Natural N abundances of inorganic nitrogen in soil treated with fertilizer and compost under changing soil moisture regimes

This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (δ¹⁵N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the δ¹⁵N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the δ¹⁵N of NH₄⁺ and NO₃⁻ in soils treated with two levels (0 and 150 mg N kg⁻¹) of ammonium sulfate (δ¹⁵N=−2.3‰) and compost (+13.9‰) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using ¹⁵N-labeled ammonium sulfate (3.05 ¹⁵N atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The δ¹⁵N values of NH₄⁺ and NO₃⁻ were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the δ¹⁵N of NH₄⁺ in soils treated with fertilizer progressively increased from +2.9‰ at 0.5 week to +18.9‰ at 6 weeks due to nitrification. During the same period, NO₃⁻ concentrations were consistently low and the corresponding δ¹⁵N increased from +16.3 to +39.2‰ through denitrification. Under subsequent water-unsaturated conditions, the NO₃⁻ concentrations increased through nitrification, which resulted in the decrease in the δ¹⁵N of NO₃⁻. In soils, which were unsaturated for the first 6-weeks incubation, the δ¹⁵N of NH₄⁺ increased sharply at 0.5 week due to fast nitrification. On the other hand, the δ¹⁵N of NO₃⁻ showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between 1 and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the δ¹⁵N of NO₃⁻ progressively with a concurrent decrease in NO₃⁻ concentration through denitrification. The differences in δ¹⁵N of NO⁻₃ between compost and fertilizer treatments were consistent throughout the incubation period. The δ¹⁵N of NO₃⁻ increased with the addition of compost (range: +13.0 to +35.4‰), but decreased with the addition of fertilizer (−10.8 to +11.4‰), thus resulting in intermediate values in soils receiving both fertilizer and compost (−3.5 to +20.3‰). Therefore, such differences in δ¹⁵N of NO₃⁻ observed in this study suggest a possibility that the δ¹⁵N of upland-grown plants receiving compost would be higher than those treated with fertilizer because NO₃⁻ is the most abundant N for plant uptake in upland soils.     

Microbial populations immobilizing NH4-N and NO3-N differ in their sensitivity to sodium chloride salinity in soil

An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH₄⁺- and NO₃⁻-N using glucose as an easily oxidizable C source. Immobilization of NH₄⁺-N was faster than that of NO₃⁻-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO₃⁻-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH₄⁺- and NO₃⁻-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg⁻¹ soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH₄⁺-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO₃⁻. However, remineralization of N from NO₃⁻-assimilating microbial population was less affected by NaCl salinity compared to NH₄⁺-assimilating population.     

Nitrate leaching in soil: Tracing the NO3 sources with the help of stable N and O isotopes

Legumes increase the plant-available N pool in soil, but might also increase NO₃⁻ leaching to groundwater. To minimize NO₃⁻ leaching, N-release processes and the contribution of legumes to NO₃⁻ concentrations in soil must be known. Our objectives were (1) to quantify NO₃⁻-N export to >0.3 m soil depth from three legume monocultures (Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO₃⁻ in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO₃⁻ concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO₃⁻ concentrations and δ¹⁵N and δ¹⁸O of NO₃⁻ for >11.5 mg NO₃⁻-N l⁻¹. We incubated soil samples to assess potential N release by mineralization and determined δ¹⁵N and δ¹⁸O signals of NO₃⁻ derived from mineralization of non-leguminous and leguminous organic matter.Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO₃⁻-N m⁻²; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO₃⁻-N m⁻²). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO₃⁻-N m⁻²). The averaged NO₃⁻-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture.The δ¹⁵N and δ¹⁸O values in NO₃⁻ of rainfall (δ¹⁵N: 3.3±0.8‰; δ¹⁸O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ¹⁵N and δ¹⁸O values in NO₃⁻ of soil solution during winter 2003, we calculated 18-41% to originate from rainfall, 38-57% from mineralization of non-leguminous SOM, and 18-40% from mineralization of leguminous SOM.Our results demonstrate that (1) even under legumes NO₃⁻-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO₃⁻ based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO₃⁻ sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO₃⁻-N loss.     

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing,China

Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO₂ concentrations and global warming through soil respiration. We conducted a comparative study to determine NH₄⁺ and NO₃⁻ concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO₃⁻-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH₄⁺-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg⁻¹ d⁻¹) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH₄⁺ and, NO₃⁻ concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.     

