DEVELOPMENT OF A SOIL N TEST FOR FERTILIZER REQUIREMENTS FOR WHEAT

Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO^? ₃)- N, soil ammonium (NH ⁺ ₄)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO^? ₃-N and NH ⁺ ₄-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH ⁺ ₄-N and NO^? ₃ -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO^? ₃-N.     

Soil inorganic nitrogen composition and plant functional type determine forage crops nitrogen uptake preference in the temperate cultivated grassland,Inner Mongolia

ABSTRACT

Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ ¹⁵N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO₃^?-N than NH₄⁺-N regardless of growth stages, and the NH₄⁺/NO₃^? uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH₄⁺-N rather than NO₃^?-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH₄⁺ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH₄⁺/NO₃^? uptake ratio was positively correlated with soil NH₄⁺/NO₃^? ratio. Our results suggest that the major forage crops prefer to absorb soil NO₃^?-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO₃^?-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.     

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.     

Leaching Methods Can Underestimate Mineralization Potential of Soils

Aerobic incubations to estimate net nitrogen (N) mineralization typically involve periodic leaching of soil with 0.01 M calcium chloride (CaCl₂), so as to remove mineral N that would otherwise be subject to immobilization. A study was conducted to evaluate the accuracy of leaching for analysis of exchangeable ammonium (NH₄⁺)-N and nitrate + nitrite (NO₃^?+ NO₂^–)-N, relative to conventional extractions using 2 M potassium chloride (KCl). Ten air-dried soils were used, five each from Illinois and Brazil, that had been amended with NH₄⁺-N (1 g kg^?1) and NO₃^–-N (0.6 g kg^?1). Both methods were in good agreement for inorganic N analysis of the Brazilian Oxisols, whereas leaching was significantly lower by 12–48% in recovering exchangeable NH₄⁺-N from Illinois Alfisols, Mollisols, and Histosols. The potential for underestimating net N mineralization was confirmed by a 12-wk incubation experiment showing 9–86% of mineral N recoveries from three temperate soils as exchangeable NH₄⁺.     

Comparison of NH4 pool dilution techniques to measure gross N fluxes in a coarse textured soil

We compared gross N fluxes by ¹⁵N pool dilution in a coarse-textured agricultural soil when ¹⁵N was applied to the soil NH₄⁺ pool by either: (i) mixing a ¹⁵NH₄NO₃ solution into disturbed soil or (ii) injection of ¹⁵NH₃ gas into intact soil cores. The two techniques produced similar results for gross N mineralization rates indicating that NH₄⁺ production in soil was not altered by soil disturbance, method of application (gas vs. solution), or amount of N applied. This was not the case for immobilization rates, which were twofold higher when ¹⁵N label was applied to the soil NH₄⁺ pool with the mixing technique compared to the injection technique. This was attributed to the fact that more NH₄⁺ was applied with the mixing technique. Estimates of gross nitrification were accompanied by large error terms meaning differences between ¹⁵N labeling methods could not be accurately assessed for this process rate.     

Nitrification and denitrification in an acidic soil of tea (Camellia sinensis L.) field estimated by δ15N values of leached nitrogen from the soil columns treated with ammonium nitrate in the presence or absence of a nitrification inhibitor and with slow-release fertilizers

Forty-two-day-old wheat (Triticum aestivum L. var. Asakazekomugi) plants were treated with complete, K-free (—K), Ca-limited (—Ca), and Mg-free (—Mg) nutrient solutions for 10 days using 2 mM NH₄NO₃ as the nitrogen source, which was replaced with 4 mM ¹⁵ NH₄C1 or Na¹⁵NO₃ for the subsequent 2 days to investigate the absorption, translocation, and assimilation of inorganic nitrogen in relation to the mineral supply. In another experiment plants were grown on NO₃ ^?, NH₄ ⁺, NH₄N0₃, and K-free and Ca-limited NH₄N0₃ nutrient solutions for 10 days, and then in the latter three treatments the nitrogen source was replaced with NO₃ ^? and half of the —K plants received K for 6 days to examine the changes in the nitrate reductase activity (NRA).

Wheat plants absorbed NH₄ ^?N and NO₃-N at a similar rate. Influence of K on the absorption of N0₃-N was stronger than that on the absorption of NH₄-N in wheat plants. The supply of K to the —K plants increased the absorption of NO₃-N, while the absorption of NH₄-N still remained at a lower rate in spite of the addition of K. A limited supply of Ca and lack of Mg in nutrient media slightly affected the absorption of NH₄-N. The influence of K was stronger on the translocation of nitrogen from roots to shoots, while Ca and Mg had little effect. When K was supplied again to the —K plants the translocation of NO₃,-N was more accelerated than that of NH₄-N. Incorporation of NH₄-N into protein was higher than that of NO₃-N in all the tissues; root, stem, and leaf. Assimilation of NH₄-N and NO₃-N decreased by the —K and —Mg treatments.

