Ammonium Fixation Research Articles

Tracing the contribution and fate of synthetic nitrogen fertilizer in young apple orchard agrosystems

Excessive synthetic nitrogen (N) inputs in intensive orchard agrosystems of developing countries are a growing concern regarding their adverse impacts on fruit production and the environment. Quantifying the distribution and contribution of fertilizer N is essential for increasing N use efficiency and minimizing N loss in orchards. A 15N tracer experiment was performed in a young dwarf apple orchard over two growing seasons to determine the fertilizer N transformation and fate. Fertilizer N primarily contributed to 25 % to 75 % of soil nitrate in the top 60 cm, but the contribution to soil microbial biomass N and fixed ammonium was <8 %, with the contribution to plant N ranging from 9 % to 19 %. In most growth periods, soil nitrate and fixed ammonium contents derived from native soil with N fertilization were higher than those not receiving N fertilizer. The N use efficiency of plants was only 2.6 % and 4.9 % in the first and second seasons, respectively, in contrast to 56.6 % and 54.0 % of N recovered in soil. Meanwhile, N assimilated into microbial biomass accounted for 0.8 %, and the proportion fixed by clay minerals was 3.5 %–5.2 %. One season after N fertilization, the nitrate below the 1 m soil layers accounted for 4.6 % of the applied N fertilizer, and the proportion increased to 22.5 % after two seasons. The N loss rate via N2O emission was 0.4 % over two years. The application of N fertilizer facilitated indigenous soil N mineralization, and abiotic ammonium fixation more efficiently retained synthetic N than microbial immobilization. These findings provide new insight into orchard N cycling, and attention should be given to the improvement of soil N retention and turnover capacity regulated by soil microbial and abiotic processes, as well as the potential environmental impacts of additional soil N mineralization resulting from prolonged chemical N fertilization.

Ammonia emission and nitrogen use efficiency with application of nitrification inhibitor and plant growth regulator in a calcareous soil (Karaj, Iran)

Ammonia volatilization (AV) is one of the main potential pathways of nitrogen (N) loss and re-deposited ammonia acts as an indirect source of N2O. The question arises how environmentally friendly inputs such as nitrification inhibitors (NI) and plant growth regulators (PGR) can affect AV in calcareous soils, while increasing nitrogen use efficiency (NUE) and crop yield. To answer this question, the effects of applying urea in combination with nitrapyrin (NP) as a NI and gibberellic acid (GA3) as a PGR on daily AV, winter wheat yield and NUE were evaluated. The treatments included a control treatment, urea and urea+NP + GA3 with five replications. Urea and urea coated with NP were applied in three equal split applications, and GA3 was foliar sprayed once. By using 15N-labeled urea and setting up three 15N microplots inside each plot, the nitrogen in the plant derived from fertilizer (Ndff) and the fate of N fertilizer were directly investigated. Ammonia emission factors and estimated values of indirect N2O-N produced from AV in urea and urea+NP + GA3 were 27% and 33%, and 0.38 and 0.45 kg N ha−1, respectively. The grain yield and 1000-grain weight in urea+NP + GA3 treatment were significantly increased by 15% and 13%, respectively, compared to urea. However, urea coated with NP increased AV and decreased Ndff by about 22% and 15%, respectively. The increase in AV caused by the addition of NP was relatively lower than amounts reported in previous studies. This could be due to the split application of N fertilizer on dry soil, the immediate incorporation and the ammonium fixation capacity of illite and vermiculite clay minerals presence in this soil and the reduction of readily available NH4+. The N difference method displayed 21% increase in NUE when urea was applied with NP and GA3 but tracking 15N showed that the combination of urea+NP + GA3 reduced NUE by about 8%. This suggests that NP may not be a viable option for increasing NUE in calcareous soils due to its ability to increase AV. Further research is needed to study the combination of other NIs with GA3 and investigate their effect on AV to simultaneously improve yield and NUE.

