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1.
In many forested wetlands of Louisiana, surface water quality is being deteriorated by nutrient input from adjacent agricultural production areas. This field study assesses the input of fertilizer N, applied to sugarcane fields, to forested wetlands. The potential use of natural abundance variations in 15N14N ratios for identification and tracing surface water N sources (NH 4 + - and NO3 --N) was evaluated. Runoff and surface water samples were collected from sugarcane fields and bordering forested wetlands (6 stations) over a 16 month period and analyzed for NH 4 + -N, NO 3 - -N, and associated NH 4 + -δ15N and NO 3 - -δ15N ratios. Fertilizer N draining into adjacent forested wetland was estimated to be only a small fraction of the amount applied. Concentrations of NH 4 + - and NO 3 - -N in the collected water samples were low and ranged from 0.02 to 1.79 mg L-1. Isotopic analysis revealed NH 4 + -δ15N and NO 3 - -δ15N means were distinctive and may have the potential to be used as tracers of N contamination. The mean NH 4 + -δ15N value was +18.6 ± 7.1‰ and the NO 3 - -δ15N mean was +8.3 ± 3.1‰. Anomalously high NO 3 - -δ15N values (>30‰) were attributed to denitrification.  相似文献   

2.
We investigated the expansion of NO3 ?-contaminated groundwater in the Sichuan Basin, China. Nitrogen concentrations and isotopic ratios of NH4 + and NO3 ? were analyzed in groundwater and rain collected from four areas in this basin in order to evaluate the sources of nitrogen pollution. NH4 + in rain, for which δ15N values are strongly negative to slightly positive ?13.4 to + 2.3‰, appears to originate from fertilizers and excretory waste. NO3 ? in rain (δ15N: ?10.2 to ?4.4‰) was attributed to NO x from automobile exhaust gas. In the studied area, well water sampled from farmyards was found to have the highest δ15NNO3 (average: +9.7 ± 4.7‰), indicating contamination by domestic sewage as animal excrement. The lowest δ15NNO3 (?0.2 ± 3.7‰), found in spring water, indicates that the studied groundwater samples are widely affected by air contaminants (mainly as nitrogen oxides) resulted from fuel combustions. The δ15NNO3 (+3.7 ± 2.1‰) values of well water from farmland are between these levels, suggesting that NO3 ? contamination results primarily from cultivation using nitrogen fertilizers, although the contribution from animal excrement cannot be excluded. These results demonstrate that the studied groundwater is widely polluted by locally derived nitrogen sources.  相似文献   

3.
Temporal variations in δ15N of NH4+ and NO3− in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ15N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg−1. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH4+-N concentration decreased from 164.8 to 34.4 mg kg−1, and the δ15N of NH4+ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO3−-N concentration and lower δ15N of NO3− (product) than that of NH4+ (substrate) at each sampling time. In saturated soil, the concentration of NH4+-N decreased gradually from 162.4 to 24.2 mg kg−1, and the δ15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO3− concentration was not observed due to loss of NO3− through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (αs/p) associated with decrease in NH4+ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH4+ were relatively more dominant than nitrification under the anaerobic conditions.  相似文献   

