community structure and function beyond initial responses. Now a mesocosm experiment shows that although warming initially increased aboveground net primary productivity in grassland ecosystems, the response diminished progressively over time. Plant growth often responds rapidly to experimentally simulated climate change1,2. Feedbacks can modulate the initial responses3, but these feedbacks are difficult to detect when they operate on long timescales4. We transplanted intact plant–soil mesocosms down an elevation gradient to expose them to a warmer climate and used collectors and interceptors to simulate changes in precipitation. Here, we show that warming initially increased aboveground net primary productivity in four grassland ecosystems, but the response diminished progressively over nine years. Warming altered the plant community, causing encroachment by species typical of warmer environments and loss of species from the native environment—trends associated with the declining response of plant productivity. Warming stimulated soil nitrogen turnover, which dampened but did not reverse the temporal decline in the productivity response. Warming also enhanced N losses, which may have weakened the expected biogeochemical feedback where warming stimulates N mineralization and plant growth1,5,6. Our results, describing the responses of four ecosystems to nearly a decade of simulated climate change, indicate that short-term experiments are insufficient to capture the temporal variability and trend of ecosystem responses to environmental change and their modulation through biogeochemical and ecological feedbacks.

Nitrogen turnover in soil and global change

Nitrogen management in soils has been considered as key to the sustainable use of terrestrial ecosystems and a protection of major ecosystem services. However, the microorganisms driving processes like nitrification, denitrification, N-fixation and mineralization are highly influenced by changing climatic conditions, intensification of agriculture and the application of new chemicals to a so far unknown extent. In this review, the current knowledge concerning the influence of selected scenarios of global change on the abundance, diversity and activity of microorganisms involved in nitrogen turnover, notably in agricultural and grassland soils, is summarized and linked to the corresponding processes. In this context, data are presented on nitrogen-cycling processes and the corresponding microbial key players during ecosystem development and changes in functional diversity patterns during shifts in land use. Furthermore, the impact of increased temperature, carbon dioxide and changes in precipitation regimes on microbial nitrogen turnover is discussed. Finally, some examples of the effects of pesticides and antibiotics after application to soil for selected processes of nitrogen transformation are also shown.

Modelling of microbial carbon and nitrogen turnover in soil with special emphasis on N-trace gases emission

We present a new model unifying state-of-the-art descriptions of microbial processes for denitrification, nitrification and decomposition of soil organic matter. The model is of medium complexity, filling a gap between simplistic model approaches with low predictive power and complex models, which are difficult to verify experimentally. The model Microbial Carbon and Nitrogen Turnover in soils (MiCNiT) is written in Ansi C++ and embedded into a modelling framework (MoBiLE) that provides initial conditions and accompanying ecosystem processes such as N uptake by plants, litterfall, soil water and soil temperature with established model approaches. The MiCNiT model explicitly calculates decomposition, dynamics of microbial biomass, denitrification, autotrophic and heterotrophic nitrification, applying the microbial activity concept, as well as transport of gases and solutes between anaerobic and aerobic soil fractions and through the soil profile. The model was tested against N2O and CO2 emission as well as C and N pool data from the Hoglwald Forest, Germany. Due to a detailed description of the soil biochemistry and gaseous transfers, MiCNiT is capable of simulating soil air NO, N2O and N2 concentrations and the net exchange of these gases at the soil-atmosphere interface, including a possible net uptake of N2O by soils.