NH4+ Fixation and Ionic Competition with K+ in a Clayey Soil from Ocumare Del Tuy, Venezuela

Ammonium and potassium are essential for plant production, both share an ionic potential capable of entering the structure of some 2:1 type clay minerals, remaining fixed. Although, in Venezuela there are abundant areas with the presence of this type of clays, ammonium fixation (NH4f) has been little studied. The soil studied located in Los Valles del Tuy, Miranda State, presented clayey texture, some textural homogenization, and little differentiation in horizons, which is characteristic of Inceptisols. That soil also evidenced by XRD the abundant presence of nontronite / montmorillonite, resulting in a semi-quantification of 20% and 16.4% for the surface and subsoil horizons, respectively. The fixation of ammonium and the competition with potassium to occupy the fixed sites of these clays were studied by means of an incubation experiment, varying the concentration of K+ and keeping the concentration of NH4+ fixed, then extracting and determining the fixed ammonium by a photocolorimetry method. The soil presented a great ammonium fixing capacity, 49% of the total added was fixed in the absence of the competitive ion; however, NH4f fell drastically when both ions were added simultaneously, decreasing exponentially under increasing amounts of the competitive ion and staying constant around 8%. It seems possible that, the high toxicity induced by excessive fertilisation with ammonium sulfate could be counteracted in soils with capacity to retain K, if, at the expense of Kf, the levels of K in the soil solution are increased causing a decrease in the concentrations of N in the tissue.

Control of paddy soil redox condition on gross and net ammonium fixation and defixation

Abiotic fixation and defixation of ammonium (NH4+) in silicate interlayers are common processes in paddy soils, owing to their often high levels of 2:1 type clay minerals. Fixed NH4+ hence forms a buffer during the supply and loss of plant-available N. The soil redox potential (Eh) is suspected to influence NH4+ (de)fixation by its impact on the negative charge of Fe3+-containing clay minerals, yet experimental confirmations are scarce. We examined the effect of a fluctuating or generally increased Eh on dynamics of NH4+ (de)fixation in a Bangladeshi paddy soil (total N: 1.9 g kg−1 | CEC: 43 cmol kg−1) during two incubation experiments. In those, we either 15N-labelled the fixed NH4+ pool (with initially 87 mg kg−1 exchangeable versus 393 mg kg−1 fixed NH4+-N) or the exchangeable pool (with initially 141 mg kg−1 exchangeable versus 249 mg kg−1 fixed NH4+-N), and this under three Eh-controlling treatments: continuous flooding (CF), CF with extra MnO2 added (3.5 g kg−1 Mn4+) (CF-MnO2), and alternate wetting and drying (AWD). The CF treatment overall led to somewhat higher net NH4+ fixation than AWD and, less clearly, also compared to the CF-MnO2 treatment, resulting from higher gross NH4+ fixation as well as lower gross defixation. These fluxes always led to a dynamic equilibrium between the exchangeable and fixed NH4+ pool within a few days of incubation, and this equilibrium was the same for both experiments but depended on the Eh treatment. However, by altering other NH4+ transformation processes, Eh treatments also indirectly impacted NH4+ (de)fixation fluxes, for which the equilibrium ratio between exchangeable versus fixed NH4+ turns out to be the determining factor.

Stover mulching and inhibitor application maintain crop yield and decrease fertilizer N input and losses in no-till cropping systems in Northeast China