Leaf NRA of wheat plants decreased in the —K and —Ca plants. Higher leaf NRA was found when K was given again to the —K plants than when the plants were continuously grown in K-free media. Replacement of NO₃ ^? with NH₄ ⁺ as the nitrogen source caused a decline of leaf NRA, while the supply of both NH₄ ^?N and NO₃-N slightly affected the leaf NRA.     

Development of a Soil N Test for Fertilizer Requirements for Corn Production in Quebec

Corn requires high nitrogen (N) fertilizer use, but no soil N test for fertilizer N requirement is yet available in Quebec. Objectives of this research were (1) to determine the effects of soil nitrate (NO₃ ^?)-N, soil ammonium (NH₄ ⁺)-N, and N fertilizer rates on corn yields and (2) to determine soil sampling times and depths most highly correlated with yields and fertilizer N response under Quebec conditions. Soil samples were taken from 0- to 30-cm and 30- to 60-cm depths at seeding and postseeding (when corn height reached 20 cm) to determine soil NH₄ ⁺ and NO₃ ^? in 44 continuous corn sites fertilized with four rates of N in two replications using a quick test (N-Trak) and a laboratory method. The N-Trak method overestimated soil NO₃ ^?-N in comparison with the laboratory method. Greater coefficients of determination were observed for soil NO₃ ^?-N analyses at postseeding compared with seeding.     

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.     

Use of diffusion for automated nttrogen‐15 analysis of soil extracts

Abstract

Studies to evaluate the use of diffusion for automated ¹⁵N analysis of inorganic N in soil extracts showed that serious error can arise from use of the Devarda's alloy recommended for steam distillations and that the error can be avoided by using a commercial product of higher purity. These studies showed that serious error can also arise when NO₃ ^‐‐N is diffused following NH₄ ⁺‐N and that separate diffusions should be performed for NH₄ ⁺‐N and (NH₄ ⁺ + NO₃‐)‐N. Other work demonstrated that the plastic specimen containers employed for diffusion can be reused if acid‐washed, that diffusions can be performed using either light or heavy MgO without ignition to decompose carbonate, and that labeled NO₂‐is completely removed from soil extracts by treatment with sulfamic acid before diffusion. A comparison of ¹⁵N analyses by steam distillation and diffusion using extracts from two soils revealed better agreement for the soil having a lower content of organic matter. Substantial differences in analyses by the two techniques for the soil having a higher organic‐matter content were attributed to enzymatic conversions of inorganic N during the 6‐d diffusion period.     

Characteristics of ammonium and nitrate fluxes along the roots of Picea asperata

Nitrogen (N), ammonium (NH₄⁺) and nitrate (NO₃^?), is one of the key determinants for plant growth. The interaction of both ions displays a significant effect on their uptake in some species. In the current study, net fluxes of NH₄⁺ and NO₃^? along the roots of Picea asperata were determined using a Non-invasive Micro-test Technology (NMT). Besides, we examined the interaction of NH₄⁺ and NO₃^? on the fluxes of both ions, and the plasma membrane (PM) H⁺-ATPases and nitrate reductase (NR) were taken into account as well. The results demonstrated that the maximal net NH₄⁺ and NO₃^? influxes were detected at 13–15?mm and 8–10.5?mm from the root apex, respectively. Net NH₄⁺ influx was significantly stimulated with the presence of NO₃^?, whereas NH₄⁺ exhibited a markedly negative effect on NO₃^? uptake in the roots of P. asperata. Also, our results indicated that PM H⁺-ATPases and NR play a key role in the control of N uptake.     

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.     