Soil C and N pools in patchy shrublands of the Negev and Chihuahuan Deserts

Patchy distribution of vegetation within semi-arid shrublands is normally mirrored in the soil beneath perennial shrubs (macrophytic patches), compared to inter-shrub areas (microphytic patches). To determine impacts of (1) litterfall inputs within vegetation patches and (2) rainfall distribution on soil C and N, we investigated soil C and N pools and associated soil properties in two semi-arid shrublands, in the Negev Desert of Israel (Lehavim), which receives >90% of annual rainfall during winter and in the Chihuahuan Desert, USA (FHMR) that experiences a bimodal (Summer-Winter) annual rainfall pattern. We also evaluated grazing effects on soil C and N pools at Lehavim. More distinct differences in soil properties existed between patch types at the Negev site, where the soils contained higher soil organic C and N, amino acids and sugars, asparaginase activity and plant-available N than those at FHMR. Soil organic C (0-5 cm) in macrophytic patches was 39 g/kg at Lehavim and 13 g/kg at FHMR, and asparaginase activity was as high as 70 μg N/g 2 h in macrophytic patches at Lehavim, two times higher than at FHMR. The soil (0-5 cm) δ¹³C was −15 to −18‰ at Lehavim and −18 to −19‰ at FHMR, with significantly lower δ¹³C in macrophytic patches at both sites. The δ¹³C suggested that considerable macrophytic patch soil C was derived from cyanobacteria at Lehavim and C4 grasses at FHMR. Plant litter δ¹⁵N was 0.9‰ at Lehavim and 0.6‰ at FHMR, suggesting that much plant N was derived from N fixation. Concentrations of inorganic soil N (NH₄⁺+NO₃⁻) were up to 37 mg N/kg at Lehavim and <9 mg N/kg at FHMR. Grazing at Lehavim resulted in lower soil CH, AA, and AS. We conclude that differences between the sites are due largely to (i) higher amounts of litterfall C and N inputs within macrophytic patches at Lehavim and (ii) the different precipitation patterns, with summer precipitation at FHMR promoting increased organic matter mineralization compared to Lehavim, which experiences Winter precipitation only. Furthermore, greater differences in soil properties between patch types at Lehavim compared to FHMR can likely be attributed to the increasing importance of physical processes of resource dispersion at the more humid site in Arizona.     

Effects of 4-amino 1,2,4-triazole, dicyandiamide and encapsulated calcium carbide on nitrification inhibition in a subtropical soil under upland and flooded conditions

 Nitrification inhibition of soil and applied fertilizer N is desirable as the accumulation of nitrates in soils in excess of plant needs leads to enhanced N losses and reduced fertilizer N-use efficiency. In a growth chamber experiment, we studied the effects of two commercial nitrification inhibitors (NIs), 4-amino 1,2,4-triazole (ATC) and dicyandiamide (DCD), and a commonly available and economical material, encapsulated calcium carbide (CaC₂) (ECC) on the nitrification of soil and applied NH₄ ⁺-N in a semiarid subtropical Tolewal sandy loam soil under upland [60% water-filled pore space (WFPS)] and flooded conditions (120% WFPS). Nitrification of the applied 100 mg NH₄ ⁺-N kg^–1 soil under upland conditions was retarded most effectively (93%) by ECC for up to 10 days of incubation, whereas for longer periods, ATC was more effective. After 20 days, only 16% of applied NH₄ ⁺-N was nitrified with ATC as compared to 37% with DCD and 98% with ECC. Under flooded soil conditions, nitrates resulting from nitrification quickly disappeared due to denitrification, resulting in a tremendous loss of fertilizer N (up to 70% of N applied without a NI). Based on four indicators of inhibitor effectiveness, namely, concentration of NH₄ ⁺-N and NO₃ ^–-N, percent nitrification inhibition, ratio of NH₄ ⁺-N/NO₃ ^–-N, and total mineral N, ECC showed the highest relative efficiency throughout the 20-day incubation under flooded soil conditions. At the end of the 20-day incubation, 96%, 58% and 38% of applied NH₄ ⁺-N was still present in the soil where ECC, ATC and DCD were used, respectively. Consequently, nitrification inhibition of applied fertilizer N in both arable crops and flooded rice systems could tremendously minimize N losses and help enhance fertilizer N-use efficiency. These results suggest that for reducing the nitrification rate and resultant N losses in flooded soil systems (e.g. rice lowlands), ECC is more effective than costly commercial NIs. Received: 25 May 2000     

Soil N transformation mechanisms can effectively conserve N in soil under saturated conditions compared to unsaturated conditions in subtropical China