Microbial immobilization of nitrogen and mineral fixation of ammonium play important roles in the retention of fertilizer N in soil, and the subsequent microbial mineralization and abiotic release regulate N availability to crops. However, how fertilization management practices temporally influence the biological and abiotic processes of fertilizer N transformation remain unclear. Therefore, we conducted a field micro-plot experiment with 15N-labeled-fertilizer application in a long-term no-till maize cropping system in Northeast China to evaluate the effects of maize stover mulching alone and in combination with inhibitors on the conversion, crop use efficiency, and loss risk of fertilizer N. The following four treatments were established in our trial: (1) conventional NPK fertilization (NPK); (2) NPK plus maize stover mulching (NPKS); (3) 20 % reduced NPK plus maize stover mulching (RNPKS); and (4) RNPKS plus urease and nitrification inhibitors (RNPKSI). The results showed that the retention of fertilizer N was dependent on both soil depth and management practices at the seedling stage of maize. At the soil depth of 0−20 cm, maize stover mulching significantly enhanced the transformation of fertilizer N into fixed NH4+ and organic N pools by 49.9 % and 65.6 % compared with NPK. The reduction of NPK with maize stover mulching further promoted fertilizer N conversion into the organic N pool by 43.2 %, with a reduction in the fixed NH4+ pool by 32.0 %, while the combined with inhibitors application in RNPKSI treatment resulted in the opposite trend due to the extended residence of fertilizer-derived NH4+-N. Meanwhile, at a depth of 20−40 cm, maize stover mulching in NPKS and RNPKS treatments significantly improved the transformation of fertilizer N to the fixed NH4+ pool compared with NPK, and maize stover mulching and inhibitors application in RKPNSI treatment further increased this conversion compares with RNPKS. The subsequent mineralization and release of fertilizer-derived organic N and fixed NH4+ affected crop uptake of fertilizer N. The fertilizer N use efficiency (NUE) in the three maize stover mulching treatments was significantly improved by an average of 16.1 %, while its gaseous losses were remarkably reduced by an average of 32.8 % compared with NPK. These findings suggest that maize stover mulching and inhibitors application in RNPKS and RNPKSI treatments are efficient management strategies in no-till maize cropping system in Northeast China, reducing N input, maintaining crop yield, enhancing NUE, and potentially minimizing environmental pollution risk by regulating fertilizer N conversion between fixed NH4+ and organic N pools.

Biotic and abiotic nitrogen immobilization in soil incorporated with crop residue

Incorporating crop residue favors biotic and abiotic nitrogen (N) immobilization, which can effectively conserve active N in soil. Here we summarize the occurrence characteristics of biotic and abiotic N immobilization and affecting factors, and then identify a general pattern of the relative importance of the two processes in soil incorporated with crop residue. When microorganisms decompose crop residue for a source of energy and carbon (C) to support their metabolism, they need N to build cellular components, resulting in N immobilization in biomass. Alternatively, N can be incorporated into soil organic matter (SOM) through the following known mechanisms: ammonium (NH4+) fixation by clay minerals, condensation of ammonia (NH3) with phenol or quinone rings, and nitrosation of nitrite (NO2-) with phenolic compounds. The biotic and abiotic contribution to N immobilization is significantly controlled by soil properties and crop residual quality. When a large fraction of C is in the form of labile organic compounds, biotic N immobilization may be important. In contrast, high fraction of lignin-derived phenolic compounds or other recalcitrant organic compounds facilitates abiotic N immobilization. In the N-limited soil, the increase in N availability contributes to microbial activity, which would stimulate biotic processes. However, when microbial metabolism switches from N-limited to C-limited due to the increase in N availability, abiotic processes become important. Abiotic processes even predominate over biotic processes under N-enriched conditions. The relative roles of biotic or abiotic N immobilization in soil incorporated with crop residue are dependent on the availability of C relative to N and soil N availability. Carbon and N availability in turn is related to their forms. Moreover, N availability is affected by soil pH. The mechanisms by which N is immobilized determine the fate of immobilized N and are of vital importance for the management of N availability and losses through combining N fertilizer with crop residue. More investigation is necessary to quantify the contribution of biotic and abiotic processes during the decomposition of crop residue and to determine the factors affecting N immobilization in the field.

Effects of a nitrification inhibitor on nitrogen species in the soil and the yield and phosphorus uptake of maize.

