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1.
 Rapid nitrate leaching losses due to current agricultural N management practices under the humid tropical environmental conditions of the Pacific island of Guam may contaminate fresh and salt water resources. Potential environmental contamination of the Northern Guam aquifer, which is overlain by shallow limestone-derived soils, is a major public concern because the aquifer is the sole underground source of fresh water for the island. The objectives of this study were to examine the use of waste office paper as a possible management alternative for reducing nitrate leaching due to N fertilizer applications in northern Guam while also providing sufficient N for crop growth. In a laboratory study, increasing rates of waste paper application reduced NO3 -N leaching up to approximately 200 days after incorporation of N fertilizer and paper treatments. Subsequent mineralization of immobilized N from paper applications was also observed, although cumulative NO3 -N leaching at the highest rate of paper addition was lower than the control after 394 days of incubation. The effect of waste paper on N availability and NO3 -N leaching after application of N fertilizer at rates up to 500 kg N ha–1 was also evaluated in two field experiments planted with sweet corn (Zea mays var. rugosa Bonaf.) during consecutive dry and wet periods. Leaching losses of NO3 -N were higher during the wet cropping season, leading to lower crop yields and crop N uptake. Combining paper with N fertilizer reduced NO3 -N leaching losses but also decreased crop ear yields up to N fertilizer application rates of 250 kg N ha–1 during the dry cropping season and up to rates of 100 kg N ha–1 during the wet period. Although combining waste paper with N fertilizer reduced NO3 -N leaching losses, no improvements in fertilizer N recovery were observed during the field experiments. This lack of crop response may be due to the importance of early season N availability for the short-season horticultural crops grown on Guam. We suggest that the application of waste paper may be a useful management practice to reduce NO3 -N leaching losses when high soil NO3 -N levels remain after cropping due either to crop failure or to over-application of N fertilizer. Received: 11 May 1999  相似文献   

2.
 N2O emission from a wetland rice soil as affected by the application of three controlled-availability fertilizers (CAFs) and urea was investigated through a pot experiment. N2O fluxes from the N fertilized paddy soil averaged 44.8–69.3 μg N m–2 h–1 during the rice growing season, accounting for 0.28–0.51% of the applied N. The emission primarily occurred during the mid-season aeration (MSA) and the subsequent re-flooding period. Fluxes were highly correlated with the NO3 and N2O concentrations in the soil water. As there were relatively large amounts of NH4 +-N present in the soil of the CAF treatments at the beginning of MSA, leading to large amounts of NO3 -N during the MSA and the subsequent re-flooding period, the tested CAFs were not effective in reducing N2O emission from this paddy soil. The potential of applied CAFs to reduce N2O emissions from paddy soil is discussed. Received: 25 May 1999  相似文献   

3.
 In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragmites australis) are increasingly used to purify waste water. Particularly in the rhizosphere of the reed, nitrification and denitrification processes take place varying locally and temporally, and the question arises to what extent this type of waste-water treatment plant may contribute to the release of N2O. In situ N2O measurements were carried out in the two reed beds of the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste water from a cheese dairy and 70 local inhabitants (12 m3 waste water or 6 kg BOD5 or 11 kg chemical O2 demand (CODMn) day–1). During November 1995 to March 1996, the release of N2O was measured weekly at 1 m distances using eight open chambers and molecular-sieve traps to collect and absorb the emitted N2O. Simultanously, the N2O trapped in the soil, the soil temperature, as well as the concentrations of NH4 +-N, NO3 -N, NO2 -N, water-soluble C and the pH were determined at depths of 0–20, 20–40 and 40–60 cm. In the waste water from the in- and outflow the concentrations of CODMn, BOD5, NH4 +-N, NO3 -N, NO2 -N, as well as the pH, were determined weekly. Highly varying amounts of N2O were emitted at all measuring dates during the winter. Even at soil temperatures of –1.5  °C in 10 cm depth of soil or 2  °C at a depth of 50 cm, N2O was released. The highest organic matter and N transformation rates were recorded in the upper 20 cm of soil and in the region closest to the outflow of the constructed wetland. Not until a freezing period of several weeks did the N2O emissions drop drastically. During the period of decreasing temperatures less NO3 -N was formed in the soil, but the NH4 +-N concentrations increased. On average the constructed wetlands of Friedelhausen emitted about 15 mg N2O-N inhabitant equivalent–1 day–1 during the winter period. Nitrification-denitrification processes rather than heterotrophic denitrification are assumed to be responsible for the N2O production. Received: 28 October 1998  相似文献   