The connection between moisture and nitrogen (N) transformation in soils is key to understanding N losses, particularly nitrate (NO₃^?) losses, and also provides a theoretical framework for appropriate water management in agricultural systems. Thus, we designed this study to provide a process-based background for management decision. We collected soil samples from the long-term field experiment in subtropical China, which was designed to examine tobacco and rice rotations under a subtropical monsoon climate. The field experiment was established in 2008 with four treatments: (1) no fertilization as control; (2) N, phosphorus (P), and potassium (K) fertilizers applied at recommended rates; (3) N fertilizers applied at rates 50% higher than the recommended amounts and P and K fertilizers applied at recommended rates; and (4) N, P, and K fertilizers applied at recommended rates with straw incorporated (NPKS). Soil samples were collected during the unsaturated tobacco-cropping season and saturated rice-cropping season and were incubated at 60% water holding capacity and under saturated conditions, respectively. Two ¹⁵N tracing treatments (¹⁵NH₄NO₃ and NH₄¹⁵NO₃) and a numerical modeling method were used to quantify N transformations and gross N dynamics. Autotrophic nitrification was stimulated by N fertilizer both under unsaturated and saturated conditions. The rate of NO₃^? consumption (via immobilization and denitrification) increased under the NPKS treatment under saturated conditions. Secondly, the rates of processes associated with ammonium (NH₄⁺) cycling, including mineralization of organic N, NH₄⁺ immobilization, and dissimilatory NO₃^? reduction to NH₄⁺, were all increased under saturated conditions relative to unsaturated conditions, except for autotrophic nitrification. Consequently, NO₃^?-N and NH₄⁺-N concentrations were significantly lower under saturated conditions relative to unsaturated conditions, which resulted in reduced risks of N losses via runoff or leaching. Our results suggest that under saturated conditions, there is a soil N conservation mechanism which alleviates the potential risk of N losses by runoff or leaching.     

Isotopomer signatures of soil-emitted N2O under different moisture conditions—A microcosm study with arable loess soil

Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N₂O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to evaluate isotopomer analysis of N₂O (intramolecular distribution of ¹⁵N) as well as conventional nitrogen and oxygen isotope ratios (i) as a tool to identify N₂O production processes in soils and (ii) to constrain the isotopic fingerprint of soil-derived N₂O. We conducted a microcosm study with arable loess soil fertilized with 20 mg N kg⁻¹ of ¹⁵NO₃⁻-labeled or non-labeled ammonium nitrate. Soils were incubated for 16 d at varying moisture (55%, 75% and 85% water-filled pore space (WFPS)) in order to establish different levels of nitrification and denitrification. Dual isotope and isotopomer ratios of emitted N₂O were determined by mass spectrometric analysis of δ¹⁸O, average δ¹⁵N (δ¹⁵N^bulk) and ¹⁵N site preference (SP=difference in δ¹⁵N between the central and peripheral N-positions of the asymmetric N₂O molecule). Total rates and N₂O emission of denitrification and nitrification were determined by ¹⁵N analysis of headspace gases and soil extracts of the ¹⁵NO₃⁻ treatment. N₂O emission and denitrification increased with moisture whereas gross nitrification was almost constant. In the 55% WFPS treatment, more than half of the N₂O flux was derived from nitrification, whereas denitrification was the dominant N₂O source in the 75% WFPS and 85% WFPS treatments. Moisture conditions were reflected by the isotopic signatures since highly significant differences were observed for average δ¹⁵N^bulk, SP and δ¹⁸O. Experiment means of the 75% WFPS and 85% WFPS treatments gave negative δ¹⁵N^bulk (−18.0‰ and −34.8‰, respectively) and positive SP (8.6‰ and 15.3‰, respectively), which we explained by the fractionation during N₂O production and partial reduction to N₂. In the 55% WFPS treatment, mean SP was relatively low (1.9‰), which suggests that nitrification produced N₂O with low or negative SP. The observed influence of process condition on isotopomer signatures suggests that the isotopomer approach might be suitable for identifying N₂O source processes. However, more research is needed to determine the impact from process rates and microbial community structure. Isotopomer signatures were within the range reported from previous soil studies which supports the assumption that SP of soil-derived N₂O is lower than SP of tropospheric N₂O.     

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.     

Effects of repeated fertilizer and cattle slurry applications over 38 years on N dynamics in a temperate grassland soil

The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m³ ha⁻¹ yr⁻¹ over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m³ ha⁻¹ yr⁻¹, 170 kg N ha⁻¹), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a ¹⁵N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH₄⁺ and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH₄⁺ production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH₄⁺ production in the control treatment, toward a predominant NO₃⁻ production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH₄⁺ oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO₃⁻ production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO₃⁻ rather than NH₄⁺ production. The ¹⁵N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.     

