Gross N mineralization–immobilization rates in heterogeneous intact soil cores can be estimated without marked error

Gross rates of N mineralization–immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a ¹⁵NH₄⁺ pool dilution technique, under the assumption that native ¹⁴NH₄⁺, applied ¹⁴NH₄⁺ and microbial processes are uniformly distributed in the soil. In a laboratory investigation we compared gross N fluxes obtained from two labelling techniques applied to an arable sandy loam: (1) injection of ¹⁵NH₄⁺ solution into intact soil cores, and (2) mixing ¹⁵NH₄⁺ solution into disturbed soil. It was assumed that MIT obtained with the mixing technique reflected the true rates, since the assumption of uniformity was thought to be satisfied by this technique. MIT from the two techniques were not significantly different, thus non-uniform distribution of native ¹⁴NH₄⁺, injected ¹⁵NH₄⁺ and microbial processes in the intact-core technique did not cause a marked error in the MIT rates. In contrast the gross nitrification rates were twofold higher with the mixing technique than with the intact-core technique. Gross nitrification rates are likely to increase with the mixing technique because mixing redistributes nitrifiers and added ¹⁵NH₄⁺, and thereby increases the contact between NH₄⁺ and nitrifiers.     

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH₄⁺) and nitrate (NO₃⁻). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃⁻ assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH₄⁺ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO₃⁻ and NH₄⁺ was estimated based on the uptake of ¹⁵N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH₄⁺ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO₃⁻ assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO₃⁻ than NH₄⁺. Heterotrophic microbes assimilated less NH₄⁺ than NO₃⁻ probably because NH₄⁺ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH₄⁺ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO₃⁻ immobilization.     

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.     

A method to assess whether ‘preferential use’ occurs after N ammonium addition; implication for the N isotope dilution technique

The robustness of the assumption of equilibrium between native and added N during ¹⁵N isotope dilution has recently been questioned by Watson et al. (Soil Biol Biochem 32 (2000) 2019-2030). We re-analyzed their raw data using equations that consider the added and native NH₄⁺ and NO₃⁻ pools as separate state variables. Gross mineralization rates and first-order rate constants for NH₄⁺ and NO₃⁻ consumption were obtained by combining analytical integration of the differential equations with a non-linear fitting procedure. The first-order rate constants for NH₄⁺ consumption and NO₃⁻ immobilization for the added NH₄⁺ and NO₃⁻ pool were used to estimate gross mineralization rates and first-order rate constants for nitrification of native NH₄⁺. The latter were 2-4 times lower than the first-order rate constants derived from the added N pool. This discrepancy between first-order rate constants for nitrification implies that one or more process rates estimated for the added N pools cannot be applied to the native N pools. Preferential use of the added N resulted in an overestimation of the gross mineralization by 1.5-2.5-fold, emphasizing the need for critical evaluation of the assumption of equilibrium before gross mineralization rates are calculated.     

Mineralization of soil organic nitrogen solubilized by aqua ammonia fertilizer

Organic N solubilized by NH_3(aq) was extracted from ¹⁵N-labelled or unlabelled soil, concentrated and added to non-extracted soil, which was incubated under aerobic conditions at 27±1°C. Gross N mineralization, gross N immobilization, and nitrification in soils with or without addition of unlabelled soluble organic N were estimated by models based on the dilution of the NH ₄ ⁺ or NO _inf3 ^sup- pools, which were labelled with ¹⁵N at the beginning of incubation. Mineralization of labelled organic N was measured by the appearance of label in the mineral N pool. Although gross N mineralization and gross N immobilization were increased in two soils between day 0 and day 7 following addition of unlabelled organic N solubilized by NH_3(aq), there was no increase in net N mineralization. Solubilization of ¹⁵N-labelled organic N increased and the ¹⁵N enrichment of the soluble organic N decereased as the concentration of NH_3(aq) added increased. A constant proportion of approximately one-quarter of the labelled organic N added at different rates to non-extracted soil was recovered in the mineral N pool after an incubation period of 14 days, and the availability ratios calculated from net N mineralization data were 1.1:1 and 2.1:1 for 111 and 186 mg added organic-N kg^-1 soil, respectively, indicating that the mineralization of organic N was increased by solubilization.     

Influence of NH4-application on gross N turnover rates in soil

Determination of gross N mineralization rate in soil, by use of the isotopic pool dilution approach implies that ¹⁵NH₄⁺ is applied homogeneously to soil. Since the labeling is applied to the product of the mineralization, the application of ¹⁵NH₄⁺ should in theory not alter the mineralization rate. However, recent studies have indicated inverse relation between the amounts of ¹⁵NH₄⁺ applied and the determined gross N mineralization rates, due to overestimated rates when ‘low’ amounts of ¹⁵NH₄⁺ were added, as a result of preferential ¹⁵NH₄⁺ consumption. We present here results from a similar study. We observed no effect from the amount of applied NH₄⁺ on the measured gross N mineralization rates. Our results indicate, that the inverse relation as described earlier, probably was due to underestimated rates when ‘high’ amounts of ¹⁵NH₄⁺ were added, as a result of preferential ¹⁴NH₄⁺ consumption, when the applied ¹⁵NH₄⁺ was incomplete distribution in the soil.     