Phosphorus (P) resource availability is declining and the efficiency of applied nutrients in agricultural soils is becoming increasingly important. This is especially true for P fertilizers from recycled materials, which often have low plant availability. Specific co-fertilization with ammonium can enhance P plant availability in soils amended with these P fertilizers, and thus the yield of plants. To investigate this effect, we performed a pot experiment with maize in slightly acidic soil (pH6.9) with one water-soluble (triple superphosphate [TSP]) and two water-insoluble (sewage sludge-based and hyperphosphate [Hyp]) P fertilizers and an ammonium sulfate nitrate with or without a nitrification inhibitor (NI). The dry matter yield of maize was significantly increased by the NI with the Hyp (from 14.7 to 21.5g/pot) and TSP (from 40.0 to 45.4g/pot) treatments. Furthermore, P uptake was slightly increased in all three P treatments with the NI, but not significantly. Olsen-P extraction and P K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy showed that apatite-P of the water-insoluble P fertilizers mobilized during the plant growth period. In addition, novel nitrogen (N) K-edge micro-XANES spectroscopy and the Mogilevkina method showed that the application of an NI increased the fixation of ammonium in detectable hot spots in the soil. Thus, the delay in the nitrification process by the NI and the possible slow-release of temporarily fixed ammonium in the soil resulted in a high amount of plant available ammonium in the soil solution. This development probably decreases the rhizosphere pH due to release of H+ by plants during ammonium uptake, which mobilizes phosphorus in the amended soil and increases the dry matter yield of maize. This is especially important for water-insoluble apatite-based P fertilizers (conventional and recycled), which tend to have poor plant availability.

Subsurface Clay Soil Application Improved Aggregate Stability, Nitrogen Availability, and Organic Carbon Preservation in Degraded Ultisols with Cover Crop Mixtures

Core Ideas Subsurface clay soil amendment improved soil aggregation and its stability. Microbial activities were suppressed by clay soil applications. Clay soil amendment enhanced labile organic C pools. Additions of clay soil promoted ammonium fixation and reduced nitrate leaching. Coastal Plain soils of the southeastern United States are highly degraded, with low soil organic carbon (C), meager soil fertility, and poor soil structure. We tested the effectiveness of clay soil amendment to promote the formation of soil aggregates, improve organic C stability, and enhance nutrient availability in field. Subsurface clay soils (B horizon, 25% clay) were applied (∼25 T ha–1) to the top of the soil surface, followed by the planting of winter cover crop mixtures. Crop growth and nutrient uptake and relevant soil biogeochemical properties were analyzed at the end of the winter season. Additions of subsurface clay soils prompted the formation of water‐stable macroaggregates (250–2000 mm) from 32 to 54% and increased mean weight diameter of the aggregates from 390 to 592 mm. Additions of subsurface clay soils increased clay content from 1.5 to 3.8% but did not change soil texture, pH, water holding capacity, and bulk density. The stock of dissolved organic nitrogen was not affected by the amendment, whereas nitrate leaching from field was reduced by 61% because of clay soil application. Clay soil additions inhibited the activities of C‐cycling (N‐acetyl‐β‐glucosaminidase, β‐d‐cellubiosidase, β‐glucosidase, and β‐xylosidase) and N‐cycling (leucine aminopeptidase) enzymes but did not modify soil microbial community level substrate utilization patterns, resulting in a 63% reduction in C turnover rate and 19% increases in concentrations of labile organic C. Our results indicated that subsurface clay soil amendment can improve soil structure, nutrient availability, and the stability of labile C in the field, at least in the short term, and may be a promising management alternative to enhance the productivity and sustainability of the degraded Coastal plain soils.

Ammonia Volatilization in Acidic Mine Soils Applied with Livestock Manure Compost and Liming Materials

To investigate the possibility of ammonia volatilization in mine lands limed, we conducted an incubation experiment with treating livestock manure compost as an organic fertilizer and dolomite and oyster shell as a liming material. For trapping ammonia volatilized, installing boric acid in a closed system was efficient, with a recovery of 101.2±3.7%. Treating compost and liming material increased soil pH to a range of 5.75-5.88 for a metal-mine soil and 6.02-6.06 for a coal-mine soil. Ammonium-N in soils decreased rapidly with time during early stage of incubation, but nitrate-N changed little. Accumulated ammonia volatilization during incubation was negligible, less than 0.3 mg N kg-1, and only 0.4-0.5% of ammonium-N was volatilized during incubation. Ammonium fixation by clay minerals and immobilization by soil microbes possibly account for the decrease in concentration of ammonium-N. From these results, we concluded that nitrogen loss by ammonium volatilization would be significantly low when acidic mine soils treated with livestock manure compost and liming materials were neutralized to slightly acidic condition. Cumulative ammonia volatilization in (a) metal and (b) coal mine soils after treating compost (Com), compost and dolomite (Com+Dol), and compost and oyster shell (Com+Oys).

