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1.
 The influence of fertilizer N applied through nitrate and ammoniacal sources on the availability of nitrate, supply of C, and gaseous N losses via denitrification (using acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) was investigated in a growth chamber simulating upland [60% water-filled pore space (WFPS)], nearly saturated (90% WFPS), and flooded (120% WFPS) conditions. The rate of denitrification was very low in the upland soil conditions, irrespective of fertilizer N treatments. Increasing water content to nearly saturated and flooded conditions resulted in four- to sixfold higher rates of denitrification within 2 days, suggesting that the denitrifying activity commences quickly. Results of this study reveal that (1) under restricted aeration, these soils could support high rates of denitrification (∼6 mg N kg–1 day–1) for short periods when nitrate is present; (2) application of fertilizer N as nitrate enhances N losses via denitrification (∼10 mg N kg–1 day–1) – however, the supply of available C determines the intensity and duration of denitrification; (3) when fertilizer N is applied as an ammoniacal form, nitrification proceeds slowly and nitrate availability limits denitrification in flooded soil; (4) the nearly saturated soil, being partially aerobic, supported greater nitrification of applied ammoniacal fertilizer N than flooded soil resulting in higher relative rates of denitrification; and (5) under aerobic soil conditions, 26 mg mineral N kg–1 accumulated in control soil over a 16-day period, demonstrating a modest capacity of such semiarid subtropical soils, low in organic matter, to supply N to growing plants. Received: 7 June 1999  相似文献   

2.
 Short-term changes in N availability in a sandy soil in response to the dissolved organic carbon (DOC) from a poultry manure (application rate equivalent to approximately 250 kg N ha–1) were evaluated in a 44-day aerobic incubation experiment. The treatments included poultry manure alone and two treatments in which an extra source of C, of low water solubility, was added with the poultry manure in the form of a low (1.05 g kg–1) and a high (4.22 g kg–1) amount of cellulose. All treatments were fertilised with the equivalent of 60 kg N ha–1 of (15NH4)2SO4 in solution. A control treatment consisted of sieved field-moist soil plus 60 kg N ha–1 of (15NH4)2SO4 in solution. Measurements were made of N2O and CO2 emissions, inorganic N, DOC, biomass N, biomass C and labelled N contained in the inorganic N and biomass N pools. The dynamics of N turnover in this study were driven mainly by processes of mineralisation–immobilisation with little significant loss of N by volatilisation or denitrification. The DOC supplied with the poultry manure played a more important role in N2O emissions than differences in C/N ratio. Changes in DOC and cumulative CO2-C production during the first 11 days were also highly correlated (R 2=0.88–0.66, P<0.01). An initial net immobilisation of N, with significant increases in biomass C and biomass N (P<0.05) for all treatments over the control at day 11, indicated a high availability of C from the DOC fraction. The presence of additional C from the applied cellulose did not enable a massive N immobilisation. Total inorganic N and unlabelled inorganic N concentrations were highest in soils treated with poultry manure alone (P<0.05), indicating that an active gross mineralisation of the added poultry manure and a possible positive priming effect were taking place during the incubation. Received: 29 May 1998  相似文献   

3.
Nutrient concentrations in the soil and crop uptake from incorporated green manure and urea in flooded rice was studied in field experiments. Release of plant-available nitrogen (NH4 +-N) from green manure was slightly delayed compared with that from prilled urea (PU) because Sesbania rostrata L. and Aeschynomene afraspera L. released the N gradually after their decomposition, whereas N became available immediately after PU application. Exchangeable NH4 +-N concentration in soil peaked at 163 mg kg–1 in the transplanted rice (TPR) and 198 mg kg—1 in broadcast-seeded rice (BSR) at 0 and 1 week after PU application. Broadcast-seeded rice depleted NH4 +-N faster than did TPR because of the crop‘s vigorous growth in the former during the early stage. Soil solution NH4 +-N followed a similar trend to that of soil NH4 +-N. Incorporation of S. rostrata and A. afraspera increased the concentration of P, K+, Fe2+ and Mn2+ in soil solution more than did the application of PU. However, zinc concentration decreased in all treatments. Both PU and green manure increased the N status of the rice plants and enhanced the uptake of P, K, Fe, Mn and Zn by the rice crop. This suggests that application of green manures improves the uptake of these nutrients by the crop. The highest apparent N recovery was obtained with PU followed by green manure. Received: 11 November 1996  相似文献   