Isolation, genetic and functional characterization of novel soil nirK-type denitrifiers

Denitrification, the reduction of nitrogen oxides (NO 3 − and NO 2 −) to N 2 via the intermediates NO and N 2O, is crucial for nitrogen turnover in soils. Cultivation-independent approaches that applied nitrite reductase genes ( nirK/ nirS) as marker genes to detect denitrifiers showed a predominance of genes presumably derived from as yet uncultured organisms. However, the phylogenetic affiliation of these organisms remains unresolved since the ability to denitrify is widespread among phylogenetically unrelated organisms. In this study, denitrifiers were cultured using a strategy to generally enrich soil microorganisms. Of 490 colonies screened, eight nirK-containing isolates were phylogenetically identified (16S rRNA genes) as members of the Rhizobiales. A nirK gene related to a large cluster of sequences from uncultured bacteria mainly retrieved from soil was found in three isolates classified as Bradyrhizobium sp. Additional isolates were classified as Bradyrhizobium japonicum and Bosea sp. that contained nirK genes also closely related to the nirK from these strains. These isolates denitrified, albeit with different efficiencies. In Devosia sp., nirK was the only denitrification gene detected. Two Mesorhizobium sp. isolates contained a nirK gene also related to nirK from cultured Mesorhizobia and uncultured soil bacteria but no gene encoding nitric oxide or nitrous oxide reductase. These isolates accumulated NO under nitrate-reducing conditions without growth, presumably due to the lethal effects of NO. This showed the presence of a functional nitrite reductase but lack of a nitric oxide reductase. In summary, similar nirK genotypes recurrently detected mainly in soils likely originated from Rhizobia, and functional differences were presumably strain-dependent.

Effects of platinum from vehicle exhaust catalyst on carbon and nitrogen mineralization in soils

There is strong evidence of continuously increasing contamination of soils with platinum group elements (PGE), in particular with platinum (Pt) from vehicle exhaust catalysts in roadside soils. However, knowledge about the effects of Pt contamination on soil processes is very limited. The objective of this study was to investigate whether the contamination of soils with Pt at realistic environmental levels leads to observable adverse effects on selected indicator parameters of the carbon and nitrogen turnover in soils. Incubation experiments with artificially contaminated soils and solutions containing dissolved organic matter (DOM) were carried out by the use of milled material from a Pt-containing vehicle exhaust catalyst. Interaction of the catalyst material with the soil resulted in a mobilization of Pt into the dissolved phase reaching up to 0.1% of the added Pt. The amount of Pt mobilization seemed to be mainly driven by the pH of the soil. Mineralization of carbon and nitrogen did not reveal any significant adverse effect of the Pt addition as compared to the control samples. Future studies dealing with Pt effects on soil processes should focus on environmental conditions favoring Pt mobilization, e.g. such as very low pH values or large concentrations of DOM.

Microbial nitrogen turnover in soils under different types of natural forest

Microbial nitrogen (N) turnover was compared among 12 forest stands with a natural vegetation composition. They comprised six typical forest types in the eastern part of Austria, including oak and beech forests, spruce–fir–beech forests, floodplain and pine forests. The aim was to provide a reference basis for measurements in managed, especially disturbed or damaged forest ecosystems. We measured total C- and N-concentrations in soil, extractable NO3− and NH4+ concentrations, microbial biomass N, nitrogen mineralisation potential and potential N2O production in soil. As indicators of NH4+ availability and NO3− translocation into deeper soil horizons, accumulation of NH4+ and NO3− on resin bags was evaluated. Differences between the forest stands in potential N turnover were related to the size of the total N stores in soil and to vegetation composition. N mineralisation potential and microbial biomass were generally higher in the spruce–fir–beech, floodplain and pine forests with large N pools than in the oak and beech forests with lower N pools. Fastest N turnover was found in soils of a virgin spruce–fir–beech forest situated on nutrient-rich bedrock. Mineralisation potentials in these soils doubled averages in the other soils. N2O production rates measured by an intact soil core-incubation method varied widely among the forest stands and were highest (up to 170 μg N2O-N m−2 h−1) in soils of an oak–hornbeam and a woodruff–beech forest situated close to the city of Vienna, probably as a result of high atmospheric N input. Within the study sites, the soil moisture content was the factor mainly responsible for seasonal and microscale variability in N2O production rates.