Natural N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ¹⁵N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg⁻¹ soil (SC3). Plant-N uptake linearly increased with compost application (r² = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg⁻¹ of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r² = 0.998, P < 0.01) with a slope of 122 g N kg⁻¹ of compost-N. Plant-δ¹⁵N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ¹⁵N of soil inorganic-N (particularly NO₃⁻-N) increased with compost application. We found that plant-δ¹⁵N reflected the N isotope signal of soil NO₃⁻-N at each measurement during plant growth, and that δ¹⁵N of inner leaves and soil NO₃⁻-N was similar when initial NO₃⁻ in the compost was abundant. Therefore, we concluded that δ¹⁵N of whole plant (more obviously in newer plant parts) and soil NO₃⁻-N could reveal whether compost application was excessive, suggesting a possible use of δ¹⁵N in plants and soil as evidence of excess compost application.     

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.     

模拟土柱条件下黑土中肥料氮素的迁移转化特征

为明确肥料氮素在土壤中的迁移转化动态特征,利用模拟土柱方法,研究了3倍常规施肥量条件下不同肥料处理(尿素、硫铵)黑土的矿质氮变化。结果表明:不同氮肥处理的氮素养分迁移转化特征有明显差异。对照处理(不施肥)土柱内各层次间NH4+-N和NO3--N含量差异不明显;施用尿素或硫铵后,表层0~50mm土层的NH4+-N和NO3--N含量比不施肥对照分别升高100.8~3408.1mg·kg-1、113.4~388.0mg·kg-1和126.7~4671.1mg·kg-1、51.4~63.3mg·kg-1,且在培养前14d内变化最大。在整个培养期内,施用硫铵处理各层次NH4+-N平均含量比尿素处理高2.54~1423.7mg·kg-1,NO3--N平均含量低4.38~335.1mg·kg-1;而尿素处理各层次的硝化率是硫铵处理的0.79~9.12倍。表明肥料氮素的迁移与转化集中在0~50mm土层内,尿素处理的氮素转化速率较硫铵处理高。     

Methane oxidation in a temperate coniferous forest soil: effects of inorganic N

Methane oxidation rates were measured in soils obtained from a coniferous forest in northern England. The effects of depth and added K⁺ (K₂SO₄), NH₄⁺ ((NH₄)₂SO₄) and NO₃⁻ (KNO₃) on potential CH₄ oxidation were investigated in a series of laboratory incubations. The humus (H) layer soil showed much greater CH₄ oxidation rates than the other soil layers, with maximal rates of 53 and 226 ng CH₄ gdw⁻¹ h⁻¹ when incubated with initial 10 and 1000 μl CH₄ l⁻¹, respectively. Additions of the solutes K⁺, NH₄⁺ and NO₃⁻ showed differing degrees of inhibition on CH₄ oxidation, which varied with the initial CH₄ concentration, the ion added, and the ion concentration. In general, inhibition by the ions was slightly greater for incubations with an initial concentration of 1000 μl CH₄ l⁻¹ than for 10 μl CH₄ l⁻¹ under otherwise identical conditions. For K⁺ and NH₄⁺ treatments, inhibitory rates were usually less than 15%, but at high K⁺ and NH₄⁺ concentrations inhibition could reach 50%, the inhibitory effects of NH₄⁺ were consistently slightly greater than those of K⁺ at the same concentration. In marked contrast to NH₄⁺, NO₃⁻ showed a very strong inhibitory effect. Added NO₃⁻ and NO₂⁻ produced via added NO₃⁻ reduction in anaerobic ‘microsites’ are probably toxic to CH₄-oxidizing bacteria. These results, together with those from other reports, suggest that NO₃⁻ may have a greater importance in the inhibition of CH₄ oxidation in forest soils than that attributed to NH₄⁺ and needs to be investigated in a wide range of soil types from various forests.     

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.     

The potential of using 15N natural abundance in changing ammonium-N and nitrate-N pools for studying in situ soil N transformations

Purpose

This study examined the usefulness of ¹⁵N natural abundance (δ¹⁵N) with in situ core incubation to quantify the predominant N transformation processes in a natural suburban forest of subtropical Australia, which was subjected to prescribed burning.