4.
 The objectives of this work were to evaluate the inhibitory action on nitrification of 3,4-dimethylpyrazole phosphate (DMPP) added to ammonium sulphate nitrate [(NH4)2SO4 plus NH4NO3; ASN] in a Citrus-cultivated soil, and to study its effect on N uptake. In a greenhouse experiment, 2 g N as ASN either with or without 0.015 g DMPP (1% DMPP relative to NH4 +-N) was applied 6 times at 20-day intervals to plants grown in 14-l pots filled with soil. Addition of DMPP to ASN resulted in higher levels of NH4 +-N and lower levels of NO3 -N in the soil during the whole experimental period. The NO3 -N concentration in drainage water was lower in the ASN plus DMPP (ASN+DMPP)-treated pots. Also, DMPP supplementation resulted in greater uptake of the fertilizer-N by citrus plants. In another experiment, 100 g N as ASN, either with or without 0.75 g DMPP (1% DMPP relative to NH4 +-N) was applied to 6-year-old citrus plants grown individually outdoors in containers. Concentrations of NH4 +-N and NO3 -N at different soil depths and N distribution in the soil profile after consecutive flood irrigations were monitored. In the ASN-amended soil, nitrification was faster, whereas the addition of the inhibitor led to the maintenance of relatively high levels of NH4 +-N and NO3 -N in soil for longer than when ASN was added alone. At the end of the experiment (120 days) 68.5% and 53.1% of the applied N was leached below 0.60 m in the ASN and ASN+DMPP treatments, respectively. Also, leaf N levels were higher in plants fertilized with ASN+DMPP. Collectively, these results indicate that the DMPP nitrification inhibitor improved N fertilizer efficiency and reduced NO3 leaching losses by retaining the applied N in the ammoniacal form. Received: 31 May 1999  相似文献   

5.
Nitrous oxide emission from herbicide-treated soybean   总被引:5,自引:0,他引:5  
 The emission of N2O from soybean plants treated with the herbicides dichlorophenoxyacetic acid (2,4-D) and bromoxynil was studied. The N2O flux from 2,4-D- and bromoxynil-treated soybean was 14.1 ng N2O-N g–1 fresh weight h–1 and 19.7 ng N2O-N g–1 fresh weight h–1, respectively, i.e. approximately twice that of the controls. The NO2 -N concentration in 2,4-D- and in bromoxynil-treated soybean was about 8 μg N g–1 fresh weight, i.e. fivefold the concentration found in control plants. The NO3 content in herbicide-treated soybean did not differ significantly from that of the control plants. Consequently, the accumulation of NO2 -N during the assimilation of NO3 -N was thought to cause the observed N2O release. Probably, N2O is a by-product produced during either the reaction of NO2 -N with plant metabolites or NO2 -N decomposition. Final conclusions must await further experiments. Received: 5 November 1999  相似文献   