Microbial nitrogen turnover in soils under different types of natural forest

Microbial nitrogen (N) turnover was compared among 12 forest stands with a natural vegetation composition. They comprised six typical forest types in the eastern part of Austria, including oak and beech forests, spruce–fir–beech forests, floodplain and pine forests. The aim was to provide a reference basis for measurements in managed, especially disturbed or damaged forest ecosystems. We measured total C- and N-concentrations in soil, extractable NO 3 − and NH 4 + concentrations, microbial biomass N, nitrogen mineralisation potential and potential N 2O production in soil. As indicators of NH 4 + availability and NO 3 − translocation into deeper soil horizons, accumulation of NH 4 + and NO 3 − on resin bags was evaluated. Differences between the forest stands in potential N turnover were related to the size of the total N stores in soil and to vegetation composition. N mineralisation potential and microbial biomass were generally higher in the spruce–fir–beech, floodplain and pine forests with large N pools than in the oak and beech forests with lower N pools. Fastest N turnover was found in soils of a virgin spruce–fir–beech forest situated on nutrient-rich bedrock. Mineralisation potentials in these soils doubled averages in the other soils. N 2O production rates measured by an intact soil core-incubation method varied widely among the forest stands and were highest (up to 170 μg N 2O-N m −2 h −1) in soils of an oak–hornbeam and a woodruff–beech forest situated close to the city of Vienna, probably as a result of high atmospheric N input. Within the study sites, the soil moisture content was the factor mainly responsible for seasonal and microscale variability in N 2O production rates.

Testing the adequacy of measured data for evaluating nitrogen turnover models by the dot‐to‐dot method

SummaryDynamic models of nitrogen turnover in soil are being used increasingly in agricultural science. To be of value, a model must be thoroughly evaluated, and the expected accuracy of simulated values must be defined. Frequent field measurements are time‐consuming and costly, and so models are often evaluated against only few data. However, this mismatch between the measured and simulated values can result in error statistics that are themselves subject to large errors and therefore unreliable. The dot‐to‐dot method quantifies the error associated with having too few measured values by interpolating linearly between measured data (hence, dot‐to‐dot) and evaluating the simulation at each time step using interpolated data where actual measurements are unavailable. A large error will be seen if the measured data do not capture the fluctuations predicted by the model. Other quantitative methods can be used to evaluate the performance of a model, but the dot‐to‐dot method can be used in conjunction with these to estimate whether or not the data are adequate for testing the model. If the performance of the model at the measured points is within the acceptable error, then the dot‐to‐dot method is used to judge whether there are too few points to evaluate the model's performance, and so to determine whether the evaluation of the model is valid.

When is a measured soil organic matter fraction equivalent to a model pool?

SummaryDynamic simulation models are important tools for rationalizing complex changes in soil organic matter. Most such models for organic matter can be described as having either a pool structure or a continuous one. A pool structure can offer advantages in ease of use and transferability. Some pools are easily measured, whereas others cannot be measured directly. New methods of fractionation are being developed in an attempt to base models on measurable fractions. A requirement for such models is a demonstration that the measured fraction and model pool are equivalent. A measured fraction is equivalent to a model pool only if, within acceptable limits, it is unique as well as non‐composite. If the measured fraction is not unique, describing it as a separate pool adds no extra information, while the added complexity can increase propagation of errors. If it is composite then the characteristics of the fraction will change with changing environment as a result of changing proportions of subpools. This will produce a model that cannot be applied without deriving parameters afresh: such a model is of greatly reduced value. Here we develop methods to examine if a fraction is both unique and non‐composite. The tests for unique and composite pools were applied to the SUNDIAL (simulation of nitrogen dynamics in arable land) model of organic matter and nitrogen turnover in soil. Results suggest that the debris, biomass and humus pools are unique, but biomass and humus are composed of two or more subpools. This worked example illustrates how, given suitable data, any pool‐based model can be tested by these methods.