Materials and methods

In situ core incubation for 3 days with 20 ml water, or 160.79 ml of 60 mg L^?1 NO₃^?-N surface application, and in situ core with 160.79 ml water but without incubation were set up in Toohey forest for sampling three times as before (once) and after (twice) a prescribed burning. The δ¹⁵N of NH₄⁺-N and NO₃^?-N in the top 5 cm soil before and after the incubation, and δ¹⁵N of NO₃^?-N in the 5–10 cm soil before incubation were compared with each other to examine the soil N mineralisation, nitrification, denitrification, and nitrate leaching processes.

Results and discussion

The significant decrease in δ¹⁵N of NH₄⁺-N after incubation under 20 ml water treatment was ascribed to soil N mineralisation, and the significant decrease in δ¹⁵N of NH₄⁺-N and significant increase in δ¹⁵N of NO₃^?-N after incubation with elevated water and nitrate inputs were associated with N mineralisation and nitrification, respectively, 2 months after the burning. The 160.79 ml water treatment also triggered nitrification in the baseline soil cores in both samplings after the burning. Water was crucial to stimulate soil N mineralisation and nitrification, but excessive water depleted labile N pools and reduced N mineralisation and nitrification. Burning effects were hard to separate from the seasonal impacts on soil N cycling processes.

Conclusions

The δ¹⁵N in soil mineral N pools was sensitive to indicate soil N mineralisation and nitrification processes. Soil water and labile N were determining factors for N transformations in the soil. It is suggested that δ¹⁵N combined with soil inorganic N concentrations and net N transformation rates could be used to identify primary N transformation processes. More frequent samplings would be needed to differentiate burning impacts from the seasonal impacts on soil N cycling processes.

     

Impact of nitrogen and phosphorus additions on soil gross nitrogen transformations in a temperate desert steppe

Nutrient addition has a significant impact on plant growth and nutrient cycling. Yet, the understanding of how the addition of nitrogen (N) or phosphorus (P) significantly affects soil gross N transformations and N availability in temperate desert steppes is still limited. Therefore, a ¹⁵N tracing experiment was conducted to study these processes and their underlying mechanism in a desert steppe soil that had been supplemented with N and P for 4 years in northwestern China. Soil N mineralization was increased significantly by P addition, and N and P additions significantly promoted soil autotrophic nitrification, rather than NH₄⁺-N immobilization. The addition of N promoted dissimilatory NO₃⁻ reduction to NH₄⁺, while that of P inhibited it. Soil NO₃⁻-N production was greatly increased by N added alone and by that of N and P combined, while net NH₄⁺-N production was decreased by these treatments. Soil N mineralization was primarily mediated by pH, P content or organic carbon, while soil NH₄⁺-N content regulated autotrophic nitrification mainly, and this process was mainly controlled by ammonia-oxidizing bacteria rather than archaea and comammox. NH₄⁺-N immobilization was mainly affected by functional microorganisms, the abundance of narG gene and comammox Ntsp-amoA. In conclusion, gross N transformations in the temperate desert steppe largely depended on soil inorganic N, P contents and related functional microorganisms. Soil acidification plays a more key role in N mineralization than other environmental factors or functional microorganisms.     

Controls on nitrification in a water-limited ecosystem: experimental inhibition of ammonia-oxidising bacteria in the Patagonian steppe

We studied controls on nitrification in an undisturbed water-limited ecosystem by inhibiting autotrophic nitrifying bacteria in soils with varying levels of vegetative cover. The activity of nitrifying bacteria was disrupted using nitrapyrin, 2-chloro-6-(trichloromethyl)-pyridine, under field conditions in three microenvironments (underneath shrubs, next to grasses and in bare soil). Ammonia-oxidising bacteria were detected by PCR analysis of DNA in soils. The inhibition of nitrification changed the concentrations of NO₃⁻ and NH₄⁺ in the soil, while the microenvironment was most important in determining the response of bacteria to the inhibitor. Nitrapyrin application resulted in a significant (p<0.05) reduction in soil NO₃⁻ concentration (39%) and a significant increase (p<0.001) in soil NH₄⁺ concentration (41%). Untreated bare-soil microenvironments had the lowest concentrations of NH₄⁺ (1.57 μg/g of dry soil) and NO₃⁻ (0.49 μg/g of dry soil) when compared to the other microenvironments, and showed the highest impacts of nitrification inhibition. For example, NH₄⁺ concentrations increased 288% and NO₃⁻ concentrations decreased 60% in inhibited bare-soil microenvironments. In contrast, untreated microenvironments underneath shrubs had the highest levels of NH₄⁺ (10.01 μg/g of dry soil) and NO₃⁻ (0.69 μg/g of dry soil), but showed no significant effects of inhibition of nitrification on soil nitrogen concentrations.     

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.