6.
 N2O emissions were periodically measured using the static chamber method over a 1-year period in a cultivated field subjected to different agricultural practices including the type of N fertilizer (NH4NO3, (NH4)2SO4, CO(NH2)2 or KNO3 and the type of crop (rapeseed and winter wheat). N2O emissions exhibited the same seasonal pattern whatever the treatment, with emissions between 1.5 and 15 g N ha–1 day–1 during the autumn, 16–56 g N ha–1 day–1 in winter after a lengthy period of freezing, 0.5–70 g N ha–1 day–1 during the spring and lower emissions during the summer. The type of crop had little impact on the level of N2O emission. These emissions were a little higher under wheat during the autumn in relation to an higher soil NO3 content, but the level of emissions was similar over a 7-month period (2163 and 2093 g N ha–1 for rape and wheat, respectively). The form of N fertilizer affected N2O emissions during the month following fertilizer application, with higher emissions in the case of NH4NO3 and (NH4)2SO4, and a different temporal pattern of emissions after CO(NH2)2 application. The proportion of applied N lost as N2O varied from 0.42% to 0.55% with the form of N applied, suggesting that controlling this agricultural factor would not be an efficient way of limiting N2O emissions under certain climatic and pedological situations. Received: 1 December 1997  相似文献   

7.
Understanding the fate of different forms of nitrogen (N) fertilizers applied to soils is an important step in enhancing N use efficiency and minimizing N losses. The growth and N uptake of two citrus rootstocks, Swingle citrumelo (SC), and Cleopatra mandarin (CM), seedlings were evaluated in a pot experiment using a Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamments) without N application or with 400 mg N kg–1 applied as urea or controlled-release fertilizers (CRF; either as Meister, Osmocote, or Poly-S). Meister and Osmocote are polyolefin resin-coated urea with longevity of N release for 270 days (at 25°C). Poly-S is a polymer and sulfur-coated urea with release duration considerably shorter than that of either Meister or Osmocote. The concentrations of 2 M KCl extractable nitrate nitrogen (NO3 -N) and ammonium nitrogen (NH4 +-N) in the soil sampled 180 days and 300 days after planting were greater in the soil with SC than with CM rootstock seedlings. In most cases, the extractable NH4 + and NO3 concentrations were greater for the Osmocote treatment compared to the other N sources. For the SC rootstock seedlings, dry weight was greater with Meister or Poly-S compared with either Osmocote or urea. At the end of the experiment, ranking of the various N sources, with respect to total N uptake by the seedlings, was: Meister = Osmocote > Poly-S > Urea > no N for CM rootstock, and Meister = Poly-S = Osmocote > Urea > no N for SC rootstock. The study demonstrated that for a given rate of N application the total N uptake by seedlings was greater for the CRF compared to urea treatment. This suggests that various N losses were lower from the CRF source as compared to those from soluble fertilizers. Received: 11 April 1997  相似文献   

8.
 The short-term (24 h) and medium-term (30 day) influence of N salts (NH4Cl, NaNO3 and NaNO2) and a non-N salt (NaCl) on first-order rate constants, k (h–1) and thresholds (CTh) for atmospheric CH4 oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g–1 dry weight (dw) soil. Additions of NH4Cl, NaNO3 and NaCl to 0.5 μmol g–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH4Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH4 +-N (≤1 μmol g–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH4 oxidation by exogenous NH4 +-N or NH4 +-N released through cation exchange. The decrease in k was consistently less for NaNO3 than for NH4Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl counter-anion. Thresholds for CH4 oxidation were less sensitive to salt addition than k for these three salts, as significant increases in CTh relative to controls were only observed at concentrations ≥1.0 μmol g–1 dw soil. Both soils were more sensitive to NaNO2 than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO2 g–1 dw soil that was clearly attributable to NO2 . Soils showed no recovery from NaCl, NH4 +-N or NaNO3 addition with respect to atmospheric CH4 oxidation after 30 days. However, soils amended with NaNO2 to 1.0 μmol NaNO2 g–1 dw showed values of k that were not significantly different from controls. Recovery of CH4-oxidizing ability was due to complete oxidation of NO2 -N to NO3 -N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH4 oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH4 budget. Received: 30 April 1999  相似文献   