4.
Previous studies in lowland tropical rainforests of French Guiana showed that, among non-N2-fixing trees, two groups of late successional species contrasting in their leaf 15N natural abundance coexist, suggesting two different main ways of nitrogen acquisition. Two abundant late-successional species typically co-occurring in rainforests in French Guiana, namely Eperua falcata and Dicorynia guianensis, were chosen as representative of each group. Stable isotope techniques and measurements of potentials of microbial N transformation were performed to assess to what extent leaf 15N natural abundance of these species could be related to (i) δ15N signatures of soil mineral N sources and (ii) the capacity of soil to express nitrification and denitrification (both processes being directly involved in the balance between NH4+ and NO3–). Soil δ15N-NH4+ was roughly similar to leaf δ15N of D. guianensis (around 3.5‰), suggesting a preferential use of NH4+, whereas in E. falcata, leaf δ15N values were closer to root δ15N-NO3? values (0.2 and ?2.0‰, respectively), suggesting a preferential use of NO3?. These differences in N source utilization were not accompanied by differences in availability in soil NO3? or in intensity of microbial functions responsible for soil N mineral evolution. However, (i) under both tree species, these functions showed clear spatial partitioning, with denitrification occurring potentially in soil and nitrification in the litter layer, and (ii) E. falcata fine roots colonized the litter layer much more strongly than D. guianensis fine roots. This strongly suggests that (i) the contrasted leaf δ15N values found in the two late-successional species reveal distinct N acquisition strategies and (ii) the ability of roots to predominantly exploit the litter layer (E. falcata) or the soil (D. guianensis) may constitute an important explanation of the observed differences. A complementarity between tree species, based on mineral N resource partitioning (itself resulting from a spatially structured location of the microbial functions responsible for the balance between NH4+ and NO3–), can thus be supposed.  相似文献   

5.
In summer 1994, stream water, moss and humus samples were collected for sulphur isotopic analysis from eight catchments located in the western Kola Peninsula region, where several industrial centres emit high loads of SO2 and other elements to the atmosphere. Three potential sources of sulphur and their isotopic signatures were identified: (1) marine (δ 34S+20 to +21‰ CDT), (2) anthropogenic emissions (<+10‰), and (3) geogenic (variableδ 34S, mostly <+10‰). Averaged per catchment, the sulphur isotopic composition varies between +6.0 and +16.3‰ for stream water sulphate, +6.0 and +8.4‰ for moss sulphur, and +5.2 and +12.2‰ for humus sulphur. Theδ 34S composition of stream water from the more remote catchments is quite variable, reflecting several natural (geogenic) sources, but it becomes restricted to the range +8 to +10‰ near the pollution sources. A plot ofδ 34S vs. 1:SO4 in stream water suggests that sulphate originating from the smelters has aδ 34S value ≈+9.5‰, and is a dominant source. Sulphur isotope values for moss and humus are consistent with the deduced composition for the emitted sulphur, though for humus a component of geogenic sulphur incorporated via vegetation uptake may play a role. Further isotopic characterisation of atmospheric emissions, together with environmental samples, is needed to better understand sulphur sources and sinks in the area.  相似文献   

6.
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.  相似文献   

7.
府河-白洋淀硝酸盐来源判定及迁移转化规律   总被引：7，自引：3，他引：4  
近年来白洋淀流域内经济高速发展、人口增加,生活污水排放量增大,严重威胁府河和白洋淀水质,其中硝酸盐浓度过高引发的水体富营养化是河流系统面临的重要难题。以白洋淀和唯一一条常年有水的入淀河流——府河为研究对象,结合水化学、水中氢氧同位素(δ~2H、δ~(18)O)和硝酸盐氮同位素(δ~(15)N)的方法 ,分析2008—2016年水化学特征和水化学类型变化,明确府河-白洋淀淀区硝酸盐污染来源以及沿程迁移转化规律,为其水质富营养化管理提供参考。研究结果表明:府河2008年硝酸盐δ~(15)N值10‰,2014年硝酸盐δ~(15)N值的变化范围是2.07‰~18.49‰,府河硝酸盐主要来自于保定市和沿府河村落的生活污水;但2009年硝酸盐δ~(15)N值的变化范围是-3.7‰~4‰,府河硝酸盐主要来源于工业废水。白洋淀淀区2008年和2014年硝酸盐δ~(15)N值的变化范围分别是5.8‰~11.7‰和3.31‰~12.53‰,2009年δ~(15)N值的变化范围是-3.8‰~0.7‰,说明府河的生活污水和工业废水是白洋淀淀区硝酸盐的主要来源。2008—2014年Cl~-和SO_4~(2-)浓度比例逐渐减小,工业废水和生活污水的排入受到控制;2009年因工业废水的排放NO3-浓度超过50 mg·L~(-1),2014年和2016年NO_3~-浓度未超标;控制硝酸盐浓度变化的主要因素是降水稀释、外源输入及反硝化脱氮作用,当溶解氧(DO)小于2 mg·L~(-1)时,硝酸盐的减少主要受反硝化作用影响。  相似文献   