Does the potential ammonium fixation of soils have an impact on the optimum nitrogen fertilizer rate?

ABSTRACTField experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH4) fixation capacity [high = 161 mg fixed NH4-N kg−1 soil, medium = 31.5 mg fixed NH4-N kg−1 soil and no = nearly no fixed NH4-N kg−1 soil]. On high NH4+ fixing soil, 80 kg N ha−1 Urea+ ammonium nitrate [NH4NO3] or 240 kg N ha−1 ammonium sulfate [(NH4)2SO4]+(NH4)2SO4, was required to obtain the maximum yield. Urea + NH4NO3 generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH4+ fixing soil, 80 kg N ha−1 urea+NH4NO3 was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH4+ fixing soil than in the others. Grain protein was highly affected by NH4+ fixation capacities and N doses. Harvest index was affected by the NH4+ fixation capacity at the 1% significance level.

Nitrogen isotopic composition of sediments from the eastern Tethys during Oceanic Anoxic Event 2

Low nitrogen isotopic compositions (δ15N) have been reported in deposits from the proto-North Atlantic and western Tethys during Oceanic Anoxic Event 2 (OAE2) at the Cenomanian-Turonian (C-T) transition. It has been proposed that the low δ15N values may have resulted from enhancement of N2 fixation and/or incomplete assimilation of the upwelled ammonium due to expansion of anoxic/euxinic conditions. However, few δ15N measurements are available for the sediments from the eastern Tethys and the Pacific and therefore the global significance of the N cycle and its environmental implications remains to be tested. Here, we present δ15N data from OAE2 deposits in the Gongzha section in the Tingri area of southern Tibet, which was located on the southern margin of the eastern Tethys during the mid-Cretaceous. The δ15N data reveal a large perturbation of the N cycle during OAE2. The pronounced negative shift in δ15N prior to OAE2, as well as the persistent low δ15N values during OAE2, imply the significant contribution of N2 fixation. The changes in the marine N cycle suggest that the expansion of an anoxic environment may also have occurred in the southern continental margin areas of eastern Tethys. The coincidence of the marine faunal turnover and the perturbation of the N cycle suggests that the expansion of anoxic water masses may have played an important role in the extinction of marine organisms across the C-T boundary in eastern Tethys.

Kinetics of Competitive Fixation of Potassium and Ammonium Ions by Silt Component of Soils from Different Agro-Climatic Regions

ABSTRACTPotassium (K+) and nitrogen [N, as the form of ammonium (NH4+)] are major nutrients for plant growth. Although there have been a number of studies on the kinetic fixation of potassium and ammonium ions in soils and clays, however, investigations on the kinetics of competitive fixation of these ions have been few, if any, especially by taking into account silt component of the soils. In this study, the kinetics of potassium and ammonium fixation were examined in the silt components of several soil samples. The results revealed that considerable amounts of K+ and in lesser amounts, NH4+ ions were fixed by silt components of the soils. Potassium fixation was strongly preferred over ammonium. To describe the fixation kinetics, seven mathematically models were evaluated. A comparison of the models showed that pseudo-second-order equation properly described the fixation of these ions by the silt components.