Effect of elevated CO2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO₂ is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO₂ Enrichment (FACE) for six years. A ¹⁵N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO₂ on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a ¹⁵N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH₄⁺ and from organic-N to NO₃⁻ and analysed the rate of dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO₂. At the same time the gross rate of NH₄⁺ immobilization increased by 20%, while NH₄⁺ oxidation to NO₃⁻ was reduced by 25% under elevated CO₂. The NO₃⁻ dynamics under elevated CO₂ were characterized by a 52% increase in NO₃⁻ immobilization and a 141% increase in the DNRA rate, while NO₃⁻ production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH₄⁺ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH₄⁺ under elevated CO₂. The advantage of such a shift is that NH₄⁺ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO₂.     

Soil nitrogen conservation mechanisms in a pristine south Chilean Nothofagus forest ecosystem

A ¹⁵N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a ¹⁵N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The ¹⁵N tracing model included five different N pools (ammonium (NH₄⁺), nitrate (NO₃⁻), labile (N_lab) and recalcitrant (N_rec) soil organic matter and adsorbed NH₄⁺), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of N_lab, while immobilization of NH₄⁺ predominantly entered the N_rec pool. A fast exchange between the NH₄⁺ and the adsorbed NH₄⁺ pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH₄⁺. Moreover, high NH₄⁺ immobilization rates into the N_rec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH₄⁺ oxidation and organic N oxidation. More than 86% of the NO₃⁻ produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO₃⁻ consumption. DNRA rather than NO₃⁻ immobilization ensures that NO₃⁻ is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.     

Measuring gross nitrogen mineralization, and nitrification by 15 N isotopic pool dilution in intact soil cores

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either ¹⁵NH₄⁺ or ¹⁵NO₄^?; gross mineralization rates were calculated from the decline in “N enrichment of the NH: pool during a 24-h incubation; gross nitrification rates were calculated from the decline in ¹⁵N enrichment of the NO^?₃ pool; gross rates of NH₄⁺ and NO₃^? consumption were calculated from disappearance of the ¹⁵N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added “N with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non-random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial ¹⁵N and ¹⁴N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added ¹⁵NH₄⁺ within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size estimates. Gross immobilization is probably stimulated by addition of inorganic ¹⁵N substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non-linear equation is given for calculating the gross immobilization rate from the appearance of ¹⁵N in chloroform-labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores.     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

Effects of annual grass senescence on NH4 and N-glycine uptake by blue oak (Quercus douglasii) seedlings and soil microorganisms in California oak woodland

Annual grasses are stronger competitors for available soil N than blue oak seedlings and soil microorganisms. However, little is known about the dynamics of N competition during annual grass senescence. We conducted a field experiment in a California oak woodland to study effects of annual grass senescence on N uptake by grasses, blue oak seedlings and soil microorganisms. Labeled N was applied at the beginning of April, May and of June in the form of ¹⁵NH₄⁺ or ¹⁵N-glycine. Plants and soils were harvested after 5 days (¹⁵NH₄⁺ and ¹⁵N-glycine treatments) and after 26 days (¹⁵NH₄⁺ treatment only). We evaluated effects of N form, season and labeling period on N competition among oak seedlings, annual grasses and soil microorganisms. N forms did not affect competition among grasses, oak seedlings and soil microorganisms, but more ¹⁵N was incorporated into the soil organic N pool in the ¹⁵N-glycine treatments than in the ¹⁵NH₄⁺ treatments. There were no seasonal (May vs June) effects on ¹⁵N recovery in blue oak seedlings and soil microorganisms. Plant samples from April harvest were lost. In June, when grasses were senescing, more ¹⁵N was found in the soil inorganic pool than in May. Extremely dry soils in June may have limited inorganic N availability to oak seedlings and soil microorganisms. After 26-day labeling period, ¹⁵N recovery in blue oak seedlings and the soil organic N pool significantly increased, while ¹⁵N recovery in both the soil microbial and inorganic N pools decreased compared to the 5-day labeling period. Although blue oak seedling biomass changed little from early May to late June, N concentrations in oak roots increased 53%. In contrast, annual grass biomass peaked in May, and then decreased rapidly. Our results suggest that blue oak seedlings and annual grasses have different temporal competitive abilities. Blue oak seedlings appear to have a long-term strategy for N competition. Blue oaks take up N slowly but steadily, increasing N uptake from 5 to 26 days. This extended time period has a greater positive effect on N uptake than does reduced grass uptake caused by senescence.     