4.
Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N2O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N2O emission. Residues were added at the rate of 3 t C ha−1 to soil with, and without, 150 kg urea N ha−1. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO3−–N, which reached minimum values of 2.8 mg N kg−1 for sugarcane (at day 28), 10.3 mg N kg−1 for maize (day 7), and 5.9 mg N kg−1 for sorghum (day 7), compared to 22.7 mg N kg−1 for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg−1 in sugarcane, 34 mg kg−1 in maize, 29 mg kg−1 in sorghum, and 16 mg kg−1 in cotton amended soil compared to soil + N fertilizer, although soil NO3−–N increased by 7 mg kg−1 in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N2O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N2O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N2O emission was significantly and positively correlated with NO3−–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO2 in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).  相似文献   

5.
The solubility and forms of phosphorus (P) were investigated in manures from chicken and pigs, eight whole soil samples and clay-, silt-, and sand-size separates from an arable and a grassland soil. Total P (Pt) in liquid pig manure (16.2 g kg–1) and dry chicken manure (26.2 g kg–1) was distributed between residual P (39–41% Pt), H2SO4–P (17–27% Pt), labile resin- and NaHCO3–P (24–39% Pt), and NaOH-P (3–10% Pt). Most soils had larger proportions of NaOH-P and residual P, indicating reactions of manure-derived P compounds with pedogenic oxides and humic substances. Clay-size separates had the highest P-concentrations in all fractions and were particularly enriched in exchangeable and labile P forms. Solution 31P-nuclear magnetic resonance (NMR) spectra of 0.5 M NaOH extracts from manures and some soil samples showed greater signal intensities for orthophosphate and monoester P than 0.1 M NaOH extracts. This can be explained by alkaline hydrolysis phosphate diesters at higher NaOH concentrations and/or by preferential extraction of diesters at lower concentrations. The 31P-NMR spectra showed differences between the two manures and confirmed that increasing proportions of ester-P can be expected if they are spread to soils. The NaOH extracts of soil samples were characterized by large proportions of orthophosphate-P (mean 77% of assigned P compounds), which seemed to be slightly enriched in clay fractions whereas the extracts from silt contained more ester-P. Sequential extractions and 31P-NMR spectroscopy both showed that these excessively manured soils are likely to lose large amounts of P. Received: 15 July 1996  相似文献   

6.
A study was conducted to investigate the effects of cow manure and sewage sludge application on the activity and kinetics of soil l-glutaminase. Soil samples were collected from a farm experiment in which 0, 25, and 100 Mg ha−1 of either cow manure or sewage sludge had been applied annually for 4 consecutive years to a clay loam soil (Typic Haplargid). A chemical fertilizer treatment had also been applied. Results indicated that the effects of chemical fertilizer and the solid waste application on pH in the 18 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different treatments, being the greatest in soils treated with 100 Mg ha−1 cow manure, and the least in the control treatment. l-Glutaminase activity was generally greater in solid-waste applied soils and was significantly correlated (r = 0.939, P < 0.001) with organic C content of soils. The values of l-glutaminase maximum velocity (Vmax) ranged from 331 to 1,389 mg NH4 +–N kg−1 2 h−1. Values of the Michaelis constant (K m) ranged from 35.1 to 71.7 mM. Organic C content of the soils were significantly correlated with V max (r = 0.919, P < 0.001) and K m (r = 0.763, P < 0.001) values. These results demonstrate the considerable influence that solid waste application has on this enzymatic reaction involved in N mineralization in soil.  相似文献   