Effect of relative humidity on non-refractory submicron aerosol evolution during summertime in Hangzhou, China

Relative humidity (RH) has a significant and complex effect on aerosols because of the aqueous phase process and gas-particle partition. The mass concentration and size distribution of organic aerosols, sulfate, nitrate, ammonium, and chloride were measured using high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). These measurements were recorded from Aug. 5 to Sept. 23, 2016 in Binjiang District, Hangzhou, China, during which period more than 78% of the readings showed an RH over 60%, while the average temperature was 26 °C. Correlation analysis was applied to inorganic aerosol measurements while positive matrix factorization (PMF) was applied for source apportionment of organic aerosols (OA). The pattern of fixation of ammonium in aerosols changed as the RH increased, suggesting that RH enhances nitrate participation in particles, while sulfate is scavenged by droplets. All species of non-refractory submicron particles (NR-PM1) showed an increase in their peak size as the RH increased. Primary OA (POA) continuously accumulated as the RH increased. When RH 60%, there was a relatively slow decrease in OOA, dominated by the wet removal effect rather than oxidation. The degree of oxidation of OA decreased as RH increased; this can be explained by most of the OOA with higher hygroscopicity being removed as droplets.

Three mitochondrial transporters of Saccharomyces cerevisiae are essential for ammonium fixation and lysine biosynthesis in synthetic minimal medium.

The nuclear genes of Saccharomyces cerevisiae YHM2, ODC1 and ODC2 encode three transporters that are localized in the inner mitochondrial membrane. In this study, the roles of YHM2, ODC1 and ODC2 in the assimilation of nitrogen and in the biosynthesis of lysine have been investigated. Both the odc1Δodc2Δ double knockout and the yhm2Δ mutant grew similarly as the YPH499 wild-type strain on synthetic minimal medium (SM) containing 2% glucose and ammonia as the main nitrogen source. In contrast, the yhm2Δodc1Δodc2Δ triple knockout exhibited a marked growth defect under the same conditions. This defect was fully restored by the individual expression of YHM2, ODC1 or ODC2 in the triple deletion strain. Furthermore, the lack of growth of yhm2Δodc1Δodc2Δ on 2% glucose SM was rescued by the addition of glutamate, but not glutamine, to the medium. Using lysine-prototroph YPH499-derived strains, the yhm2Δodc1Δodc2Δ knockout (but not the odc1Δodc2Δ and yhm2Δ mutants) also displayed a growth defect in lysine biosynthesis on 2% glucose SM, which was rescued by the addition of lysine and, to a lesser extent, by the addition of 2-aminoadipate. Additional analysis of the triple mutant showed that it is not respiratory-deficient and does not display mitochondrial DNA instability. These results provide evidence that only the simultaneous absence of YHM2, ODC1 and ODC2 impairs the export from the mitochondrial matrix of i) 2-oxoglutarate which is necessary for the synthesis of glutamate and ammonium fixation in the cytosol and ii) 2-oxoadipate which is required for lysine biosynthesis in the cytosol. Finally, the data presented allow one to suggest that the yhm2Δodc1Δodc2Δ triple knockout is suitable in complementation studies aimed at assessing the pathogenic potential of human SLC25A21 (ODC) mutations.

Soil mineral nitrogen dynamics following repeated application of dairy slurry

Repeated applications of animal slurry to soil can lead to residual nitrogen (N) effects from mineralization of organic N carried over from the previous year and from remineralization of previously immobilized N. We studied the effect of repeated slurry applications on soil mineral N (SMNt: nitrate‐N plus soluble, exchangeable and non‐exchangeable ammonium‐N) dynamics in a simplified, aerobic laboratory incubation. The experiment evaluated the effects of up to four applications (84‐day intervals) of two different liquid cow slurries, ammonium sulphate and water (unfertilized control, CON) to sandy loam and clay loam soils. The slurries came from heifers (HEI) and lactating dairy cows (COW). Both soil types showed net N mineralization in HEI during each 84‐day interval after application (3–6% of slurry‐N), whereas decomposition of COW induced net N immobilization at 16% of slurry‐N. The effect observed for COW might have come from its larger C to organic‐N ratio. After each application to the clay loam soil, 36–64% of the ammonium applied was not recoverable at day 0 because of ammonium fixation by clay minerals, and an average of 20% of fertilizer‐N was measured as non‐exchangeable ammonium at day 84. Recovery of N applied with both HEI and COW at day 84 increased significantly with subsequent applications to clay loam soil, but not to sandy loam soil. Residual effects in clay loam soil ranged from 2 to 11% of applied N, which probably resulted from slow mineralization of recalcitrant organic fractions in the slurry and partial stabilization of microbial by‐products within the soil.HighlightsA novel incubation approach was used to study residual N effects of ammonium sulphate and slurries. Fertilizers were applied one, two, three or four times to a sandy loam (SL) and a clay loam (CL) soil. Residual N effects were small; less slurry NH4‐N was available in CL than SL because of clay fixation. Mineralization of residual slurry‐N and stabilization of microbial by‐products were slow.