Evidence of carbon stimulated N transformations in grassland soil after slurry application

High nitrification rates which convert ammonium (NH₄⁺) to the mobile ions NO₂⁻ and NO₃⁻ are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH₄⁺) or organic N. Selective nitrification inhibitors and ¹⁵N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH₄⁺-N and organic-N by labelling the NH₄⁺ or NO₃⁻ pools separately or both together with ¹⁵N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a ¹⁵N tracing model was developed. This model consists of the three N-pools NH₄⁺, NO₃⁻ and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH₄⁺ to NO₃⁻ and not the conversion of organic N to NO₃⁻. There was evidence that slurry-enhanced oxidation of NH₄⁺ to NO₃⁻ was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH₄⁺ by approximately a factor of three.     

Nitrogen Transformation as Influenced by Soil Microbial Nitrogen under Carbon Dioxide Enrichment Induced in Created Riparian Wetlands

Limited research has been conducted on how atmospheric carbon dioxide (CO₂) affects water and soil nitrogen (N) transformation in wetland ecosystems. A stable isotope technique is suitable for conducting a detailed investigation of mechanistic nutrient transformations. Nutrient ammonium sulfate (NH₄)₂SO₄ input in culture water under elevated CO₂ (700 μL L^?1) and ambient CO₂ (380 μL L^?1) was studied to analyze N transformations with N blanks for both water and soil. It was measured by ¹⁵N pool dilution using analytical equations in a riparian wetland during a 3-month period. Soil gross ammonium (NH₄ ⁺) mineralization and consumption rates increased significantly by 22% and 404%, Whereas those of water decreased greatly by??57% and??57% respectively in enriched CO₂. In contrast, gross nitrate (NO₃ ^?) consumption and nitrification rates of soil decreased by??11% and??14% and those of water increased by 29% and 27% respectively in enrichment CO₂. These may be due to the extremely high soil microbial biomass nitrogen (MBN), which increased by 94% in elevated soil. The results can show when CO₂ concentrations are going to rise in the future. Consequently soil microbial activity initiates the decreased N concentration in sediment and increased N concentration in overlying water in riparian wetland ecosystems.     

Ammonium immobilisation during chloroform fumigation

The assumption in using the chloroform fumigation technique for microbial biomass determination is that microbes are killed or at least inactivated by the treatment. Problems associated with transformations of the N released on or during fumigation have so far only been associated with the fumigation-incubation method. A laboratory and a field study were carried out to investigate the possible N transformations during biomass determination by the fumigation-extraction method. Labelled NH₄NO₃ (either the NO₃⁻, NH₄⁺ or both pools were ¹⁵N enriched) was applied to the soil and biomass determinations made at intervals subsequently. The size and enrichment of the ammonium (NH₄⁺), and nitrate (NO₃⁻) pools were determined before and after chloroform fumigation. The ¹⁵N enrichment of the NH₄⁺ pool after fumigation could only be explained if immobilisation of ammonium occurred at some time during the 24 h fumigation period. The extent of this immobilisation was calculated. In addition, there was evidence that nitrification occurred during the fumigation procedure at the start of the laboratory study and throughout the field study. The laboratory and field study differed mainly in the dynamics related to NO₃⁻ uptake and release. There was evidence for uptake of NO₃⁻ by the microbial biomass with and without utilization. We conclude that the ¹⁵N enrichment in the microbial biomass cannot be accurately determined when N transformations and release of non-utilized N occurs during fumigation. The possible immobilisation of mineral N during fumigation will affect the magnitude of the factor used to convert measured microbial biomass N to actual microbial biomass N in soil.     

Cold storage and laboratory incubation of intact soil cores do not reflect in-situ nitrogen cycling rates of tropical forest soils

Measurements of N transformation rates in tropical forest soils are commonly conducted in the laboratory from disturbed or intact soil cores. On four sites with Andisol soils under old-growth forests of Panama and Ecuador, we compared N transformation rates measured from laboratory incubation (at soil temperatures of the sites) of intact soil cores after a period of cold storage (at 5 °C) with measurements conducted in situ. Laboratory measurements from stored soil cores showed lower gross N mineralization and NH₄⁺ consumption rates and higher gross nitrification and NO₃⁻ immobilization rates than the in-situ measurements. We conclude that cold storage and laboratory incubation change the soils to such an extent that N cycling rates do not reflect field conditions. The only reliable way to measure N transformation rates of tropical forest soils is in-situ incubation and mineral N extraction in the field.     

Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years

Plant response to increasing atmospheric CO₂ partial pressure (pCO₂) depends on several factors, one of which is mineral nitrogen availability facilitated by the mineralisation of organic N. Gross rates of N mineralisation were examined in grassland soils exposed to ambient (36 Pa) and elevated (60 Pa) atmospheric pCO₂ for 7 years in the Swiss Free Air Carbon dioxide Enrichment experiment. It was hypothesized that increased below-ground translocation of photoassimilates at elevated pCO₂ would lead to an increase in immobilisation of N due to an excess supply of energy to the roots and rhizosphere. Intact soil cores were sampled from Lolium perenne and Trifolium repens swards in May and September, 2000. The rates of gross N mineralisation (m) and NH₄⁺ consumption (c) were determined using ¹⁵N isotopic dilution during a 51-h period of incubation. The rates of N immobilisation were estimated either as the difference between m and the net N mineralisation rate or as the amount of ¹⁵N released from the microbial biomass after chloroform fumigation. Soil samples from both swards showed that the rates of gross N mineralisation and NH₄⁺ consumption did not change significantly under elevated pCO₂. The lack of a significant effect of elevated pCO₂ on organic N turnover was consistent with the similar size of the microbial biomass and similar immobilisation of applied ¹⁵N in the microbial N pool under ambient and elevated pCO₂. Rates of m and c, and microbial ¹⁵N did not differ significantly between the two sward types although a weak (p<0.1) pCO₂ by sward interaction occurred. A significantly larger amount of NO₃⁻ was recovered at the end of the incubation in soil taken from T. repens swards compared to that from L. perenne swards. Eleven percent of the added ¹⁵N were recovered in the roots in the cores sampled under L. perenne, while only 5% were recovered in roots of T. repens. These results demonstrate that roots remained a considerable sink despite the shoots being cut at ground level prior to incubation and suggest that the calculation of N immobilisation from gross and net rates of mineralisation in soils with a high root biomass does not reflect the actual immobilisation of N in the microbial biomass. The results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated pCO₂. It is suggested that differences in plant N demand, rather than major changes in soil N mineralisation/immobilisation, are the long-term driving factors for N dynamics in these grassland systems.     

Technique of soil sample pretreatment for analysis of 15N:14N isotope ratio

Abstract

The technique of simultaneous quantitative determination of mineral N soil forms (nitrates, exchangeable and non‐exchangeable ammonium, and total amount of these compounds) and sample pretreatment for the analysis of ¹⁵N:¹⁴N ratio is suggested. The technique is based on the selective association of NH₄ ⁺‐ions into indophenol complex and subsequent ethyl‐acetate extraction of this complex from solution. The mineralization of indophenol is carried out in alkaline medium with simultaneous NH₃ distillation into H₂SO₄ titrant. The application of given technique allows us to shorten significantly the time required for analysis and to increase the accuracy of analytical determination.     

Presubmergence and green manure affect the transformations of nitrogen‐15‐labeled urea under lowland soil conditions

The effect of presubmergence and green manuring on various processes involved in [¹⁵N]‐urea transformations were studied in a growth chamber after [¹⁵N]‐urea application to floodwater. Presubmergence for 14 days increased urea hydrolysis rates and floodwater pH, resulting in higher NH₃ volatilization as compared to without presubmergence. Presubmergence also increased nitrification and subsequent denitrification but lower N assimilation by floodwater algae caused higher gaseous losses. Addition of green manure maintained higher NH₄⁺‐N concentration in floodwater mainly because of lower nitrification rates but resulted in highest NH₃ volatilization losses. Although green manure did not affect the KCl extractable NH₄⁺‐N from applied fertilizer, it maintained higher NH₄⁺‐N content due to its decomposition and increased mineralization of organic N. After 32 days about 36.9 % (T₁), 23.9 % (T₂), and 36.4 % (T₃) of the applied urea N was incorporated in the pool of soil organic N in treatments. It was evident that the presubmergence has effected the recovery of applied urea N.     

Pathways and dynamics of NO3 and NH4 applied in a mountain Picea abies forest and in a nearby meadow in central Switzerland

To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of ¹⁵N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce (Picea abies) and a nearby meadow, at the Alptal research site in central Switzerland. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha⁻¹ y⁻¹ equally divided between NH₄⁺ and NO₃⁻. Pulses of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were applied separately as tracers on plots of 2.25 m². Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground biomass (excluding trees), litter layer, soil LF horizon (approx. 5-0 cm), A horizon (approx. 0-5 cm) and gleyic B horizon (5-20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to 1 week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added ¹⁵NO₃⁻ was immobilised just after 1 h, and probably chemically bound to soil organic matter. 16% of the NH₄⁺ tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost ¹⁵N over time, a long-term increase in ¹⁵N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and 1 year later were surprisingly small. Overall, more NO₃⁻ tracer than NH₄⁺ tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH₄⁺ by the ground vegetation, especially by mosses.