7.
 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  相似文献   

8.
Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mineralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusion of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial biomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg–1 soil in a pearl millet-wheat-fallow rotation to 256 mg kg–1 soil in a pearl millet-wheat-green manure rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soil enzyme activity. There was a good correlation between microbial biomass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatase activity of the soil under different crop rotations was little affected. The results indicate the green manuring improved the organic matter status of the soil and soil microbial activity vital for the nutrient turnover and long-term productivity of the soil. Received: 7 January 1996  相似文献   

9.
 Nitrogen (N) mineralization and availability from neem seed residue after oil extraction was studied in a laboratory incubation and greenhouse cropping. Several decomposition models were tested for estimating potentially mineralizable N and mineralization rates from the residue. Net N mineralization was best described by a Gompertz function and a mixed-order rate model with R 2=0.996 for each and residual mean square error (RMSE)=8.3 for the Gompertz function and 8.8 for the mixed-order rate model. A consecutive reaction model also fitted the data closely (R 2=0.983; RMSE=16.6) and is preferable to a Gompertz function or a mixed-order rate model because of its mechanistic basis. Potentially mineralizable N estimated by the decomposition models ranged from 335 to 489 mg N kg–1 representing between 32% and 43% of total N applied. Actual cumulative N mineralized in a 98-day incubation period was 339 mg N kg–1 soil. Bio-available N from neem residue and inorganic N (urea) with maize as a test crop in a greenhouse cropping gave similar biomass yield and N uptake, suggesting rapid N mineralization from neem residue to meet plant nutrition. Received: 15 July 1998  相似文献   

10.
 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   

11.
Water quantity and quality were monitored for 3 years in a 360-m-long wetland with riparian fences and plants in a pastoral dairy farming catchment. Concentrations of total nitrogen (TN), total phosphorus (TP) and Escherichia coli were 210–75,200 g N m−3, 12–58,200 g P m−3 and 2–20,000 most probable number (MPN)/100 ml, respectively. Average retentions (±standard error) for the wetland over 3 years were 5 ± 1%, 93 ± 13% and 65 ± 9% for TN, TP and E. coli, respectively. Retentions for nitrate–N, ammonium–N, filterable reactive P and particulate C were respectively −29 ± 5%, 32 ± 10%, −53 ± 24% and 96 ± 19%. Aerobic conditions within the wetland supported nitrification but not denitrification and it is likely that there was a high conversion rate from dissolved inputs of N and P in groundwater, to particulate N and P and refractory dissolved forms in the wetland. The wetland was notable for its capacity to promote the formation of particulate forms and retain them or to provide conditions suitable for retention (e.g. binding of phosphate to cations). Nitrogen retention was generally low because about 60% was in dissolved forms (DON and NOX–N) that were not readily trapped or removed. Specific yields for N, P and E. coli were c. 10–11 kg N ha−1 year−1, 0.2 kg P ha−1 year−1 and ≤109 MPN ha−1 year−1, respectively, and generally much less than ranges for typical dairy pasture catchments in New Zealand. Further mitigation of catchment runoff losses might be achieved if the upland wetland was coupled with a downslope wetland in which anoxic conditions would promote denitrification.  相似文献   