9.
 Nitrous oxide (N2O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha–1. During the maize season (24 August–26 October), N2O emissions ranged between –0.94 and 1.53 g N ha–1 h–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha–1 h–1. N2O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha–1 h–1. N2O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha–1 h–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha–1 h–1. N2O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha–1 h–1. Total N2O emissions were 0.16 and 0.49 kg N ha–1, whereas the total N2O sink activity was 0.04 and 0.06 kg N ha–1 during the maize and wheat seasons, respectively. N2O emissions under maize were significantly correlated with denitrification rate and soil NO3 -N but not with soil NH4 +-N or soil temperature. Under wheat, however, N2O emissions showed a strong correlation with soil NH4 +-N, soil NO3 -N and soil temperature but not with the denitrification rate. Under either crop, N2O emissions did not show a significant relationship with water-filled pore space or soil respiration. Received: 11 June 1997  相似文献   

10.
 Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(15NO3)2 was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot–1, 200 mg N pot–1 or 300 mg N pot–1 were applied at each N application date. At crop maturity, 15N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total 15N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (Nmic) was determined for cropped plots at GS 31. Although Nmic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in Nmic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low. Received: 6 July 1999  相似文献   

11.
 Nitrogen excretion rates of 15N-labeled earthworms and contributions of 15N excretion products to organic (dissolved organic N) and inorganic (NH4-N, NO3-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with 15N by providing earthworms with 15N-labeled organic substrates for 5–6 weeks. The quantity of 15N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g–1 earthworm fresh weight day–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH4-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted 15N was found in the 15N-mineral N (NH4-N+NO3-N) pool, and 13–23% was in the 15N-DON pool. Other fates of excreted 15N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999  相似文献   

12.
Nitrogen dynamics in different types of pasture in the Austrian Alps   总被引:7,自引:0,他引:7  
 Soil N dynamics were compared in Alpine pastures on two mountains. N-pool sizes and N fluxes were measured relative to N losses via leaching and denitrification in summer. On each mountain, four types of pasture were studied: (1) forest pastures, (2) recently developed pastures formed by forest clearance ("new pastures"), (3) older established pastures, and (4) pastures planted with clover. At both study sites (Scheuchegg and Teufelstein) we obtained similar results. Compared with forest pasture soils, open pasture soils were found to have greater microbial biomass and faster mineralisation potentials, but net field mineralisation rates were slower. In the forest pastures, highest N losses via denitrification were found. Higher potential leaching of NO3 , estimated by accumulation of NO3 on ion-exchange resins, in the forest pasture soils suggests lower N uptake by microbes and herbaceous plants compared with open pastures. N2O-production rates of the forest pasture soils at the Scheuchegg site (11.54 μg N2O-N m–2 h–1) were of similar magnitude to those reported for spruce forests without pastures, but at Teufelstein (53.75 μg N2O-N m–2 h–1) they were higher. However, if forest pastures are not overgrazed, no elevated N loss through N2O production and leaching of NO3 is expected. Denitrification rates in the open pastures (0.83–7.50 μg N2O-N m–2 h–1) were low compared with reports on lowland pastures. In soils of the new pastures, rates of microbial N processes were similar to those in the established pastures, indicating a high capacity of soils to restore their internal N cycle after forest clearance. Received: 19 August 1999  相似文献   

13.
 This study was conducted to determine effects of long-term winter cover cropping with hairy vetch, cereal rye and annual ryegrass on soil N availability and corn productivity. From 1987 to 1995, with the exception of the first year of the study, the cover crops were seeded each year in late September or early October after the corn harvest and incorporated into the soil in late April or early May. Corn was seeded 10 days to 2 weeks after the cover crop residues had been incorporated, and N fertilizer was applied as a side-dressing at rates of 0, 67, 134, or 201 kg N ha–1 each year. While the average annual total N input from the above-ground biomass of the cover crops was highest for hairy vetch (72.4 kg N ha–1), the average annual total C input was highest for cereal rye (1043 kg C ha–1) compared with the other cover crops. Hairy vetch was the only cover crop that significantly increased pre-side-dressed NO3 -N (Ni) corn biomass and N uptake at 0 N. At an N fertilizer rate of 134 kg N ha–1 or higher, the cover crops had a minimal effect on corn biomass. This indicated that even after 9 years of winter cover cropping, the effect of the cover crops on corn growth resulted primarily from their influence on soil N availability. The amount of available N estimated from the cover crops (Nac) was significantly correlated with relative corn biomass production (r 2=0.707, P<0.001). The total amount of available N, comprising Nac and N added from fertilizer (Nf), was strongly correlated (r 2=0.820, P<0.001)) with relative corn biomass production. The correlation was also high for the available N comprising Ni and Nf (r 2=0.775, P<0.001). Although cereal rye and annual ryegrass did not improve corn biomass production in the short term, they benefited soil organic N accumulation and gradually improved corn biomass production compared with the control over the long term. Received: 10 August 1999  相似文献   