Uptake,transport and assimilation of 15N-nitrate and 15N-ammonium in tea (Camellia sinensis L.) plants

¹⁵N studies were conducted using hydroponically grown tea (Camellia sinensis L.) plants to clarify the characteristics of uptake, transport and assimilation of nitrate and ammonium. From the culture solution containing 50 mg L^-1 N0₃-N and 50 mg L^-1 NH.-N, the uptake of NH₃-N after 24 h was twice as high as that of NO₃-N, while the uptake of N0₃-N from the culture solution containing 90 mg N0₃-N and 10 mg NH₃-N was twice that of NH₄-N. The presence of 0.4 mM Al had no significant effect on the N0₃-N and NH₄-N uptake from the culture solutions containing 50 mg L^-1 N0₃-N and 50 mg L^-1 NH₄-N, 90 mg L^-1 N0₃-N and 10 mg L^-1 NH₄-N or 99 mg L^-1 N0₃-N and 1 mg L^-1 NH₄-N. Transport of N0₃-derived N to young leaves was much more rapid than that of NH₄-derived NO₃ and NH₄-derived N was largely retained in the roots and lower stem. Young and mature shoots separated from the roots absorbed more N0₃-N than intact plants. Nitrate assimilation occurred in both, roots and young as well as mature leaves. Internal cycling of N0₃-derived Nand NH₄-derived N from one root part to another part was not appreciable after 28 h, suggesting that a longer of time is required for cycling in woody plants.     

Comparison of Factors Affecting Soil Nitrate Nitrogen and Ammonium Nitrogen Extraction

Extraction of soil nitrate nitrogen (NO₃ ^?-N) and ammonium nitrogen (NH₄ ⁺-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO₃ ^?-N and NH₄ ⁺-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L^?1 calcium chloride (CaCl₂), 1 mol L^?1 potassium chloride (KCl), and 0.5 mol L^?1 potassium sulfate (K₂SO₄)] and concentration (0.5, 1, and 2 mol L^?1 KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (?18 ^oC, 4 ^oC, and 25 ^oC) of extracted solution. The recovery of soil NO₃ ^?-N and NH₄ ⁺-N was also measured to compare the differences of three extracting reagents (CaCl₂, KCl, and K₂SO₄) for NO₃ ^?-N and NH₄ ⁺-N extraction. Air drying decreased NO₃ ^?-N but increased NH₄ ⁺-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO₃ ^?-N and NH₄ ⁺-N concentrations compared to other treatments. The concentrations of extracted NO₃ ^?-N and NH₄ ⁺-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO₃ ^?-N and NH₄ ⁺-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO₃ ^?-N and 94.9% for NH₄ ⁺-N. K₂SO₄ was not found suitable for NO₃ ^?-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl₂ was only 57.3% for NH₄ ⁺-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L^?1 KCl could fully extract NO₃ ^?-N. A ratio of 10:1 of solution to soil was adequate for NO₃ ^?-N extraction, whereas the NH₄ ⁺-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at ?18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO₃ ^?-N concentration, whereas the NH₄ ⁺-N concentration varied greatly with storage temperature. Storing the extracted solution at ?18 ^oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO₃ ^?-N and NH₄ ⁺-N concentrations. Compared with the immediate extraction, the averaged NO₃ ^?-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH₄ ⁺-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L^?1 KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO₃ ^?-N, whereas the concentration of extracted NH₄ ⁺-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at ?18 ^oC could improve the reliability of NO₃ ^?-N and NH₄ ⁺-N results.     

The significance of plant energy status for the uptake and incorporation of NH4-nitrogen by young rice plants

Uptake and assimilation of inorganic N in young rice plants has been studied with labelled N (N-15). Depletion of the plants' carbohydrate content, obtained by a preceding dark period, resulted in a drastic reduction of NH₄ ⁺-N uptake. Plants exposed to low light intensity showed diminishing NH₄ ⁺-N uptake rates as compared with plants exposed to full light intensity, the latter showing constant NH₄ ⁺-N uptake rates during the whole experimental period. The percentage of labelled insoluble N in total labelled N was not significantly affected by a preceding dark period, whereas the low light intensity resulted in a lower proportion of insoluble N in roots and shoots. The incorporation of labelled N into the insoluble fraction (proteins, nucleic acids) was higher in plants fed with NH₄ ⁺-N than in those fed with NO₃ ^-.

The uptake of NH₄ ⁺-N was not significantly affected by NO₃ ^-, whereas the NO₃- uptake rate was considerably reduced in the presence of NH^{4 +}-N. Low energy status of plants affected the nitrate uptake more than the uptake of NH₄ ⁺-N. The results show that uptake and assimilation of inorganic N depend much on the energetic status of plants. Nitrate uptake and assimilation is more sensitive to low energy conditions than NH₄ ⁺-N.     