12.
Denitrification plays an important role in N-cycling. However, information on the rates of denitrification from horticultural growing media is rare in literature. In this study, the effects of pH, N, C, and moisture contents on denitrification were investigated using four moderately decomposed peat types (oligotrophic, mesotrophic, eutrophic, and transitional). Basal and potential denitrification rates (20°C, 18 h) from the unlimed peat samples varied widely from 2.0 to 21.8 and from 118.9 to 306.6 μg (N2O + N2)–N L−1 dry peat h−1, respectively, with the highest rates from the eutrophic peat and the lowest from the transitional one. Both basal and potential denitrification rates were substantially increased by 3.6–14- and 1.4–2.3-fold, respectively, when the initial pH (4.3–4.8) was raised to 5.9–6.5 units. Emissions of (N2O + N2)–N from oligotrophic, mesotrophic, and transitional peats were markedly increased by the addition of 0.15 g NO3–N L−1 dry peat but further additions had no effect. Denitrification rates were increased by increasing glucose concentration suggesting that the activity of denitrifiers in all peat types was limited by the low availability of easily decomposable C source. Increasing moisture contents of all peats from 40 to 50% water-filled pore space (WFPS) did not significantly (p > 0.05) increase (N2O + N2)–N emissions. However, a positive effect was observed when the moisture contents were increased from 60% to 70% WFPS in the eutrophic peat, from 70% to 80% in the transitional, from 80% to 90% in the oligotrophic and from 70% to 90% in the mesotrophic peat. It can be concluded that liming, N-fertilization, availability of easily decomposable C, and moist condition above 60% WFPS could encourage denitrification from peats although the rates are greatly influenced by the peat-forming environments (eutrophic > mesotrophic > oligotrophic > transitional types).  相似文献   

13.
A 12-week incubation experiment was conducted to determine the pattern and rate of N mineralized from organic materials. Treatments consisted of sheep manure (SM), cattle manure (CM), poultry manure (PM), sewage sludge (SS) at 1% (W/W) level, and unfertilized treatment with three replications. The concentrations of nitrate (NO3)- nitrogen (N) and ammonium (NH4)-N were determined in day 1 and 1, 2, 4, 8, and 12 weeks after the beginning of incubation. Results indicated that the magnitude of N mineralized during the incubation time periods was in the order of CM (134 mg kg?1) > PM (83 mg kg?1) > SS (56 mg kg?1) > SM (55 mg kg?1), and different management is required for obtaining optimum N-use efficiency. In conclusion, improving N-uptake efficiency in manure- and SS-amended soils depends on the pattern and rate of N mineralization to synchronize N released with crop N demand periods.  相似文献   

14.
 The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg–1) and lowest under natural fallow (6.3 mg kg–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg–1) and lowest in continuously cropped plots (107 mg N kg–1). After addition of all nutrients except N, a highly significant linear relationship (R 2=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R 2 increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability. Received: 9 April 1999  相似文献   

15.
 Rates of methane uptake were measured in incubation studies with intact cores from adjacent fenland peats that have been under arable management and woodland management for at least the past 30 years. On two separate occasions the woodland peat showed greater rates of uptake than the arable peat. These rates ranged from 23.1 to 223.3 μg CH4 m–2 day–1 for the woodland peat and from 29.6 to 157.6 μg CH4 m–2 day–1 for the arable peat. When the peats were artificially flooded there was a decrease in the rate of methane oxidation, but neither site showed any net efflux of methane. 15N isotopic dilution was used to characterise nitrogen cycling within the two peats. Both showed similar rates of gross nitrogen mineralisation (3.58 mg N kg–1 day–1, arable peat; 3.54 N kg–1 day–1, woodland peat) and ammonium consumption (4.19 arable peat and 4.70 mg N kg–1 day–1 woodland peat). There were significant differences in their inorganic ammonium and nitrate pool sizes, and the rate of gross nitrification was significantly higher in the woodland peat (4.90 mg N kg–1 day–1) compared to the arable peat (1.90 mg N kg–1 day–1). These results are discussed in the light of high atmospheric nitrogen deposition. Received: 1 December 1997  相似文献   