14.
 The high input of nutrients through the use of fertilizers, manure and animal feed make it possible to reach high levels of agricultural production. However, high nutrient inputs may also result in large nutrient losses and thus have adverse effects on groundwater, surface water, and the atmosphere. To minimize nutrient emissions from agriculture, the Dutch government has introduced regulations on nutrient use. These include: (1) a ban on spreading animal manure on agricultural land during the winter, (2) the obligation to cover storage facilities for animal manure, (3) compulsory low-emission applications of animal manure to land, and (4) applying levies when the maximum permissible annual N and P surpluses for farms are exceeded. The nutrient surplus is the difference between nutrient input into the farm and nutrient output from the farm. The maximum permissible N surpluses for 2000 are 250 kg N ha–1 year–1 and 125 kg N ha–1 year–1 for grassland and arable land, respectively, and for P, 35 kg P2O5 ha–1 year–1 for both grassland and arable land. When the annual permissible levels are exceeded, farmers are charged with a levy. Results obtained at the experimental dairy farm "De Marke" showed that a reduction in nutrient inputs via fertilizers and purchased food, in combination with restricted grazing, reduced the N surplus in such a way that the NO3 concentration in the groundwater decreased to about the maximum permissible level of 50 mg NO3 l–1. Since these results were obtained on a sandy soil that is very sensitive to NO3 leaching, it is suggested that all dairy farmers should be able to sufficiently reduce NO3 leaching by improving their farm management. Received: 13 July 1999  相似文献   

15.
 The effects of salt type and its concentration on nitrification, N mineralization and N2O emission were examined under two levels of moisture content in Yellow soil and Andosol samples as simulated to agriculture under arid/semi-arid conditions and under heavy application of fertilizer in a glass-house, respectively. The salt mixtures were composed of chlorides (NaCl and NH4Cl) or sulphates [Na2SO4 and (NH4)2SO4] and were added at various concentrations (0, 0.1, 0.2, 0.4 and 0.6 M as in the soil solution). These salts were added to non-saline Yellow soil at different moisture contents (45 or 40 and 65% of maximum water-holding capacity; WHC) and their effects on the changes in mineral N (NH4 +-N and NO3 -N) concentration as well as N2O emission were examined periodically during laboratory incubation. We also measured urease activities to know the effect of salts on N mineralization. Furthermore, Ca(NO3)2 solution was added at various concentrations (0, 0.1, 0.3, 0.5 and 0.8 M as in the soil solution) to a non-saline Andosol taken from the subsurface layer in a glass-house and incubated at different moisture contents (50% and 70% of WHC) to examine their effects on changes in mineral N. Nitrification was inhibited by high, but remained unaffected by low, salt concentrations. These phenomena were shown in both the model experiments. It was considered that the salinity level for inhibition of nitrification was an electric conductivity (1 : 5) of 1 dS m–1. This level was independent of the type of salts or soil, and was not affected by soil moisture content. The critical level of salts for urease activities was about 2 dS m–1. The emission rate of N2O was maximum at the beginning of the incubation period and stabilized at a low level after an initial peak. There was no significant difference in N2O emission among the treatments at different salt concentrations, while higher moisture level enhanced N2O emission remarkably. Received: 29 July 1998  相似文献   