Responses of biotic and abiotic effects on conservation and supply of fertilizer N to inhibitors and glucose inputs

Fertilizer N can be conserved through immobilization by microorganisms (biotic process) and fixation by soil clay minerals (abiotic process), and then subsequently remineralized and released. These processes are significantly affected by inhibitors, and available C application. In this study, a 96-day incubation experiment was conducted to assess the effects of microbial immobilization and ammonium fixation on conservation and supply of urea-N with the nitrification inhibitor (DMPP: 3,4-dimethylpyrazole phosphate), urease inhibitor (NBPT: N-(n-butyl) thiophosphoric triamide), and glucose additions. The results showed that urea-derived soil microbial biomass nitrogen (SMBN) consistently increased with DMPP input, whereas NBPT increased urea-derived SMBN in the absence of glucose but decreased it in the presence of glucose. Both inhibitors enhanced the effects of fixed NH4+ on conservation and supply of urea-N in all cases, and retarded the release of fixed NH4+. Glucose addition intensified the competition for NH4+ between microbial immobilization and mineral fixation, as well as reduced the availability of urea-N and native soil N, resulting in a negative added N interaction at the initial incubation stage. From 12 to 96 days, the release of fixed NH4+ was 2.6-fold greater than the mineralization of organic N (including SMBN and non-microbial organic N) in the non-glucose treatments, whereas the latter was 2.7-fold greater than the former in the glucose treatments. Taken together, our study indicates both microbial immobilization and mineral fixation are important processes by which N is stabilized in soil. Clarification of fertilizer N transformation induced by these biotic and abiotic processes can provide helpful implications for quantifying N cycle and optimizing agricultural nutrient management.

Measurement and simulation of soluble, exchangeable, and non-exchangeable ammonium in three soils

Clay minerals can fix high amounts of ammonium in non-exchangeable form, which can be of management importance when some ammonium applied with fertilisers is sequestered and not available to the crop. Non-exchangeable ammonium can be released slowly and increase soil N availability. The percentage of ammonium fixed depends, among other factors, on the ammonium dose applied. We applied increasing amounts of ammonium to three soils and then measured (after 48h) the soil equilibrium concentration of non-exchangeable, exchangeable, and soluble ammonium. At a 70mg NH4–Nkg−1 dose, non-exchangeable ammonium measured 60, 24, and 17% on the three soils, while at 2800mg NH4–Nkg−1 the fixation percentage decreased non-linearly to 12, 4, and 5%. Ammonium equilibrium concentrations in non-exchangeable, exchangeable, and soluble fractions as a function of ammonium applied were simulated with two innovative analytically-solved models using two Langmuir equations, one for each pair of pools. Both models predicted well the three ammonium fractions in all soils, with average relative root mean squared errors ranging from 2 to 5%. Sensitivity analysis demonstrated the importance of all four model parameters; however, to extend the model to other soils requires measurement of two parameters — cation exchange capacity and maximum fixation of ammonium in non-exchangeable form. The other two parameters are equilibrium constants that vary little among soils, according to our preliminary results. Therefore, to simplify model use, averaged optimised constant values on multiple soils might suffice. Additional experiments on more soils are needed to verify this hypothesis.