16.
 Pot experiments were carried out to evaluate the response of rice to Sesbania rostrata green manure N as compared to urea fertilizer N under flooded conditions. After growing S. rostrata for 21 days with a 15N-labelled N source, the labelled Sesbania was applied to wetland rice as a green manure and the uptake of 15N from this substrate was compared to that from labelled urea. Rice was cultivated twice in the same pots. The rice was grown for a period of 49 days in each case, separated by a period of 21 days when the soil was allowed to dry. The 15N content of the soil and shoots and roots of rice was determined and 15N balances established. The total N content of the shoots and roots of rice was determined by a non-tracer method. The percentage recovery of 15N from shoot material which was derived from urea N was more than twice that from S. rostrata. The recovery of 15N from the pots receiving both green manure and urea was low, and not significantly different from that recovered from the green manure treatment. As much as 64.5–73.5% and 40.1–41% of the 15N remained in the soil which had received green manure or urea, respectively. The overall recoveries of 15N varied between 86.5% and 94.4%. At the second harvest, the oven-dry weight of shoots was significantly (P<0.05) higher in green-manure treated pots, but the total N content did not differ significantly. Labelled N remaining in the soil after amendment with the green manure was much more available to the rice crop than that remaining after the addition of urea-N. The total recovery of labelled N (shoots plus roots) amounted to 65.5% and 74%, respectively of the residual labelled N in the two S. rostrata treatments (i.e. 19.55 mg 15N pot–1 and 39.10 mg 15N pot–1) and 23.2% and 23.2% of the residual labelled N in the two urea treatments (i.e. 19.55 mg 15N pot and 39.10 mg 15N pot–1), respectively. Received: 8 December 1997  相似文献   

17.
A field trial was conducted on an inceptisol to assess the effect of different bio-manures on sugarcane yield, cane quality, and changes in soil physico-chemical and microbial properties in plant–ratoon system. Seven treatments, viz. control, vermicompost, farmyard manure (FYM), biogas slurry, sulphitation pressmud cake (SPMC), green manuring with intercropped Sesbania, and recommended dose of NPK (150:60:60 kg ha−1), were randomized within a block and replicated three times. Improvement in bulk density and infiltration rates was recorded after the addition of various bio-manures. The highest organic C was recorded in the vermicompost (0.54%) and pressmud (0.50%) treatments. The highest increase in soil microbial biomass C (185.5%) and soil microbial biomass N (220.2%) over its initial value was recorded with the addition of FYM. Dry matter production in plant, as well as ratoon crop, was significantly higher by bio-manure application over the control. Plant N uptake was highest in the pressmud treatment (227.7 kg ha−1), whereas P and K uptake were highest (41.4 and 226.50 kg ha−1) in vermicompost treatment. The highest number of millable canes (95.6 and 101.0 thousand ha−1) in plant and ratoon crop were obtained with the addition of pressmud. The highest yield (76.7 t ha−1) was recorded in planted cane with vermicompost application, whereas ratoon yield was highest (78.16 t ha−1) with pressmud application. In both planted and ratoon crop, organic amendments produced yields statistically similar to those with recommended NPK (76.1 and 78.1 t ha−1 for plant and ratoon cane).  相似文献   

18.
 N turnover in flooded rice soils is characterized by a tight coupling between nitrification and denitrification. Nitrification is restricted to the millimetre-thin oxic surface layer while denitrification occurs in the adjacent anoxic soil. However, in planted rice soil O2 released from the rice roots may also support nitrification within the otherwise anoxic bulk soil. To locate root-associated nitrification and denitrification we constructed a new multi-channel microelectrode that measures NH4 +, NO2 –, and NO3 – at the same point. Unfertilized, unplanted rice microcosms developed an oxic-anoxic interface with nitrification taking place above and denitrification below ca. 1 mm depth. In unfertilized microcosms with rice plants, NH4 +, NO2 – and NO3 – could not be detected in the rhizosphere. Assimilation by the rice roots reduced the available N to a level where nitrification and denitrification virtually could not occur. However, a few hours after injecting (NH4)2HPO4 or urea, a high nitrification activity could be detected in the surface layer of planted microcosms and in a depth of 20–30 mm in the rooted soil. O2 concentrations of up to 150 μM were measured at the same depth, indicating O2 release from the rice roots. Nitrification occurred at a distance of 0–2 mm from the surface around individual roots, and denitrification occurred at a distance of 1.5–5.0 mm. Addition of urea to the floodwater of planted rice microcosms stimulated nitrification. Transpiration of the rice plants caused percolation of water resulting in a mass flow of NH4 + towards the roots, thus supporting nitrification. Received: 23 July 1999  相似文献   