Root Carbon Enrichment Alleviates Ammonium Toxicity in Cucumber Plants

ABSTRACT

The addition of carbonates to a nutrient solution to alleviate ammonium (NH₄ ⁺) toxicity in hydroponically-grown cucumber (Cucumis sativus L.) plants was investigated. Stable isotopes [nitrogen (¹⁵N) and carbon (¹³C)] were used to assess the uptake of nitrogen [NH₄ ⁺ or nitrate (NO₃ ^?)] as well as carbon [bicarbonate (HCO₃ ^?)/carbonate (CO₃ ^2?)] by the roots. Ammonium as the sole N source at 5 mM decreased plant fresh weights compared to NO₃ ^?. However, at lower concentrations of NH₄ ⁺ (25% of 5 mM total N), growth was increased compared to NO₃ ^? alone. Inorganic C enrichment [calcium carbonate (CaCO₃)] of the nutrient solution increased the fresh weight of NH₄ ⁺ grown plants with up to 150% relative to control plants receiving calcium hydroxide [Ca(OH)₂] for pH regulation. Root ¹⁵N enrichment was lower in ¹⁵NH₄ ⁺ supplied plants compared to ¹⁵NO₃ ^?, while the ¹³C enrichment in leaves was increased by NH₄ ⁺ nutrition compared to NO₃ ^? or NH₄NO_3. The enhanced C capture was associated with high PEPCase activity in the roots. It is concluded that inorganic carbon enrichment of the root medium may alleviate NH₄ ⁺ toxicity via increased synthesis of C skeletons and, accordingly, increased capacity for NH₄ ⁺ assimilation and N export to the shoots.     

A N tracing model to analyse N transformations in old grassland soil

A new ¹⁵N tracing model was developed to analyse nitrogen (N) transformations in old grassland soil. There was a need to develop a new model because existing models such as FLUAZ were not able to simulate the observed N dynamics. The new features of the model are: (a) simulation of heterotrophic nitrification, (b) simulation of dissimilatory nitrate (NO₃⁻) reduction to ammonium (NH₄⁺) (DNRA), (c) release of adsorbed or stored fertiliser N into the available mineral N pools and (d) immobilisation of NH₄⁺ and NO₃⁻ into two separate organic N pools with different re-mineralisation characteristics. The tracing model contains six N pools and nine simultaneous N transformations either at zero- or first-order kinetics. The model is set up in the modelling software ModelMaker which contains non-linear optimisation routines based on the Marquardt-Levenberg algorithm. The model is able to simulate data obtained from triple labelling studies where either the NH₄⁺, the NO₃⁻ or both pools were labelled with ¹⁵N. The flexible modelling environment allows the user to develop the model further.     

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.     

脲酶抑制剂/硝化抑制剂对土壤中尿素氮转化及形态分布的影响

运用^15N尿素示踪技术,以壤质草甸棕壤和春小分别作供试土壤和作物进行盆栽试验,结果表明,与单^15N尿素或^15N尿素＋氢素＋氢醌（HQ）相比,配施双氰胺（DCD）尤其与HQ组合可使土壤保持较高的^15N回收率,其中有^15N占相当大的比例,在小麦孕穗期前,上述两处理可有效地保持尿素解后土壤中NH^ 4 -^15N含量,并显著降低（NO^ 3 NO^-2)-^15N的富集;促进肥料^15N固持－矿化的周转,HQ与DCD配合施用对尿素施用后土壤中残留^15N量及有机^15N的再矿化和NH^ 4-^15N含量具有一定的协同作用。     

农田土壤中氮同位素分级与硝化能力的关系研究

A laboratory-based aerobic incubation was conducted to investigate nitrogen(N) isotopic fractionation related to nitrification in five agricultural soils after application of ammonium sulfate((NH4)2SO4). The soil samples were collected from a subtropical barren land soil derived from granite(RGB),three subtropical upland soils derived from granite(RQU),Quaternary red earth(RGU),Quaternary Xiashu loess(YQU) and a temperate upland soil generated from alluvial deposit(FAU). The five soils varied in nitrification potential,being in the order of FAU YQU RGU RQU RGB. Significant N isotopic fractionation accompanied nitrification of NH+4. δ15N values of NH+4 increased with enhanced nitrification over time in the four upland soils with NH+4 addition,while those of NO-3 decreased consistently to the minimum and thereafter increased. δ15N values of NH+4 showed a significantly negative linear relationship with NH+4-N concentration,but a positive linear relationship with NO-3-N concentration. The apparent isotopic fractionation factor calculated based on the loss of NH+4 was 1.036 for RQU,1.022 for RGU,1.016 for YQU,and 1.020 for FAU,respectively. Zero- and first-order reaction kinetics seemed to have their limitations in describing the nitrification process affected by NH+4 input in the studied soils. In contrast,N kinetic isotope fractionation was closely related to the nitrifying activity,and might serve as an alternative tool for estimating the nitrification capacity of agricultural soils.     

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.