Decoupled carbon and nitrogen mineralization in soil particle size fractions of a forest topsoil

To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long-term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000–20 μm), silt (20–2 μm), and clay (<2 μm). Bulk soil and particle size fractions were incubated in replicate, allowing periodic destructive sampling of triplicates at day 0, 14, 42, 84, 140, 210, and 280. We monitored CO2–C respiration, NH3–N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.01 M CaCl2) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. The chemical composition of selected samples was further characterized by 13C-NMR spectroscopy. Fractionation did not influence carbon mineralization (∑ fractions ≈ bulk soil), which decreased in the order sand > clay > silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure (∑ fractions < bulk soil) and followed the order clay > silt > sand. Fractionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N-alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach.

Ammonium fixation and release by clay minerals as influenced by potassium

It is postulated that stabilized ammonium fertilizers improve fertilizer-N utilization by crops, leading thus to higher yields with the same fertilizer rate, especially on sandy soils. However, it must be taken into consideration that in clayey soil at least a part of the NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; ions may be fixed by 2:1 clay minerals, thus delaying the effect of the N fertilizer. Because NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; and K&lt;sup&gt;+&lt;/sup&gt; have similar size and valence properties and therefore compete for the same non-exchangeable sites of 2:1 clay minerals, we investigated the influence of time and K&lt;sup&gt;+&lt;/sup&gt; application rate on both fixation and release of NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;. Fixation of NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; ions was higher when K&lt;sup&gt;+&lt;/sup&gt; was applied after NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;, while the influence of the K&lt;sup&gt;+&lt;/sup&gt; application rate was less pronounced. Mobilization of non-exchangeable NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; was retarded when K&lt;sup&gt;+&lt;/sup&gt; was applied at the high rate after NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;. At the first harvest yield formation of ryegrass was neither influenced by the amount as well as the application time of K&lt;sup&gt;+&lt;/sup&gt;, because plant available N was not growth limiting, while yield of the second harvest was significantly higher with the low K&lt;sup&gt;+&lt;/sup&gt; application rate after NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;. After the second harvest the blocking effect of K&lt;sup&gt;+&lt;/sup&gt; on the release of non-exchangeable NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; was attenuated and the highest yields of the third cut were reached in the treatments with the high K&lt;sup&gt;+&lt;/sup&gt; application rate after NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;. Total dry matter yield was highest when K&lt;sup&gt;+&lt;/sup&gt; was applied at the low rate after NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;. Our results show that K&lt;sup&gt;+&lt;/sup&gt; governs fixation and release of non-exchangeable NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;, which should be taken into consideration when applying ammonium containing N fertilizers like ammonium sulfate, ammonium sulfate nitrate and ENTEC. Thus K&lt;sup&gt;+&lt;/sup&gt; can affect N availability when N is applied as NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; in both the short and long term.

Empirical and Mechanistic Evaluation of NH 4 + Release Kinetic in Calcareous Soils

Release, fixation, and distribution of ammonium (NH4(+)) as a source of nitrogen can play an important role in soil fertility and plant nutrition. In this study, ten surface soils, after addition of 1,000 mg NH4(+) kg(-1,) were incubated for 1 week at the field capacity moisture and 25 ± 2 °C temperature, and then NH4(+) release kinetic was investigated by sequential extractions with 10 mM CaCl2. Furthermore, NH4(+) distribution among three fractions, including water-soluble, exchangeable, and non-exchangeable, was determined in all soil samples. NH4(+) release was initially rapid followed by a slower reaction, and this was described well with the Elovich equation as an empirical model. The cumulative NH4(+) concentration released in spiked soil samples had a positive significant correlation with sand content and negative ones with pH, exchangeable Ca(2+)m and K(+), cation exchange capacity (CEC), equivalent calcium carbonate (ECC), and clay content. The cation exchange model in the PHREEQC program was successful in mechanistic simulation of the release trend of native and added NH4(+) in all control and spiked soil samples. The results of fractionation experiments showed that the non-exchangeable fraction in control and spiked soil samples was greater than that in water-soluble and exchangeable fractions. Soil properties, such as pH, exchangeable Ca(2+) and K(+), CEC, ECC, and contents of sand and clay, had significant influences on the distribution of NH4(+) among three measured fractions. This study indicated that both native and recently fixed NH4(+), added to soil through the application of fertilizers, were readily available for plant roots during 1 week after exposure.