19.
Little information is available on soil microbial and biochemical properties, important for understanding nutrient cycling and organic matter dynamics, as affected by different peanut cropping systems and how they relate to soil functioning. Thus, we studied a Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) in Georgia, which is first in peanut production in USA, after 5 and 8 years under continuous cotton (Gossypium hirsutum, L) (CtCtCt), cotton–cotton–peanut (CtCtPt), corn (Zea mays L.)–peanut–cotton (CrPtCt), peanut–peanut–cotton (PtPtCt), and continuous peanut (PtPtPt). Soil organic carbon (OC) at 0–20 cm was already higher under PtPtPt (average, 8.7 g C kg−1 soil), PtPtCt (average, 7.7 g C kg−1 soil), and CrPtCt (average, 7.8 g C kg−1 soil) compared with CtCtPt (average, 4.7 g C kg−1 soil) and CtCtCt (average, 3.3 g C kg−1 soil). Similarly, alkaline phosphatase, acid phosphatase, and phosphodiesterase as a group showed higher activities under PtPtPt, PtPtCt, and CrPtCt than under CtCtPt and CtCtCt. The activities of glycosidases (α-galactosidase, β-glucosidase, and β-glucosaminidase) as a group were more sensitive to the cropping systems than phosphastases and showed a distinctive cropping system separation as follows: PtPtPt = CrPtCt > PtPtCt > CtCtPt > CtCtCt. Similar to OC and microbial biomass C trends, distinctive differences were found in the microbial community structure of this sandy soil after 8 years between peanut-based cropping systems (CrPtCt, PtPtCt, and PtPtPt) and cotton-based cropping systems (CtCtCt and CtCtPt) as indicated by the fatty acid methyl esters profiles. Trade names and company names are included for the benefit of the reader and do not infer any endorsement or preferential treatment of the product by USDA-ARS.  相似文献   

20.
 The combined effects of soil compaction and soil waterlogging on the growth of two rice cultivars (Oryza sativa L., cultivars Kanto 168 and Koshihikari) and soil N transformations were studied in pots. Although waterlogging eliminated initial differences in mechanical resistance between compacted and loose soils, Kanto 168 and Koshihikari roots had, respectively, less biomass and a lower porosity if soil was compacted prior to waterlogging. The cause for this was probably established before waterlogging. Redox values showed that upland soils were well aerated. Loose waterlogged soils contained oxic sites, but compacted waterlogged soils did not. Potential denitrification was stimulated by waterlogging and, to a larger extent, by plant presence. Waterlogging lowered potential nitrifying capacities, by competition between plants and micro-organisms for NH4 + rather than by oxygen shortage. Compaction prior to waterlogging benefited the potential nitrifying capacity of soils with either cultivar and the potential denitrifying capacity for soils with Koshihikari. Compaction had no effect on nitrification or denitrification in upland soils. N recoveries were low, especially in pots without plants, as a result from sampling strategy and N loss. On day 42/43 after potting, total δ15N values of waterlogged pots were positive, whereas after 22 days all pots had negative total δ15N values. Final δ15N values of plant parts from waterlogged and upland soils were positive and negative, respectively. Although the δ15N values generally accorded well with the other results, they did not support higher N losses from compacted waterlogged soils than from loose waterlogged soils with plants, as suggested by potential denitrifying activities. Received: 4 February 2000  相似文献   

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