16.
 High molecular weight, anionic polyacrylamide (PAM) is currently being used as an irrigation water additive to significantly reduce soil erosion associated with furrow irrigation. PAM contains amide-N, and PAM application to soils has been correlated with increased activity of soil enzymes, such as urease and amidase, involved in N cycling. Therefore we investigated potential impacts of PAM treatment on the rate at which fertilizer N is transformed into NH4 + and NO3 in soil. PAM-treated and untreated soil microcosms were amended with a variety of fertilizers, ranging from common rapid-release forms, such as ammonium sulfate [(NH4)2SO4] and urea, to a variety of slow-release formulations, including polymerized urea and polymer-encapsulated urea. Ammonium sulfate was also tested together with the nitrification inhibitor dicyandiamide (DCD). The fertilizers were applied at a concentration of 1.0 mg g–1, which is comparable to 100 lb acre–l, or 112 kg ha–1. Potassium chloride-extractable NH4 +-N and NO3 -N were quantified periodically during 2–4 week incubations. PAM treatment had no significant effect on NH4 + release rates for any of the fertilizers tested and did not alter the efficacy of DCD as a nitrification inhibitor. However, the nitrification rate of urea and encapsulated urea-derived NH4 +-N was slightly accelerated in the PAM-treated soil. Received: 16 January 1998  相似文献   

17.
 We hypothesized that the integration of trees and shrubs in agricultural landscapes can reduce NO3 leaching and increase utilization of subsoil N. A field survey was conducted on 14 farms on acid soils in the subhumid highlands of Kenya, where there is little use of fertilizers, to determine the effect of vegetation types (VT) on soil NH4 + and NO3 to 4 m depth. The VT included maize (Zea mays) with poor growth and good growth, Markhamia lutea trees scattered in maize, natural weed fallow, banana (Musa spp.), hedgerow, and eucalyptus woodlot. The effect of VT on NH4 + was small (<1 mg N kg–1). NO3 within a VT was about constant with depth below 0.25 m, but subsoil NO3 varied greatly among VT. Mean NO3 -N concentrations at 0.5–4 m depth were low beneath hedgerow and woodlot (<0.2 mg kg–1), intermediate beneath weed fallow (0.2–0.7 mg kg–1), banana (0.5–1.0 mg kg–1) and markhamia (0.5–1.6 mg kg–1), and high beneath both poor (1.0–2.1 mg kg–1) and good (1.9–3.1 mg kg–1) maize. Subsoil NO3 (0.5–4 m) was agronomically significant after maize harvest with 37 kg N ha–1 m–1 depth of subsoil beneath good maize and 27 kg N ha–1 m–1 depth beneath poor maize. In contrast, subsoil NO3 was only 2 kg N ha–1 m–1 depth beneath woodlot and hedgerow. These results demonstrate that the integration of perennial vegetation and the rotation of annual and perennial crops can tighten N cycling in agricultural landscapes. Received: 8 July 1999  相似文献   

18.
 When comparing nitrite (NO2 ) and nitrate (NO3 ) toxicity to maize (Zea mays L.) growth, it is important to know the fate of applied nitrogen (N). A pot experiment, using potassium nitrite (K15NO2) and potassium nitrate (K15NO3) was conducted to determine the fate of N (0, 75, 150, and 225 mg N kg–1 soil) applied to a sandy loam soil collected from Gistel (Belgium). The total dry weight of the plants treated with NO2 was lower than that of the plants treated with NO3 at 15 and 26 days after N application (harvest 1 and harvest 2, respectively). Shoot and root biomass reduction started at a relatively low NO2 application rate (75 mg NO2 -N kg–1). Biomass reduction increased, at both harvests with increasing amounts of NO2 to more than 55% at the highest application rate (225 mg NO3 -N kg–1). In the NO3 treatment, a reduction of 16% in total plant dry biomass was recorded only at the highest application rate (225 mg NO2 -N kg–1), at both harvest times. The 15N plant uptake (shoots plus roots) at harvest 1 decreased with increasing N application rates of both N forms (KNO2 and KNO3). Twenty-six days after the N application, the total 15N taken up by the plant increased in all treatments in comparison with 15 days after the N application. However, only at higher rates of N application (150 and 225 mg N kg–1) was the 15N uptake by the NO2 fed plants significantly lower than by the NO3 fed plants. The percentage of immobilized N from the applied N was low (0–17.7%) at both harvests, irrespective of the N source. However, with relatively low N application rates (75 mg N kg–1), the immobilized N in the soil decreased with time. This may be due to the re-mineralization of the applied N. The percentage of inorganic 15N in the soil in NO2 treatments was slightly lower than in equivalent doses of NO3 . This might be due to higher losses of N as N-oxides. Unaccounted for N from the applied N ranged from 21% to 52% for the NO2 treatments and from 3% to 38% for the NO3 treatments. Received: 17 July 1997  相似文献   