8.
Abstract

Very low recovery of NH4+‐N was observed in total N determination of (NH4)2SO4 in KC1 solutions by a semimicro Kjeldahl method using permanganate and reduced iron to recover NO3‐ and NO2‐, whereas complete recovery was obtained in analysis of NH4+‐N in water, and of NO3 ?‐N or NO2 ?‐N in either water or KC1 solutions. The loss of NH4 +‐N observed with KC1 was attributed to the formation of NCl3 upon reaction of NH4 + with Cl2 generated during oxidation of Cl? by MnO4 ?. This difficulty is avoided by using K2SO4 instead of KC1 for extraction of inorganic N from soil. Complete recovery was obtained by adding 15N‐labeled NH4+, NO3‐, or NO2‐ to 0.5 M K2SO4 soil extracts, and total 15N analyses of the labeled extracts were in good agreement with values calculated from the additions of 15N and the total N contents of the soil extracts.  相似文献   

9.
Displacement of NH4+ fixed in clay minerals by fertilizer 15NH4+ is seen as one mechanism of apparent added nitrogen interactions (ANI), which may cause errors in 15N tracer studies. Pot and incubation experiments were carried out for a study of displacement of fixed NH4+ by 15N‐labeled fertilizer (ammonium sulfate and urea). A typical ANI was observed when 15N‐labeled urea was applied to wheat grown on soils with different N reserves that resulted from their long‐term fertilization history: Plants took up more soil N when receiving fertilizer. Furthermore, an increased uptake of 15N‐labeled fertilizer, induced by increasing unlabeled soil nitrogen supply, was found. This ANI‐like effect was in the same order of magnitude as the observed ANI. All causes of apparent or real ANI can be excluded as explanation for this effect. Plant N uptake‐related processes beyond current concepts of ANI may be responsible. NH4+ fixation of fertilizer 15NH4+ in sterilized or non‐sterile, moist soil was immediate and strongly dependent on the rate of fertilizer added. But for the tested range of 20 to 160 mg 15NH4+‐N kg–1, the NH4+ fixation rate was low, accounting for only up to 1.3 % of fertilizer N added. For sterilized soil, no re‐mobilization of fixed 15NH4+ was observed, while in non‐sterile, biologically active soil, 50 % of the initially fixed 15NH4+ was released up to day 35. Re‐mobilization of 15NH4+ from the pool of fixed NH4+ started after complete nitrification of all extractable NH4+. Our results indicate that in most cases, experimental error from apparent ANI caused by displacement of fixed NH4+ in clay is unlikely. In addition to the low percentage of only 1.3 % of applied 15N, present in the pool of fixed NH4+ after 35 days, there were no indications for a real exchange (displacement) of fixed NH4+ by 15N.  相似文献   

10.
High rice (Oryza sativa L.) yields are closely related to plant absorption of a large amount of nitrogen (N). However, there is little information on the fate of N applied at the middle growth stages of rice. Labeled 15N ammonium sulfate was applied at the panicle formation stage in Experiment I, and 10 d after heading in Experiment II. Zeolite was also added at the concentration of 0, 0.01, and 0.1 kg kg-1 to increase the cation exchange capacity (CEC) of the soil. The amount of 15N fertilizer in the soil surface water decreased exponentially and the fertilizer disappeared within 2 d after application. The soil that received zeolite at 0.1 kg kg-1 exhibited significantly less 15NH4 +-N in the surface water and in the soil solution than the soil without the zeolite amendment. A significantly larger amount of exchangeable 15NH4 +-N was observed in the high zeolite-treatment of soil compared to the low zeolite-treatment of soil. The amount of exchangeable 15NH4 +-N increased initially, and thereafter decreased to traces 4 d after application in Experiment I, while 6 or 9 d after application in Experiment II. The disappearance of exchangeable 15NH4 +-N could be attributed mainly to the uptake by plants. The zeolite amendment or the time of N application did not significantly affect the amount of immobilized N. The rate of N adsorption was inhibited with increasing zeolite application. Moreover, zeolite application did not increase the recovery percentage of ammonium sulfate by rice plants. The total recovery of applied N ranged from 65 to 75%, irrespective of the zeolite treatments or the time of N application.  相似文献   

11.
In cultivated soils, total soil N, organic C and C-to-N ratios were in the range of 0.24–0.49%, 3.1–5.8% and 10.7–15.0, respectively in the surface horizons and decreased with depth. Native fixed NH+4-N accounted for 2.3–3.0% of total soil N in surface horizons but while the quantities of fixed NH+4-N decreased with depth, the proportion to total soil N increased. Exchangeable NH+4-N ranged from 15 to 32 and NO?3-N from 26 to 73 μg g?1 soil in surface horizons, and both decreased with depth. Exchangeable-N accounted for 1.1–2.4% of total soil N. Over 97% of total soil N was organically bound.Of the total soil N in the surface horizons, 29.0–79.0% was acid hydrolysable and 21.0–71.0% was nonhydrolysable. The range of proportions of each of hydrolysable NH+4-N, hexosamine-N, serine plus threonine α-amino acid-N, identified-N, and unidentified-N to total soil N in the surface horizons were 14.5–22.4, 4.8–9.2, 0.2–5.8, 4.0–16.7, 23.3–48.8, and 0.3–41.5%, respectively. Hydrolysable NH+4-N constituted the largest proportion of the identified-N fraction. Distribution patterns of the organic-N fractions in the profiles varied from soil to soil. Sixteen amino acids were identified which accounted for 82–100% of the α-amino acid-N fraction in the soils; glycine and alanine alone accounted for 35–40%. All the organic-N fractions were transformed to varying degree during aerobic incubation.  相似文献   

12.
The effects of different forms and concentrations of N in the rooting medium on the CO2/H2O gas exchange of leaves of the pedunculate oak (Quercus robur L.) were investigated. Two-year-old seedlings were grown in nutrient solutions containing low (1.8 mM) or high (4.8 mM) concentrations of NH4+, 3.6 mM NO3?, or both NH4+ and NO3? (1.8 mM + 1.8 mM). In various sets of plants subjected to these N treatments, the following parameters were determined: biomasses of leaves and fine roots, leaf area-related net photosynthesis at light saturation (A) and leaf conductance (g), foliar concentrations of chlorophylls, N, Ca2+, Mg2+ and K+ and the ash alkalinity of the leaves (as a measure of the carboxylate content). In all treatments, the leaves were equally well supplied with nutrients. Oaks grown in high NH4+ concentrations produced significantly smaller leaf and root biomasses. Compared to oaks cultivated with both N forms or with low NH4+ concentration, oaks grown with high NH4+ supply showed lower values of A and g, but no significant differences in ash alkalinity and leaf area-related chlorophyll concentrations. Oaks fed with NO3? as the only N form had an intermediate biomass production, but low values of A and g. The time courses of A in the different treatments closely followed the patterns of g. In all N treatments, the same linear relationship was found between A and g, indicating that, within a rather wide range, the variation in the form and amount of supplied N does not affect the instantaneous water use efficiency of young pedunculate oaks.  相似文献   

13.
Abstract

Yield response of Idaho Russet Burbank potatoes to nitrogen fertilizer was related to soil test for inorganic N (NO3 ‐ and NH4 +) in a total of 27 field experiments over a 3‐year period using polynomial correlation and regression analysis. Nitrate plus ammonium nitrogen content in the surface foot of soil was found to be useful in predicting yield response of potatoes to applied nitrogen.