19.
 A low efficiency of use of N fertilisers has been observed in mid-Wales on permanent pasture grazed intensively by cattle. Earlier laboratories studies have suggested that heterogeneity in redox conditions at shallow soil depths may allow nitrification and denitrification to occur concurrently resulting in gaseous losses of N from both NH4 + and NO3 . The objective of the investigation was to test the hypothesis that both nitrification and denitrification can occur simultaneously under simulated field capacity conditions (∼5 kPa matric potential). Intact soil cores were taken from grassland subjected to both grazing and amenity use. The fate of applied NH4 + was examined during incubation. 15N was used as a tracer. Nitrapyrin was used as a nitrification inhibitor and acetylene was used to block N2O reductase. More than 50% of N applied as NH4 + disappeared over a period of 42 days from the soil mineral-N pool. Some of this N was evolved as N2O. Accumulation of NO3 –N in the surface 0–2.5 cm indicated active nitrification. Addition of nitrapyrin increased N recovery by 26% and inhibited both the accumulation of NO3–N and emission of N2O. When intact field cores were incubated after addition of 15N-urea, all of the N2O evolved was derived from added urea-N. It was concluded that nitrification and denitrification do occur simultaneously in the top 7.5 cm or so, of the silty clay loam grassland topsoils of mid-Wales at moisture contents typical of field capacity. The quantitative importance of these concurrent processes to N loss from grassland systems has not yet been assessed. Received: 15 December 1998  相似文献   

20.
 Microcosms were used to determine the influence of N additions on active bacterial and active fungal biomass, cellulose degradation and lignin degradation at 5, 10 and 15 weeks in soils from blackwater and redwater wetlands in the northern Florida panhandle. Blackwater streams contain a high dissolved organic C concentration which imparts a dark color to the water and contain low concentrations of nutrients. Redwater streams contain high concentrations of suspended clays and inorganic nutrients, such as N and P, compared to blackwater streams. Active bacterial and fungal biomass was determined by direct microscopy; cellulose and lignin degradation were measured radiometrically. The experimental design was a randomized block. Treatments were: soil type (blackwater or redwater forested wetlands) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha–1 as NH4NO3). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, the active bacterial biomass in redwater soils was lower than in blackwater soils; the active bacterial biomass in blackwater soils was lower when 400 kg N ha–1, but not when 200 kg N ha–1, was added. The active fungal biomass in blackwater soils was higher when 400 kg N ha–1, but not when 200 kg N ha–1, was added. The active fungal biomass in redwater wetland soils was lower when 200 kg N ha–1, but not when 400 kg N ha–1, was added. Cellulose and lignin degradation was higher in redwater than in blackwater soils. After 10 and 15 weeks of incubation, the addition of 200 or 400 kg N as NH4NO3 ha–1 decreased cellulose and lignin degradation in both wetland soils to similar levels. This study indicated that the addition of N may slow organic matter degradation and nutrient mineralization, thereby creating deficiencies of other plant-essential nutrients in wetland forest soils. Received: 7 April 1999  相似文献   

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