Correlations between yield and extractable N were considerably better when the data from each cropping system were analyzed separately than when all locations were analyzed as one group. Additional improvement was obtained by including extractable ammonium nitrogen and nitrogen in the second foot of soil respectively. The best correlation with yield was found using (NO3 ‐ + NH4 +)‐N in the surface foot following grain and the top two feet following non‐grain crops with R2 values of 0.875 and 0.821 respectively.  相似文献   

14.
Curt  M. D.  Aguado  P.  Sánchez  G.  Bigeriego  M.  Fernández  J. 《Water, air, and soil pollution》2004,151(1-4):135-142
This work aims to identify the sources of nitrogencontamination in nitrate vulnerable zones (NVZs) of Spain by means of the nitrogen isotope method. Three categories of nitrogen sources (synthetic fertilisers, animal wastes, and sludges and effluents from waste-water treatment plants) from three NVZs were analysed for their nitrogen isotopic composition (δ15N) in order to assess the applicability of the method to the identification of these N sources. The mean δ15N values were: +1.48‰ for synthetic fertilisers, +15.98‰ for animal wastes and +11.52‰ for sludges and effluents. The synthetic fertiliser sources were significantly different from the organic sources and so, the method can be used for their identification. The highest variability was found within the animal waste category. The range of values found for the different kinds of animal wastes (+5.86 to +36.74) was very wide and overlappedthe range found for sludges and effluents from waste-water treatmentplants (+4.57 to +20.18). Accordingly, these two nitrogen sources areisotopically indistinguishable.  相似文献   

15.
坡缕石包膜对尿素氮行为的影响   总被引：2，自引：1，他引：2  
采用静态吸收和土柱淋溶试验方法,分析对比了3种不同用量坡缕石包膜尿素与普通尿素施入土壤后对尿素氮行为的影响,结果表明:在土壤中施用坡缕石包膜尿素较普通尿素减少10.38%～26.24%的氨挥发损失,减少5.88%～27.74%的氮素(NO3--N+NH4+-N)淋溶损失,20%的坡缕石包膜尿素能显著提高土柱土壤NH4+-N含量,3种坡缕石包膜尿素都能极显著提高土柱土壤NO3--N含量.坡缕石包膜后能减少尿素氨的挥发,降低NH4+-N和NO3--N的淋失,提高土壤NH4+-N和NO3--N含量,以20%的坡缕石包膜尿素的综合生态效应最好.  相似文献   

16.
Nitrate reduction to NH4+ and incorporation into organic matter was investigated in sediment-water systems composed of a 2.4-cm layer of estuarine sediment covered by 2 cm of water. Between 15 and 28% of the 15N-labelled NO3? added to the sediment or floodwater of the systems was recovered as NH4+ and organic N. The results indicated that NO3? was reduced to NH4+ by a dissimilatory mechanism. A separate experiment examined the influence of redox potential (+300, 0 and ?200 mV) on NO3? reduction in sediment suspensions maintained at pH 7.5. Conversion of NO3? to NH4+ and organic N increased markedly with decreasing redox potential. The results suggested that although the reaction accounted for 35–42% of the NO3? reduced under intensely-reduced conditions (?200 mV), the significance of the reaction in nature was dependent upon NO3? movement into zones sufficiently reduced for reduction of NO3? to NH4+ instead of denitrification. Under conditions in which NO3? moved downward through a sediment-water column into a reduced sediment zone approximately 15% of the NO3? was reduced to NH4+ and recycled in the sediment.  相似文献   

17.
Zeolitites (ZTs) are rocks containing more than 50% of zeolite minerals and are known to be a suitable material for agricultural purposes by improving soil physicochemical properties and nitrogen (N) use efficiency. However, little is known about their effects on soil microbial biomass. This study aimed to evaluate short-term effects of different chabazite-rich ZT (CHAZT) amendments on soil microbial biomass and activity. A silty-clay agricultural soil was amended in three different ways, including the addition of natural (5% and 15%) and NH_4~+-enriched (10%) CHAZT. Soil dissolved organic carbon (C), total dissolved N, NH_4~+, NO_3~-, NO_2~-, microbial biomass C and N, and ergosterol were measured periodically over 16 d in a laboratory incubation. To verify the microbial immobilization of the N derived from NH_4~+-enriched CHAZT, a high15N source was used for enriching the mineral to measure the microbial biomass δ15N signature. An increase in the ergosterol content was observed in the soil amended with 5% natural CHAZT. However, no similar result was observed in the soil amended with 15% natural CHAZT, suggesting that the fungal biomass was favored at a lower CHAZT application rate. In the soil amended with NH+ 4-enriched CHAZT, microbial biomass N was related to NO_3~-production over time and inversely related to NH_4~+, suggesting high nitrification process. Isotopic measurements on microbial biomass confirmed immediate assimilation of N derived from NH_4~+-enriched CHAZT. These results suggested that the NH_4~+-enriched CHAZT used in this study supplied an immediately available N pool to the microbial biomass.  相似文献   

18.
A pot experiment was conducted to compare the uptake and dry matter production potential of NH inf4 sup+ and NO inf3 sup- and to study the effect of Baythroid, a contact poison for several insect pests of agricultural crops, on growth and N uptake of maize (Zea mays L.). Nitrogen was applied as (15NH4)2SO4, K15NO3, or 15NH4NO3 and in one treatment Baythroid was combined with 15NH4NO3. Source of N had, in general, a nonsignificant effect on dry matter and N yield, but uptake of NO inf3 sup- was significantly higher than that of NH inf4 sup+ when both N sources were applied together. Substantial loss of N occurred from both the sources, with NH inf4 sup+ showing greater losses. Baythroid was found to have a significant positive effect on dry matter yield of both root and shoot; N yield also increased significantly. Uptake of N from both the applied and native sources increased significantly in the presence of Baythroid and a substantial added nitrogen interaction (ANI) was determined. The positive effect of Baythroid was attributed to: (1) a prolonged availability of NH inf4 sup+ due to inhibition of nitrification, (2) an increased availability of native soil N through enhanced mineralization, and (3) an enhanced root proliferation.  相似文献   

19.
A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH4+), nitrate (NO3−), labile (Nlab) and recalcitrant (Nrec) soil organic matter and adsorbed NH4+), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of Nlab, while immobilization of NH4+ predominantly entered the Nrec pool. A fast exchange between the NH4+ and the adsorbed NH4+ pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH4+. Moreover, high NH4+ immobilization rates into the Nrec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH4+ oxidation and organic N oxidation. More than 86% of the NO3− produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO3− consumption. DNRA rather than NO3− immobilization ensures that NO3− is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.  相似文献   

20.
Uptake of blue-green algal nitrogen (N) and total N uptake by lowland rice (Oryza sativa) was affected by tubificid (Oligochaeta) presence in submerged soils. Recovery of algal 15N by the first crop was 24–43% but only 4–7% for the second, and recovery was determined by both the method of algal application (surface vs buried) and the presence of tubificids. Tubificid activities reduced recovery of algal N by rice, increased its total N content and doubled losses of 15N to the atmosphere. Soil N uptake by rice was increased by tubificid presence. Soil N mineralization, measured as NH4+ production, was doubled over 7 days by their activities and algal mineralization was also apparently enhanced. The NH4+ release rate of Limnodrilus sp. was 4.11 ± 0.06 ng NH4+-N mg ash free dry wt?1 h?1. Effects of tubificids on rice nutrition are discussed.  相似文献   

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